Draft: draft-schaad-cose-rfc8152bis-struct

Commentary: These are the schaad-00 versions that were published a while ago .
This document along with I-D.schaad-cose-rfc8152bis-algs obsoletes
RFC8152.
The key is determined by the key and algorithm in the recipient
structure.  The encryption algorithm and size of the key to be
used are inputs into the KDF used for the recipient.  (For
direct, the KDF can be thought of as the identity operation.)
Examples of these algorithms are found in Sections !!!DIRECT-
KDF!!!  and !!! ECDH !!! .
This document defines a set of common parameters for a COSE Key
object.  table-key-labels provides a summary of the parameters
defined in this section.  There are also parameters that are defined
for specific key types.  Key-type-specific parameters can be found in
I-D.schaad-cose-rfc8152bis-algs.
This parameter is used to identify the family of keys for this
structure and, thus, the set of key-type-specific parameters to be
found.  The set of values defined in this document can be found in
!!!! TABLE_KEY_TYPES !!! .  This parameter MUST be present in a
key object.  Implementations MUST verify that the key type is
appropriate for the algorithm being processed.  The key type MUST
be included as part of the trust decision process.
ECDSA DSS defines a signature algorithm using ECC.  Implementations
SHOULD use a deterministic version of ECDSA such as the one defined
in RFC6979.  The use of a deterministic signature algorithm allows
for systems to avoid relying on random number generators in order to
avoid generating the same value of 'k' (the per-message random
value).  Biased generation of the value 'k' can be attacked, and
collisions of this value leads to leaked keys.  It additionally
allows for doing deterministic tests for the signature algorithm.
The use of deterministic ECDSA does not lessen the need to have good
random number generation when creating the private key.
The ECDSA signature algorithm is parameterized with a hash function
(h).  In the event that the length of the hash function output is
greater than the group of the key, the leftmost bytes of the hash
output are used.
The algorithms defined in this document can be found in table_ecdsa.
This document defines ECDSA to work only with the curves P-256,
P-384, and P-521.  This document requires that the curves be encoded
using the 'EC2' (2 coordinate elliptic curve) key type.
Implementations need to check that the key type and curve are correct
when creating and verifying a signature.  Other documents can define
it to work with other curves and points in the future.
In order to promote interoperability, it is suggested that SHA-256 be
used only with curve P-256, SHA-384 be used only with curve P-384,
and SHA-512 be used with curve P-521.  This is aligned with the
recommendation in Section 4 of RFC5480.
The signature algorithm results in a pair of integers (R, S).  These
integers will be the same length as the length of the key used for
the signature process.  The signature is encoded by converting the
integers into byte strings of the same length as the key size.  The
length is rounded up to the nearest byte and is left padded with zero
bits to get to the correct length.  The two integers are then
concatenated together to form a byte string that is the resulting
signature.
The 'kty' field MUST be present, and it MUST be 'EC2'.
If the 'alg' field is present, it MUST match the ECDSA signature
algorithm being used.
If the 'key_ops' field is present, it MUST include 'sign' when
creating an ECDSA signature.
If the 'key_ops' field is present, it MUST include 'verify' when
verifying an ECDSA signature.
The security strength of the signature is no greater than the minimum
of the security strength associated with the bit length of the key
and the security strength of the hash function.
Note: Use of this technique is a good idea even when good random
number generation exists.  Doing so both reduces the possibility of
having the same value of 'k' in two signature operations and allows
for reproducible signature values, which helps testing.
There are two substitution attacks that can theoretically be mounted
against the ECDSA signature algorithm.
Changing the curve used to validate the signature: If one changes
the curve used to validate the signature, then potentially one
could have two messages with the same signature, each computed
under a different curve.  The only requirement on the new curve is
that its order be the same as the old one and it be acceptable to
the client.  An example would be to change from using the curve
secp256r1 (aka P-256) to using secp256k1.  (Both are 256-bit
curves.)  We currently do not have any way to deal with this
version of the attack except to restrict the overall set of curves
that can be used.
Change the hash function used to validate the signature: If one
either has two different hash functions of the same length or can
truncate a hash function down, then one could potentially find
collisions between the hash functions rather than within a single
hash function (for example, truncating SHA-512 to 256 bits might
collide with a SHA-256 bit hash value).  As the hash algorithm is
part of the signature algorithm identifier, this attack is
mitigated by including a signature algorithm identifier in the
protected header.
RFC8032 describes the elliptic curve signature scheme Edwards-curve
Digital Signature Algorithm (EdDSA).  In that document, the signature
algorithm is instantiated using parameters for edwards25519 and
edwards448 curves.  The document additionally describes two variants
of the EdDSA algorithm: Pure EdDSA, where no hash function is applied
to the content before signing, and HashEdDSA, where a hash function
is applied to the content before signing and the result of that hash
function is signed.  For EdDSA, the content to be signed (either the
message or the pre-hash value) is processed twice inside of the
signature algorithm.  For use with COSE, only the pure EdDSA version
is used.  This is because it is not expected that extremely large
contents are going to be needed and, based on the arrangement of the
message structure, the entire message is going to need to be held in
memory in order to create or verify a signature.  This means that
there does not appear to be a need to be able to do block updates of
the hash, followed by eliminating the message from memory.
Applications can provide the same features by defining the content of
the message as a hash value and transporting the COSE object (with
the hash value) and the content as separate items.
The algorithms defined in this document can be found in table-eddsa-
algs.  A single signature algorithm is defined, which can be used for
multiple curves.
RFC8032 describes the method of encoding the signature value.
The 'kty' field MUST be present, and it MUST be 'OKP' (Octet Key
Pair).
The 'crv' field MUST be present, and it MUST be a curve defined
for this signature algorithm.
If the 'alg' field is present, it MUST match 'EdDSA'.
If the 'key_ops' field is present, it MUST include 'sign' when
creating an EdDSA signature.
If the 'key_ops' field is present, it MUST include 'verify' when
verifying an EdDSA signature.
How public values are computed is not the same when looking at EdDSA
and Elliptic Curve Diffie-Hellman (ECDH); for this reason, they
should not be used with the other algorithm.
If batch signature verification is performed, a well-seeded
cryptographic random number generator is REQUIRED.  Signing and non-
batch signature verification are deterministic operations and do not
need random numbers of any kind.
HMAC RFC2104 RFC4231 was designed to deal with length extension
attacks.  The algorithm was also designed to allow for new hash
algorithms to be directly plugged in without changes to the hash
function.  The HMAC design process has been shown as solid since,
while the security of hash algorithms such as MD5 has decreased over
time; the security of HMAC combined with MD5 has not yet been shown
to be compromised RFC6151.
The HMAC algorithm is parameterized by an inner and outer padding, a
hash function (h), and an authentication tag value length.  For this
specification, the inner and outer padding are fixed to the values
set in RFC2104.  The length of the authentication tag corresponds to
the difficulty of producing a forgery.  For use in constrained
environments, we define a set of HMAC algorithms that are truncated.
There are currently no known issues with truncation; however, the
security strength of the message tag is correspondingly reduced in
strength.  When truncating, the leftmost tag length bits are kept and
transmitted.
The algorithms defined in this document can be found in table-hmac.
Some recipient algorithms carry the key while others derive a key
from secret data.  For those algorithms that carry the key (such as
AES Key Wrap), the size of the HMAC key SHOULD be the same size as
the underlying hash function.  For those algorithms that derive the
key (such as ECDH), the derived key MUST be the same size as the
underlying hash function.
If the 'alg' field is present, it MUST match the HMAC algorithm
being used.
If the 'key_ops' field is present, it MUST include 'MAC create'
when creating an HMAC authentication tag.
If the 'key_ops' field is present, it MUST include 'MAC verify'
when verifying an HMAC authentication tag.
Implementations creating and validating MAC values MUST validate that
the key type, key length, and algorithm are correct and appropriate
for the entities involved.
AES-CBC-MAC is parameterized by the key length, the authentication
tag length, and the IV used.  For all of these algorithms, the IV is
fixed to all zeros.  We provide an array of algorithms for various
key lengths and tag lengths.  The algorithms defined in this document
are found in table-aes-mac.
Keys may be obtained either from a key structure or from a recipient
structure.  Implementations creating and validating MAC values MUST
validate that the key type, key length, and algorithm are correct and
appropriate for the entities involved.
If the 'alg' field is present, it MUST match the AES-MAC algorithm
being used.
If the 'key_ops' field is present, it MUST include 'MAC create'
when creating an AES-MAC authentication tag.
If the 'key_ops' field is present, it MUST include 'MAC verify'
when verifying an AES-MAC authentication tag.
A number of attacks exist against Cipher Block Chaining Message
Authentication Code (CBC-MAC) that need to be considered.
Cipher Block Chaining (CBC) mode, if the same key is used for both
encryption and authentication operations, an attacker can produce
messages with a valid authentication code.
If the IV can be modified, then messages can be forged.  This is
addressed by fixing the IV to all zeros.
The Galois/Counter Mode (GCM) mode is a generic authenticated
encryption block cipher mode defined in AES-GCM.  The GCM mode is
combined with the AES block encryption algorithm to define an AEAD
cipher.
The GCM mode is parameterized by the size of the authentication tag
and the size of the nonce.  This document fixes the size of the nonce
at 96 bits.  The size of the authentication tag is limited to a small
set of values.  For this document however, the size of the
authentication tag is fixed at 128 bits.
