FIPS 203
Federal Information Processing Standards Publication

Module-Lattice-Based
Key-Encapsulation Mechanism Standard
Category: Computer Security

Subcategory: Cryptography

Information Technology Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899-8900
This publication is available free of charge from:https://doi.org/10.6028/NIST.FIPS.203
Published August 13, 2024

U.S. Department of Commerce
Gina M. Raimondo, Secretary
National Institute of Standards and Technology
Laurie E. Locascio, NIST Director and Under Secretary of Commerce for Standards and Technology

Foreword
The Federal Information Processing Standards (FIPS) Publication Series of the National Institute of Standards and Technology is the official series of publications relating to standards and guidelines developed
under 15 U.S.C. 278g-3, and issued by the Secretary of Commerce under 40 U.S.C. 11331.
Comments concerning this Federal Information Processing Standard publication are welcomed and should
be submitted using the contact information in the “Inquiries and Comments” clause of the announcement
section.
Kevin M. Stine, Director
Information Technology Laboratory

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Abstract
A key-encapsulation mechanism (KEM) is a set of algorithms that, under certain conditions, can beused by two parties to establish a shared secret key over a public channel. A shared secret key thatis securely established using a KEM can then be used with symmetric-key cryptographic algorithms
to perform basic tasks in secure communications, such as encryption and authentication. This
standard specifies a key-encapsulation mechanism called ML-KEM. The security of ML-KEM isrelated to the computational difficulty of the Module Learning with Errors problem. At present,
ML-KEM is believed to be secure, even against adversaries who possess a quantum computer.
This standard specifies three parameter sets for ML-KEM. In order of increasing security strengthand decreasing performance, these are ML-KEM-512, ML-KEM-768, and ML-KEM-1024.
Keywords: computer security; cryptography; encryption; Federal Information Processing Standards; key-encapsulation mechanism; lattice-based cryptography; post-quantum; public-key
cryptography.

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Federal Information Processing Standards Publication 203
Published: August 13, 2024
Effective: August 13, 2024

Announcing theModule-Lattice-Based Key-Encapsulation
Mechanism Standard
Federal Information Processing Standards (FIPS) publications are developed by the National
Institute of Standards and Technology (NIST) under 15 U.S.C. 278g-3 and issued by the Secretaryof Commerce under 40 U.S.C. 11331.
1. Name of Standard. Module-Lattice-Based Key-Encapsulation Mechanism Standard (FIPS203).
2. Category of Standard. Computer Security. Subcategory. Cryptography.
3. Explanation. A cryptographic key (or simply “key”) is represented in a computer as a string ofbits. A shared secret key is a cryptographic key that is computed jointly by two parties (e.g.,
Alice and Bob) using a set of algorithms. Under certain conditions, these algorithms ensurethat both parties will produce the same key and that this key is secret from adversaries. Sucha shared secret key can then be used with symmetric-key cryptographic algorithms (specified
in other NIST standards) to perform tasks such as encryption and authentication of digital
information.
This standard specifies a set of algorithms for establishing a shared secret key. While there
are many methods for establishing a shared secret key, the particular method described inthis standard is a key-encapsulation mechanism (KEM).
In a KEM, the computation of the shared secret key begins with Alice generating a decapsulation key and an encapsulation key. Alice keeps the decapsulation key private and makes
the encapsulation key available to Bob. Bob then uses Alice’s encapsulation key to generate
one copy of a shared secret key along with an associated ciphertext. Bob then sends the
ciphertext to Alice. Finally, Alice uses the ciphertext from Bob along with Alice’s private
decapsulation key to compute another copy of the shared secret key.
The security of the particular KEM specified in this standard is related to the computationaldifficulty of solving certain systems of noisy linear equations, specifically the Module Learning With Errors (MLWE) problem. At present, it is believed that this particular method of
establishing a shared secret key is secure, even against adversaries who possess a quantum
computer. In the future, additional KEMs may be specified and approved in FIPS publications
or in NIST Special Publications.
4. Approving Authority. Secretary of Commerce.
5. Maintenance Agency. Department of Commerce, National Institute of Standards and Technology, Information Technology Laboratory (ITL).
iFIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

6. Applicability. Federal Information Processing Standards apply to information systems used or
operated by federal agencies or by a contractor of an agency or other organization on behalfof an agency. They do not apply to national security systems as defined in 44 U.S.C. 3552.
This standard, or other FIPS or NIST Special Publications that specify alternative mechanisms,
shall be used wherever the establishment of a shared secret key (or shared secret from which
keying material can be generated) is required for federal applications, including the use ofsuch a key with symmetric-key cryptographic algorithms, in accordance with applicable Officeof Management and Budget and agency policies.
The adoption and use of this standard are available to private and commercial organizations.
7. Implementations. A key-encapsulation mechanism may be implemented in software, firmware,
hardware, or any combination thereof. For every computational procedure that is specified
in this standard, a conforming implementation may replace the given set of steps with any
mathematically equivalent set of steps. In other words, different procedures that produce
the correct output for every input are permitted.
NIST will develop a validation program to test implementations for conformance to thealgorithms in this standard. Information about validation programs is available at https://csrc.nist.gov/projects/cmvp. Example values will be available at https://csrc.nist.gov/proj
ects/cryptographic-standards-and-guidelines/example-values.
8. Other Approved Security Functions. Implementations that comply with this standard
shall employ cryptographic algorithms that have been approved for protecting Federal
Government-sensitive information. Approved cryptographic algorithms and techniques
include those that are either:(a) Specified in a Federal Information Processing Standards (FIPS) publication,(b) Adopted in a FIPS or NIST recommendation, or(c) Specified in the list of approved security functions in SP 800-140C.
9. Export Control. Certain cryptographic devices and technical data regarding them are subject
to federal export controls. Exports of cryptographic modules that implement this standard
and technical data regarding them must comply with all federal laws and regulations andbe licensed by the Bureau of Industry and Security of the U.S. Department of Commerce.
Information about export regulations is available at https://www.bis.doc.gov.
10. Patents. NIST has entered into two patent license agreements to facilitate the adoption of
NIST’s announced selection of the PQC key-encapsulation mechanism CRYSTALS-KYBER. NIST
and the licensing parties share a desire, in the public interest, the licensed patents be freely
available to be practiced by any implementer of the ML-KEM algorithm as published by NIST.
ML-KEM is the name given to the algorithm in this standard derived from CRYSTALS-KYBER.
For a summary and extracts from the license, please see https://csrc.nist.gov/csrc/media/P
rojects/post-quantum-cryptography/documents/selected-algos-2022/nist-pqc-license-sum
mary-and-excerpts.pdf. Implementation of the algorithm specified in the standard may becovered by U.S. and foreign patents of which NIST is not aware.

