SoK: Secure Messaging1
Nik Unger∗ , Sergej Dechand†
Joseph Bonneau‡§ , Sascha Fahl¶ , Henning Perl¶
Ian Goldberg∗ , Matthew Smith†∗ University of Waterloo, † University of Bonn, ‡ Stanford University, § Electronic Frontier Foundation, ¶ Fraunhofer FKIE

insecure ways. However, as will become clear over the courseof this paper, the academic research community is also failingto learn some lessons from tools in the wild.
Furthermore, there is a lack of coherent vision for the futureof secure messaging. Most solutions focus on specific issues
and have different goals and threat models. This is compounded by differing security vocabularies and the absence ofa unified evaluation of prior work. Outside of academia, many
products mislead users by advertising with grandiose claimsof “military grade encryption” or by promising impossible
features such as self-destructing messages [7]–[10]. The recent
EFF Secure Messaging Scorecard evaluated tools for basic
indicators of security and project health [11] and found many
purportedly “secure” tools do not even attempt end-to-end
encryption.
We are motivated to systematize knowledge on secure
messaging due to the lack of a clear winner in the race forwidespread deployment and the persistence of many lingering
unsolved research problems. Our primary goal is to identify where problems lie and create a guide for the researchcommunity to help move forward on this important topic. A
further goal in this work is to establish evaluation criteria formeasuring security features of messaging systems, as well as
their usability and adoption implications. We aim to provide
a broad perspective on secure messaging and its challenges,
as well as a comparative evaluation of existing approaches,in order to provide context that informs future efforts. Our
primary contributions are: (1) establishing a set of common
security and privacy feature definitions for secure messaging;(2) systematization of secure messaging approaches based bothon academic work and “in-the-wild” projects; (3) comparativeevaluation of these approaches; and (4) identification anddiscussion of current research challenges, indicating future
research directions.
After defining terminology in Section II, we present oursystematization methodology in Section III. In subsequent
sections (Sections IV–VI), we evaluate each of the proposed
problem areas (namely trust establishment, conversation security and transport privacy) in secure messaging. Our findings
are discussed and concluded in Section VII.

Abstract—Motivated by recent revelations of widespread statesurveillance of personal communication, many products now
claim to offer secure and private messaging. This includes both alarge number of new projects and many widely adopted tools thathave added security features. The intense pressure in the past two
years to deliver solutions quickly has resulted in varying threat
models, incomplete objectives, dubious security claims, and a lackof broad perspective on the existing cryptographic literature on
secure communication.
In this paper, we evaluate and systematize current secure
messaging solutions and propose an evaluation framework fortheir security, usability, and ease-of-adoption properties. We consider solutions from academia, but also identify innovative andpromising approaches used “in the wild” that are not consideredby the academic literature. We identify three key challenges
and map the design landscape for each: trust establishment,
conversation security, and transport privacy. Trust establishment
approaches offering strong security and privacy features perform
poorly from a usability and adoption perspective, whereas somehybrid approaches that have not been well studied in theacademic literature might provide better trade-offs in practice.
In contrast, once trust is established, conversation security canbe achieved without any user involvement in most two-party
conversations, though conversations between larger groups stilllack a good solution. Finally, transport privacy appears to be
the most difficult problem to solve without paying significant
performance penalties.

I. I NTRODUCTION
Most popular messaging tools used on the Internet do
not offer end-to-end security. Even though protocols suchas OpenPGP and S/MIME have been available for decades,
they have failed to achieve widespread adoption and havebeen plagued by usability issues [2]–[5]. However, recent
revelations about mass surveillance by intelligence services
have highlighted the lack of security and privacy in messaging tools and spurred demand for better solutions. A recent
Pew Research poll found that 80% of Americans are now
concerned about government monitoring of their electronic
communications. A combined 68% of respondents reported
feeling “not very secure” or “not at all secure” when using
online chat and 57% felt similarly insecure using email [6].
Consequently, many new applications claiming to offer securecommunication are being developed and adopted by end users.
Despite the publication of a large number of secure messaging protocols in the academic literature, tools are being
released with new designs that fail to draw upon this knowledge, repeat known design mistakes, or use cryptography in

II. BACKGROUND AND D EFINITIONS
Secure messaging systems vary widely in their goals andcorresponding design decisions. Additionally, their target audiences often influence how they are defined. In this section, we

1 This is an extended version of our paper in the 2015 IEEE Symposium

on Security and Privacy [1]. This document was last updated on 2015-04-14.

1

define terminology to differentiate these designs and provide
a foundation for our discussion of secure messaging.

vulnerable window

secureEtcompromise

A. Types of specification(a) Forward Secrecy

Secure messaging systems can be specified at three different
broad levels of abstraction:
Chat protocols: At the most abstract level, chat protocols
can be defined as sequences of values exchanged between
participants. This mode of specification deals with highlevel data flows and often omits details as significant as the
choice of cryptographic protocols (e.g., key exchanges) to use.
Academic publications typically specify protocols this way.
Wire protocols: Complete wire protocols aim to specify
a binary-level representation of message formats. A wire
protocol should be complete enough that multiple parties canimplement it separately and interoperate successfully. Oftenthese are specific enough that they have versions to ensurecompatibility as changes are made. Implicitly, a wire protocol
implements some higher-level chat protocol, though extracting
it may be non-trivial.
Tools: Tools are concrete software implementations thatcan be used for secure messaging. Implicitly, a tool contains
a wire protocol, though it may be difficult and error-prone toderive it, even from an open-source tool.

vulnerable window

secureEtcompromise(b) Backward Secrecy
Fig. 1. Session keys are protected from long-term key compromise.

We can distinguish between message repudiation, in which
Alice denies sending a specific message, and participation
repudiation in which Alice denies communicating with Bob
at all. The high-level goal of repudiable messaging systems is
to achieve deniability similar to real-world conversations.
A fundamental problem of deniability is that Justin may
simply trust Bob even with no technical evidence due to Bob’s
reputation or perceived indifference. In a group chat, thisproblem may be even worse as Alice may need to convince
Justin that a number of accusers are all colluding to frame
her. It is not possible to construct a messaging system that
overcomes this fundamental social problem; the best that canbe done is to provide no stronger evidence than the word of theaccusers. Some technical systems clearly offer more evidence;for example, signed PGP emails offer strong evidence that
Alice really was the sender.
The cryptographic literature has produced many definitionsof “deniability” since deniable encryption was first formally
proposed [12]. For example, we can draw a distinction between
an offline and online judge: in the offline case, the accuser
attempts to convince the judge of an event after the conversation has already concluded; in the online case, the judge
exchanges private communications with the accuser while theconversation is still taking place. Existing work defines online
repudiation in incompatible ways, and very few protocols
attempt to achieve meaningful online repudiation [13], [14].
Thus, in this work we only consider the offline setting.

B. Synchronicity
A chat protocol can be synchronous or asynchronous. Synchronous protocols require all participants to be online andconnected at the same time in order for messages to betransmitted. Systems with a peer-to-peer architecture, where
the sender directly connects to the recipient for message
transmission, are examples of synchronous protocols. Asynchronous protocols, such as SMS (text messaging) or email, do
not require participants to be online when messages are sent,utilizing a third party to cache messages for later delivery.
Due to social and technical constraints, such as switchedoff devices, limited reception, and limited battery life, synchronous protocols are not feasible for many users. Mobile
environments are also particularly prone to various transmission errors and network interruptions that preclude the use ofsynchronous protocols. Most popular instant messaging (IM)solutions today provide asynchronicity in these environments
by using a store-and-forward model: a central server is used
to buffer messages when the recipient is offline. Secure
messaging protocols designed for these environments need toconsider, and possibly extend, this store-and-forward model.

D. Forward/Backward Secrecy
In systems that use the same static keys for all messages,
a key compromise allows an attacker to decrypt the entire
message exchange. A protocol provides forward secrecy if thecompromise of a long-term key does not allow ciphertexts
encrypted with previous session keys to be decrypted (Figure 1a). If the compromise of a long-term key does not allow
subsequent ciphertexts to be decrypted by passive attackers,
then the protocol is said to have backward secrecy (Figure 1b).
However, tools with backward secrecy are still vulnerable toactive attackers that have compromised long-term keys. In thiscontext, the “self-healing” aspect of backward secrecy has also
been called future secrecy. The terms are controversial andvague in the literature [15]–[17].

C. Deniability
Deniability, also called repudiability, is a common goal for
secure messaging systems. Consider a scenario where Bob
accuses Alice of sending a specific message. Justin, a judge,
must decide whether or not he believes that Alice actually did
so. If Bob can provide evidence that Alice sent that message,
such as a valid cryptographic signature of the message under
Alice’s long-term key, then we say that the action is nonrepudiable. Otherwise, the action is repudiable or deniable.

2III. S YSTEMATIZATION M ETHODOLOGY

existing secure messaging systems. Each section then definesand evaluates these approaches, as well as several possible
variations, in terms of the already-defined properties. Concrete
examples of protocols or tools making use of each approach
are given whenever possible. The sections then conclude by
discussing the implications of these evaluations.
In each section, we include a table (Tables I, II, and III)visualizing our evaluation of approaches within that problem
area. Columns in the tables represent the identified properties, while rows represent the approaches. Groups of rows
begin with a generic concept, specified as a combinationof cryptographic protocols, followed by extension rows that
add or modify components of the base concept. Wheneverpossible, rows include the name of a representative protocol
or tool that uses the combination of concepts. Representatives
may not achieve all of the features that are possible using
the approach; they are merely included to indicate whereapproaches are used in practice. Each row is rated as providing
or not providing the desired properties. In some cases, a row
might only partially provide a property, which is explained in
the associated description.
For each problem area, we identify desirable properties inthree main categories:
1) Security and Privacy Properties: Most secure messaging
systems are designed using standard cryptographic primitives
such as hash functions, symmetric encryption ciphers, anddigital signature schemes. When evaluating the security andprivacy features of a scheme, we assume cryptographic primitives are securely chosen and correctly implemented. We
do not attempt to audit for software exploits which may
compromise users’ security. However, if systems allow end
users to misuse these cryptographic primitives, the scheme ispenalized.
2) Usability Properties: Usability is crucial for the use andadoption of secure messaging services. Human end users needto understand how to use the system securely and the effort
required to do so must be acceptable for the perceived benefits.
In previous research, various secure messaging tools havebeen evaluated and weaknesses in the HCI portion of theirdesign have been revealed. The seminal paper “Why Johnny
Can’t Encrypt” [2] along with follow-up studies evaluating
PGP tools [3], [4] and other messaging protocols [18]–[22]have also showed users encountering severe problems using
encryption securely. However, these studies focused on UI
issues unique to specific implementations. This approach
results in few generic insights regarding secure messenger
protocol and application design. Given the huge number of
secure messaging implementations and academic approaches
considered in our systematization, we opted to extract genericconcepts. Because we focus on usability consequences imposed by generic concepts, our results hold for any tool that
implements these concepts.
To evaluate the usability of secure messaging approaches,
we examine the additional user effort (and decisions), securityrelated errors, and reduction in reliability and flexibility thatthey introduce. Our usability metrics compare this extra effort

Over the years, hundreds of secure messaging systems havebeen proposed and developed in both academia and industry.
An exhaustive analysis of all solutions is both infeasible andundesirable. Instead, we extract recurring secure messaging
techniques from the literature and publicly available messaging
tools, focusing on systematization and evaluation of the underlying concepts and the desirable secure messaging properties.
In this section, we explain our precise methodology.
A. Problem Areas
While most secure messaging solutions try to deal with all
possible security aspects, in our systematization, we divide
secure messaging into three nearly orthogonal problem areas
addressed in dedicated sections: the trust establishment problem (Section IV), ensuring the distribution of cryptographic
long-term keys and proof of association with the owning
entity; the conversation security problem (Section V), ensuring
the protection of exchanged messages during conversations;and the transport privacy problem (Section VI), hiding thecommunication metadata.
While any concrete tool must decide on an approach foreach problem area, abstractly defined protocols may only
address some of them. Additionally, the distinction between
these three problem areas is sometimes blurred since techniques used by secure messaging systems may be part of theirapproach for multiple problem areas.
B. Threat Model
When evaluating the security and privacy properties insecure messaging, we must consider a variety of adversaries.
Our threat model includes the following attackers:
Local Adversary (active/passive): An attacker controlling
local networks (e.g., owners of open wireless access points).
Global Adversary (active/passive): An attacker controlling
large segments of the Internet, such as powerful nation statesor large Internet service providers.
Service providers: For messaging systems that require centralized infrastructure (e.g., public-key directories), the service
operators should be considered as potential adversaries.
Note that our adversary classes are not necessarily exclusive.
In some cases, adversaries of different types might collude.
We also assume that all adversaries are participants in themessaging system, allowing them to start conversations, send
messages, or perform other normal participant actions. We
assume that the endpoints in a secure messaging system aresecure (i.e., malware and hardware attacks are out of scope).
C. Systematization Structure
Sections IV–VI evaluate trust establishment, conversation
security, and transport privacy approaches, respectively. For
each problem area, we identify desirable properties divided
into three main groups: security and privacy features, usability
features, and adoption considerations. Each section starts
by defining these properties, followed by the extraction ofgeneric approaches used to address the problem area from3

to a baseline approach with minimal security or privacy
features. This is a challenging task and conventional user
studies are not well suited to extract such high-level usability
comparisons between disparate tools. We opted to employ
expert reviews to measure these usability properties, which isconsistent with previous systematization efforts for security
schemes in other areas [23], [24]. To consider usability andadoption hurdles in practice, we combined these expert reviews
with cognitive walkthroughs of actual implementations basedon Nielsen’s usability principles [25]–[27] and already known
end-user issues discovered in previous work [2]–[5], [19]–[21],[28]. These usability results supplement our technical systematization and highlight potential trade-offs between security
and usability.
3) Ease of Adoption: Adoption of secure messaging
schemes is not only affected by their usability and security
claims, but also by requirements imposed by the underlying
technology. Protocols might introduce adoption issues byrequiring additional resources or infrastructure from end users
or service operators. When evaluating the adoption propertiesof an approach, we award a good score if the system does
not exceed the resources or infrastructure requirements of abaseline approach that lacks any security or privacy features.