The set of algorithms defined in this document are in table-AES-GCM.
If the 'alg' field is present, it MUST match the AES-GCM algorithm
being used.
The key and nonce pair MUST be unique for every message encrypted.
The total amount of data encrypted for a single key MUST NOT
exceed 2^39 - 256 bits.  An explicit check is required only in
environments where it is expected that it might be exceeded.
Consideration was given to supporting smaller tag values; the
constrained community would desire tag sizes in the 64-bit range.
Doing so drastically changes both the maximum messages size
(generally not an issue) and the number of times that a key can be
used.  Given that Counter with CBC-MAC (CCM) is the usual mode for
constrained environments, restricted modes are not supported.
CCM is a generic authentication encryption block cipher mode defined
in RFC3610.  The CCM mode is combined with the AES block encryption
algorithm to define a commonly used content encryption algorithm used
in constrained devices.
The CCM mode has two parameter choices.  The first choice is M, the
size of the authentication field.  The choice of the value for M
involves a trade-off between message growth (from the tag) and the
probability that an attacker can undetectably modify a message.  The
second choice is L, the size of the length field.  This value
requires a trade-off between the maximum message size and the size of
the Nonce.
It is unfortunate that the specification for CCM specified L and M as
a count of bytes rather than a count of bits.  This leads to possible
misunderstandings where AES-CCM-8 is frequently used to refer to a
version of CCM mode where the size of the authentication is 64 bits
and not 8 bits.  These values have traditionally been specified as
bit counts rather than byte counts.  This document will follow the
convention of using bit counts so that it is easier to compare the
different algorithms presented in this document.
We define a matrix of algorithms in this document over the values of
L and M.  Constrained devices are usually operating in situations
where they use short messages and want to avoid doing recipient-
specific cryptographic operations.  This favors smaller values of
both L and M.  Less-constrained devices will want to be able to use
larger messages and are more willing to generate new keys for every
operation.  This favors larger values of L and M.
This limits messages to 2^16 bytes (64 KiB) in length.  This is
sufficiently long for messages in the constrained world.  The
nonce length is 13 bytes allowing for 2^(13*8) possible values of
the nonce without repeating.
This limits messages to 2^64 bytes in length.  The nonce length is
7 bytes allowing for 2^56 possible values of the nonce without
repeating.
This produces a 64-bit authentication tag.  This implies that
there is a 1 in 2^64 chance that a modified message will
authenticate.
This produces a 128-bit authentication tag.  This implies that
there is a 1 in 2^128 chance that a modified message will
authenticate.
If the 'alg' field is present, it MUST match the AES-CCM algorithm
being used.
The key and nonce pair MUST be unique for every message encrypted.
Note that the value of L influences the number of unique nonces.
The total number of times the AES block cipher is used MUST NOT
exceed 2^61 operations.  This limitation is the sum of times the
block cipher is used in computing the MAC value and in performing
stream encryption operations.  An explicit check is required only
in environments where it is expected that it might be exceeded.
RFC3610 additionally calls out one other consideration of note.  It
is possible to do a pre-computation attack against the algorithm in
cases where portions of the plaintext are highly predictable.  This
reduces the security of the key size by half.  Ways to deal with this
attack include adding a random portion to the nonce value and/or
increasing the key size used.  Using a portion of the nonce for a
random value will decrease the number of messages that a single key
can be used for.  Increasing the key size may require more resources
in the constrained device.  See Sections 5 and 10 of RFC3610 for more
information.
ChaCha20 and Poly1305 combined together is an AEAD mode that is
defined in RFC7539.  This is an algorithm defined to be a cipher that
is not AES and thus would not suffer from any future weaknesses found
in AES.  These cryptographic functions are designed to be fast in
software-only implementations.
The ChaCha20/Poly1305 AEAD construction defined in RFC7539 has no
parameterization.  It takes a 256-bit key and a 96-bit nonce, as well
as the plaintext and additional data as inputs and produces the
ciphertext as an option.  We define one algorithm identifier for this
algorithm in Table-CHACHA.
If the 'alg' field is present, it MUST match the ChaCha20/Poly1305
algorithm being used.
The same KDF can be set up to deal with the first two types of
secrets in a different way.  The KDF defined in !!! HDKF !!!  is such
a function.  This is reflected in the set of algorithms defined for
the HMAC-based Extract-and-Expand Key Derivation Function (HKDF).
The HKDF key derivation algorithm is defined in RFC5869.
secret -- a shared value that is secret.  Secrets may be either
previously shared or derived from operations like a Diffie-Hellman
(DH) key agreement.
salt -- an optional value that is used to change the generation
process.  The salt value can be either public or private.  If the
salt is public and carried in the message, then the 'salt'
algorithm header parameter defined in HKDF_Alg_Params is used.
While RFC5869 suggests that the length of the salt be the same as
the length of the underlying hash value, any amount of salt will
improve the security as different key values will be generated.
This parameter is protected by being included in the key
computation and does not need to be separately authenticated.  The
salt value does not need to be unique for every message sent.
length -- the number of bytes of output that need to be generated.
context information -- Information that describes the context in
which the resulting value will be used.  Making this information
specific to the context in which the material is going to be used
ensures that the resulting material will always be tied to that
usage.  The context structure defined in context is used by the
KDFs in this document.
PRF -- The underlying pseudorandom function to be used in the HKDF
algorithm.  The PRF is encoded into the HKDF algorithm selection.
HKDF is defined to use HMAC as the underlying PRF.  However, it is
possible to use other functions in the same construct to provide a
different KDF that is more appropriate in the constrained world.
Specifically, one can use AES-CBC-MAC as the PRF for the expand step,
but not for the extract step.  When using a good random shared secret
of the correct length, the extract step can be skipped.  For the AES
algorithm versions, the extract step is always skipped.
The extract step cannot be skipped if the secret is not uniformly
random, for example, if it is the result of an ECDH key agreement
step.  This implies that the AES HKDF version cannot be used with
ECDH.  If the extract step is skipped, the 'salt' value is not used
as part of the HKDF functionality.
The algorithms defined in this document are found in table-hkdf.
The context information structure is used to ensure that the derived
keying material is "bound" to the context of the transaction.  The
context information structure used here is based on that defined in
SP800-56A.  By using CBOR for the encoding of the context information
structure, we automatically get the same type and length separation
of fields that is obtained by the use of ASN.1.  This means that
there is no need to encode the lengths for the base elements, as it
is done by the encoding used in JOSE (Section 4.6.2 of RFC7518).
The context information structure refers to PartyU and PartyV as the
two parties that are doing the key derivation.  Unless the
application protocol defines differently, we assign PartyU to the
entity that is creating the message and PartyV to the entity that is
receiving the message.  By doing this association, different keys
will be derived for each direction as the context information is
different in each direction.
Fields can be defined by the application.  This is commonly used
to assign fixed names to parties, but it can be used for other
items such as nonces.
Fields can be defined by usage of the output.  Examples of this
are the algorithm and key size that are being generated.
Fields can be defined by parameters from the message.  We define a
set of parameters in KDF_Context_Alg_Params that can be used to
carry the values associated with the context structure.  Examples
of this are identities and nonce values.  These parameters are
designed to be placed in the unprotected bucket of the recipient
structure; they do not need to be in the protected bucket since
they already are included in the cryptographic computation by
virtue of being included in the context structure.
This field indicates the algorithm for which the key material will
be used.  This normally is either a key wrap algorithm identifier
or a content encryption algorithm identifier.  The values are from
the "COSE Algorithms" registry.  This field is required to be
present.  The field exists in the context information so that if
the same environment is used for different algorithms, then
completely different keys will be generated for each of those
algorithms.  This practice means if algorithm A is broken and thus
is easier to find, the key derived for algorithm B will not be the
same as the key derived for algorithm A.
This contains the identity information for party U.  The
identities can be assigned in one of two manners.  First, a
protocol can assign identities based on roles.  For example,
the roles of "client" and "server" may be assigned to different
entities in the protocol.  Each entity would then use the
correct label for the data they send or receive.  The second
way for a protocol to assign identities is to use a name based
on a naming system (i.e., DNS, X.509 names).
We define an algorithm parameter 'PartyU identity' that can be
used to carry identity information in the message.  However,
identity information is often known as part of the protocol and
can thus be inferred rather than made explicit.  If identity
information is carried in the message, applications SHOULD have
a way of validating the supplied identity information.  The
identity information does not need to be specified and is set
to nil in that case.
This contains a nonce value.  The nonce can either be implicit
from the protocol or be carried as a value in the unprotected
headers.
We define an algorithm parameter 'PartyU nonce' that can be
used to carry this value in the message; however, the nonce
value could be determined by the application and the value
determined from elsewhere.
This option does not need to be specified and is set to nil in
that case.
This contains other information that is defined by the
protocol.  This option does not need to be specified and is set
to nil in that case.
This field holds information about party V.  The content of the
structure is the same as for the PartyUInfo but for party V.
This field contains public information that is mutually known to
both parties.
This is set to the number of bits of the desired output value.
This practice means if algorithm A can use two different key
lengths, the key derived for longer key size will not contain
the key for shorter key size as a prefix.
This field contains the protected parameter field.  If there
are no elements in the protected field, then use a zero-length
bstr.
This field is for free form data defined by the application.
An example is that an application could define two different
strings to be placed here to generate different keys for a data
stream versus a control stream.  This field is optional and
will only be present if the application defines a structure for
this information.  Applications that define this SHOULD use
CBOR to encode the data so that types and lengths are correctly
included.