ii

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

11. Implementation Schedule. This standard becomes effective immediately upon final publication.
12. Specifications. Federal Information Processing Standards (FIPS) 203, Module-Lattice-Based
Key-Encapsulation Mechanism Standard (affixed).
13. Qualifications. In applications, the security guarantees of a KEM only hold under certain
conditions (see SP 800-227 [1]). One such condition is the secrecy of several values, including
the randomness used by the two parties, the decapsulation key, and the shared secret key
itself. Users shall, therefore, guard against the disclosure of these values.
While it is the intent of this standard to specify general requirements for implementing
ML-KEM algorithms, conformance to this standard does not ensure that a particular implementation is secure. It is the responsibility of the implementer to ensure that any modulethat implements a key establishment capability is designed and built in a secure manner.
Similarly, the use of a product containing an implementation that conforms to this standard
does not guarantee the security of the overall system in which the product is used. The responsible authority in each agency or department shall ensure that an overall implementation
provides an acceptable level of security.
NIST will continue to follow developments in the analysis of the ML-KEM algorithm. As with
its other cryptographic algorithm standards, NIST will formally reevaluate this standard every
five years.
Both this standard and possible threats that reduce the security provided through the use ofthis standard will undergo review by NIST as appropriate, taking into account newly available
analysis and technology. In addition, the awareness of any breakthrough in technology or
any mathematical weakness of the algorithm will cause NIST to reevaluate this standard andprovide necessary revisions.
14. Waiver Procedure. The Federal Information Security Management Act (FISMA) does not allow
for waivers to Federal Information Processing Standards (FIPS) that are made mandatory by
the Secretary of Commerce.
15. Where to Obtain Copies of the Standard. This publication is available by accessing https://csrc.nist.gov/publications. Other computer security publications are available at the same
website.
16. How to Cite This Publication. NIST has assigned NIST FIPS 203 as the publication identifier
for this FIPS, per the NIST Technical Series Publication Identifier Syntax. NIST recommends
that it be cited as follows:
National Institute of Standards and Technology (2024) Module-Lattice-Based KeyEncapsulation Mechanism Standard. (Department of Commerce, Washington,
D.C.), Federal Information Processing Standards Publication (FIPS) NIST FIPS 203.
https://doi.org/10.6028/NIST.FIPS.203
17. Inquiries and Comments. Inquiries and comments about this FIPS may be submitted tofips-203-comments@nist.gov.

iii

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Federal Information Processing Standards Publication 203
Specification for the

Module-Lattice-Based Key-Encapsulation
Mechanism Standard
Table of Contents
1 Introduction1

1.1

Purpose and Scope1

1.2

Context

1

2 Terms, Acronyms, and Notation2

2.1

Terms and Definitions2

2.2

Acronyms

4

2.3

Mathematical Symbols

5

2.4

Interpreting the Pseudocode
2.4.1 Data Types
2.4.2 Loop Syntax
2.4.3 Arithmetic With Arrays of Integers
2.4.4 Representations of Algebraic Objects
2.4.5 Arithmetic With Polynomials and NTT Representations
2.4.6 Matrices and Vectors
2.4.7 Arithmetic With Matrices and Vectors
2.4.8 Applying Algorithms to Arrays, Examples

6
7
7
7
8
9
9
10
11

3 Overview of the ML-KEM Scheme

12

3.1

Key-Encapsulation Mechanisms

12

3.2

The ML-KEM Scheme

13

3.3

Requirements for ML-KEM Implementations

15

4 Auxiliary Algorithms

18

4.1

Cryptographic Functions

18

4.2

General Algorithms
4.2.1 Conversion and Compression Algorithms
4.2.2 Sampling Algorithms
The Number-Theoretic Transform

20
20
22
24

4.3

iv

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

4.3.1

Multiplication in the NTT Domain

5 The K-PKE Component Scheme

27
28

5.1

K-PKE Key Generation

28

5.2

K-PKE Encryption

29

5.3

K-PKE Decryption

31

6 Main Internal Algorithms

32

6.1

Internal Key Generation

32

6.2

Internal Encapsulation

32

6.3

Internal Decapsulation

33

7 The ML-KEM Key-Encapsulation Mechanism

35

7.1

ML-KEM Key Generation

35

7.2

ML-KEM Encapsulation

36

7.3

ML-KEM Decapsulation

37

8 Parameter Sets

39

References

41

Appendix A — Precomputed Values for the NTT

44

Appendix B — SampleNTT Loop Bounds

46

Appendix C — Differences From the CRYSTALS-KYBER Submission

47

C.1

Differences Between CRYSTALS-KYBER and FIPS 203 Initial Public Draft

47

C.2

Changes From FIPS 203 Initial Public Draft

47v

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

List of Tables
Table 1
Table 2
Table 3
Table 4

Decapsulation failure rates for ML-KEM
Approved parameter sets for ML-KEM
Sizes (in bytes) of keys and ciphertexts of ML-KEM
While-loop limits and probabilities of occurrence for SampleNTT

15
39
39
46

List of Figures
Figure 1

A simple view of key establishment using a KEM

12

List of Algorithms
Algorithm 1
Algorithm 2
Algorithm 3
Algorithm 4
Algorithm 5
Algorithm 6
Algorithm 7
Algorithm 8
Algorithm 9
Algorithm 10
Algorithm 11
Algorithm 12
Algorithm 13
Algorithm 14
Algorithm 15
Algorithm 16
Algorithm 17
Algorithm 18
Algorithm 19
Algorithm 20
Algorithm 21