B. Usability Properties
Most trust establishment schemes require key management:user agents must generate, exchange, and verify other participants’ keys. For some approaches, users may be confronted
with additional tasks, as well as possible warnings and errors,compared to classic tools without end-to-end security. If a
concept requires little user effort and introduces no new error
types, we award a mark for the property to denote good usability. We only consider the minimum user interaction requiredby the protocol instead of rating specific implementations.
Automatic Key Initialization: No additional user effort isrequired to create a long-term key pair.
Low Key Maintenance: Key maintenance encompasses recurring effort users have to invest into maintaining keys. Somesystems require that users sign other keys or renew expired
keys. Usable systems require no key maintenance tasks.
Easy Key Discovery: When new contacts are added, no
additional effort is needed to retrieve key material.
Easy Key Recovery: When users lose long-term key material, it is easy to revoke old keys and initialize new keys (e.g.,
simply reinstalling the app or regenerating keys is sufficient).
In-band: No out-of-band channels are needed that require
users to invest additional effort to establish.
No Shared Secrets: Shared secrets require existing social
relationships. This limits the usability of a system, as not allcommunication partners are able to devise shared secrets.
Alert-less Key Renewal: If other participants renew theirlong-term keys, a user can proceed without errors or warnings.
Immediate Enrollment: When keys are (re-)initialized, otherparticipants are able to verify and use them immediately.
Inattentive User Resistant: Users do not need to carefully
inspect information (e.g., key fingerprints) to achieve security.

IV. T RUST E STABLISHMENT
One of the most challenging aspects of messaging security is trust establishment, the process of users verifyingthat they are actually communicating with the parties they
intend. Long-term key exchange refers to the process whereusers send cryptographic key material to each other. Longterm key authentication (also called key validation and key
verification) is the mechanism allowing users to ensure thatcryptographic long-term keys are associated with the correct
real-world entities. We use trust establishment to refer to the
combination of long-term key exchange and long-term key
authentication in the remainder of this paper. After contact
discovery (the process of locating contact details for friends
using the messaging service), end users first have to perform
trust establishment in order to enable secure communication.

C. Adoption Properties
Multiple Key Support: Users should not have to invest
additional effort if they or their conversation partners use
multiple public keys, making the use of multiple devices withseparate keys transparent. While it is always possible to share
one key on all devices and synchronize the key between them,this can lead to usability problems.
No Service Provider Required: Trust establishment does not
require additional infrastructure (e.g., key servers).
No Auditing Required: The approach does not require
auditors to verify correct behavior of infrastructure operators.
No Name Squatting: Users can choose their names and canbe prevented from reserving a large number of popular names.
Asynchronous: Trust establishment can occur asynchronously without all conversation participants online.
Scalable: Trust establishment is efficient, with resource
requirements growing logarithmically (or smaller) with the thetotal number of participants in the system.

A. Security and Privacy Features
A trust establishment protocol can provide the following
security and privacy features:
Network MitM Prevention: Prevents Man-in-the-Middle(MitM) attacks by local and global network adversaries.
Operator MitM Prevention: Prevents MitM attacks executedby infrastructure operators.
Operator MitM Detection: Allows the detection of MitM
attacks performed by operators after they have occurred.
Operator Accountability: It is possible to verify that operators behaved correctly during trust establishment.
Key Revocation Possible: Users can revoke and renew keys
(e.g., to recover from key loss or compromise).
Privacy Preserving: The approach leaks no conversation
metadata to other participants or even service operators.

D. Evaluation
1) Opportunistic Encryption (Baseline): We consider opportunistic encryption, in which an encrypted session is established without any key verification, as a baseline. For

4

TABLE I
T RADE - OFFS FOR COMBINATIONS OF TRUST ESTABLISHMENT APPROACHES . S ECURE APPROACHES OFTEN SACRIFICE USABILITY AND ADOPTION .
ExampleSecurity Features

Usability

Adoption

Ne
O two
p r
O era k M
pe to ir r t
O ato Mi M P
pe r tM r
ev
Ke ra Mitt P enPr y R or A M rev ted
iva ev cc De ency oca ou tec ted
Pr tion nta ted
es P biAu
er o lit
vi ss y
t
o
ng ib
Lo m
le
at
w
Ea K ic
sy ey Ke
Ea Ke Ma y Iny
In sy K Di inte itias n l
No Ban ey R cov anc izat
S d eco ery e ion
Al h arveeery
Im rt-l d See
In med ss K cre
at ia e ts
ten te ytiv En Re
e ro new
Us ll aM
er me lul
No tipRe ntlsis
No Ser e Ktant
Au vic ey
No di e P Su
p
t
r
i
p
As Na ng ovi ormd t
Sc ynch e S Req er
al ro q ui
ab n ua rele ou tti ds ng

SchemeOpportunistic Encryption†*
TCPCrypt- - +TOFU (Strict)†+TOFU†*TextSecure
Key Fingerprint Verification†*
Threema+Short Auth Strings (Out-of-Band)†*
SilentText+Short Auth Strings (In-Band/Voice/Video)†*
ZRTP+Socialist Millionaire (SMP)†*OTR+Mandatory Verification†*
SafeSlinger
Key Directory†*
iMessage- +Certificate Authority†*
S/MIME- +Transparency Log+Extended Transparency Log†
+Self-Auditable Log†CONIKS
Web-of-Trust†*
PGP+Trust Delegation†*
GnuNS+Tracking*
KeybasePure IBC†
SIM-IBC-KMS- +Revocable IBC†
- Blockchains*
Namecoin
Key Directory+TOFU+Optional Verification†* TextSecure
Opportunistic Encryption+SMP†*
OTR= provides property;

- -

-

-

-

- - -

-

-

-

- - -

- -

-

- - -

- - - -- -- - - - -

-

-

-

- - -

-

-

-

-

-

= partially provides property; - = does not provide property; † has academic publication; * end-user tool available

instance, this could be an OTR encryption session without any
authentication. The main goal of opportunistic encryption is tocounter passive adversaries; active attackers can easily execute
MitM attacks. From a usability perspective, this approach isthe baseline since it neither places any burden on the user nor
generates any new error or warning messages.

For instance, TextSecure shows a warning that a user’s key haschanged and the user must either confirm the new key or apply
manual verification to proceed (shown in Figure 2). If theuser chooses to accept the new key immediately, it is possible
to perform the verification later. The motivation behind thisapproach is to provide more transparency for more experienced
or high-risk users, while still offering an “acceptable” solution
for novice end users. Critically, previous work in the related
domain of TLS warnings has shown that frequent warning
messages leads to higher click-through rates in dangerous
situations, even with experienced users [30].
From an adoption perspective, TOFU performs similarly tothe baseline, except for key recovery in the strict version andmultiple key support in both versions. The multiple key support
problem arises from the fact that if multiple keys are used, theprotocol cannot distinguish between devices. An attacker canclaim that a new device, with the attacker’s key, is being used.

2) TOFU: Trust-On-First-Use (TOFU) extends opportunistic encryption by remembering previously seen key material [29]. The network MitM prevented and infrastructure
MitM prevented properties are only partially provided due tothe requirement that no attacker is present during the initialconnection. TOFU requires no service provider since keys canbe exchanged by the conversation participants directly. TOFU
does not define a mechanism for key revocation. TOFU can be
implemented in strict and non-strict forms. The strict form fails
when the key changes, providing inattentive user resiliencebut preventing easy key recovery. The non-strict form prompts
users to accept key changes, providing easy key recovery at
the expense of inattentive user resilience.
TOFU-based approaches, like the baseline, do not require
any user interaction during the initial contact discovery. This
yields good scores for all user-effort properties except for the
key revocation property, which is not defined, and alert-lesskey renewal, since users cannot distinguish benign key changes
from MitM attacks without additional verification methods.

3) Key Fingerprint Verification: Manual verification requires users to compare some representation of a cryptographic
hash of their partners’ public keys out of band (e.g., in person
or via a separate secure channel).
Assuming the fingerprint check is performed correctly by
end users, manual verification provides all desirable security
properties with the exception of only partial key revocation
support, as this requires contacting each communication part-5

Fig. 2. TextSecure warning for key changes: the user must either accept the
new key by selecting “complete”, or perform manual verification [31].

ner out-of-band. The approaches differ only in their usability
and adoption features.
Fingerprint verification approaches introduce severe usability and adoption limitations: users have to perform manualverification before communicating with a new partner (and getthem to do the same) to ensure strong authentication. Thus,
manual verification does not offer automatic key initialization,easy key discovery, or immediate enrollment. In addition,
new keys introduce an alert on key renewal, resulting in a
key maintenance effort. Fingerprints complicate multiple key
support since each device might use a different key.
While it is possible to improve the usability of key fingerprint verification by making it optional and combining it withother approaches, we postpone discussion of this strategy until
the discussion.
4) Short Authentication Strings: To ease fingerprint verification, shorter strings can be provided to the users forcomparison. A short authentication string (SAS) is a truncatedcryptographic hash (e.g., 20–30 bits long) of all public partsof the key exchange. It is often represented in a format aimed
to be human friendly, such as a short sequence of words.
All participants compute the SAS based on the key exchange
they observed, and then compare the resulting value witheach other. The method used for comparison of the SAS
must authenticate the entities using some underlying trust
establishment mechanism.
Several encrypted voice channels, including the ZRTP protocol and applications like RedPhone, Signal, and SilentPhone,
use the SAS method by requiring participants to read strings
aloud [32], [33]. Figure 3 shows an example of SAS verification during establishment of a voice channel in RedPhone.
For usability reasons, RedPhone and SilentPhone use random
dictionary words to represent the hash. Because these tools
require the user to end the established call manually if theverification fails, they are not inattentive user resistant.
SAS systems based on voice channels anchor trust in theability of participants to recognize each other’s voices. Users
who have never heard each other’s voices cannot authenticate

Fig. 3. Users read random words during SAS verification in RedPhone [31].

using this method. Even for users that are familiar with eachother, the security provided by voice identification has been thesubject of controversy [34], [35]. Recent work [36] suggests
that, with even a small number of samples of a target user’s
speaking voice, audio samples can be synthesized that are
indistinguishable from the genuine user’s voice with typicallevels of background noise. We should expect that artificial
voice synthesis will improve in cost and accuracy, while
human auditory recognition will not improve.
For this reason, we consider voice-based SAS verification
to be obsolescent from a security standpoint. In Table I, we
assume that users verify the SAS with a method providing
stronger security (e.g., using audio and video channels withcareful inspection during the SAS verification). If the communication channel (e.g., text messaging) does not support amechanism to establish trust, the SAS must be compared outof band (e.g., as recommended by SilentText).
The SAS approach sacrifices asynchronicity, since mutual
authentication must be done with all users at the same time.
Due to the short size of the SAS, the naı̈ve approach isvulnerable to a MitM attack by an adversary that attempts
to select key exchange values that produce a hash collision
for the two connections. To mitigate this problem, the attacker
can be limited to a single guess by forcing them to reveal theirchosen keys before observing the keys of the honest parties.
This can be accomplished by requiring that the initiator of
the key exchange release a commitment to their key, and thenopen the commitment after the other party reveals theirs.
5) Secret-based Zero-Knowledge Verification: The Socialist
Millionaire Protocol (SMP) is a zero-knowledge proof ofknowledge protocol that determines if secret values held by
two parties are equal without revealing the value itself. This
protocol is used in OTR as the recommended method for user
verification [37], [38]. Alice poses a question based on sharedknowledge to Bob in-band and secretly records her answer.