This field contains private information that is mutually known
private information.  An example of this information would be a
preexisting shared secret.  (This could, for example, be used in
combination with an ECDH key agreement to provide a secondary
proof of identity.)  The field is optional and will only be
present if the application defines a structure for this
information.  Applications that define this SHOULD use CBOR to
encode the data so that types and lengths are correctly included.
The following CDDL fragment corresponds to the text above.
This recipient algorithm is the simplest; the identified key is
directly used as the key for the next layer down in the message.
There are no algorithm parameters defined for this algorithm.  The
algorithm identifier value is assigned in table-direct.
When this algorithm is used, the protected field MUST be zero length.
The key type MUST be 'Symmetric'.
These keys need to have some method to be regularly updated over
time.  All of the content encryption algorithms specified in this
document have limits on how many times a key can be used without
significant loss of security.
These keys need to be dedicated to a single algorithm.  There have
been a number of attacks developed over time when a single key is
used for multiple different algorithms.  One example of this is
the use of a single key for both the CBC encryption mode and the
CBC-MAC authentication mode.
Breaking one message means all messages are broken.  If an
adversary succeeds in determining the key for a single message,
then the key for all messages is also determined.
These recipient algorithms take a common shared secret between the
two parties and applies the HKDF function (HKDF), using the context
structure defined in context to transform the shared secret into the
CEK.  The 'protected' field can be of non-zero length.  Either the
'salt' parameter of HKDF or the 'PartyU nonce' parameter of the
context structure MUST be present.  The salt/nonce parameter can be
generated either randomly or deterministically.  The requirement is
that it be a unique value for the shared secret in question.
If the salt/nonce value is generated randomly, then it is suggested
that the length of the random value be the same length as the hash
function underlying HKDF.  While there is no way to guarantee that it
will be unique, there is a high probability that it will be unique.
If the salt/nonce value is generated deterministically, it can be
guaranteed to be unique, and thus there is no length requirement.
A new IV must be used for each message if the same key is used.  The
IV can be modified in a predictable manner, a random manner, or an
unpredictable manner (i.e., encrypting a counter).
The IV used for a key can also be generated from the same HKDF
functionality as the key is generated.  If HKDF is used for
generating the IV, the algorithm identifier is set to "IV-
GENERATION".
When these algorithms are used, the key type MUST be 'symmetric'.
The set of algorithms defined in this document can be found in table-
direct-kdf.
If the 'alg' field is present, it MUST match the algorithm being
used.
If the 'key_ops' field is present, it MUST include 'deriveKey' or
'deriveBits'.
The shared secret needs to have some method to be regularly updated
over time.  The shared secret forms the basis of trust.  Although not
used directly, it should still be subject to scheduled rotation.
While these methods do not provide for perfect forward secrecy, as
the same shared secret is used for all of the keys generated, if the
key for any single message is discovered, only the message (or series
of messages) using that derived key are compromised.  A new key
derivation step will generate a new key that requires the same amount
of work to get the key.
The AES Key Wrap algorithm is defined in RFC3394.  This algorithm
uses an AES key to wrap a value that is a multiple of 64 bits.  As
such, it can be used to wrap a key for any of the content encryption
algorithms defined in this document.  The algorithm requires a single
fixed parameter, the initial value.  This is fixed to the value
specified in Section 2.2.3.1 of RFC3394.  There are no public
parameters that vary on a per-invocation basis.  The protected header
field MUST be empty.
If the 'alg' field is present, it MUST match the AES Key Wrap
algorithm being used.
The mathematics for ECDH can be found in RFC6090.  In this document,
the algorithm is extended to be used with the two curves defined in
RFC7748.
Curve Type/Curve: The curve selected controls not only the size of
the shared secret, but the mathematics for computing the shared
secret.  The curve selected also controls how a point in the curve
is represented and what happens for the identity points on the
curve.  In this specification, we allow for a number of different
curves to be used.  A set of curves are defined in table-ec-
curves.
The math used to obtain the computed secret is based on the curve
selected and not on the ECDH algorithm.  For this reason, a new
algorithm does not need to be defined for each of the curves.
Computed Secret to Shared Secret: Once the computed secret is
known, the resulting value needs to be converted to a byte string
to run the KDF.  The x-coordinate is used for all of the curves
defined in this document.  For curves X25519 and X448, the
resulting value is used directly as it is a byte string of a known
length.  For the P-256, P-384, and P-521 curves, the x-coordinate
is run through the I2OSP function defined in RFC8017, using the
same computation for n as is defined in ECDSA.
Ephemeral-Static or Static-Static: The key agreement process may
be done using either a static or an ephemeral key for the sender's
side.  When using ephemeral keys, the sender MUST generate a new
ephemeral key for every key agreement operation.  The ephemeral
key is placed in the 'ephemeral key' parameter and MUST be present
for all algorithm identifiers that use ephemeral keys.  When using
static keys, the sender MUST either generate a new random value or
create a unique value.  For the KDFs used, this means either the
'salt' parameter for HKDF (HKDF_Alg_Params) or the 'PartyU nonce'
parameter for the context structure (KDF_Context_Alg_Params) MUST
be present (both can be present if desired).  The value in the
parameter MUST be unique for the pair of keys being used.  It is
acceptable to use a global counter that is incremented for every
static-static operation and use the resulting value.  When using
static keys, the static key should be identified to the recipient.
The static key can be identified either by providing the key
('static key') or by providing a key identifier for the static key
('static key id').  Both of these parameters are defined in table-
ecdh-es-parameter-table.
Key Derivation Algorithm: The result of an ECDH key agreement
process does not provide a uniformly random secret.  As such, it
needs to be run through a KDF in order to produce a usable key.
Processing the secret through a KDF also allows for the
introduction of context material: how the key is going to be used
and one-time material for static-static key agreement.  All of the
algorithms defined in this document use one of the HKDF algorithms
defined in HKDF with the context structure defined in context.
Key Wrap Algorithm: No key wrap algorithm is used.  This is
represented in table-ecdh-es-table as 'none'.  The key size for
the context structure is the content layer encryption algorithm
size.
The set of direct ECDH algorithms defined in this document are found
in table-ecdh-es-table.
This document defines these algorithms to be used with the curves
P-256, P-384, P-521, X25519, and X448.  Implementations MUST verify
that the key type and curve are correct.  Different curves are
restricted to different key types.  Implementations MUST verify that
the curve and algorithm are appropriate for the entities involved.
There is a method of checking that points provided from external
entities are valid.  For the 'EC2' key format, this can be done by
checking that the x and y values form a point on the curve.  For the
'OKP' format, there is no simple way to do point validation.
Consideration was given to requiring that the public keys of both
entities be provided as part of the key derivation process (as
recommended in Section 6.1 of RFC7748).  This was not done as COSE is
used in a store and forward format rather than in online key
exchange.  In order for this to be a problem, either the receiver
public key has to be chosen maliciously or the sender has to be
malicious.  In either case, all security evaporates anyway.
A proof of possession of the private key associated with the public
key is recommended when a key is moved from untrusted to trusted
(either by the end user or by the entity that is responsible for
making trust statements on keys).
These algorithms are defined in table-ecdh-es-table-wrap.
Key Wrap Algorithm: Any of the key wrap algorithms defined in
key_wrap_algs are supported.  The size of the key used for the key
wrap algorithm is fed into the KDF.  The set of identifiers are
found in table-ecdh-es-table-wrap.
The COSE_Key object defines a way to hold a single key object.  It is
still required that the members of individual key types be defined.
This section of the document is where we define an initial set of
members for specific key types.
For each of the key types, we define both public and private members.
The public members are what is transmitted to others for their usage.
Private members allow for the archival of keys by individuals.
However, there are some circumstances in which private keys may be
distributed to entities in a protocol.  Examples include: entities
that have poor random number generation, centralized key creation for
multi-cast type operations, and protocols in which a shared secret is
used as a bearer token for authorization purposes.
Two different key structures are defined for elliptic curve keys.
One version uses both an x-coordinate and a y-coordinate, potentially
with point compression ('EC2').  This is the traditional EC point
representation that is used in RFC5480.  The other version uses only
the x-coordinate as the y-coordinate is either to be recomputed or
not needed for the key agreement operation ('OKP').
Applications MUST check that the curve and the key type are
consistent and reject a key if they are not.
The traditional way of sending ECs has been to send either both the
x-coordinate and y-coordinate or the x-coordinate and a sign bit for
the y-coordinate.  The latter encoding has not been recommended in
the IETF due to potential IPR issues.  However, for operations in
constrained environments, the ability to shrink a message by not
sending the y-coordinate is potentially useful.
This contains an identifier of the curve to be used with the key.
The curves defined in this document for this key type can be found
in table-ec-curves.  Other curves may be registered in the future,
and private curves can be used as well.
This contains the x-coordinate for the EC point.  The integer is
converted to an octet string as defined in SEC1.  Leading zero
octets MUST be preserved.
This contains either the sign bit or the value of the y-coordinate
for the EC point.  When encoding the value y, the integer is
converted to an octet string (as defined in SEC1) and encoded as a
CBOR bstr.  Leading zero octets MUST be preserved.  The compressed
point encoding is also supported.  Compute the sign bit as laid
out in the Elliptic-Curve-Point-to-Octet-String Conversion
function of SEC1.  If the sign bit is zero, then encode y as a
CBOR false value; otherwise, encode y as a CBOR true value.  The
encoding of the infinity point is not supported.