ForExample() . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SHAKE128example(str1 , … , str𝑚 , 𝑏1 , … , 𝑏ℓ ) . . . . . . . . . . . . .
BitsToBytes(𝑏) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
BytesToBits(𝐵) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ByteEncode𝑑 (𝐹 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ByteDecode𝑑 (𝐵) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SampleNTT(𝐵) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SamplePolyCBD𝜂 (𝐵) . . . . . . . . . . . . . . . . . . . . . . . . .
NTT(𝑓) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
NTT−1 (𝑓)̂ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MultiplyNTTs(𝑓,̂ 𝑔)̂ . . . . . . . . . . . . . . . . . . . . . . . . . .
BaseCaseMultiply(𝑎0 , 𝑎1 , 𝑏0 , 𝑏1 , 𝛾) . . . . . . . . . . . . . . . . . .
K-PKE.KeyGen(𝑑) . . . . . . . . . . . . . . . . . . . . . . . . . . .
K-PKE.Encrypt(ekPKE , 𝑚, 𝑟) . . . . . . . . . . . . . . . . . . . . . . .
K-PKE.Decrypt(dkPKE , 𝑐) . . . . . . . . . . . . . . . . . . . . . . . .
ML-KEM.KeyGen_internal(𝑑, 𝑧) . . . . . . . . . . . . . . . . . . . .
ML-KEM.Encaps_internal(ek, 𝑚) . . . . . . . . . . . . . . . . . . .
ML-KEM.Decaps_internal(dk, 𝑐) . . . . . . . . . . . . . . . . . . . .
ML-KEM.KeyGen() . . . . . . . . . . . . . . . . . . . . . . . . . . .
ML-KEM.Encaps(ek) . . . . . . . . . . . . . . . . . . . . . . . . . .
ML-KEM.Decaps(dk, 𝑐) . . . . . . . . . . . . . . . . . . . . . . . .

vi

8
19
20
20
22
22
23
23
26
26
27
27
29
30
31
32
33
34
35
37
38

FIPS 203

1.

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Introduction

1.1 Purpose and Scope
This standard specifies the Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM). A
key-encapsulation mechanism (KEM) is a set of algorithms that can be used to establish a sharedsecret key between two parties communicating over a public channel. A KEM is a particular typeof key establishment scheme. Other NIST-approved key establishment schemes are specified
in NIST Special Publication (SP) 800-56A, Recommendation for Pair-Wise Key-Establishment
Schemes Using Discrete Logarithm-Based Cryptography [2], and SP 800-56B, Recommendation
for Pair-Wise Key Establishment Schemes Using Integer Factorization Cryptography [3].
The key establishment schemes specified in SP 800-56A and SP 800-56B are vulnerable toattacks that use sufficiently-capable quantum computers. ML-KEM is an approved alternativethat is presently believed to be secure, even against adversaries in possession of a large-scale
fault-tolerant quantum computer. ML-KEM is derived from the round-three version of the
CRYSTALS-KYBER KEM [4], a submission in the NIST Post-Quantum Cryptography Standardization
project. For the differences between ML-KEM and CRYSTALS-KYBER, see Appendix C.
This standard specifies the algorithms and parameter sets of the ML-KEM scheme. It aims
to provide sufficient information to implement ML-KEM in a manner that can pass validation(see https://csrc.nist.gov/projects/cryptographic- module- validation- program). For
general definitions and properties of KEMs, including requirements for the secure use of KEMs
in applications, see SP 800-227 [1].
This standard specifies three parameter sets for ML-KEM that offer different trade-offs in security
strength versus performance. All three parameter sets of ML-KEM are approved to protect
sensitive, non-classified communication systems of the U.S. Federal Government.

1.2 Context
Over the past several years, there has been steady progress toward building quantum computers.
If large-scale quantum computers are realized, the security of many commonly used public-key
cryptosystems will be at risk. This would include key-establishment schemes and digital signatureschemes whose security depends on the difficulty of solving the integer factorization and discrete
logarithm problems (both over finite fields and elliptic curves). As a result, in 2016, NIST initiateda public Post-Quantum Cryptography (PQC) Standardization process to select quantum-resistantpublic-key cryptographic algorithms. A total of 82 candidate algorithms were submitted to NIST
for consideration.
After three rounds of evaluation and analysis, NIST selected the first four algorithms for standardization. These algorithms are intended to protect sensitive U.S. Government information
well into the foreseeable future, including after the advent of cryptographically-relevant quantum computers. This standard specifies a variant of the selected algorithm CRYSTALS-KYBER, alattice-based key-encapsulation mechanism (KEM) designed by Peter Schwabe, Roberto Avanzi,
Joppe Bos, Léo Ducas, Eike Kiltz, Tancrède Lepoint, Vadim Lyubashevsky, John Schanck, Gregor
Seiler, Damien Stehlé, and Jintai Ding [4]. Throughout this standard, the KEM specified here will
be referred to as ML-KEM, as it is based on the Module Learning With Errors assumption.
1

FIPS 203

2.

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Terms, Acronyms, and Notation

2.1 Terms and Definitions
approved

FIPS-approved and/or NIST-recommended. An algorithm or techniquethat is either 1) specified in a FIPS or NIST recommendation, 2) adopted
in a FIPS or NIST recommendation, or 3) specified in a list of NIST-approved
security functions.