6

After Bob answers the question, the two parties perform the
SMP to determine if their answers match, without revealing
any additional information. Users are expected to choose
secure questions with answers based on shared knowledge that
attackers would be unable to know or guess.
The SMP used in OTR is performed on a cryptographic hashof the session identifier, the two parties’ fingerprints, and theirsecret answers. This prevents MitM and replay attacks.
Since a MitM must perform an online attack and can
only guess once, even low min-entropy secrets achieve strong
security [38], [39]. However, use of the SMP sacrifices asynchronicity since all participants must be online during theverification. If the protocol fails, the end users do not knowwhether their answers did not match, or if a MitM attacker
exists and has made an incorrect guess.
6) Mandatory Verification: The previously defined verification methods are prone to inattentive users. Mandatory
verification approaches counter user negligence by requiringthat users enter the correct fingerprint strings instead of merely
confirming that they are correct. Of course, entering thefingerprints takes user effort. In practice, QR-Codes and NFC
are popular methods to ease this process.
In SafeSlinger the user must choose the correct answer
among three possibilities to proceed [40]. Physically colocated users form a group and exchange ephemeral keys.
Each device hashes all received information and displays thehash as a sequence of three common words. Two additionalsequences are randomly generated. The users communicate
to determine the sequence that is common to all devices andselect it to verify the ephemeral keys, preventing users from
simply clicking an “OK” button. These keys are then used to
exchange contact information within the group with security
guarantees including confidentiality and authenticity.
Mandatory verification is a technique that is applied toanother trust establishment scheme; the resulting approach
inherits the usability properties of the underlying scheme.
Incorporating mandatory verification sacrifices asynchronicity
to ensure inattentive user resistance.
7) Authority-based Trust: In authority-based trust schemes,
public keys must be vouched for by one or more trusted
authorities.
During key initialization, authorities can verify ownershipof public keys by the claimed subjects through various means,such as password-based authentication or email validation. The
authority then asserts the keys’ validity to other users. Twowell-known examples of authority-based trust establishment
are public-key directories and certificate authority schemes.
A Certificate Authority (CA) may issue signed certificatesof public keys to users, who can then present them directly toother users without needing to communicate further with theauthority. This model has been widely deployed on the web
with the X.509 Public Key Infrastructure (PKIX) for HTTPS.
While the S/MIME standard uses this model for secure email,it has seen less widespread deployment than PGP.
Alternatively, users may look up keys directly from an
online public-key directory over a secure channel. This iscommon in several proprietary messaging applications suchas Apple iMessage and BlackBerry Protected Messenger.
In contrast to CA schemes, where the conversation partner
directly provides an ownership assertion from the CA, theauthority is directly asked for ownership assertions in key
directory schemes.
From the security point of view, the two schemes only differ
in key revocation and privacy preservation. While key updates
in key directories imply the revocation of old keys, in the
CA approach, certificates signed by the authority are trusted
by default; revocation lists have to be maintained separately.
However, CA-based revocation lists used in web browsers are
known to have issues with effectiveness and practicality [24],[41], [42]. Since certificates may be exchanged by peers
directly, the CA-based approach can be privacy preserving.
With either system, users are vulnerable to MitM attacks
by the authority, which can vouch for, or be coerced to vouchfor, false keys. This weakness has been highlighted by recent
CA scandals [43], [44]. Both schemes can also be attacked if
the authority does not verify keys before vouching for them.
Authorities in messaging services often rely on insecure SMS
or email verification, enabling potential attacks.
The two approaches both support good usability. Wellknown systems using public-key directories, such as iMessage,
work without any user involvement.
8) Transparency Logs: A major issue with trusted authorities is that they can vouch for fraudulent keys in an attack. The
Certificate Transparency protocol [45] requires that all issued
web certificates are included in a public log.
This append-only log is implemented using a signed Merkle
tree with continual proofs of consistency [45]. Certificates are
only trusted if they include cryptographic proof that they arepresent in the log. This ensures that any keys the authority
vouches for will be visible in the log and evidence will exist
that the authority singed keys used in an attack.
Certificate Transparency is a specific proposal for logging
PKIX certificates for TLS, but the general idea can be applied
to authority-based trust establishment in secure messaging.
We refer to the general concept as transparency logs for theremainder of the paper. While there are no known deployments
to date, Google plans to adapt transparency logs for user keys
in End-to-End, its upcoming email encryption tool [46]. In
the absence of a concrete definition, we evaluate transparency
logs based on the certificate transparency protocol.
The main security improvement of the two schemes consistsof operator accountability and the detection of operator MitM
attacks after the fact. The remaining security features are
inherited from authority-based trust systems.
However, these schemes introduce new and unresolved usability and adoption issues. For instance, the logs must be audited to ensure correctness, negating the no auditing required
property. The auditing services require gossip protocols tosynchronize the view between the monitors and prevent attack
bubbles (e.g., where different views are presented to different
geographical regions) [45]. Also, since only identity owners
are in a position to verify the correctness of their long-term7

keys, they share responsibility for verifying correct behavior of
the log. Previous research has shown that users often neglectsuch security responsibilities [30], [47], so this task shouldbe performed automatically by client applications. However,
if a client detects a certificate in the log that differs from theirversion, it is not clear whether the authorities have performed
an attack, an adversary has successfully impersonated thesubject of the certificate to the authorities, or if the subject
actually maintains multiple certificates (e.g., due to installingthe app on a second device). Ultimately, end users have to copewith additional security warnings and errors, and it remains
to be seen whether they can distinguish between benign
and malicious log discrepancies without training. In addition,transparency logs might hamper immediate enrollment due todelays in log distribution.
Ryan [48] proposed extending the transparency logs concept
using two logs: one of all certificates in chronological orderof issuance, and one of currently valid certificates sorted
lexicographically. This enables a form of revocation by making
it efficient to query which certificates are currently valid for agiven username.
Melara et al. [49] proposed CONIKS, using a series ofchained commitments to Merkle prefix trees to build a key
directory that is self-auditing, that is, for which individual
users can efficiently verify the consistency of their own entry
in the directory without relying on a third party. This “selfauditing log” approach makes the system partially have no
auditing required (as general auditing of non-equivocationis still required) and also enables the system to be privacy
preserving as the entries in the directory need not be madepublic. This comes at a mild bandwidth cost not reflected in
our table, estimated to be about 10 kilobytes per client per
day for self-auditing.
Both Ryan’s Extended Certificate Transparency and
CONIKS also support a proof-of-absence, which guarantees
the absence of an identifier or key in the log.
9) Web of Trust: In a web of trust scheme, users verify
each other’s keys using manual verification and, once they
are satisfied that a public key is truly owned by its claimed
owner, they sign the key to certify this. These certification
signatures might be uploaded to key servers. If Alice hasverified Bob’s key, and Bob certifies that he has verified
Carol’s key, Alice can then choose to trust Carol’s key basedon this assertion from Bob. Ideally, Alice will have multiple
certification paths to Carol’s key to increase her confidence inthe key’s authenticity.
The user interface for web of trust schemes tends to berelatively complex and has never been fully standardized. The
scheme also requires a well-connected social graph, hence
the motivation for “key-signing parties” to encourage users
to form many links within a common social context.
Assuming that the web of trust model performs correctly,
MitM attacks by network and operator adversaries are limited
due to distribution of trust. However, since key revocations
and new keys might be withheld by key servers, the model
offers only partial operator accountability and key revocation.

Since the web of trust model produces a public social graph,
it is not privacy preserving.
The key-initialization phase requires users to get their keys
signed by other keys, so the system does not offer automatic
key initialization, alert-less key renewal, or immediate enrollment, and is not inattentive user resistant. Because users must
participate in key-signing parties to create many paths for trust
establishment, users have a high key maintenance overhead
and a need for an out-of-band channel. Even worse, users
must understand the details of the PKI and be able to decide
whether to trust a key.
PGP typically uses a web of trust for email encryption
and signing. In practice, the PGP web of trust consists of
one strongly connected component and many unsigned keys
or small connected components, making it difficult for thoseoutside the strongly connected component to verify keys [50].
A simplification of the general web of trust framework is
SDSI [51] (Simple Distributed Security Infrastructure) later
standardized as SPKI [52], [53] (Simple Public Key Infrastructure). With SDSI/SPKI, Bob can assert that a certain key
belongs to “Carol” and, if Alice has verified Bob’s key as
belonging to “Bob”, that key will be displayed to Alice as“Bob’s Carol” until Alice manually verifies Carol’s key herself(which she can then give any name she wants, such as “CarolN.”). We refer to these approaches as trust delegation. A
modern implementation is the GNU Name System (GNS) [54],[55], which implements SDSI/SPKI-like semantics with a key
server built using a distributed hash table to preserve privacy.
10) Keybase: Keybase is a trust establishment scheme
allowing users to find public keys associated with social
network accounts. It is designed to be easily integrated into
other software to provide username-based trust establishment.
If a user knows a social network username associated with apotential conversation partner, they can use Keybase to findthe partner’s public key.
During key initialization, all users register for accounts with
the Keybase server. They then upload a public key and proof
that they own the associated private key. Next, the user can
associate accounts on social networks or other services with
their Keybase account. Each external service is used to posta signature proving that the account is bound to the named
Keybase account.
When looking up the key associated with a given user, theKeybase server returns the public key, a list of associatedaccounts, and web addresses for the external proofs. The client
software requests the proofs from the external services andverifies the links. The user is then prompted to verify that the
key belongs to the expected individual, based on the verifiedsocial network usernames. To avoid checking these proofs
for every cryptographic operation, the user can sign the setof accounts owned by their partner. This signature is storedby the Keybase server so that all devices owned by the user
can avoid verifying the external proofs again. This process isknown as tracking. Tracking signatures created by other users
are also accessible, providing evidence of account age. Oldtracking signatures provide confidence that a user’s accounts

8have not been recently compromised, but does not protect
against infrastructure operator attacks.
Keybase provides partial operator MitM protection sinceattacks require collusion between multiple operators. The
scheme also provides easier key initialization and key maintenance than web-of-trust methods.
11) Identity-Based Cryptography: In identity-based cryptography (IBC), plaintext identifiers (such as email or IP
addresses) are mapped to public keys. A trusted third party,
the Private Key Generator (PKG), publishes a PKG public key
that is distributed to all users of the system. Public keys for an
identifier are computed using a combination of the identifier
and the PKG public key. The owner of the identity requests
the private key for that identity from the PKG while providing
proof that they own the identity. The advantage of this systemis that users do not need to contact any other entity in orderto retrieve the public key of a target user, since the public key
is derived from the identifier.
There are two main problems with basic IBC schemes: theylack any operator MitM prevention and key revocation is notpossible. Since the PKG can generate private keys for any user,
the operator of the PKG can break the security properties of allconversations. While this fundamental problem cannot be overcome without using hybrid encryption schemes, key revocation
support can be added. Revocable IBC approaches [56]–[58]add timestamps to the public key derivation process, regularly
refreshing key material.
IBC schemes are normally deployed in situations where thetrustworthiness of the PKG operator is assumed, such as inenterprise settings. Few pure-IBC schemes have been proposed
for end-user messaging [59], [60].
12) Blockchains: The Bitcoin cryptocurrency utilizes anovel distributed consensus mechanism using pseudonymous“miners” to maintain an append-only log [61]. Voting power
is distributed in proportion to computational resources byusing a probabilistic proof-of-work puzzle. For the currency
application, this log records every transaction to prevent
double-spending. Miners are rewarded (and incentivized tobehave honestly) by receiving money in proportion to theamount of computation they have performed. The success of
Bitcoin’s consensus protocol has led to enthusiasm that similar
approaches could maintain global consensus on other types ofdata, such as a mapping of human-readable usernames to keys.
Namecoin, the first fork of Bitcoin, allows users to claim
identifiers, add arbitrary data (e.g., public keys) as records for
those identifiers, and even sell control of their identifiers toothers [62]. Namecoin and similar name-mapping blockchainsare denoted by the blockchain entry in Table I. Unlike mostother schemes, Namecoin is strictly “first-come, first-served”,
with any user able to purchase ownership of any number of
unclaimed names for a small, fixed fee per name. This priceis paid in Namecoins—units of currency that are an inherent
part of the system. A small maintenance fee is required tomaintain control of names, and small fees may be charged byminers to update data or transfer ownership of names.
From the security perspective, blockchain schemes achieve