For public keys, it is REQUIRED that 'crv', 'x', and 'y' be present
in the structure.  For private keys, it is REQUIRED that 'crv' and
'd' be present in the structure.  For private keys, it is RECOMMENDED
that 'x' and 'y' also be present, but they can be recomputed from the
required elements and omitting them saves on space.
A new key type is defined for Octet Key Pairs (OKP).  Do not assume
that keys using this type are elliptic curves.  This key type could
be used for other curve types (for example, mathematics based on
hyper-elliptic surfaces).
This contains an identifier of the curve to be used with the key.
The curves defined in this document for this key type can be found
in table-ec-curves.  Other curves may be registered in the future
and private curves can be used as well.
This contains the x-coordinate for the EC point.  The octet string
represents a little-endian encoding of x.
For public keys, it is REQUIRED that 'crv' and 'x' be present in the
structure.  For private keys, it is REQUIRED that 'crv' and 'd' be
present in the structure.  For private keys, it is RECOMMENDED that
'x' also be present, but it can be recomputed from the required
elements and omitting it saves on space.
Occasionally it is required that a symmetric key be transported
between entities.  This key structure allows for that to happen.
This contains the value of the key.
This key structure does not have a form that contains only public
members.  As it is expected that this key structure is going to be
transmitted, care must be taken that it is never transmitted
accidentally or insecurely.  For symmetric keys, it is REQUIRED that
'k' be present in the structure.
The restriction applies to the encoding of the COSE_KDF_Context,
the Sig_structure, the Enc_structure, and the MAC_structure.
The registeries and registrations listed below were created during
processing of RFC 8152 RFC8152.  The only known action at this time
is to update the references.
IANA has created a registry titled "COSE Header Parameters".  The
registry has been created to use the "Expert Review Required"
registration procedure RFC8126.  Guidelines for the experts are
provided in review.  It should be noted that, in addition to the
expert review, some portions of the registry require a specification,
potentially a Standards Track RFC, be supplied as well.
The initial contents of the registry is ...
IANA has created a registry titled "COSE Header Algorithm
Parameters".  The registry uses the "Expert Review Required"
registration procedure.  Expert review guidelines are provided in
review.
IANA has created a new registry titled "COSE Algorithms".  The
registry has been created to use the "Expert Review Required"
registration procedure.  Guidelines for the experts are provided in
review.  It should be noted that, in addition to the expert review,
some portions of the registry require a specification, potentially a
Standards Track RFC, be supplied as well.
IANA has created a registry titled "COSE Algorithms".  The registry
has been created to use the "Expert Review Required" registration
procedure.  Guidelines for the experts are provided in review.  It
should be noted that, in addition to the expert review, some portions
of the registry require a specification, potentially a Standards
Track RFC, be supplied as well.
The initial contents of the registry can be found in Tables
table_ecdsa, table-eddsa-algs, table-hmac, table-aes-mac, table-AES-
GCM, table-AES-CCM, Table-CHACHA, table-direct, table-direct-kdf,
table_aes_keywrap, table-ecdh-es-table, and table-ecdh-es-table-wrap.
All of the entries in the "References" column of this registry point
to this document.  All of the entries in the "Recommended" column are
set to "Yes".
IANA has created a registry titled "COSE Key Common Parameters".  The
registry has been created to use the "Expert Review Required"
registration procedure.  Guidelines for the experts are provided in
review.  It should be noted that, in addition to the expert review,
some portions of the registry require a specification, potentially a
Standards Track RFC, be supplied as well.
IANA has created a new registry titled "COSE Key Type Parameters".
The registry has been created to use the "Expert Review Required"
registration procedure.  Expert review guidelines are provided in
review.
IANA has created a registry titled "COSE Key Type Parameters".  The
registry has been created to use the "Expert Review Required"
registration procedure.  Expert review guidelines are provided in
review.
This registry has been initially populated by the values in Tables
table-ec2-keys, table-ec1-keys, and table-symmetric-keys.  All of the
entries in the "References" column of this registry point to this
document.
This registry has been initially populated by the values in
table_key_types.  The specification column for all of these entries
will be this document.
IANA has created a new registry titled "COSE Elliptic Curves".  The
registry has been created to use the "Expert Review Required"
registration procedure.  Guidelines for the experts are provided in
review.  It should be noted that, in addition to the expert review,
some portions of the registry require a specification, potentially a
Standards Track RFC, be supplied as well.
IANA has created a registry titled "COSE Elliptic Curves".  The
registry has been created to use the "Expert Review Required"
registration procedure.  Guidelines for the experts are provided in
review.  It should be noted that, in addition to the expert review,
some portions of the registry require a specification, potentially a
Standards Track RFC, be supplied as well.
This registry has been initially populated by the values in table-ec-
curves.  All of the entries in the "References" column of this
registry point to this document.  All of the entries in the
"Recommended" column are set to "Yes".
Concise Binary Object Representation (CBOR) is a data format designed
for small code size and small message size.  There is a need for the
ability to have basic security services defined for this data format.
This document defines the CBOR Object Signing and Encryption (COSE)
protocol.  This specification describes how to create and process
signatures, message authentication codes, and encryption using CBOR
for serialization.  This specification additionally describes how to
represent cryptographic keys using CBOR.
Concise Binary Object Representation (CBOR) is a data format designed
for small code size and small message size.  There is a need for the
ability to have basic security services defined for this data format.
This document defines the CBOR Object Signing and Encryption (COSE)
protocol.  This specification describes how to create and process
signatures, message authentication codes, and encryption using CBOR
for serialization.  COSE additionally describes how to represent
cryptographic keys using CBOR.
In this specification the conventions for the use of a number of
cryptographic algorithms with COSE.  The details of the structure of
COSE are defined in I-D.schaad-cose-rfc8152bis-struct.
This document along with I-D.schaad-cose-rfc8152bis-struct obsoletes
RFC8152.
CBOR has capabilities that are not present in JSON and are
appropriate to use.  One example of this is the fact that CBOR has
a method of encoding binary directly without first converting it
into a base64-encoded string.
COSE is not a direct copy of the JOSE specification.  In the
process of creating COSE, decisions that were made for JOSE were
re-examined.  In many cases, different results were decided on as
the criteria were not always the same.
Define a single top message structure so that encrypted, signed,
and MACed messages can easily be identified and still have a
consistent view.
Signed messages distinguish between the protected and unprotected
parameters that relate to the content from those that relate to
the signature.
MACed messages are separated from signed messages.
MACed messages have the ability to use the same set of recipient
algorithms as enveloped messages for obtaining the MAC
authentication key.
Use binary encodings for binary data rather than base64url
encodings.
Combine the authentication tag for encryption algorithms with the
ciphertext.
The set of cryptographic algorithms has been expanded in some
directions and trimmed in others.
When the words appear in lowercase, this interpretation does not
apply.
There is currently no standard CBOR grammar available for use by
specifications.  The CBOR structures are therefore described in
prose.
any -- non-specific value that permits all CBOR values to be
placed here.
bool -- a boolean value (true: major type 7, value 21; false:
major type 7, value 20).
bstr -- byte string (major type 2).
int -- an unsigned integer or a negative integer.
nil -- a null value (major type 7, value 22).
nint -- a negative integer (major type 1).
tstr -- a UTF-8 text string (major type 3).
uint -- an unsigned integer (major type 0).
FOO / BAR -- indicates that either FOO or BAR can appear here.
As well as the prose description, a version of a CBOR grammar is
presented in CDDL.  Since CDDL has not been published in an RFC, this
grammar may not work with the final version of CDDL.  The CDDL
grammar is informational; the prose description is normative.
CDDL expects the initial non-terminal symbol to be the first symbol
in the file.  For this reason, the first fragment of CDDL is
presented here.
The non-terminal Internal_Types is defined for dealing with the
automated validation tools used during the writing of this document.
It references those non-terminals that are used for security
computations but are not emitted for transport.
In JSON, maps are called objects and only have one kind of map key: a
string.  In COSE, we use strings, negative integers, and unsigned
integers as map keys.  The integers are used for compactness of
encoding and easy comparison.  The inclusion of strings allows for an
additional range of short encoded values to be used as well.  Since
the word "key" is mainly used in its other meaning, as a
cryptographic key, we use the term "label" for this usage as a map
key.
The presence of a label in a COSE map that is not a string or an
integer is an error.  Applications can either fail processing or
process messages with incorrect labels; however, they MUST NOT create
messages with incorrect labels.
A CDDL grammar fragment defines the non-terminal 'label', as in the
previous paragraph, and 'values', which permits any value to be used.
The core COSE specification consists of two documents.  I-D.schaad-
cose-rfc8152bis-struct contains the serialization structures and the
procedures for using the different cryptographic algorithms.  This
document provides for an initial set of algorithms that are then use
with those structures.
Authenticated Encryption (AE) RFC5116 algorithms are those encryption
algorithms that provide an authentication check of the contents
algorithm with the encryption service.
Authenticated Encryption (AE) RFC5116 algorithms are those encryption
algorithms that provide an authentication check of the plain text
contents as part of the encryption service.
The set of protected header parameters wrapped in a bstr.
The set of unprotected header parameters as a map.
The content of the message.  The content is either the plaintext
or the ciphertext as appropriate.  The content may be detached,
but the location is still used.  The content is wrapped in a bstr
when present and is a nil value when detached.