(KEM) ciphertext

A bit string that is produced by encapsulation and used as an input todecapsulation.

cryptographic
moduleThe set of hardware, software, and/or firmware that implements approved cryptographic functions (including key generation) that are contained within the cryptographic boundary of the module.

decapsulation

The process of applying the Decaps algorithm of a KEM. This algorithm
accepts a KEM ciphertext and the decapsulation key as input and produces a shared secret key as output.

decapsulation key

A cryptographic key produced by a KEM during key generation and usedduring the decapsulation process. The decapsulation key must be kept
private and must be destroyed after it is no longer needed. (See Section
3.3.)

decryption key

A cryptographic key that is used with a PKE in order to decrypt ciphertexts into plaintexts. The decryption key must be kept private and must
be destroyed after it is no longer needed.

destroy

An action applied to a key or other piece of secret data. After a pieceof secret data is destroyed, no information about its value can be recovered.

encapsulation

The process of applying the Encaps algorithm of a KEM. This algorithm
accepts the encapsulation key as input, requires private randomness,and produces a shared secret key and an associated ciphertext as output.

encapsulation key

A cryptographic key produced by a KEM during key generation and usedduring the encapsulation process. The encapsulation key can be madepublic. (See Section 3.3.)

encryption key

A cryptographic key that is used with a PKE in order to encrypt plaintexts
into ciphertexts. The encryption key can be made public.

equivalent process

Two processes are equivalent if the same output is produced when thesame values are input to each process (either as input parameters, asvalues made available during the process, or both).

fresh random value An output that was produced by a random bit generator and has notbeen previously used.
2

FIPS 203

hash function

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

A function on bit strings in which the length of the output is fixed.
Approved hash functions (such as those specified in FIPS 180 [5] andFIPS 202 [6]) are designed to satisfy the following properties:
1. (One-way) It is computationally infeasible to find any input thatmaps to any new pre-specified output.
2. (Collision-resistant) It is computationally infeasible to find any two
distinct inputs that map to the same output.

input checking

Examination of a potential input to an algorithm for the purpose ofdetermining whether it conforms to certain requirements.

key

A bit string that is used in conjunction with a cryptographic algorithm,
such as the encapsulation and decapsulation keys (of a KEM), the sharedsecret key (produced by a KEM), and the encryption and decryption
keys (of a PKE). (See Section 3.3.)

key-encapsulation
mechanism (KEM)

A set of three cryptographic algorithms (KeyGen, Encaps, and Decaps)that can be used by two parties to establish a shared secret key over a
public channel.

key establishment

A procedure that results in secret keying material that is shared among
different parties.

key pair

A set of two keys with the property that one key can be made public
while the other key must be kept private. In this standard, this could
refer to either the (encapsulation key, decapsulation key) key pair of a
KEM or the (encryption key, decryption key) key pair of a PKE.

little-endian

The property of a byte string having its bytes positioned in order ofincreasing significance. In particular, the leftmost (first) byte is theleast significant, and the rightmost (last) byte is the most significant.
The term “little-endian” may also be applied in the same manner to
bit strings (e.g., the 8-bit string 11010001 corresponds to the byte
20 + 21 + 23 + 27 = 139).

party

An individual person, organization, device, or process. In this specification, there are two parties (e.g., Party A and Party B, or Alice and Bob)
who jointly perform the key establishment process using a KEM.

pseudorandom

A process (or data produced by a process) is said to be pseudorandom
when the outcome is deterministic yet also appears random as longas the internal action of the process is hidden from observation. For
cryptographic purposes, “effectively random” means “computationallyindistinguishable from random within the limits of the intended security
strength.”

public channel

A communication channel between two parties. Such a channel can beobserved and possibly also corrupted by third parties.

3FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

public-key
encryption scheme(PKE)

A set of three cryptographic algorithms (KeyGen, Encrypt, and Decrypt)
that can be used by two parties to send secret data over a public channel.
Also known as an asymmetric encryption scheme.

shared secretA secret value that has been computed during a key-establishment
scheme, is known by both participants, and is used as input to a keyderivation method to produce keying material.

shared secret key

A shared secret that can be used directly as a cryptographic key insymmetric-key cryptography. It does not require additional key derivation. The shared secret key must be kept private and must be destroyedwhen no longer needed.

security category

A number associated with the security strength of a post-quantum
cryptographic algorithm, as specified by NIST (see [7]).

security strength

A number associated with the amount of work (i.e., the number of operations) that is required to break a cryptographic algorithm or system.

shallUsed to indicate a requirement of this standard.

shouldUsed to indicate a strong recommendation but not a requirement of
this standard. Ignoring the recommendation could lead to undesirable
results.

2.2 Acronyms
AES

Advanced Encryption Standard

CBD

Centered Binomial Distribution

FIPS

Federal Information Processing Standard

KEM

Key-Encapsulation Mechanism

LWE

Learning with Errors

MLWE

Module Learning with Errors

NIST

National Institute of Standards and Technology

NISTIRNIST Interagency or Internal ReportNTT

Number-Theoretic Transform

PKE

Public-Key Encryption

PQC

Post-Quantum Cryptography

PRF

Pseudorandom Function

RBG

Random Bit Generator

SHA

Secure Hash Algorithm
4FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISMSHAKESecure Hash Algorithm KECCAKSPSpecial Publication

XOF

Extendable-Output Function

2.3 Mathematical Symbols𝑛Denotes the integer 256 throughout this document.

𝑞

Denotes the prime integer 3329 = 28 ⋅ 13 + 1 throughout this document.

𝜁

Denotes the integer 17, which is a primitive 𝑛-th root of unity modulo 𝑞.

𝔹

The set {0, 1, … , 255} of unsigned 8-bit integers (bytes).

ℚ

The set of rational numbers.

ℤ

The set of integers.

ℤ𝑚

The ring of integers modulo 𝑚 (i.e., the set {0, 1, … , 𝑚 − 1} equipped withthe operations of addition and multiplication modulo 𝑚.)ℤ𝑛𝑚

The set of 𝑛-tuples over ℤ𝑚 equipped with ℤ𝑚 -module structure. As a data
type, this is the set of length-𝑛 arrays whose entries are in ℤ𝑚 .

𝑅𝑞

The ring ℤ𝑞 [𝑋]/(𝑋 𝑛 + 1) consisting of polynomials of the form 𝑓 = 𝑓0 +𝑓1 𝑋 + ⋯ + 𝑓255 𝑋 255 , where 𝑓𝑗 ∈ ℤ𝑞 for all 𝑗. The ring operations are addition and multiplication modulo 𝑋 𝑛 + 1.

𝑇𝑞

The image of 𝑅𝑞 under the number-theoretic transform. Its elements arecalled “NTT representations” of polynomials in 𝑅𝑞 . (See Section 4.3.)