similar results to manual verification, except that instead of
exchanging keys, the trust relies on the username only. Once
users have securely exchanged usernames, they can reliably
fetch the correct keys.
However, various shortcomings arise from a usability andadoption perspective. The primary usability limitation is that
if users ever lose the private key used to register their name(which is not the same as the communication key bound tothat name), they will permanently lose control over that name(i.e., key recovery is not possible). Similarly, if the key iscompromised, the name can be permanently and irrevocably
hijacked. Thus, the system requires significant key management effort and burdens users with high responsibility. If
users rely on a web-based service to manage private keys forthem, as many do with Bitcoin in practice, the system is nolonger truly end-to-end. The system requires users to pay toreserve and maintain names, sacrificing low key maintenanceand automatic key initialization. Users also cannot instantly
issue new keys for their identifiers (i.e., there is no immediate
enrollment) but are required to wait for a new block to be
published and confirmed. In practice, this can take 10–60
minutes depending on the desired security level.
On the adoption side, for the system to be completely
trustless, users must store the entire blockchain locally andtrack its progress. Experience from Bitcoin shows that the vast
majority of users will not do this due to the communicationand storage requirements and will instead trust some otherparty to track the blockchain for them. This trusted party
cannot easily insert spurious records, but can provide stale
information without detection. In any case, the system is nothighly scalable since the required amount of storage and traffic
consumption increases linearly with the number of users.
Finally, there are serious issues with name squatting, whichhave plagued early attempts to use the system. Because anybody can register as many names as they can afford, a numberof squatters have preemptively claimed short and common
names. Given the decentralized nature of blockchains, this is
hard to address without raising the registration fees, which
increases the burden on all users of the system.
E. Discussion
As Table I makes evident, no trust establishment approachis perfect. While it is common knowledge that usability andsecurity are often at odds, our results show exactly where
the trade-offs lie. Approaches either sacrifice security andprovide a nearly ideal user experience, or sacrifice user experience to achieve nearly ideal security scores. Authoritybased trust (whether in the form of a single authority or
multiple providers) and TOFU schemes are the most usable
and well-adopted, but only offer basic security properties. Not
surprisingly, authority-based trust (particularly app-specific
key directories) is predominant among recently developed
apps in the wild, as well as among apps with the largest
userbases (e.g., iMessage, BlackBerry Protected, TextSecure,and Wickr). By contrast, no approach requiring specific user
action to manage keys, such as web-of-trust, Keybase, GNS,

9

or blockchains, has seen significant adoption among nontechnically-minded users.
In practice, we may be faced with the constraint that none of
the usability properties can be sacrificed in a system that willachieve mass adoption. Higher-security schemes may be useful
within organizations or niche communities, but defending
against mass surveillance requires a communication systemthat virtually all users can successfully use. Thus, it may
be wise to start from the basic user experience of today’s
widely deployed communication apps and try to add as muchsecurity as possible, rather than start from a desired security
level and attempt to make it as simple to use as possible. The
recent partnership between WhatsApp and TextSecure [95] isan example of a successful application of this approach.
There appears to be considerable room for security improvements over authoritative key directories even without changes
to the user experience. Transparency logs might provide moreaccountability with no interaction from most users. Because
this approach has not yet been deployed, it remains to be
seen how much security is gained in practice. The insertionof new keys in the log does not provide public evidence ofmalicious behavior if insecure user authentication methods(e.g., passwords) are used to authorize key changes, as we fully
expect will be the case. Still, the possible loss of reputation
may be enough to keep the server honest.
Another promising strategy is a layered design, with basic
security provided by a central key directory, additional trust
establishment methods for more experienced users (e.g., visual
fingerprint verification or QR-codes), and TOFU warning
messages whenever contacts’ keys have changed. TextSecure
and Threema, among others, take such a layered approach(represented by the second-to-last row in Table I). In contrast,
OTR uses opportunistic encryption with the ability to performthe SMP to ensure trust (represented by the last row in Table I).
Conversely, the approaches with good security properties
should focus on improving usability. There has been little
academic work studying the usability of trust establishment.
Further research focusing on end-users’ mental models andperception for trust establishment could help to develop moresophisticated and understandable approaches.

A. Security and Privacy Features
Confidentiality: Only the intended recipients are able toread a message. Specifically, the message must not be readable
by a server operator that is not a conversation participant.
Integrity: No honest party will accept a message that has
been modified in transit.
Authentication: Each participant in the conversation receives proof of possession of a known long-term secret fromall other participants that they believe to be participating in
the conversation. In addition, each participant is able to verify
that a message was sent from the claimed source.
Participant Consistency: At any point when a message is
accepted by an honest party, all honest parties are guaranteed
to have the same view of the participant list.
Destination Validation: When a message is accepted by anhonest party, they can verify that they were included in the setof intended recipients for the message.
Forward Secrecy: Compromising all key material does notenable decryption of previously encrypted data.
Backward Secrecy: Compromising all key material does notenable decryption of succeeding encrypted data.
Anonymity Preserving: Any anonymity features provided
by the underlying transport privacy architecture are notundermined (e.g., if the transport privacy system provides anonymity, the conversation security level does notdeanonymize users by linking key identifiers).
Speaker Consistency: All participants agree on the sequenceof messages sent by each participant. A protocol might perform consistency checks on blocks of messages during theprotocol, or after every message is sent.
Causality Preserving: Implementations can avoid displaying a message before messages that causally precede it.
Global Transcript: All participants see all messages in thesame order.
Not all security and privacy features are completely independent. If a protocol does not authenticate participants, then it
offers participation repudiation (since no proof of participation
is ever provided to anyone). Similarly, no authentication of
message origin implies message repudiation as well as message unlinkability. Note that the implications are only one
way: repudiation properties might be achieved together withauthentication. Additionally, a global transcript order implies
both speaker consistency and causality preservation since alltranscripts are identical.
Conversation security protocols may provide several different forms of deniability. Based on the definitions from Section II-C, we define the following deniability-related features:
Message Unlinkability: If a judge is convinced that a
participant authored one message in the conversation, this does
not provide evidence that they authored other messages.
Message Repudiation: Given a conversation transcript andall cryptographic keys, there is no evidence that a given
message was authored by any particular user. We assume thatthe accuser has access to the session keys because it is trivial
to deny writing a plaintext message when the accuser cannot
demonstrate that the ciphertext corresponds to this plaintext.

V. C ONVERSATION S ECURITY
After trust establishment has been achieved, a conversation
security protocol protects the security and privacy of theexchanged messages. This encompasses how messages are encrypted, the data and metadata that messages contain, and what
cryptographic protocols (e.g., ephemeral key exchanges) are
performed. A conversation security scheme does not specify
a trust establishment scheme nor define how transmitted datareaches the recipient.
In Table II, we compare the features of existing approaches
for conversation security. Rows without values in the “groupchat” columns can only be used in a two-party setting.

10

TABLE IIC ONVERSATION SECURITY PROTOCOLS AND THEIR USABILITY AND ADOPTION IMPLICATIONS . N O APPROACH REQUIRES ADDITIONAL USER EFFORT.
ExampleSecurity and Privacy

AdoptionGroup Chat

Co
In nfidt e
Au egr ntia
th ity lityPa enticr
De tici atiop
Fo stin ant n
rw ati C
Ba ard on VonsiAnckw Sec alid sten
r
a
a c
o
e
Sp ny rdc tio yea mi Se y n
Ca ker ty Pcrec
G usa Con rese ylo lir
M bal ty P siste vin
es T rnc ges
M age ran serv ysePa ssa Unl crip ing
rti ge ink t
cip Re ab. R pu iliO
ep dia ty
ut
ud tioDr -ofia ntiooOAs pp rdnyn ed er
M chr Me Reo s suNo lti- nic sag ilienD
Ad ev ity e R t
es
di ice
ili
tio S
en
Co
na up
t
po
l
m
Se rt
Tr pu
rv
us ta
Su t tiice
bg Eq onCo rou uali al E
p t
Ex ntra M y qua
lit
pa ct essynd ab ag
l
i
e
ab
ng
le

SchemeTLS+Trusted Server†*Skype- - Static Asymmetric Crypto†*
OpenPGP, S/MIME+IBE†
Wang et al.
+Short Lifetime Keys
OpenPGP Draft+Non-Interactive IBE†
Canetti et al.
+Puncturable Encryption†
Green and Miers
Key Directory+Short Lifetime Keys†
IMKE+Long-Term Keys†
SIMPP
Authenticated DH†*
TLS-EDH-MA+Naı̈ve KDF Ratchet*SCIMP+DH Ratchet†*
OTR+Double Ratchet†*
Axolotl+Double Ratchet+3DH AKE†*+Double Ratchet+3DH AKE+Prekeys†*
TextSecure
Key Directory+Static DH+Key Transport† Kikuchi et al.
+Authenticated EDH+Group MAC†GROK
GKA+Signed Messages+Parent IDs†
OldBlue
Authenticated MP DH+Causal Blocks†*
KleeQ
OTR Network+Star Topology†
GOTR (2007)+Pairwise Topology†+Pairwise Axolotl+Multicast Encryption*
TextSecure
DGKE+Shutdown Consistency Check†
mpOTR
Circle Keys+Message Consistency Check† GOTR (2013)= provides property;-- - - -

- -

-

- -

- - - - -

- -

-

- - -

-

-

-

-

-

-

- -

- - - - -

- -

-

-

-

- - - - - - -

- -

-

- -

= partially provides property; - = does not provide property; † has academic publication; * end-user tool available

We also assume that the accuser does not have access to theaccused participant’s long-term secret keys because then it issimple for the accuser to forge the transcript (and thus any
messages are repudiable).
Participation Repudiation: Given a conversation transcript
and all cryptographic key material for all but one accused (honest) participant, there is no evidence that the honest participant
was in a conversation with any of the other participants.
Several additional features are only meaningful for groupprotocols (i.e., protocols supporting chats between three ormore participants):
Computational Equality: All chat participants share an
equal computational load.
Trust Equality: No participant is more trusted or takes onmore responsibility than any other.
Subgroup messaging: Messages can be sent to a subset ofparticipants without forming a new conversation.
Contractible Membership: After the conversation begins,participants can leave without restarting the protocol.
Expandable Membership: After the conversation begins,participants can join without restarting the protocol.
When a participant joins a secure group conversation, it isdesirable for the protocol to compute new cryptographic keys
so that the participant cannot decrypt previously sent mes-

sages. Likewise, keys should be changed when a participant
leaves so that they cannot read new messages. This is trivial to
implement by simply restarting the protocol, but this approach
is often computationally expensive. Protocols with expandable/ contractible membership achieve this without restarts.
There are many higher-level security and privacy design
issues for secure group chat protocols. For example, themechanisms for inviting participants to chats, kicking users
out of sessions, and chat room moderation are all important
choices that are influenced by the intended use cases. We do
not cover these features here because they are implemented at
a higher level than the secure messaging protocol layer.
B. Usability and Adoption
In classic messaging tools, users must only reason about
two simple tasks: sending and receiving messages. However,in secure communication, additional tasks might be added.
In old secure messaging systems, often based on OpenPGP,
users could manually decide whether to encrypt and/or sign
messages. Many studies have shown that this caused usability
problems [2]–[5], [21]. However, during our evaluation, we
found that most recent secure messenger apps secure all
messages by default without user interaction. Since all implementations can operate securely once the trust establishment11

is complete, we omit the user-effort columns in Table II.
However, we take other usability and adoption factors, suchas resilience properties, into account:
Out-of-Order Resilient: If a message is delayed in transit,
but eventually arrives, its contents are accessible upon arrival.
Dropped Message Resilient: Messages can be decrypted
without receipt of all previous messages. This is desirable forasynchronous and unreliable network services.
Asynchronous: Messages can be sent securely to disconnected recipients and received upon their next connection.
Multi-Device Support: A user can participate in the conversation using multiple devices at once. Each device must beable to send and receive messages. Ideally, all devices haveidentical views of the conversation. The devices might use asynchronized long-term key or distinct keys.
No Additional Service: The protocol does not require any
infrastructure other than the protocol participants. Specifically,
the protocol must not require additional servers for relaying
messages or storing any kind of key material.