Elements after this point are dependent on the specific message type.
COSE messages are also built using the concept of layers to separate
different types of cryptographic concepts.  As an example of how this
works, consider the COSE_Encrypt message (EnvelopedData).  This
message type is broken into two layers: the content layer and the
recipient layer.  In the content layer, the plaintext is encrypted
and information about the encrypted message is placed.  In the
recipient layer, the content encryption key (CEK) is encrypted and
information about how it is encrypted for each recipient is placed.
A single layer version of the encryption message COSE_Encrypt0
(EnvelopedData0) is provided for cases where the CEK is pre-shared.
The specific message type is known from the context.  This may be
defined by a marker in the containing structure or by restrictions
specified by the application protocol.
The message type is identified by a CBOR tag.  Messages with a
CBOR tag are known in this specification as tagged messages, while
those without the CBOR tag are known as untagged messages.  This
document defines a CBOR tag for each of the message structures.
These tags can be found in CBOR-Tags.
When a COSE object is carried in a media type of 'application/
cose', the optional parameter 'cose-type' can be used to identify
the embedded object.  The parameter is OPTIONAL if the tagged
version of the structure is used.  The parameter is REQUIRED if
the untagged version of the structure is used.  The value to use
with the parameter for each of the structures can be found in
CBOR-Tags.
When a COSE object is carried as a CoAP payload, the CoAP Content-
Format Option can be used to identify the message content.  The
CoAP Content-Format values can be found in CoAP_content_type.  The
CBOR tag for the message structure is not required as each
security message is uniquely identified.
The following CDDL fragment identifies all of the top messages
defined in this document.  Separate non-terminals are defined for the
tagged and the untagged versions of the messages.
The structure of COSE has been designed to have two buckets of
information that are not considered to be part of the payload itself,
but are used for holding information about content, algorithms, keys,
or evaluation hints for the processing of the layer.  These two
buckets are available for use in all of the structures except for
keys.  While these buckets are present, they may not all be usable in
all instances.  For example, while the protected bucket is defined as
part of the recipient structure, some of the algorithms used for
recipient structures do not provide for authenticated data.  If this
is the case, the protected bucket is left empty.
Both buckets are implemented as CBOR maps.  The map key is a 'label'
(label).  The value portion is dependent on the definition for the
label.  Both maps use the same set of label/value pairs.  The integer
and string values for labels have been divided into several sections
including a standard range, a private range, and a range that is
dependent on the algorithm selected.  The defined labels can be found
in the "COSE Header Parameters" IANA registry (cose-header-key-
table).
Contains parameters about the current layer that are to be
cryptographically protected.  This bucket MUST be empty if it is
not going to be included in a cryptographic computation.  This
bucket is encoded in the message as a binary object.  This value
is obtained by CBOR encoding the protected map and wrapping it in
a bstr object.  Senders SHOULD encode a zero-length map as a zero-
length string rather than as a zero-length map (encoded as h'a0').
The zero-length binary encoding is preferred because it is both
shorter and the version used in the serialization structures for
cryptographic computation.  After encoding the map, the value is
wrapped in the binary object.  Recipients MUST accept both a zero-
length binary value and a zero-length map encoded in the binary
value.  The wrapping allows for the encoding of the protected map
to be transported with a greater chance that it will not be
altered in transit.  (Badly behaved intermediates could decode and
re-encode, but this will result in a failure to verify unless the
re-encoded byte string is identical to the decoded byte string.)
This avoids the problem of all parties needing to be able to do a
common canonical encoding.
Contains parameters about the current layer that are not
cryptographically protected.
Only parameters that deal with the current layer are to be placed at
that layer.  As an example of this, the parameter 'content type'
describes the content of the message being carried in the message.
As such, this parameter is placed only in the content layer and is
not placed in the recipient or signature layers.  In principle, one
should be able to process any given layer without reference to any
other layer.  With the exception of the COSE_Sign structure, the only
data that needs to cross layers is the cryptographic key.
The buckets are present in all of the security objects defined in
this document.  The fields in order are the 'protected' bucket (as a
CBOR 'bstr' type) and then the 'unprotected' bucket (as a CBOR 'map'
type).  The presence of both buckets is required.  The parameters
that go into the buckets come from the IANA "COSE Header Parameters"
registry (cose-header-key-table).  Some common parameters are defined
in the next section, but a number of parameters are defined
throughout this document.
Labels in each of the maps MUST be unique.  When processing messages,
if a label appears multiple times, the message MUST be rejected as
malformed.  Applications SHOULD verify that the same label does not
occur in both the protected and unprotected headers.  If the message
is not rejected as malformed, attributes MUST be obtained from the
protected bucket before they are obtained from the unprotected
bucket.
The following CDDL fragment represents the two header buckets.  A
group "Headers" is defined in CDDL that represents the two buckets in
which attributes are placed.  This group is used to provide these two
fields consistently in all locations.  A type is also defined that
represents the map of common headers.
This section defines a set of common header parameters.  A summary of
these parameters can be found in Header-Table.  This table should be
consulted to determine the value of label and the type of the value.
This parameter is used to indicate the algorithm used for the
security processing.  This parameter MUST be authenticated where
the ability to do so exists.  This support is provided by AEAD
algorithms or construction (COSE_Sign, COSE_Sign0, COSE_Mac, and
COSE_Mac0).  This authentication can be done either by placing the
header in the protected header bucket or as part of the externally
supplied data.  The value is taken from the "COSE Algorithms"
registry (see cose-algorithm-registry).
Integer labels in the range of 0 to 8 SHOULD be omitted.
Integer labels in the range -1 to -128 can be omitted as they
are algorithm dependent.  If an application can correctly
process an algorithm, it can be assumed that it will correctly
process all of the common parameters associated with that
algorithm.  Integer labels in the range -129 to -65536 SHOULD
be included as these would be less common parameters that might
not be generally supported.
Labels for parameters required for an application MAY be
omitted.  Applications should have a statement if the label can
be omitted.
The header parameter values indicated by 'crit' can be processed
by either the security library code or an application using a
security library; the only requirement is that the parameter is
processed.  If the 'crit' value list includes a value for which
the parameter is not in the protected bucket, this is a fatal
error in processing the message.
This parameter is used to indicate the content type of the data in
the payload or ciphertext fields.  Integers are from the "CoAP
Content-Formats" IANA registry table COAP.Formats.  Text values
following the syntax of "<type-name>/<subtype-name>" where <type-
name> and <subtype-name> are defined in Section 4.2 of RFC6838.
Leading and trailing whitespace is also omitted.  Textual content
values along with parameters and subparameters can be located
using the IANA "Media Types" registry.  Applications SHOULD
provide this parameter if the content structure is potentially
ambiguous.
This parameter identifies one piece of data that can be used as
input to find the needed cryptographic key.  The value of this
parameter can be matched against the 'kid' member in a COSE_Key
structure.  Other methods of key distribution can define an
equivalent field to be matched.  Applications MUST NOT assume that
'kid' values are unique.  There may be more than one key with the
same 'kid' value, so all of the keys associated with this 'kid'
may need to be checked.  The internal structure of 'kid' values is
not defined and cannot be relied on by applications.  Key
identifier values are hints about which key to use.  This is not a
security-critical field.  For this reason, it can be placed in the
unprotected headers bucket.
This parameter holds the Initialization Vector (IV) value.  For
some symmetric encryption algorithms, this may be referred to as a
nonce.  The IV can be placed in the unprotected header as
modifying the IV will cause the decryption to yield plaintext that
is readily detectable as garbled.
This parameter holds a part of the IV value.  When using the
COSE_Encrypt0 structure, a portion of the IV can be part of the
context associated with the key.  This field is used to carry a
value that causes the IV to be changed for each message.  The IV
can be placed in the unprotected header as modifying the IV will
cause the decryption to yield plaintext that is readily detectable
as garbled.  The 'Initialization Vector' and 'Partial
Initialization Vector' parameters MUST NOT both be present in the
same security layer.
Left-pad the Partial IV with zeros to the length of IV.
XOR the padded Partial IV with the context IV.
This parameter holds one or more counter signature values.
Counter signatures provide a method of having a second party sign
some data.  The counter signature parameter can occur as an
unprotected attribute in any of the following structures:
COSE_Sign1, COSE_Signature, COSE_Encrypt, COSE_recipient,
COSE_Encrypt0, COSE_Mac, and COSE_Mac0.  These structures all have
the same beginning elements, so that a consistent calculation of
the counter signature can be computed.  Details on computing
counter signatures are found in counter_signature.
The CDDL fragment that represents the set of headers defined in this
section is given below.  Each of the headers is tagged as optional
because they do not need to be in every map; headers required in
specific maps are discussed above.
COSE supports two different signature structures.  COSE_Sign allows
for one or more signatures to be applied to the same content.
COSE_Sign1 is restricted to a single signer.  The structures cannot
be converted between each other; as the signature computation
includes a parameter identifying which structure is being used, the
converted structure will fail signature validation.
The COSE_Sign structure allows for one or more signatures to be
applied to a message payload.  Parameters relating to the content and
parameters relating to the signature are carried along with the
signature itself.  These parameters may be authenticated by the
signature, or just present.  An example of a parameter about the
content is the content type.  Examples of parameters about the
signature would be the algorithm and key used to create the signature
and counter signatures.