D𝜂 (𝑅𝑞 )A certain distribution of polynomials in 𝑅𝑞 with small coefficients, fromwhich noise is sampled. The distribution is parameterized by 𝜂 ∈ {2, 3}. (See
Section 4.2.2.)

𝑆∗

If 𝑆 is a set, this denotes the set of finite-length tuples (or arrays) of elementsfrom the set 𝑆, including the empty tuple (or empty array).

𝑆𝑘

If 𝑆 is a set, this denotes the set of 𝑘-tuples (or length-𝑘 arrays) of elementsfrom the set 𝑆.

𝑓𝑗

The coefficient of 𝑋 𝑗 of a polynomial 𝑓 = 𝑓0 + 𝑓1 𝑋 + ⋯ + 𝑓255 𝑋 255 ∈ 𝑅𝑞 .

𝑓̂The element of 𝑇𝑞 that is equal to the NTT representation of a polynomial𝑓 ∈ 𝑅𝑞 . (See Sections 2.4.4 and 4.3.)

𝐯𝑇 , 𝐀𝑇The transpose of a row or column vector 𝐯. In general, the transpose of amatrix 𝐀.

5

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM∘

Denotes linear-algebraic composition with coefficients in 𝑅𝑞 or 𝑇𝑞 (e.g.,𝐀 ∘ 𝐯 denotes the vector resulting from applying matrix 𝐀 to vector 𝐯). (See
Section 2.4.7.)

×𝑇𝑞

Denotes the operation on coefficient arrays that implements product in thering 𝑇𝑞 . (See Sections 2.4.5 and 4.3.1.)

𝐴‖𝐵

The concatenation of two arrays or bit strings 𝐴 and 𝐵.

𝐵[𝑖]The entry at index 𝑖 in the array 𝐵. All arrays have indices that begin at zero.

𝐵[𝑘 ∶ 𝑚]The subarray (𝐵[𝑘], 𝐵[𝑘 + 1], … , 𝐵[𝑚 − 1]) of the array 𝐵.

|𝐵|

If 𝐵 is a number, this denotes the absolute value of 𝐵. If 𝐵 is an array, thisdenotes its length.

⌈𝑥⌉

The ceiling of 𝑥 (i.e., the smallest integer greater than or equal to 𝑥).

⌊𝑥⌋

The floor of 𝑥 (i.e., the largest integer less than or equal to 𝑥).

⌈𝑥⌋

The rounding of 𝑥 to the nearest integer. If 𝑥 = 𝑦 + 1/2 for some 𝑦 ∈ ℤ, then⌈𝑥⌋ = 𝑦 + 1.

∶=

Denotes that the left-hand side is defined to be the expression on the righthand side.

𝑟 mod 𝑚The unique integer 𝑟′ in {0, 1, … , 𝑚 − 1} such that 𝑚 divides 𝑟 − 𝑟′ .

BitRev7 (𝑟)Bit reversal of a seven-bit integer 𝑟. Specifically, if 𝑟 = 𝑟0 + 2𝑟1 + 4𝑟2 + ⋯ +64𝑟6 with 𝑟𝑖 ∈ {0, 1}, then BitRev7 (𝑟) = 𝑟6 + 2𝑟5 + 4𝑟4 + ⋯ + 64𝑟0 .

𝑠←𝑥

In pseudocode, this notation means that the variable 𝑠 is assigned the valueof the expression 𝑥.

$𝑠 ←− 𝔹ℓ

In pseudocode, this notation means that the variable 𝑠 is assigned the valueof an array of ℓ random bytes. The bytes must be freshly generated using
randomness from an approved RBG. (See Section 3.3.)⊥

A symbol indicating failure or the lack of output from an algorithm.

2.4 Interpreting the Pseudocode
This section outlines the conventions of the pseudocode used to describe the algorithms inthis standard. All algorithms are understood to have access to two global integer constants:
𝑛 = 256 and 𝑞 = 3329. There are also five global integer variables: 𝑘, 𝜂1 , 𝜂2 , 𝑑𝑢 , and 𝑑𝑣 . Allother variables are local. The five global variables are set to particular values when a parameter
set is selected (see Section 8).
When algorithms in this specification invoke other algorithms as subroutines, all arguments (i.e.,
inputs) are passed by value. In other words, a copy of the inputs is created, and the subroutine
is invoked with the copy. There is no “passing by reference.”
Pseudocode assignments are performed using the symbol “←.” For example, the statement𝑧 ← 𝑦 means that the variable 𝑧 is assigned the value of variable 𝑦. Pseudocode comparisons
6

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

are performed via the symbol “==.” For example, the expression 𝑥 == 𝑤 is a boolean valuethat is TRUE if and only if the variables 𝑥 and 𝑤 have the same value.
In regular text (i.e., outside of the pseudocode), a different convention is applied. There, the“=” symbol is used both for assigning values and for comparisons, in keeping with standard
mathematical notation. When emphasis is needed, assignments will be made with “∶=” instead.
Variables will always have a valid value that is appropriate to their data type, with two exceptions:
1. The outputs of a random bit generator (RBG) have the byte array data type but are also
allowed to have the special value NULL. This value indicates that randomness generation
failed. This can only occur in ML-KEM.KeyGen and ML-KEM.Encaps.
2. The outputs of ML-KEM.KeyGen and ML-KEM.Encaps have the byte array data type but
are also allowed to have the special value ⊥. When ML-KEM.KeyGen or ML-KEM.Encaps
return the value ⊥, this indicates that the algorithm failed due to a failure of randomness
generation.
2.4.1 Data Types
For variables that represent the input or output of an algorithm, the data type (e.g., bit, byte,
array of bits) will be explicitly described at the start of the algorithm. For most local variablesin the pseudocode, the data type is easily deduced from context. For all other variables, thedata type will be declared in a comment. In a single algorithm, the data type of a variable isdetermined the first time that the variable is used and will not be changed. Variable names can
and will be reused across different algorithms, including with different data types.
In addition to standard atomic data types (e.g., bits, bytes) and data structures (e.g., arrays),integers modulo 𝑚 (i.e., elements of ℤ𝑚 ) will also be used as an abstract data type. It is implicit
that reduction modulo 𝑚 takes place whenever an assignment is made to a variable in ℤ𝑚 . For
example, for 𝑧 ∈ ℤ𝑚 and integers 𝑥 and 𝑦, the statement𝑧 ← 𝑥+𝑦(2.1)

means that 𝑧 is assigned the value 𝑥 + 𝑦 mod 𝑚. The pseudocode is agnostic regarding how
an integer modulo 𝑚 is represented in actual implementations or how modular reduction iscomputed.
2.4.2