warding attacks are possible [71]. Without participant consistency, identity misbinding attacks might be possible [72].
Defenses against replay attacks should also be included. These
considerations are particularly relevant since the OpenPGP
and S/MIME standards do not specify how to provide thesefeatures, and thus most implementations remain vulnerable toall of these attacks [68], [70].
To simplify key distribution, several authors have proposed
the use of identity-based cryptography in the same setting.
The SIM-IBC-KMS protocol acts as an overlay on the MSN
chat network with a third-party server acting as the PKG [60].
Messages are encrypted directly using identity-based encryption. The protocol from Wang et al. [73] operates similarly,
but distributes the PKG function across many servers with a
non-collusion assumption in order to limit the impact of a malicious PKG. These protocols partially sacrifice confidentiality
since an attacker with access to the PKG private key could
surreptitiously decrypt communications.
A second issue with naı̈ve asymmetric cryptography is thelack of forward or backward secrecy. One way to address thisissue is to use keys with very short lifetimes (e.g., changing
the key every day). Brown et al. propose several extensions to
OpenPGP based on this principle [74]. In the most extreme
proposal, conversations are started using long-term keys, buteach message includes an ephemeral public key to be used for
replies. This method provides forward and backward secrecy
for all messages except those used to start a conversation.
From a usability and adoption perspective, static key approaches achieve the same properties as the baseline. Apartfrom the non-transparent trust establishment, iMessage is aprominent example of how static asymmetric cryptography
can achieve end-to-end conversation security with no changes
to the user experience. Since the same long-term keys areused for all messages, message order resilience, dropped
message resilience, asynchronicity, and multi-device-support
are provided. No additional services are required.
3) FS-IBE: In traditional PKI cryptography, forward secrecy is achieved by exchanging ephemeral session keys or
by changing keypairs frequently. The use of key agreement
protocols makes asynchronicity difficult, whereas frequently
changing keypairs requires expensive key distribution. Forward
Secure Identity Based Encryption (FS-IBE) allows keypairs to
be changed frequently with a low distribution cost. Unlike
traditional identity-based encryption schemes, the private key
generators (PKG) in FS-IBE are operated by the end users andnot by a server. Initially, each participant generates a PKG
for an identity-based cryptosystem. Participants generate N
private keys (SKi ), one for each time period i, by using their
PKG, and then immediately destroy the PKG. Each private
key SKi is stored encrypted by the previous private key
SKi−1 [15], [75]. The participant then distributes the public
key of the PKG. Messages sent to the participant are encrypted
for the private key corresponding to the current time period.
When a time period concludes, the next secret key is decrypted
and the expired key is deleted. Thus, if intermediate keys
are compromised, the attacker can only retrieve corresponding

C. Two-party Chat Evaluation
1) Trusted central servers (baseline): The most basic conversation security features that a secure chat protocol can
provide are confidentiality and integrity. This can be easily
implemented without adversely affecting usability and adoption properties by using a central server to relay messages
and securing connections from clients to the central serverusing a transport-layer protocol like TLS. This also allows
the central server to provide presence information. Since
this approach does not negatively affect usability, it is no
surprise that this architecture has been adopted by some of themost popular messaging systems today (e.g., Skype, Facebook
Chat, Google Hangouts) [63]–[67]. We do not consider these
protocols further because they allow the central server todecrypt messages and thus do not meet our stronger end-toend definition of confidentiality—that messages cannot be readby anyone except the intended recipient(s). We include thisapproach as a baseline in Table II in order to evaluate theeffects of various designs.
Note that the baseline protocols provide all repudiation
features, since there is no cryptographic proof of any activity.
Additionally, these protocols are highly resilient to errors
since there are no cryptographic mechanisms that could cause
problems when messages are lost. The use of a trusted central
server makes asynchronicity and multi-device support trivial.
2) Static Asymmetric Cryptography: Another simple approach is to use participants’ static long-term asymmetrickeypairs for signing and encrypting.
OpenPGP and S/MIME are two well-known and widely
implemented standards for message protection, mostly used
for email but also in XMPP-based tools [63], [68]–[70].
While this approach provides confidentiality, message authentication, and integrity, it causes a loss of all forms of
repudiation. Additionally, care must be taken to ensure that
destination validation and participant consistency checks are
performed. Without destination validation, surreptitious for-12

future private keys; forward secrecy, but not backward secrecy,
is provided. In contrast to generating key pairs for each time
period, which requires distribution of N keys, only a single
public master key is published; however, the generation stillneeds to be repeated after all time periods expire.
Canetti, Halevi, and Katz were the first to construct a
non-interactive forward secrecy scheme based on hierarchical
IBE with logarithmic generation and storage costs [75]. In
addition, they showed how their scheme can be extended toan unbounded number of periods (i.e., the private keys do nothave to be generated in advance), removing the need for additional services to distribute new keys at the cost of increasing
computational requirements over time. This scheme provides
non-interactive asynchronous forward secrecy without relying
on additional services. However, if messages arrive out oforder, their corresponding private keys might have alreadybeen deleted. As a mitigation, expired keys might be briefly
retained, providing partial out-of-order resilience.
Green and Miers proposed puncturable encryption, a modification of attribute-based encryption (built using a hierarchical
IBE scheme) in which each message is encrypted with arandomly chosen “tag” and the recipient can update theirprivate key to no longer be able to decrypt messages withthat tag after receipt [76]. This approach provides arbitrary
out-of-order resilience, although to make the scheme efficientin practice requires periodically changing keys.
Computational costs and storage costs increase over time
for both FS-IBE and puncturable encryption, introducing scalability concerns. To our knowledge, neither approach has been
deployed and they thus merit further development.
4) Short lifetime key directories: Several protocols makeuse of a central server for facilitating chat session establishment. In these systems, users authenticate to the central server
and upload public keys with short lifetimes. The server acts asa key directory for these ephemeral public keys. Conversations
are initiated by performing key exchanges authenticated withthe short-term keys vouched for by the key directory. Messages
are then encrypted and authenticated using a MAC. IMKE [77]is a protocol of this type where the server authenticates users
through the use of a password. SIMPP [78]–[80] operates
similarly, but uses long-term keys to authenticate instead.
These protocols achieve confidentiality and integrity, butlack authentication of participants since the central server
can vouch for malicious short-term keys. Since session keys
are exchanged on a per-conversation basis, these protocols
achieve forward and backward secrecy between conversations.
Since SIMPP uses signatures during the login procedure, itloses participation repudiability; the accuser cannot forge their
response to the server’s challenge.
5) Authenticated Diffie-Hellman: While the use of central
servers for presence information and central authentication isfundamental to systems such as IMKE and SIMPP, there is analternative class of solutions that instead performs end-to-end
authenticated Diffie-Hellman (DH) key exchanges. By default,
the authenticated DH key agreement does not rely on central
servers. In an authenticated key exchange (AKE) such as

authenticated DH, the participants generate an ephemeral session key and authenticate the exchange using their long-term
keys. The resulting session key is used to derive symmetricencryption and MAC keys, which then protect messages using
an encrypt-then-MAC approach. This basic design provides
confidentiality, integrity, and authentication. TLS with anephemeral DH cipher suite and mutual authentication (TLSEDH-MA) is a well-known example of this approach. Notethat further protections are required during key exchange toprotect against identity misbinding attacks violating participant consistency [38], [72], such as those provided by SIGMA
protocols [82].
The use of ephemeral session keys provides forward andbackward secrecy between conversations. Message unlinkability and message repudiation are provided since messages
are authenticated with shared MAC keys rather than being
signed with long-term keys. At a minimum, messages canbe forged by any chat participants. Some protocols, such as
OTR, take additional measures, such as publication of MAC
keys and the use of malleable encryption, to expand the setof possible message forgers [83]. If the participants simply
sign all AKE parameters, then this approach does not provide
participation repudiation. However, if participants only sign
their own ephemeral keys, these signatures can be reused bytheir conversation partners in forged transcripts. Figure 4ashows the authenticated key exchange used by OTRv1 (morerecent versions use a SIGMA key exchange). Conversation
partners are able to reuse ephemeral keys signed by theother party in forged transcripts, thereby providing partial
participation repudiation. OTR users can increase the numberof possible forgers by publishing previously signed ephemeral
keys in a public location, thereby improving their participation
repudiation.
Once the AKE has been performed, the encrypt-then-MAC
approach allows messages to be exchanged asynchronouslywith out-of-order and dropped message resilience. However,since a traditional AKE requires a complete handshake before
actual messages can be encrypted, this basic approach requires
synchronicity during conversation initialization. Additionally,
since key agreements can only be performed with connected
devices, there is no trivial multi-device support.
6) Key Evolution: A desirable property is forward secrecy
for individual messages rather than for entire conversations.
This is especially useful in settings where conversations canlast for the lifetime of a device. To achieve this, the session
key from the initial key agreement can be evolved over timethrough the use of a session key ratchet [17]. A simple
approach is to use key derivation functions (KDFs) to computefuture message keys from past keys. This naı̈ve approach, as
used in SCIMP [84], provides forward secrecy. However, it
does not provide backward secrecy within conversations; if a
key is compromised, all future keys can be derived using the
KDF. Speaker consistency is partially obtained since messages
cannot be surreptitiously dropped by an adversary without
also dropping all future messages (otherwise, recipients would
not be able to decrypt succeeding messages). If messages are

13

B

ga

g be

g ae

A

sign

gb

sign
authenticated key agreementg ae(a) OTRv1 DH handshake. The session key is derived fromthe key agreement based on signed ephemeral keys: s =DH(g ae , g be )key agreements

g be

(b)
3-DH
handshake.
Thesessionkey
combination
of
all
key
agreements:
s
a
b
a
b
a
b
KDF(DH(g e , g e )||DH(g e , g )||DH(g , g e ))

is

a
=

Fig. 4. TLS/OTRv1 handshake vs. 3-DH handshake (figures derived from [81]). Gray nodes represent ephemeral keys, white nodes represent long-term keys.

agreement(A0 , B0 )A

B

Alice generates A1
DH(A1 ,B0 )KDF1
c0message counter
KDF2

k0

encrypt message with ki

Bob receives A1

send(A1 , 0, Ek (M0 ))0

DH(A1 ,B0 )

KDF1
c1

KDF2

k1

KDF1

message M1 lost

k0

KDF1

KDF2

c0

KDF1
c2

KDF2

k2

send(A1 , 2, Ek (M2 ))
2

k1

KDF1

KDF2

c1

KDF1
c3
k2

KDF2

c2

KDF1
c3
Bob generates B1
DH(A1 ,B1 )KDF1
k0
Alice receives B1

c0

KDF2

k1
k0

send(B1 , 1, Ek1 (M4 ))

KDF1
c1

c0

KDF1

send(B1 , 0, Ek0 (M3 ))DH(A1 ,B1 )
KDF1

KDF2

KDF2

c1

KDF1
c2

KDF2

k1

KDF1
c2
Fig. 5. Simplified version of Axolotl: ci denote chain keys, ki message keys, KDFi arbitrary key derivation functions, Eki an encryption function using
ki , and Ai = g ai and Bi = g bi as public DH values. Gray key nodes denote keys held in memory after Alice receives message M4 .

14

dropped or arrive out of order, the recipient will notice since
the messages are encrypted with an unexpected key. To handle
this, the recipient must store expired keys so that delayed or
re-transmitted messages can still be decrypted, leaving a larger
window of compromise than necessary. Thus, out-of-order anddropped message resilience are only partially provided.