When more than one signature is present, the successful validation
of one signature associated with a given signer is usually treated
as a successful signature by that signer.  However, there are some
application environments where other rules are needed.  An
application that employs a rule other than one valid signature for
each signer must specify those rules.  Also, where simple matching
of the signer identifier is not sufficient to determine whether
the signatures were generated by the same signer, the application
specification must describe how to determine which signatures were
generated by the same signer.  Support for different communities
of recipients is the primary reason that signers choose to include
more than one signature.
For example, the COSE_Sign structure might include signatures
generated with the Edwards-curve Digital Signature Algorithm (EdDSA)
RFC8032 and with the Elliptic Curve Digital Signature Algorithm
(ECDSA) DSS.  This allows recipients to verify the signature
associated with one algorithm or the other.  More-detailed
information on multiple signature evaluations can be found in
RFC5752.
A COSE Signed Message is defined in two parts.  The CBOR object that
carries the body and information about the body is called the
COSE_Sign structure.  The CBOR object that carries the signature and
information about the signature is called the COSE_Signature
structure.  Examples of COSE Signed Messages can be found in
SignedExamples.
This field contains the serialized content to be signed.  If the
payload is not present in the message, the application is required
to supply the payload separately.  The payload is wrapped in a
bstr to ensure that it is transported without changes.  If the
payload is transported separately ("detached content"), then a nil
CBOR object is placed in this location, and it is the
responsibility of the application to ensure that it will be
transported without changes.
Note: When a signature with a message recovery algorithm is used
(SigAlgs), the maximum number of bytes that can be recovered is
the length of the payload.  The size of the payload is reduced by
the number of bytes that will be recovered.  If all of the bytes
of the payload are consumed, then the payload is encoded as a
zero-length binary string rather than as being absent.
This field is an array of signatures.  Each signature is
represented as a COSE_Signature structure.
The CDDL fragment that represents the above text for COSE_Sign
follows.
This field contains the computed signature value.  The type of the
field is a bstr.  Algorithms MUST specify padding if the signature
value is not a multiple of 8 bits.
The CDDL fragment that represents the above text for COSE_Signature
follows.
The COSE_Sign1 signature structure is used when only one signature is
going to be placed on a message.  The parameters dealing with the
content and the signature are placed in the same pair of buckets
rather than having the separation of COSE_Sign.
The CBOR object that carries the body, the signature, and the
information about the body and signature is called the COSE_Sign1
structure.  Examples of COSE_Sign1 messages can be found in
Sign1_Examples.
This is as described in full-signature.
This field contains the computed signature value.  The type of the
field is a bstr.
The CDDL fragment that represents the above text for COSE_Sign1
follows.
One of the features offered in the COSE document is the ability for
applications to provide additional data to be authenticated, but that
is not carried as part of the COSE object.  The primary reason for
supporting this can be seen by looking at the CoAP message structure
RFC7252, where the facility exists for options to be carried before
the payload.  Examples of data that can be placed in this location
would be the CoAP code or CoAP options.  If the data is in the header
section, then it is available for proxies to help in performing its
operations.  For example, the Accept Option can be used by a proxy to
determine if an appropriate value is in the proxy's cache.  But the
sender can prevent a proxy from changing the set of values that it
will accept by including that value in the resulting authentication
tag.  However, it may also be desired to protect these values so that
if they are modified in transit, it can be detected.
If multiple items are included, applications need to ensure that
the same byte string is not produced if there are different
inputs.  This could occur by appending the strings 'AB' and 'CDE'
or by appending the strings 'ABC' and 'DE'.  This is usually
addressed by making fields a fixed width and/or encoding the
length of the field as part of the output.  Using options from
CoAP RFC7252 as an example, these fields use a TLV structure so
they can be concatenated without any problems.
If multiple items are included, an order for the items needs to be
defined.  Using options from CoAP as an example, an application
could state that the fields are to be ordered by the option
number.
Applications need to ensure that the byte stream is going to be
the same on both sides.  Using options from CoAP might give a
problem if the same relative numbering is kept.  An intermediate
node could insert or remove an option, changing how the relative
number is done.  An application would need to specify that the
relative number must be re-encoded to be relative only to the
options that are in the external data.
"Signature" for signatures using the COSE_Signature structure.
"Signature1" for signatures using the COSE_Sign1 structure.
"CounterSignature" for signatures used as counter signature
attributes.
The protected attributes from the signer structure encoded in a
bstr type.  If there are no protected attributes, a bstr of length
zero is used.  This field is omitted for the COSE_Sign1 signature
structure.
The payload to be signed encoded in a bstr type.  The payload is
placed here independent of how it is transported.
Create a Sig_structure and populate it with the appropriate
fields.
Call the signature creation algorithm passing in K (the key to
sign with), alg (the algorithm to sign with), and ToBeSigned (the
value to sign).
Place the resulting signature value in the 'signature' field of
the array.
Create a Sig_structure object and populate it with the appropriate
fields.
Call the signature verification algorithm passing in K (the key to
verify with), alg (the algorithm used sign with), ToBeSigned (the
value to sign), and sig (the signature to be verified).
In addition to performing the signature verification, the application
may also perform the appropriate checks to ensure that the key is
correctly paired with the signing identity and that the signing
identity is authorized before performing actions.
Counter signatures provide a method of associating a different
signature generated by different signers with some piece of content.
This is normally used to provide a signature on a signature allowing
for a proof that a signature existed at a given time (i.e., a
Timestamp).  In this document, we allow for counter signatures to
exist in a greater number of environments.  As an example, it is
possible to place a counter signature in the unprotected attributes
of a COSE_Encrypt object.  This would allow for an intermediary to
either verify that the encrypted byte stream has not been modified,
without being able to decrypt it, or assert that an encrypted byte
stream either existed at a given time or passed through it in terms
of routing (i.e., a proxy signature).
An example of a counter signature on a signature can be found in
Appendix_B_1_3.  An example of a counter signature in an encryption
object can be found in Appendix_B_3_3.
The creation and validation of counter signatures over the different
items relies on the fact that the objects have the same structure.
The elements are a set of protected attributes, a set of unprotected
attributes, and a body, in that order.  This means that the
Sig_structure can be used in a uniform manner to get the byte stream
for processing a signature.  If the counter signature is going to be
computed over a COSE_Encrypt structure, the body_protected and
payload items can be mapped into the Sig_structure in the same manner
as from the COSE_Sign structure.
It should be noted that only a signature algorithm with appendix (see
SigAlgs) can be used for counter signatures.  This is because the
body should be able to be processed without having to evaluate the
counter signature, and this is not possible for signature schemes
with message recovery.
COSE supports two different encryption structures.  COSE_Encrypt0 is
used when a recipient structure is not needed because the key to be
used is known implicitly.  COSE_Encrypt is used the rest of the time.
This includes cases where there are multiple recipients or a
recipient algorithm other than direct is used.
The enveloped structure allows for one or more recipients of a
message.  There are provisions for parameters about the content and
parameters about the recipient information to be carried in the
message.  The protected parameters associated with the content are
authenticated by the content encryption algorithm.  The protected
parameters associated with the recipient are authenticated by the
recipient algorithm (when the algorithm supports it).  Examples of
parameters about the content are the type of the content and the
content encryption algorithm.  Examples of parameters about the
recipient are the recipient's key identifier and the recipient's
encryption algorithm.
The same techniques and structures are used for encrypting both the
plaintext and the keys.  This is different from the approach used by
both "Cryptographic Message Syntax (CMS)" RFC5652 and "JSON Web
Encryption (JWE)" RFC7516 where different structures are used for the
content layer and for the recipient layer.  Two structures are
defined: COSE_Encrypt to hold the encrypted content and
COSE_recipient to hold the encrypted keys for recipients.  Examples
of encrypted messages can be found in EnvelopedExamples.
This field contains the ciphertext encoded as a bstr.  If the
ciphertext is to be transported independently of the control
information about the encryption process (i.e., detached content),
then the field is encoded as a nil value.
This field contains an array of recipient information structures.
The type for the recipient information structure is a
COSE_recipient.
This field contains the encrypted key encoded as a bstr.  All
encoded keys are symmetric keys; the binary value of the key is
the content.  If there is not an encrypted key, then this field is
encoded as a nil value.
This field contains an array of recipient information structures.
The type for the recipient information structure is a
COSE_recipient (an example of this can be found in three-layer).
If there are no recipient information structures, this element is
absent.
The CEK is the same as the identified previously distributed
symmetric key or is derived from a previously distributed secret.
No CEK is transported in the message.
The CEK is encrypted using a previously distributed symmetric KEK.
Also known as key wrap.
The recipient's public key and a sender's private key are used to
generate a pairwise secret, a Key Derivation Function (KDF) is
applied to derive a key, and then the CEK is either the derived
key or encrypted by the derived key.
The CEK is encrypted with the recipient's public key.  No key
transport algorithms are defined in this document.
The CEK is encrypted in a KEK that is derived from a password.  No
password algorithms are defined in this document.
The COSE_Encrypt0 encrypted structure does not have the ability to
specify recipients of the message.  The structure assumes that the
recipient of the object will already know the identity of the key to
be used in order to decrypt the message.  If a key needs to be
identified to the recipient, the enveloped structure ought to be
used.
Examples of encrypted messages can be found in EnvelopedExamples.
"Encrypt0" for the content encryption of a COSE_Encrypt0 data
structure.
"Encrypt" for the first layer of a COSE_Encrypt data structure
(i.e., for content encryption).