Loop Syntax

The pseudocode will make use of both “while” and “for” loops. The “while” syntax is selfexplanatory. In the case of “for” loops, the syntax will be in the style of the programming language
C. Two simple examples are given in Algorithm 1. The standard mathematical expression (e.g.,∑𝑛𝑖←1 (𝑖 + 3)) will be used for simple summations instead of a “for” loop.
2.4.3 Arithmetic With Arrays of Integers
This standard makes extensive use of arrays of integers modulo 𝑚 (i.e., elements of ℤℓ𝑚 ). In atypical case, the relevant values are 𝑚 = 𝑞 = 3329 and ℓ = 𝑛 = 256. Arithmetic with arrays in
7

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Algorithm 1 ForExample()
Performs two simple “for” loops.
1: for (𝑖 ← 0; 𝑖 < 10; 𝑖 ++)2:
𝐴[𝑖] ← 𝑖
3: end for
4: 𝑗 ← 0
5: for (𝑘 ← 256; 𝑘 > 1; 𝑘 ← 𝑘/2)
6:
𝐵[𝑗] ← 𝑘7:
𝑗 ← 𝑗+1
8: end for▷ 𝐴 is an integer array of length 10▷ 𝐴 now has the value (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)▷ 𝐵 is an integer array of length 8▷ 𝐵 now has the value (256, 128, 64, 32, 16, 8, 4, 2)ℓ. The statementsℤℓ𝑚 will be done as follows. Let 𝑎 ∈ ℤ𝑚 and 𝑋, 𝑌 ∈ ℤ𝑚

𝑍 ← 𝑎⋅𝑋
𝑊 ← 𝑋 +𝑌

(2.2)
(2.3)

will result in two arrays 𝑍, 𝑊 ∈ ℤℓ𝑚 , with the property that 𝑍[𝑖] = 𝑎 ⋅ 𝑋[𝑖] and 𝑊 [𝑖] = 𝑋[𝑖] + 𝑌 [𝑖]for all 𝑖. Multiplication of arrays in ℤℓ𝑚 will only be meaningful when 𝑚 = 𝑞 and ℓ = 𝑛 = 256, in
which case it corresponds to multiplication in a particular ring. This operation will be describedin (2.8).
2.4.4

Representations of Algebraic Objects

An essential operation in ML-KEM is the number-theoretic transform (NTT), which maps a polŷ an isomorphic ring 𝑇 . The rings 𝑅
nomial 𝑓 in a certain ring 𝑅𝑞 to its “NTT representation” 𝑓 in𝑞𝑞
and 𝑇𝑞 and the NTT are discussed in detail in Section 4.3. This standard will represent elementsof 𝑅𝑞 and 𝑇𝑞 in pseudocode using arrays of integers modulo 𝑞 as follows.
An element 𝑓 of 𝑅𝑞 is a polynomial of the form𝑓 = 𝑓0 + 𝑓1 𝑋 + ⋯ + 𝑓255 𝑋 255 ∈ 𝑅𝑞

(2.4)

and will be represented in pseudocode by the array(𝑓0 , 𝑓1 , … , 𝑓255 ) ∈ ℤ256
𝑞 ,

(2.5)

whose entries contain the coefficients of 𝑓. Overloading notation, the array in (2.5) will also bedenoted by 𝑓. The 𝑖-th entry of the array 𝑓 will thus contain the 𝑖-th coefficient of the polynomial
𝑓 (i.e., 𝑓[𝑖] = 𝑓𝑖 ).
An element (sometimes called “NTT representation”) 𝑔 ̂ of 𝑇𝑞 is a tuple of 128 polynomials, eachof degree at most one. Specifically,𝑔 ̂ = (𝑔0,0̂ + 𝑔0,1
̂ 𝑋, 𝑔1,0
̂ + 𝑔1,1
̂ 𝑋, … , 𝑔127,0̂+ 𝑔127,1
̂𝑋) ∈ 𝑇𝑞 .

8

(2.6)

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Such an algebraic object will be represented in pseudocode by the array(𝑔0,0
̂ , 𝑔0,1
̂ , 𝑔1,0̂ , 𝑔1,1
̂̂
̂ , … , 𝑔127,0, 𝑔127,1) ∈ ℤ256
𝑞 .

(2.7)

Overloading notation, the array in (2.7) will also be denoted by 𝑔.̂ In this case, the mapping
between array entries and coefficients is 𝑔[2𝑖]̂= 𝑔𝑖,0̂ and 𝑔[2𝑖
̂ + 1] = 𝑔𝑖,1
̂ for 𝑖 ∈ {0, 1, … , 127}.
Converting between a polynomial 𝑓 ∈ 𝑅𝑞 and its NTT representation 𝑓 ̂ ∈ 𝑇𝑞 will be done via thealgorithms NTT (Algorithm 9) and NTT−1 (Algorithm 10). These algorithms act on arrays ofcoefficients, as described above, and satisfy 𝑓 ̂ = NTT(𝑓) and 𝑓 = NTT−1 (𝑓)̂.
2.4.5