Axolotl also simplifies the use of its outer DH ratchet. In
OTR, a chain of trust, allowing trust in new DH key exchanges
to be traced back to the original AKE, is provided throughthe use of DH key advertisements and acknowledgments. To
speed up this process, Axolotl instead derives a root key
from the initial AKE in addition to the initial DH keys.
Each subsequent DH secret is derived by using the sender’slatest DH key, the latest DH key received from the otherparticipant, and the current root key. Each time the DH ratchet
is advanced, a new root key is derived in addition to a new
chain key. Since deriving the chain keys requires knowledgeof the current root key, newly received DH keys can betrusted immediately without first sending an acknowledgment.
Despite these improvements, the double ratchet still requires
synchronicity for the initial AKE.
9) 3-DH Handshake: A triple DH (3-DH) handshake isa different AKE scheme that provides stronger participation
repudiation. Specifically, transcripts of conversations between
any two participants can be forged by anyone knowing nothing
more than the long-term public keys of the participants.
Figure 4b depicts a 3-DH AKE. Triple DH is an implicitly
authenticated key agreement protocol—a category that has
been extensively examined in the literature [86]–[92]. Notethat in this simplified version, if an attacker’s ephemeral key
was used (possible since ephemeral keys are not signed),
the attacker would be able to calculate the session key retrospectively assuming the delayed possession of the corresponding long-term key. Thus, in practice the key exchange
requires further protection mechanisms against ephemeral keys
chosen by an attacker. Assuming that Alice and Bob bothhave long-term DH keys g a and g b and ephemeral keys
g ae and g be , the 3-DH shared secret s is computed ass = KDF(DH(g ae , g be )||DH(g a , g be )||DH(g ae , g b )) [85]. If a
secure key derivation function is used, a MitM attacker must
either know a and ae , or b and be . Kudla et al. have shownthat the 3-DH key exchange provides the same authenticationlevel as achieved with the authenticated versions of DH key
agreements [93]. 3-DH achieves full participation repudiation
since anybody is able to forge a transcript between any twoparties by generating both ae and be and performing DH
key exchanges with g a and g b . Assuming that Mallory uses
g m as her long-term DH value and g me as her ephemeral
key agreement value, and that she knows Alice’s long-term
DH value g a , she is able to forge a transcript by calculatings = KDF(DH(g ae , g me )||DH(g a , g me )||DH(g ae , g m )) as thecommon HMAC and encryption secrets. Mallory can do thiswithout ever actually interacting with Alice. Since the secretis partially derived from the long-term public keys, 3-DH also
provides participant consistency without the need to explicitly
exchange identities after a secure channel has been established.
Unfortunately, this also causes a partial loss of anonymity
preservation since long-term public keys are observable during
the initial key agreement (although future exchanges can beprotected by using past secrets to encrypt these identities). It
is possible to regain anonymity preservation by encrypting key
identifiers with the given ephemeral keys.

7) Diffie-Hellman Ratchet: A different ratcheting approach,introduced by OTR, is to attach new DH contributions tomessages [83]. With each sent message, the sender advertises
a new DH value. Message keys are then computed fromthe latest acknowledged DH values. This design introduces
backward secrecy within conversations since a compromised
key will regularly be replaced with new key material. Causality
preservation is partially achieved since messages implicitly
reference their causal predecessors based on which keys theyuse. The same level of speaker consistency as the naı̈ve KDF
solution can be provided by adding a per-speaker monotonic
counter to messages. A disadvantage of the DH ratchet is thatsession keys might not be renewed for every message (i.e.,
forward secrecy is only partially provided). Like the KDFbased ratchet, the DH ratchet lacks out-of-order resilience; if
a message arrives after a newly advertised key is accepted,
then the necessary decryption key was already deleted.
8) Double-Ratchet (Axolotl): To improve the forward secrecy of a DH ratchet, both ratchet approaches can be combined: session keys produced by DH ratchets are used to seed
per-speaker KDF ratchets. Messages are then encrypted using
keys produced by the KDF ratchets, frequently refreshed by
the DH ratchet on message responses. The resulting double
ratchet, as implemented by Axolotl [85], provides forward
secrecy across messages due to the KDF ratchets, but also
backward secrecy since compromised KDF keys will eventually be replaced by new seeds. To achieve out-of-orderresilience, the Axolotl ratchet makes use of a second derivation
function within its KDF ratchets. While the KDF ratchets areadvanced normally, the KDF keys are passed through a second
distinct derivation function before being used for encryption.
Figure 5 depicts the double ratchet used in Axolotl. The
secondary KDF, denoted as KDF2 , allows the chain keys(ci ) to be advanced without sacrificing forward secrecy; eachci is deleted immediately after being used to derive thesubsequent chain key ci+1 and the corresponding messagekey (ki ) for encryption. If messages arrive out of order, thissystem provides a mechanism for decrypting the messages
without compromising forward secrecy. For example, if Bob
is expecting message M1 and is storing c1 in memory, butthen receives M2 instead, he uses c1 to compute k1 , c2 , k2 ,and c3 . Bob uses k2 to decrypt the newly received message,
and then he deletes c1 and c2 from memory, leaving only k1
and c3 . When the missing M1 eventually arrives, Bob can use
k1 to decrypt it directly. However, if an attacker compromises
Bob’s system at this moment, they will be unable to derive
k2 to decrypt M2 . A similar situation is depicted in Figure 5,
where gray key nodes denote keys held in memory after Alicewas able to receive M4 .

1510) Prekeys: While a double ratchet does not provide asynchronicity by itself, it can be combined with a prekey scheme
to create an asynchronous version of the protocol. Prekeys are
one-time ephemeral public DH contributions that have been
uploaded in advance to a central server. Clients can complete
a DH key exchange with a message recipient by requesting
their next prekey from the server. When combined with a 3-DH
exchange, this is sufficient to complete an asynchronous AKE
as part of the first message. In comparison to time-window
based FS-IBE approaches (cf. Section V-C3), this approachrequires the precomputation of a number of ephemeral keys;otherwise, forward secrecy is weakened. However, this scheme
also permits the destruction of the private ephemeral values
immediately after receiving a message using them, instead of
keeping a key until a time window expires.
TextSecure [31] is a popular Android app that combines
Axolotl, prekeys, and 3-DH to provide an asynchronous user
experience while sacrificing the no additional service property.
It has gained considerable attention recently after being incorporated into WhatsApp [94], [95]. Assuming Axolotl is used
on two devices, the key material can evolve independently
for each device. However, if one of those devices remains
offline for a long time, a key compromise on that device
is problematic: if the device can use its outdated keys toread messages that were sent when it was offline, then thiscompromise defeats forward secrecy; if the device cannot
read the old messages, then the protocol does not achievecomplete multi-device support. Deciding how long a device
may be offline before it can no longer read buffered messages
is an adoption consideration requiring further study of user
behavior.

2) Key transport: Rather than relying on users to exchange
a secret password out-of-band, it is far better to automaticallyexchange a new secret for each conversation. A simple proposed method for doing this is to have one participant generatea session key and securely send it to the other participants.
These systems begin by establishing secure channels between
participants. The conversation initiator then generates a group
key and sends it to the other participants using the pairwise
channels. This design provides forward and backward secrecy
since a new group key is randomly generated for each conversation. Due to the use of a group leader, computational
and trust equality are also lost. However, groups are easily
expandable and contractible by having the initiator generateand distribute a new group key.
An early design of this type, proposed by Kikuchi etal. [97], suggests using a key directory to store static DH
public keys for users. When group chats are formed, thesekeys and are used to derive pairwise session keys for theparticipants. A modified DH exchange is used in order toallow the server to reduce the required computation for theclients. Participation repudiation is lost due to the design ofthe key exchange mechanism, whose security properties havenot been rigorously verified. An improvement, used in the
GROK protocol [98], is to use standard DH exchanges forthe pairwise channels, authenticated using long-term publickeys stored in the key directory. This improvement provides
authentication and anonymity preservation, but still suffersfrom the inherent inequality of key transport approaches.
3) Causality preservation: One issue that is rarely addressed in the design of conversation security protocols is
causality preservation. The user interface of the chat application must make design choices such as whether to display
messages immediately when they are received, or to buffer
them until causal predecessors have been received. However,the conversation security protocol must provide causality information in order to allow the interface to make these choices.
OldBlue [99] is a protocol that provides speaker consistency and causality preservation. An authenticated group key
agreement (GKA) protocol is executed at the start of theconversation. Messages are encrypted with the group key andthen signed with long-term asymmetric keys. This approach
to signatures eliminates message repudiation. To preservecausality, messages include a list of identifiers of messages thatcausally precede them. The OldBlue protocol conservatively
assumes that any message received by a user might influence
the messages they subsequently send. Therefore, all received
messages are considered to causally precede subsequently
transmitted messages. Message identifiers are hashes of thesender, the list of preceding identifiers, and the message contents. When a message has been lost, the client continuously
issues resend requests to the other clients.
A different approach is employed by KleeQ [100], a protocol designed for use by multiple trusted participants withtenuous connectivity. An authenticated multi-party DH exchange is performed to initiate the protocol. By authenticating
the parameters in a manner similar to OTR, participation

D. Group Chat Evaluation
1) Trusted central servers (baseline): The baseline protocol
described in Section V-C1, where clients simply connect to atrusted central server using TLS, can trivially support groupchats. While it is easy to add and remove group participantsin this system, the only thing preventing participants fromreading messages sent before or after they are part of thegroup is the trustworthiness of the server. This fact is indicatedby half circles for expandable / contractible membership.
SILC [96] in its default mode is an example of a protocol using
this design. While SILC’s architecture involves a networkof trusted servers similar to the IRC protocol, for analysis
purposes this network can be considered as one trusted entity.
To improve the security and privacy of these systems,
participants can simply encrypt and authenticate messages
before sending them to the server by using a pre-shared secret
key for the group. This approach is useful because it canbe applied as a layer on top of any existing infrastructure.
SILC has built-in support for this method in its “privatemode”; users can provide a password for a channel that isused to derive a pre-shared key unknown to the server. While
this design provides confidentiality and integrity, it does notprovide authentication.

16

repudiation can be provided. The group can be easily expanded
by incorporating the DH contribution of a new member into
the multi-party DH exchange, deriving a new group key.
However, the group is not contractible without restarting theprotocol. When two conversation participants can establish aconnection, they exchange the messages that the other is missing using a patching algorithm. All messages are encrypted
and authenticated with a MAC using keys derived from thegroup secret, providing message repudiation. Messages aresealed into blocks, which are sequences of messages having
the property that no messages could possibly be missing.
After each block is sealed, rekeying is performed using
the previous keys and the block contents. A mechanism isprovided to seal blocks even if some users are inactive inthe conversation. Speaker consistency is not guaranteed until
the messages have been sealed in a block. While participants
are authenticated during group formation, message contents
are not authenticated until after they have been sealed into
a block. The block sealing mechanism indirectly provides
participant consistency and destination validation. If malicious
participants send differing messages to others, this will be
uncovered during the block sealing phase. Manual resolution
is required to identify malicious participants.
4) OTR networks: Since OTR [83] provides desirable features for two-party conversations, it is natural to extend it toa group setting by using OTR to secure individual links in a
network. A basic strategy is to enlist a trusted entity to relay
messages and then secure client links to this entity using OTR.
This is the approach taken by the GOTR protocol released
in 2007 (we write the year to distinguish it from a different
protocol with the same name from 2013). GOTR (2007) [101]selects a participant to act as the relay, forming a star topologyof pairwise connections with the selected participant acting
as the hub. All authentication properties, speaker consistency,
and causality preservation are lost because they do not persist
across the relay node. Since the relay server can buffer
messages, asynchronicity is provided as long as the relay noderemains online. All other properties are inherited from OTR.
Groups can be expanded and contracted simply by establishing
new OTR connections to the relay.
Instead of using a star topology, pairwise OTR connections
between all participants can be established. This approach
restores authentication and anonymity preservation, as well as
equal trust between members. It is also possible to send messages to subgroups by only transmitting the message across
selected OTR links. The downside of this approach is that it
does not preserve causality or provide speaker consistency;
participants can send different messages to different people.
This design also incurs significant computational overhead. It
would be desirable to achieve these security properties without
this level of additional cost.
5) OTR for groups: Several protocols have been proposedto achieve OTR-like repudiation properties for group conversations. The TextSecure protocol can be naturally extended
to groups by sending messages to each recipient using thetwo-party TextSecure protocol [102]. Multicast encryption isused for performance: a single encrypted message is sent to acentral server for relaying to recipients while the decryption
key for the message is sent pairwise using TextSecure. In
practice, the wrapped decryption keys are attached to the samemessage for broadcasting. It is also possible to accomplish
this task using one of the many existing broadcast encryption
schemes [103]. This design does not provide any guarantees
of participant consistency, but it inherits the asynchronicity ofthe two-party TextSecure protocol. Speaker consistency andcausality preservation are achieved by attaching precedingmessage identifiers to messages. A message identifier is a hashof the sender, the list of preceding identifiers, and the message
contents.
A repudiable group chat scheme can also be designed byutilizing a deniable group key exchange (DGKE) protocol, as
in the mpOTR protocol [104], [105]. When completed, the
DGKE provides each participant with a shared secret group
key and individual ephemeral signing keys. This information isauthenticated with long-term keys in a manner providing participation repudiation while still authenticating participants—
participants receive proof of each other’s identities, but thisproof cannot be used to convince outsiders. All parties must
be online to complete the DGKE, so the protocol does notsupport asynchronicity. Messages are encrypted with theshared group key and signed with the ephemeral keys. The
ephemeral signatures provide proof of authorship to others in
the group but, because outsiders cannot be certain that theseephemeral signing keys correspond to specific long-term keys,
message repudiation is preserved. However, since all messages
from an individual are signed with the same (ephemeral)
key, the protocol does not have message unlinkability. Whenthe conversation has concluded, each participant hashes all
messages received from each other participant. The hashes arethen compared to ensure that everyone received the same set of
messages, providing speaker consistency. If this check fails,
messages must be individually compared to uncover discrepancies. This approach, where a consistency check is performed
only once at the conclusion of the conversation, does not work
if a network adversary disconnects users from the conversation
before the consistency check can be completed. In this worstcase scenario, the only information received by users is thatsomething went wrong at some point during the protocol, but
nothing more specific. Unfortunately, in many scenarios it isunclear how users should respond to this limited information.
In this scheme, subgroup messaging is not possible since all
messages share a single encryption key. The group is also not
expandable or contractible without performing a new DGKE.
A completely different approach is taken by the GOTR
(2013) protocol. GOTR (2013) [14] is built using a “hotpluggable” group key agreement (GKA) protocol, allowing
members to join and drop out of the conversation with little
overhead. This system involves the use of “circle keys”: setsof public keys having the property that a shared secret key
can be computed by anyone with a private key matching apublic key in the set. The key exchange mechanism in thisprotocol is relatively complex; we refer the interested reader