"Enc_Recipient" for a recipient encoding to be placed in an
COSE_Encrypt data structure.
"Mac_Recipient" for a recipient encoding to be placed in a
MACed message structure.
"Rec_Recipient" for a recipient encoding to be placed in a
recipient structure.
Encode the Enc_structure to a byte stream (Additional
Authenticated Data (AAD)), using the encoding described in CBOR-
Canonical.
The key is randomly or pseudorandomly generated.
Call the encryption algorithm with K (the encryption key), P (the
plaintext), and AAD.  Place the returned ciphertext into the
'ciphertext' field of the structure.
Encode the Enc_structure to a byte stream (AAD), using the
encoding described in CBOR-Canonical.
Call the decryption algorithm with K (the decryption key to use),
C (the ciphertext), and AAD.
The key is randomly generated.
Call the encryption algorithm with K (the encryption key to use)
and P (the plaintext).  Place the returned ciphertext into the
'ciphertext' field of the structure.
Call the decryption algorithm with K (the decryption key to use)
and C (the ciphertext).
COSE supports two different MAC structures.  COSE_MAC0 is used when a
recipient structure is not needed because the key to be used is
implicitly known.  COSE_MAC is used for all other cases.  These
include a requirement for multiple recipients, the key being unknown,
and a recipient algorithm of other than direct.
In this section, we describe the structure and methods to be used
when doing MAC authentication in COSE.  This document allows for the
use of all of the same classes of recipient algorithms as are allowed
for encryption.
When using MAC operations, there are two modes in which they can be
used.  The first is just a check that the content has not been
changed since the MAC was computed.  Any class of recipient algorithm
can be used for this purpose.  The second mode is to both check that
the content has not been changed since the MAC was computed and to
use the recipient algorithm to verify who sent it.  The classes of
recipient algorithms that support this are those that use a pre-
shared secret or do static-static (SS) key agreement (without the key
wrap step).  In both of these cases, the entity that created and sent
the message MAC can be validated.  (This knowledge of the sender
assumes that there are only two parties involved and that you did not
send the message to yourself.)  The origination property can be
obtained with both of the MAC message structures.
The multiple recipient MACed message uses two structures: the
COSE_Mac structure defined in this section for carrying the body and
the COSE_recipient structure (EnvelopedData) to hold the key used for
the MAC computation.  Examples of MACed messages can be found in
MacExamples.
This field contains the serialized content to be MACed.  If the
payload is not present in the message, the application is required
to supply the payload separately.  The payload is wrapped in a
bstr to ensure that it is transported without changes.  If the
payload is transported separately (i.e., detached content), then a
nil CBOR value is placed in this location, and it is the
responsibility of the application to ensure that it will be
transported without changes.
The CDDL fragment that represents the above text for COSE_Mac
follows.
In this section, we describe the structure and methods to be used
when doing MAC authentication for those cases where the recipient is
implicitly known.
The MACed message uses the COSE_Mac0 structure defined in this
section for carrying the body.  Examples of MACed messages with an
implicit key can be found in Mac0Examples.
This is as described in Mac_n.
A text string that identifies the structure that is being encoded.
This string is "MAC" for the COSE_Mac structure.  This string is
"MAC0" for the COSE_Mac0 structure.
The protected attributes from the COSE_MAC structure.  If there
are no protected attributes, a zero-length bstr is used.
The payload to be MACed encoded in a bstr type.  The payload is
placed here independent of how it is transported.
Create a MAC_structure and populate it with the appropriate
fields.
Place the resulting MAC in the 'tag' field of the COSE_Mac or
COSE_Mac0 structure.
Encrypt and encode the MAC key for each recipient of the message.
Create a MAC_structure object and populate it with the appropriate
fields.
Obtain the cryptographic key from one of the recipients of the
message.
Compare the MAC value to the 'tag' field of the COSE_Mac or
COSE_Mac0 structure.
A COSE Key structure is built on a CBOR map object.  The set of
common parameters that can appear in a COSE Key can be found in the
IANA "COSE Key Common Parameters" registry (cose-key-map-registry).
Additional parameters defined for specific key types can be found in
the IANA "COSE Key Type Parameters" registry (cose-key-parameter-
registry).
A COSE Key Set uses a CBOR array object as its underlying type.  The
values of the array elements are COSE Keys.  A COSE Key Set MUST have
at least one element in the array.  Examples of COSE Key Sets can be
found in COSE_KEYS.
Each element in a COSE Key Set MUST be processed independently.  If
one element in a COSE Key Set is either malformed or uses a key that
is not understood by an application, that key is ignored and the
other keys are processed normally.
The element "kty" is a required element in a COSE_Key map.
This parameter is used to restrict the algorithm that is used with
the key.  If this parameter is present in the key structure, the
application MUST verify that this algorithm matches the algorithm
for which the key is being used.  If the algorithms do not match,
then this key object MUST NOT be used to perform the cryptographic
operation.  Note that the same key can be in a different key
structure with a different or no algorithm specified; however,
this is considered to be a poor security practice.
This parameter is used to give an identifier for a key.  The
identifier is not structured and can be anything from a user-
provided string to a value computed on the public portion of the
key.  This field is intended for matching against a 'kid'
parameter in a message in order to filter down the set of keys
that need to be checked.
This parameter is defined to restrict the set of operations that a
key is to be used for.  The value of the field is an array of
values from table-key-ops.  Algorithms define the values of key
ops that are permitted to appear and are required for specific
operations.  The set of values matches that in RFC7517 and
W3C.WebCrypto.
This parameter is defined to carry the base portion of an IV.  It
is designed to be used with the Partial IV header parameter
defined in cose-headers.  This field provides the ability to
associate a Partial IV with a key that is then modified on a per
message basis with the Partial IV.
Extreme care needs to be taken when using a Base IV in an
application.  Many encryption algorithms lose security if the same
IV is used twice.
If different keys are derived for each sender, using the same Base
IV with Partial IVs starting at zero is likely to ensure that the
IV would not be used twice for a single key.  If different keys
are derived for each sender, starting at the same Base IV is
likely to satisfy this condition.  If the same key is used for
multiple senders, then the application needs to provide for a
method of dividing the IV space up between the senders.  This
could be done by providing a different base point to start from or
a different Partial IV to start with and restricting the number of
messages to be sent before rekeying.
The second scheme is signature with message recovery (an example of
such an algorithm is PVSig).  In this scheme, the message content is
processed, but part of it is included in the signature.  Moving bytes
of the message content into the signature allows for smaller
signatures; the signature size is still potentially large, but the
message content has shrunk.  This has implications for systems
implementing these algorithms and for applications that use them.
The first is that the message content is not fully available until
after a signature has been validated.  Until that point, the part of
the message contained inside of the signature is unrecoverable.  The
second is that the security analysis of the strength of the signature
is very much based on the structure of the message content.  Messages
that are highly predictable require additional randomness to be
supplied as part of the signature process.  In the worst case, it
becomes the same as doing a signature with appendix.  Finally, in the
event that multiple signatures are applied to a message, all of the
signature algorithms are going to be required to consume the same
number of bytes of message content.  This means that the mixing of
the different schemes in a single message is not supported, and if a
recovery signature scheme is used, then the same amount of content
needs to be consumed by all of the signatures.
Signature algorithms are used with the COSE_Signature and COSE_Sign1
structures.  At this time, only signatures with appendixes are
defined for use with COSE; however, considerable interest has been
expressed in using a signature with message recovery algorithm due to
the effective size reduction that is possible.  Implementations will
need to keep this in mind for later possible integration.
The document defines signature algorithm identifiers for two
signature algorithms.
Message Authentication Codes (MACs) provide data authentication and
integrity protection.  They provide either no or very limited data
origination.  A MAC, for example, can be used to prove the identity
of the sender to a third party.
MACs use the same scheme as signature with appendix algorithms.  The
message content is processed and an authentication code is produced.
The authentication code is frequently called a tag.
MAC algorithms can be based on either a block cipher algorithm (i.e.,
AES-MAC) or a hash algorithm (i.e., a Hash-based Message
Authentication Code (HMAC)).  This document defines a MAC algorithm
using each of these constructions.
MAC algorithms are used in the COSE_Mac and COSE_Mac0 structures.
This section defines the usages for two MAC algorithms.
Content encryption algorithms provide data confidentiality for
potentially large blocks of data using a symmetric key.  They provide
integrity on the data that was encrypted; however, they provide
either no or very limited data origination.  (One cannot, for
example, be used to prove the identity of the sender to a third
party.)  The ability to provide data origination is linked to how the
CEK is obtained.
Most AEAD algorithms are logically defined as returning the message
content only if the decryption is valid.  Many but not all
implementations will follow this convention.  The message content
MUST NOT be used if the decryption does not validate.
These algorithms are used in COSE_Encrypt and COSE_Encrypt0.
This docuement defines the identifier and usages for three content
encryption algorithms.
Secrets that are uniformly random: This is the type of secret that
is created by a good random number generator.
Secrets that are not uniformly random: This is type of secret that
is created by operations like key agreement.
Secrets that are not random: This is the type of secret that
people generate for things like passwords.
General KDFs work well with the first type of secret, can do
reasonably well with the second type of secret, and generally do
poorly with the last type of secret.  None of the KDFs in this
section are designed to deal with the type of secrets that are used
for passwords.  Functions like PBES2 RFC8018 need to be used for that
type of secret.