Arithmetic With Polynomials and NTT Representations

The algebraic operations of addition and scalar multiplication in 𝑅𝑞 and 𝑇𝑞 are done coordinatewise. For example, if 𝑎 ∈ ℤ𝑞 and 𝑓 ∈ 𝑅𝑞 , the 𝑖-th coefficient of the polynomial 𝑎 ⋅ 𝑓 ∈ 𝑅𝑞 is
equal to 𝑎 ⋅ 𝑓𝑖 mod 𝑞. In pseudocode, elements of both 𝑅𝑞 and 𝑇𝑞 are represented by coefficientarrays (i.e., elements of ℤ256𝑞 ). The algebraic operations of addition and scalar multiplication arethus performed by addition and scalar multiplication of the corresponding coefficient arrays,
as in (2.3) and (2.2). For example, the addition of two NTT representations in pseudocode iŝ 𝑔,̂ where ℎ,̂ 𝑓,̂ 𝑔 ̂ ∈ ℤ256 are coefficient arrays.
performed by a statement of the form ℎ̂ ← 𝑓 +𝑞
The algebraic operations of multiplication in 𝑅𝑞 and 𝑇𝑞 are treated as follows. For efficiency
reasons, multiplication in 𝑅𝑞 will not be used. The algebraic meaning of multiplication in 𝑇𝑞 is
discussed in Section 4.3.1. In pseudocode, it will be performed by the algorithm MultiplyNTTs(Algorithm 11). Specifically, if 𝑓,̂ 𝑔 ̂ ∈ ℤ256
are a pair of arrays (each representing the NTT of some𝑞polynomial), then̂ℎ̂ ← 𝑓 ×
𝑇𝑞 𝑔 ̂ℎ̂ ← MultiplyNTTs(𝑓,̂ 𝑔)̂ .

means

(2.8)

The result is an array ℎ̂ ∈ ℤ256
𝑞 .
2.4.6

Matrices and Vectors

In addition to arrays of integers modulo 𝑞, the pseudocode will also make use of arrays whose
256 3
entries are themselves elements of ℤ256
𝑞 . For example, an element 𝐯 ∈ (ℤ𝑞 ) will be a lengththree array whose entries 𝐯[0], 𝐯[1], and 𝐯[2] are themselves elements of ℤ256
(i.e., arrays). One𝑞
can think of each of these entries as representing a polynomial in 𝑅𝑞 so that 𝐯 itself represents
an element of the module 𝑅𝑞3 .
When arrays are used to represent matrices and vectors whose entries are elements of 𝑅𝑞 , they
will be denoted with bold letters (e.g., 𝐯 for vectors and 𝐀 for matrices). When arrays are used
to represent matrices and vectors whose entries are elements of 𝑇𝑞 , they will be denoted with a“hat” (e.g., 𝐯̂ and 𝐀̂). Unless an explicit transpose operation is performed, it is understood that
vectors are column vectors. One can then view vectors as the special case of matrices with only
one column.
Converting between matrices over 𝑅𝑞 and matrices over 𝑇𝑞 will be done coordinate-wise. For
9FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

𝑘example, if 𝐯 ∈ (ℤ256𝑞 ) , then the statement𝐯̂ ← NTT(𝐯)

(2.9)

𝑘
will result in 𝐯̂ ∈ (ℤ256̂ = NTT(𝐯[𝑖]) for all 𝑖. This involves running NTT a total𝑞 ) such that 𝐯[𝑖]of 𝑘 times.

2.4.7

Arithmetic With Matrices and Vectors

The following describes how to perform arithmetic with matrices over 𝑅𝑞 and 𝑇𝑞 with vectors asa special case.
Addition and scalar multiplication are performed coordinate-wise, so the addition of matrices
over 𝑅𝑞 and 𝑇𝑞 is straightforward. In the case of 𝑇𝑞 , scalar multiplication is done via (2.8). For𝑘example, if 𝑓 ̂ ∈ ℤ256
and 𝐮,̂ 𝐯̂ ∈ (ℤ256𝑞𝑞 ) , then𝐰̂ ← 𝑓 ⋅̂ 𝐮̂𝐳̂ ← 𝐮̂ + 𝐯̂(2.10)(2.11)𝑘̂
will result in 𝐰,̂ 𝐳̂ ∈ (ℤ256̂ =𝑓×
̂ and 𝐳[𝑖]
̂ = 𝐮[𝑖]̂ + 𝐯[𝑖]̂ for all 𝑖. Here, the𝑞 ) satisfying 𝐰[𝑖]𝑇𝑞 𝐮[𝑖]
multiplication and addition of individual entries are performed using the appropriate arithmetic
for coefficient arrays of elements of 𝑇𝑞 (i.e., as in (2.3)).

It will also be necessary to multiply matrices with entries in 𝑇𝑞 , which is done by using standard
matrix multiplication with the base-case multiplication (i.e., multiplication of individual entries)
being multiplication in 𝑇𝑞 . If 𝐀̂ and 𝐁̂ are two matrices with entries in 𝑇𝑞 , their matrix product̂ Some example pseudocode statements involving matrix multiplication
will be denoted 𝐀̂ ∘ 𝐁.
are given in (2.12), (2.13), and (2.14). In these examples, 𝐀̂ is a 𝑘 × 𝑘 matrix, while 𝐮̂ and 𝐯̂ arevectors of length 𝑘. All three of these objects are represented in pseudocode by arrays: a 𝑘 × 𝑘array for 𝐀̂ and length-𝑘 arrays for 𝐮̂ and 𝐯̂ . The entries of 𝐀̂, 𝐮̂ , and 𝐯̂ are elements of ℤ256
𝑞 . In(2.12) and (2.13), the pseudocode statement on the left produces a new length-𝑘 vector whose
entries are specified on the right. In (2.14), the pseudocode statement on the left computes adot product. The result is in the base ring (i.e., 𝑇𝑞 ) and is represented by an element 𝑧 ̂ of ℤ256𝑞 .
𝑘−1

𝐰̂ ← 𝐀̂ ∘ 𝐮̂̂ 𝑗] ×𝑇 𝐮[𝑗]𝐰[𝑖]
̂ = ∑ 𝐀[𝑖,̂𝑞

(2.12)

𝑗=0
𝑘−1

𝐲̂ ← 𝐀̂ ⊺ ∘ 𝐮̂̂ 𝑖] ×𝑇 𝐮[𝑗]𝐲[𝑖]
̂ = ∑ 𝐀[𝑗,̂𝑞

(2.13)

𝑗=0𝑘−1

𝑧 ̂ ← 𝐮̂ ⊺ ∘ 𝐯̂𝑧 ̂ = ∑ 𝐮[𝑗]
̂ ×𝑇𝑞 𝐯[𝑗]̂

(2.14)

𝑗=0

The multiplication ×𝑇𝑞 of individual entries above is performed using MultiplyNTTs, as describedin (2.8).