17

to the original publication for details [14]. Pairwise securechannels are set up between participants to send consistency
check messages. These consistency channels have the effectof providing global transcript order, but all participants are
required to be online to receive messages. The system otherwise provides features similar to mpOTR but with flexible
group membership and message unlinkability.

necessarily an insurmountable problem. Additional work inthis area is needed.
When it comes to group chat properties, we can identify
several areas for improvement in Table II. Classic protocols
often do not provide participant consistency or destination
validation, making them potentially vulnerable to surreptitious
forwarding or identity misbinding attacks. However, these aresometimes addressed in concrete implementations. The double
ratchet used in Axolotl improves forward secrecy with low costin performance, implementation complexity, and resilience,but it has not yet been thoroughly evaluated in an academic
context. Additionally, decentralized group chat systems inherently permit a participant to send different messages todifferent people. Due to network conditions, users can also
end up observing significantly different transcripts. Despite
these intrinsic weaknesses, surprisingly few protocols explicitly consider speaker consistency or causality preservation.
The recently proposed (n+1)sec protocol [108] is an exampleof new work in this area. (n+1)sec builds off of Abdalla etal.’s flexible group key exchange protocol [109] to provide a
DGKE and checks for transcript consistency.
Existing solutions achieve mixed results concerning repudiation. It is often debated whether repudiation is a desirable
feature and, if it is, whether or not it is worth pursuing. There
are situations in which the mere suspicion of authoring a given
message is potentially harmful to an author; in these cases,
repudiation is not useful, and participants should make useof a protocol providing anonymity instead. Even in scenarios
where repudiation is traditionally thought to be useful, such as
during criminal trials in societies requiring proof of authorship“beyond a reasonable doubt”, there are no prominent examplesof repudiable messaging properties influencing legal decisions.
Nonetheless, if it is possible to maintain repudiation within asecure messaging protocol without substantial cost, we believethat it remains a desirable property. Users typically think ofreal-world conversations as “off-the-record”, so it is natural
to desire (or expect) this property from a secure messaging
protocol. For the definitions of participation repudiation andmessage repudiation used in this work, the two-party protocols
based on authenticated DH key exchanges and the OTR-like
group protocols provide inexpensive solutions.
There are also additional adoption constraints imposed bymany modern secure group chat protocols. Group protocols
often choose to employ either a trusted participant or an
additional service to improve protocol performance, which
can lead to security concerns or introduce additional costs
for deployment. Very few group protocols support subgroupmessaging or changing group membership after the conversation has started without incurring the substantial costs ofa new protocol run. Additionally, many proposed designs
require synchronicity in order to simplify their protocols,
which largely precludes their use on current mobile devices.

E. Discussion
Similar to our study of trust establishment, Table II makes
immediately clear that no conversation security protocol provides all desired properties. Since most of the properties in thetable are not mutually exclusive, however, there is significant
room for improvement by combining protocol designs and thisshould be seen as a tangible and important call to action for
the research community.
Sadly, the most widely adopted solutions also have theworst security and privacy properties, with most non-securityfocused applications providing only basic static asymmetric
cryptography. This does not appear to be due to the usability
drawbacks of the more secure protocols: once the trust establishment has been done, all of the conversation security approaches we studied can be automated without any additional
effort for the user. An exception is enabling asynchronous
communication while still providing forward and backward
secrecy; the only solution for this problem that appears tohave any significant deployment in practice is the prekeys
approach implemented by TextSecure. This requires relatively
complicated infrastructure compared to a simple key server,
introduces problems for multi-device support, and is prone
to denial-of-service attacks if it is used in anonymous communication. This approach is poorly studied in the academic
literature. The FS-IBE scheme discussed in Section V-C3
promises to resolve the issues of server complexity and denialof service, but introduces new challenges such as scalability
and performance issues [75]. Unlike prekeys (Section V-C10),
this scheme has received a considerable amount of followup research and academic citations, but we are unaware ofany practical tool implementing it. In addition, a time-window
based FS-IBE scheme requires holding the ephemeral keys
for a certain amount of time to allow decryption of delayed
messages. One possible mitigation is to rely on an additional
server maintaining window counters where every window
number is used once, analogous to the prekeys approach. Improving the practicality of FS-IBE and puncturable encryption
schemes warrants further research.
Another outstanding concern that limits adoption of secure
conversation security protocols is the limited support for multiple devices. Despite a vast number of users owning multiple
devices, only the most insecure protocols support this property
without requiring users to perform pairing procedures. Device
pairing has proved extremely difficult for users in practice [106], [107] and allowing users to register multiple devices
with distinct keys is a major usability improvement. Althoughextremely difficult, implementing usable device pairing is not

VI. T RANSPORT P RIVACY
The transport privacy layer defines how messages are exchanged, with the goal of hiding message metadata such as

18

TABLE III
T RANSPORT PRIVACY SCHEMES . E VERY PRIVACY- ENHANCING APPROACH CARRIES USABILITY AND / OR ADOPTION COSTS .
ExamplePrivacy

Usability

AdoptionSen
Re derPa cipi An
rti ent on
Un cip. Anymit
G link An ony y
lo a on mba biil A lity ymi ty
ty
d
v
Co
.R
es
Nontacisttan
No Me Dit
M ssa scovg
Ea ess e era D yNosy I ge ela
Fe niti Dro ys
es ali ps
Re zatTo
qu ion
po
ire
No lo
d
g
Sp Ad y I
am dit nd
Lo /Fl ion epe
o a n
Low S od l Se den
to Re rv tw
L o B ras i
w an ge ista cedntCAs o wim d
Sc ynch pu th
al ro tatab n iole ou ns

SchemeStore-and-Forward†*
Email/XMPP - - +DHT Lookup†*
Kademlia
Onion Routing+Message Padding†*
Tor+Hidden Services*
Ricochet+Inbox Servers†+Random Delays†*
Mixminion+Hidden Services+Delays+Inboxes+ZKGP*
Pond
DC-Nets†*+Silent Rounds†
Anonycaster+Shuffle-Based DC-Net+Leader†
Dissent+Shuffle-Based DC-Net+Anytrust Servers†
Verdict
Message Broadcast†+Blockchain
PIR*
Pynchon Gate = provides property;

- - -

-

- -

- - - -

- -

- -

-

- - - ---= partially provides property; - = does not provide property; † has academic publication; * end-user tool available

No Message Delays: No long message delays are incurred.
No Message Drops: Dropped messages are retransmitted.
Easy Initialization: The user does not need to perform any
significant tasks before starting to communicate.
No Fees Required: The scheme does not require monetary
fees to be used.

the sender, receiver, and conversation to which the message
belongs. Some transport privacy architectures impose topological structures on the conversation security layer, while others
merely add privacy to data links between entities. The transport
privacy schemes may also be used for privacy-preserving
contact discovery. In this section, we compare approaches fortransport privacy in terms of the privacy features that they
provide, as well as usability concerns and other factors that
limit their adoption. Table III compares the various schemes.

C. Adoption Properties
Topology Independent: No network topology is imposed on
the conversation security or trust establishment schemes.
No Additional Service: The architecture does not depend on
availability of any infrastructure beyond the chat participants.
Spam/Flood Resistant: The availability of the system isresistant to denial-of-service attacks and bulk messaging.
Low Storage Consumption: The system does not require alarge amount of storage capacity for any entity.
Low Bandwidth: The system does not require a large
amount of bandwidth usage for any entity.
Low Computation: The system does not require a large
amount of processing power for any entity.
Asynchronous: Messages sent to recipients who are offline
will be delivered when the recipient reconnects, even if thesender has since disconnected.
Scalable: The amount of resources required to maintainsystem availability scales linearly with the number of users.

A. Privacy Features
We make the distinction between chat messages, which
are the user-generated payloads for the messaging protocol
to exchange, and protocol messages, which are the underlying
data transmissions dictated by the upper protocol layers. We
define following privacy properties:
Sender Anonymity: When a chat message is received, nonon-global entities except for the sender can determine which
entity produced the message.
Recipient Anonymity: No non-global entities except thereceiver of a chat message know which entity received it.
Participation Anonymity: No non-global entities except theconversation participants can discover which set of network
nodes are engaged in a conversation.
Unlinkability: No non-global entities except the conversation participants can discover that two protocol messages
belong to the same conversation.
Global Adversary Resistant: Global adversaries cannot
break the anonymity of the protocol.

D. Evaluation
1) Store-and-Forward (baseline): To evaluate the effectiveness and costs of different transport privacy architecturesin Table III, we compare the solutions to a baseline. For
the baseline protocol, we assume a simple store-and-forward
messaging protocol. This method is employed by email andtext messaging, causing minor message delays and storage