When using KDFs, one component that is included is context
information.  Context information is used to allow for different
keying information to be derived from the same secret.  The use of
context-based keying material is considered to be a good security
practice.
Content key distribution methods (recipient algorithms) can be
defined into a number of different classes.  COSE has the ability to
support many classes of recipient algorithms.  In this section, a
number of classes are listed, and then a set of algorithms are
specified for each of the classes.  The names of the recipient
algorithm classes used here are the same as those defined in RFC7516.
Other specifications use different terms for the recipient algorithm
classes or do not support some of the recipient algorithm classes.
The direct encryption class algorithms share a secret between the
sender and the recipient that is used either directly or after
manipulation as the CEK.  When direct encryption mode is used, it
MUST be the only mode used on the message.
The 'protected' field MUST be a zero-length item unless it is used
in the computation of the content key.
The 'alg' parameter MUST be present.
A parameter identifying the shared secret SHOULD be present.
The 'ciphertext' field MUST be a zero-length item.
The 'recipients' field MUST be absent.
This document defines the identifiers and usage for a number of
content key distribution methods.
In key wrap mode, the CEK is randomly generated and that key is then
encrypted by a shared secret between the sender and the recipient.
All of the currently defined key wrap algorithms for COSE are AE
algorithms.  Key wrap mode is considered to be superior to direct
encryption if the system has any capability for doing random key
generation.  This is because the shared key is used to wrap random
data rather than data that has some degree of organization and may in
fact be repeating the same content.  The use of key wrap loses the
weak data origination that is provided by the direct encryption
algorithms.
The 'protected' field MUST be absent if the key wrap algorithm is
an AE algorithm.
The 'recipients' field is normally absent, but can be used.
Applications MUST deal with a recipient field being present, not
being able to decrypt that recipient is an acceptable way of
dealing with it.  Failing to process the message is not an
acceptable way of dealing with it.
The plaintext to be encrypted is the key from next layer down
(usually the content layer).
At a minimum, the 'unprotected' field MUST contain the 'alg'
parameter and SHOULD contain a parameter identifying the shared
secret.
Key transport mode is also called key encryption mode in some
standards.  Key transport mode differs from key wrap mode in that it
uses an asymmetric encryption algorithm rather than a symmetric
encryption algorithm to protect the key.  This document does not
define any key transport mode algorithms.
The 'protected' field MUST be absent.
At a minimum, the 'unprotected' field MUST contain the 'alg'
parameter and SHOULD contain a parameter identifying the
asymmetric key.
The 'direct key agreement' class of recipient algorithms uses a key
agreement method to create a shared secret.  A KDF is then applied to
the shared secret to derive a key to be used in protecting the data.
This key is normally used as a CEK or MAC key, but could be used for
other purposes if more than two layers are in use (see three-layer).
The most commonly used key agreement algorithm is Diffie-Hellman, but
other variants exist.  Since COSE is designed for a store and forward
environment rather than an online environment, many of the DH
variants cannot be used as the receiver of the message cannot provide
any dynamic key material.  One side effect of this is that perfect
forward secrecy (see RFC4949) is not achievable.  A static key will
always be used for the receiver of the COSE object.
Ephemeral-Static (ES) DH: where the sender of the message creates
a one-time DH key and uses a static key for the recipient.  The
use of the ephemeral sender key means that no additional random
input is needed as this is randomly generated for each message.
Static-Static DH: where a static key is used for both the sender
and the recipient.  The use of static keys allows for the
recipient to get a weak version of data origination for the
message.  When static-static key agreement is used, then some
piece of unique data for the KDF is required to ensure that a
different key is created for each message.
When direct key agreement mode is used, there MUST be only one
recipient in the message.  This method creates the key directly, and
that makes it difficult to mix with additional recipients.  If
multiple recipients are needed, then the version with key wrap needs
to be used.
At a minimum, headers MUST contain the 'alg' parameter and SHOULD
contain a parameter identifying the recipient's asymmetric key.
The headers SHOULD identify the sender's key for the static-static
versions and MUST contain the sender's ephemeral key for the
ephemeral-static versions.
The 'protected' field is fed into the KDF context structure.
The 'alg' parameter MUST be present in the layer.
A parameter identifying the recipient's key SHOULD be present.  A
parameter identifying the sender's key SHOULD be present.
The rules for "Canonical CBOR" (Section 3.9 of RFC 7049) MUST be
used in these locations.  The main rule that needs to be enforced
is that all lengths in these structures MUST be encoded such that
they are using definite lengths, and the minimum length encoding
is used.
Applications MUST NOT generate messages with the same label used
twice as a key in a single map.  Applications MUST NOT parse and
process messages with the same label used twice as a key in a
single map.  Applications can enforce the parse and process
requirement by using parsers that will fail the parse step or by
using parsers that will pass all keys to the application, and the
application can perform the check for duplicate keys.
This document is designed to provide a set of security services, but
not implementation requirements for specific usage.  The
interoperability requirements are provided for how each of the
individual services are used and how the algorithms are to be used
for interoperability.  The requirements about which algorithms and
which services are needed are deferred to each application.
An example of a profile can be found in OSCOAP where two profiles are
being developed.  One is for carrying content by itself, and the
other is for carrying content in combination with CoAP headers.
It is intended that a profile of this document be created that
defines the interoperability requirements for that specific
application.  This section provides a set of guidelines and topics
that need to be considered when profiling this document.
Applications need to determine the set of messages defined in this
document that they will be using.  The set of messages corresponds
fairly directly to the set of security services that are needed
and to the security levels needed.
Applications may define new header parameters for a specific
purpose.  Applications will often times select specific header
parameters to use or not to use.  For example, an application
would normally state a preference for using either the IV or the
Partial IV parameter.  If the Partial IV parameter is specified,
then the application would also need to define how the fixed
portion of the IV would be determined.
When applications use externally defined authenticated data, they
need to define how that data is encoded.  This document assumes
that the data will be provided as a byte stream.  More information
can be found in Extern_AAD.
Applications need to determine the set of security algorithms that
are to be used.  When selecting the algorithms to be used as the
mandatory-to-implement set, consideration should be given to
choosing different types of algorithms when two are chosen for a
specific purpose.  An example of this would be choosing HMAC-
SHA512 and AES-CMAC as different MAC algorithms; the construction
is vastly different between these two algorithms.  This means that
a weakening of one algorithm would be unlikely to lead to a
weakening of the other algorithms.  Of course, these algorithms do
not provide the same level of security and thus may not be
comparable for the desired security functionality.
Advertising in the message (S/MIME capabilities) RFC5751.
Advertising in the certificate (capabilities extension)
RFC4262.
Minimum requirements for the S/MIME, which have been updated
over time RFC2633 RFC5751 (note that RFC2633 has been obsoleted
by RFC5751).
IANA has assigned the following tags from the "CBOR Tags" registry.
The tags for COSE_Sign1, COSE_Encrypt0, and COSE_Mac0 were assigned
in the 1 to 23 value range (one byte long when encoded).  The tags
for COSE_Sign, COSE_Encrypt, and COSE_Mac were assigned in the 24 to
255 value range (two bytes long when encoded).
The tags assigned are in CBOR-Tags.
The name is present to make it easier to refer to and discuss the
registration entry.  The value is not used in the protocol.  Names
are to be unique in the table.
This is the value used for the label.  The label can be either an
integer or a string.  Registration in the table is based on the
value of the label requested.  Integer values between 1 and 255
and strings of length 1 are designated as "Standards Action".
Integer values from 256 to 65535 and strings of length 2 are
designated as "Specification Required".  Integer values of greater
than 65535 and strings of length greater than 2 are designated as
"Expert Review".  Integer values in the range -1 to -65536 are
"delegated to the COSE Header Algorithm Parameters registry".
Integer values less than -65536 are marked as private use.
This contains a pointer to the registry used to contain values
where the set is limited.
The name is present to make it easier to refer to and discuss the
registration entry.  The value is not used in the protocol.
The algorithm(s) that this registry entry is used for.  This value
is taken from the "COSE Algorithms" registry.  Multiple algorithms
can be specified in this entry.  For the table, the algorithm/
label pair MUST be unique.
This is the value used for the label.  The label is an integer in
the range of -1 to -65536.
This is a descriptive name that enables easier reference to the
item.  It is not used in the encoding.
The value to be used to identify this algorithm.  Key map labels
MUST be unique.  The label can be a positive integer, a negative
integer, or a string.  Integer values between 0 and 255 and
strings of length 1 are designated as "Standards Action".  Integer
values from 256 to 65535 and strings of length 2 are designated as
"Specification Required".  Integer values of greater than 65535
and strings of length greater than 2 are designated as "Expert
Review".  Integer values in the range -65536 to -1 are "used for
key parameters specific to a single algorithm delegated to the
COSE Key Type Parameters registry".  Integer values less than
-65536 are marked as private use.
This field contains the CBOR type for the field.
This field denotes the registry that values come from, if one
exists.
This field contains a brief description for the field.
This contains a pointer to the public specification for the field
if one exists.
This section registers the 'application/cose' media type in the
"Media Types" registry.  These media types are used to indicate that
the content is a COSE message.
Applications that use this media type: IoT applications sending
security content over HTTP(S) transports.
This section registers the 'application/cose-key' and 'application/
cose-key-set' media types in the "Media Types" registry.  These media
types are used to indicate, respectively, that content is a COSE_Key
or COSE_KeySet object.