B. Usability Properties
Contact Discovery: The system provides a mechanism fordiscovering contact information.

19

requirements for intermediate servers. Since email headers
contain sender and recipient information, a simple store-andforward mechanism does not provide any privacy properties.
2) Peer-to-Peer Solutions: Instead of relying on centralized
servers for message storage and forwarding, peer-to-peer based
schemes try to establish a direct message exchange between
the participants. Since end users frequently change their IP
addresses, these systems often use Distributed Hash Tables(DHTs) to map usernames to IP addresses without a central
authority. Examples of popular DHT systems are Chord,
Kademlia (used by BitTorrent), and GNUnet [110]–[112]. In
addition to acting as an IP address lookup table, it is possible
to store exchanged messages directly in a DHT. Various query
privacy extensions have been proposed to prevent other users
from learning what data is being requested. They can be usedin advanced DHT overlays allowing anonymous queries andmessage exchange [113]–[115].
Global network adversaries are still able to see the traffic
flow between participants during message exchange. Thus,
clients have two options to protect the data flow: fake message transmissions, or use anonymization techniques. End-userclients might use services such as onion routing, which isevaluated in the next section, to hide their identities.
From the usability and adoption perspective, peer-to-peer
networks require a synchronous environment. DHTs can beused for contact discovery with easy initialization, but they
introduce message delays and message drops.
In practice, various end-user applications use the BitTorrent
or GNUnet networks for their secure messaging service. For
instance, Tox, Bleep, and other messengers use BitTorrent formessage exchange. The GNUnet Name Service (GNS) offers
privacy-preserving name queries for contact discovery [54].
3) Onion Routing: Onion routing is a method for communicating through multiple proxy servers that complicates endto-end message tracing [116]. In onion routing, senders send
messages wrapped in multiple layers of encryption throughpreselected paths—called circuits—of proxy servers. These
servers unwrap layers of encryption until the original message
is exposed, at which point it is relayed to the final destination.
Each node in the path only knows the immediate predecessor
and successor in the path. The routing process adds some latency to messages, but otherwise retains the baseline usability
features. An onion routing protocol, such as the widely used
Tor protocol [117], provides sender anonymity, participant
anonymity, and unlinkability against network attackers withlimited scope.
Global network adversaries are still able to break theanonymity properties of simple onion routing designs by performing statistical analysis incorporating features such as content size, transmission directions, counts, and timing, among
others. The success of such an adversary can be limited by individually eliminating these features. Protection can be added,
for example, by introducing random delays to transmissions.
The longer the allowed delays, the less statistical power isavailable to the adversary. Of course, this imposes potentiallylong message delays and additional storage requirements forrelays, making it unusable for synchronous instant messaging.
Unfortunately, random delays do not completely defeat
global adversaries. The only way to do so is to make transmission indistinguishable from no transmission (e.g., by saturating
the bandwidth of all connections). However, in practice, thisis likely infeasible. Additionally, concrete implementations
such as Tor often provide weaker anonymity guarantees thanidealized onion routing schemes. Several prominent attacks
against Tor have been based on implementation defects, limited resources, weaknesses introduced by performance tradeoffs, and predictability of the content being transmitted [118]–[121]. Adoption of onion routing is limited by the requirement
to establish a large network of nodes to provide a sufficientanonymity set and cover traffic.
In the default mode, onion routing systems do not attempt toprovide recipient anonymity. However, Tor can be extended toachieve this property using an extension called hidden services.
To create a Tor hidden service, the recipient uses traditional
Tor circuits to upload a set of introduction points and a public
key to a public database. The sender later uses a circuit toacquire this information from the database. The sender choosesa rendezvous point and sends it along with a nonce to therecipient through an introduction point. The recipient andsender both connect to the rendezvous point, which uses thenonce to establish a communication channel by matching up
the sender and recipient circuits. Without the aforementioned
defenses, this scheme is also vulnerable to global adversaries.
To provide asynchronous communication support, storeand-forward servers can be incorporated into the onion routing
model. Each user is associated with a Tor hidden service thatremains online. To send a message, the sender constructs a
circuit to the recipient’s server and transmits the message.
Users periodically poll their own servers to determine if
any messages are queued. Ricochet is an example of thisapproach [122].
Pond uses this design for its transmission architecture [123]but adds random delays between connections, all of whichtransmit the same amount of data, to weaken statistical analysis
by network adversaries. While some protection against global
network adversaries is provided by the onion routing model,
this protection is strictly weaker than Tor because connections
are made directly from senders to recipient mail servers.
This design requires storage commitments by servers and also
introduces very high latency.
Without additional protections, this scheme is also highly
vulnerable to denial-of-service attacks because connection
delays and fixed transmission sizes artificially limit bandwidth
to very low levels. Pond addresses this by requiring users tomaintain group lists secured by zero-knowledge-group-proof
schemes (ZKGP). This way, recipients can upload contact lists
without revealing their contacts. Simultaneously, senders canauthenticate by providing zero-knowledge proofs that they arein this list. The BBS signature scheme [124] is currently used
by Pond to achieve this. Additional work is underway toprovide a similar mechanism in more efficient manner by using
one-time delivery tokens [123].

20

The ZKGP schemes used by Pond are related to secrethandshake protocols. Secret handshakes enable authentication
between parties that share some attributes, while keeping
identities hidden from others [125].
4) DC-nets: Dining Cryptographer networks (DC-nets) areanonymity systems that are often compared to onion routing
schemes. Since they are primarily used as a general-purposetransport privacy mechanism, many varieties have been proposed [126]–[134]. In our brief overview, we focus on recently
introduced schemes that explicitly list secure messaging as anintended use case.
DC-nets are group protocols that execute in rounds. At thestart of each round, each participant either submits a secretmessage or no message. At the end of the round, all participants receive the xor of all secret messages submitted, without
knowing which message was submitted by which participant.
In this way, DC-nets provide sender anonymity while alsoachieving global adversary resilience—no statistical analysis
can reveal the sender of a message. Recipient anonymity can beachieved by using the protocol to publish an ephemeral public
key. Messages encrypted with this key are then sent and, since
the owner of the matching private key is unknown, the participant able to decrypt the messages cannot be determined. Since
messages are sent in rounds, DC-nets add message latency
and do not support asynchronous communication; dropped
messages prevent the protocol from advancing. Messages areeasily linked by observing which network nodes participate ina round. Additionally, DC-nets have limited scalability due torequiring pairwise communication.
The basic DC-net design has a problem with collisions: if
two parties submit a message in the same round, the result
will be corrupted. A malicious participant can exploit this toperform an anonymous denial-of-service attack by submitting
garbled messages each round. Worse still, an active network
attacker can also perform this attack by perturbing transmitted
bits. There are several approaches to mitigate this problem.
Anonycaster [131] adds pseudorandomly determined “silent
rounds” where all members know that no message should becontributed. Receipt of a message during a silent round indicates a denial-of-service attack by an active network attacker.
However, malicious participants can still launch attacks bysending garbled messages only during non-silent rounds.
Dissent [130], [132], [134] and Verdict [133] take a different
approach by constructing a DC-net system through the use ofa verifiable shuffle and bulk transfer protocol. Shuffle-based
DC-nets can include a blame protocol to pinpoint the entitythat caused a round to fail. Dissent appoints one participant
as a leader to manage round timing, the blame protocol,and exclusion of disconnected members from rounds, thereby
restoring support for asynchronicity. Verdict uses an alternative
approach where the DC-net protocol is executed by a setof central servers that clients connect to, providing greater
scalability and maintaining security as long as any one serveris honest.
While DC-nets are primarily a transport privacy mechanism,
they are distinguished from other schemes by their use ofrounds and the fact that every network node is also a participant in the conversation. When using DC-nets to transmit
higher-level conversation security protocols, it is important for
designers to consider how these properties affect the overall
security of the scheme (e.g., the use of synchronous rounds
creates a global transcript, and the details of the DC-net key
exchanges may cause a loss of participation repudiation).
5) Broadcast Systems: There is a simple approach to providing recipient anonymity against all attackers, including
global adversaries: distributing messages to everyone. This approach provides recipient anonymity, participation anonymity,
and unlinkability against all network attackers. It also provides
a natural way to discover contacts because requests for contact
data can be sent to the correct entity without knowledge ofany addressing information. However, there are some serious
downsides that hinder adoption: broadcasting a message toeveryone in the network requires high bandwidth, there isno support for asynchronicity, and it has extreme scalability
issues. Additionally, it is easy to attack the availability of
the network through flooding. Bitmessage [135], a broadcastbased transport system, either requires monetary fees or a
proof of work to send messages in order to limit spam, adding
computation requirements and message delays as represented
by the blockchains row in Table III. It is also possible toalleviate scalability problems by clustering users into smaller
broadcast groups, at the cost of reduced anonymity set sizes.
6) PIR: Private Information Retrieval (PIR) protocols allow
a user to query a database on a server without enabling theserver to determine what information was retrieved. Thesesystems, such as the Pynchon Gate [136], can be used to store
databases of message inboxes, as well as databases of contact
information. Recipient anonymity is provided because, while
the server knows the network node that is connecting to it, theserver cannot associate incoming connections with protocol
messages that they retrieve. For the same reason, the protocols
offer participation anonymity and unlinkability. By default,
there is no mechanism for providing sender anonymity. Thesesystems are naturally asynchronous, but they result in highlatency because inboxes must be polled. The servers also incura high storage cost and are vulnerable to flooding attacks.
PIR schemes can also be used to privately retrieve presence
information, which can be useful for augmenting synchronous
protocols lacking this capability. For example, DP5 [137] uses
PIR to privately provide presence data for a secure messaging
protocol; DP5 does not facilitate message transmission itself.
PIR implementations can be divided into computational
schemes, which rely on computational limitations of the server,
information-theoretic schemes, which rely on non-collusion ofservers, and hybrid schemes that combine properties of both.
There is also a class of PIR schemes that make use of secure
coprocessors, which require users to trust that the coprocessor
has not been compromised. PIR implementations differ in theirbandwidth, computation, and initialization costs, as well as
their scalability. PIR is not widely adopted in practice because
one or more of these costs is usually prohibitively high.

21

caution against a proliferation of a-la-carte systems for specificniches. The main purpose of communication networks is tointeract with others and there is considerable value in having asmall number of popular protocols that connect a large numberof users. Currently, many people fall back to email despite its
insecurity.
We also note that, disappointingly, most of the exciting
progress being made right now is by protocols that are either
completely proprietary (e.g., Apple iMessage) or are opensource but lack a rigorously specified protocol to facilitate
interoperable implementations (e.g., TextSecure). An open
standard for secure messaging, combining the most promisingfeatures identified by our survey, would be of immense value.
Inevitably, trade-offs have to be made. We conclude that
secure approaches in trust establishment perform poorly inusability and adoption, while more usable approaches lackstrong security guarantees. We consider the most promising
approach for trust establishment to be a combination of central
key directories, transparency logs to ensure global consistencyof the key directory’s entries, and a variety of options forsecurity-conscious users to verify keys out of band to put
pressure on the key directory to remain honest.
Our observations on the conversation security layer suggest that asynchronous environments and limited multi-device
support are not fully resolved. For two-party conversation
security, per-message ratcheting with resilience for out-oforder messages combined with deniable key exchange protocols, as implemented in Axolotl, can be employed today
at the cost of additional implementation complexity withno significant impact on user experience. The situation isless clear for secure group conversations; while no approachis a clear answer, the TextSecure group protocol provides
pragmatic security considerations while remaining practical. It
may be possible to achieve other desirable properties, such as
participant consistency and anonymity preservation, by incorporating techniques from the other systems. It remains unclear
exactly what consistency properties are required to match
users’ expectations and usability research is sorely needed to
guide future protocol design. Finally, transport privacy remainsa challenging problem. No suggested approaches managed
to provide strong transport privacy properties against global
adversaries while also remaining practical.
We consider this systematization to be a useful assessmentof published research and deployment experience. We haveuncovered many open challenges and interesting problems tobe solved by the research community. The active developmentof secure messaging tools offers a huge potential to provide
real-world benefits to millions; we hope this paper can serveas an inspiration and a basis for this important goal.

E. Discussion
If messages are secured end to end, leaving only identifiers for anonymous inboxes in the unencrypted header, thenmetadata is easily hidden from service operators. Assumingthat each message is sent using new channels, an adversary
is not able to link single messages to conversations. However,such schemes introduce adoption and usability issues; they
are prone to spam, flooding, and denial-of-service attacks,
or require expensive operations such as zero-knowledge authentication, posing barriers to adoption. Worse still, hiding
metadata from a global adversary in these schemes necessitates
serious usability problems such as long delays.
In contrast, decentralized schemes either exhibit synchronicity issues or have serious scalability problems. Most decentralized projects, especially BitTorrent-based approaches,lack detailed documentation that is required for complete
evaluation. Some tools claiming to hide metadata only do soin the absence of global network adversaries, which recent
surveillance revelations suggest may exist.
Broadcast-based schemes can achieve the best privacy
properties, but exhibit serious usability issues, such as lostor delayed messages, in addition to apparently intractablescalability issues. Even if anonymous transmission schemes
are adopted, they require a large user base to provide a highdegree of anonymity, potentially discouraging early adopters.
Finally, care must be taken when selecting a conversation
security scheme to avoid leaking cryptographic material or
identifiers that might lead to deanonymization.
VII. C ONCLUDING R EMARKS
The vast majority of the world’s electronic communication
still runs over legacy protocols such as SMTP, SMS/GSM,
and centralized messengers, none of which were designed with
end-to-end security in mind. We encourage the research community to view the high-profile NSA revelations in the UnitedStates as a golden opportunity to encourage the adoption of
secure systems in their place. As the old adage goes: “neverlet a crisis go to waste”.
Unfortunately, while we have seen considerable progress inpractical tools over the past two years, there is little evidence
suggesting that academic research on secure messaging has
dramatically increased. This is unfortunate for two reasons:First, many interesting problems of practical importance remain unresolved. In particular, apparent practical deployment
constraints, including limitations for asynchronous communication, multiple independent devices, and zero user effort,
are not fully appreciated in most published research papers.
Second, many theoretically solved problems are not consideredin practice, whether because developers are unaware of their
existence, or because they cannot immediately translate thecryptographic publications into working systems.
Our effort to systematize existing knowledge on secure messaging suggests three major problems must be resolved: trust
establishment, conversation security, and transport privacy.
The schemes can largely be chosen independently, yielding avast design space for secure messaging systems. Yet we also

ACKNOWLEDGMENTS
The authors would like to thank the anonymous reviewers
for their insightful comments, and Trevor Perrin and Henry
Corrigan-Gibbs for their discussion and excellent feedback.
Joseph Bonneau is supported by a Secure Usability Fellowshipfrom the Open Technology Fund and Simply Secure. We

22

gratefully acknowledge the support of NSERC and the Ontario
Research Fund.

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