10

FIPS 203

2.4.8

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Applying Algorithms to Arrays, Examples

In the previous examples, arithmetic over ℤ𝑚 was extended to arithmetic with arrays over ℤ𝑚and then further extended to arithmetic with matrices whose entries are themselves arrays overℤ𝑚 . Similarly, algorithms defined with a given data type as input will be applied to arrays andmatrices over that data type. When the pseudocode invokes such an algorithm on an array or
matrix input, it is implied that the algorithm is invoked repeatedly and applied to each entry of
the input.
For example, consider the function Compress𝑑 ∶ ℤ𝑞 → ℤ2𝑑 defined in Section 4. It can be invokedon an array input 𝐹 ∈ ℤ256
with the statement𝑞
𝐾 ← Compress𝑑 (𝐹 ) .

(2.15)The result will be an array 𝐾 ∈ ℤ256
such that 𝐾[𝑖] = Compress𝑑 (𝐹 [𝑖]) for every 𝑖 ∈ {0, 1, … , 255}.
2𝑑
The computation (2.15) involves running the Compress algorithm 256 times.
For a second example, consider the algorithm NTT defined in Section 4.3. It takes an array 𝑓 ∈ℤ256(representing an element of 𝑅𝑞 ) as input and outputs another array 𝑓 ̂ ∈ ℤ256
(representing
𝑞
𝑞
256 𝑘
an element of 𝑇𝑞 ). If the NTT algorithm is invoked on a vector 𝐬 ∈ (ℤ𝑞 ) (representing anelement of 𝑅𝑞𝑘 ) with the pseudocode statement𝐬 ̂ ← NTT(𝐬) ,(2.16)𝑘̂ = NTT(𝐬[𝑖]) for all 𝑖 ∈ {0, 1, … , 𝑘 − 1}. The vector
the result is a vector 𝐬 ̂ ∈ (ℤ256𝑞 ) such that 𝐬[𝑖]𝑘𝐬̂ represents an element of 𝑇𝑞 . The computation (2.16) involves running the NTT algorithm 𝑘times.

For a third example, consider line 2 of K-PKE.Encrypt in Section 5.2:𝐭̂ ← ByteDecode12 (ekPKE [0 ∶ 384𝑘]) .

(2.17)

ByteDecode12 is defined to receive a byte array of length 32 ⋅ 12 = 384 as input and produce
an integer array in ℤ256
as output. The computation (2.17) is run on the first 384𝑘 bytes of𝑞𝑘byte array ekPKE and results in 𝐭̂ ∈ (ℤ256𝑞 ) . ByteDecode12 will thus be applied 𝑘 times, once for̂ ∈ ℤ256
each subarray ekPKE [384 ⋅ 𝑗, 384 ⋅ (𝑗 + 1)], and will result in an integer array 𝐭[𝑗]such that𝑞̂𝐭[𝑗] = ByteDecode (ekPKE [384 ⋅ 𝑗, 384 ⋅ (𝑗 + 1)]) for each 𝑗 from 0 to 𝑘 − 1.
1211

FIPS 203

3.

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Overview of the ML-KEM Scheme

This section gives a high-level overview of the ML-KEM scheme.

3.1 Key-Encapsulation Mechanisms
The following is a high-level overview of key-encapsulation mechanisms (KEMs). For details, see
SP 800-227 [1].
A KEM is a cryptographic scheme that, under certain conditions, can be used to establish a sharedsecret key between two communicating parties. This shared secret key can then be used forsymmetric-key cryptography.
A KEM consists of three algorithms and a collection of parameter sets. The three algorithms are:
1. A probabilistic key generation algorithm denoted by KeyGen
2. A probabilistic ”encapsulation” algorithm denoted by Encaps
3. A deterministic ”decapsulation” algorithm denoted by Decaps
The collection of parameter sets is used to select a trade-off between security and efficiency.
Each parameter set in the collection is a list of specific (typically numerical) values, one for eachparameter required by the three algorithms.

Alice

Bob

KeyGendecapsulation key

Decapsencapsulation key

ciphertext

EncapsAlice’s copy of theshared secret key

Bob’s copy of theshared secret key

𝐾′

𝐾

Figure 1. A simple view of key establishment using a KEM

12

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

In the typical application, a KEM is used to establish a shared secret key between two parties(here referred to as Alice and Bob) as described in Figure 1. Alice begins by running KeyGen inorder to generate a (public) encapsulation key and a (private) decapsulation key. Upon obtaining
Alice’s encapsulation key, Bob runs the Encaps algorithm, which produces Bob’s copy 𝐾 of theshared secret key along with an associated ciphertext. Bob sends the ciphertext to Alice, andAlice completes the process by running the Decaps algorithm using her decapsulation key andthe ciphertext. This final step produces Alice’s copy 𝐾 ′ of the shared secret key.
After completing this process, Alice and Bob would like to conclude that their outputs satisfy
𝐾 ′ = 𝐾 and that this value is a secure, random, shared secret key. However, these properties
only hold if certain important conditions are satisfied, as discussed in SP 800-227 [1].

3.2 The ML-KEM Scheme
ML-KEM is a key-encapsulation mechanism based on CRYSTALS-KYBER [4], a scheme that was
initially described in [8]. The following is a brief and informal description of the computational
assumption underlying ML-KEM and how the ML-KEM scheme is constructed.
The computational assumption. The security of ML-KEM is based on the presumed hardnessof the so-called Module Learning with Errors (MLWE) problem [9], which is a generalization of
the Learning With Errors (LWE) problem introduced by Regev in 2005 [10]. The hardness of the
MLWE problem is itself based on the presumed hardness of certain computational problems inmodule lattices [9]. This motivates the name of the scheme ML-KEM.
In the LWE problem, the input is a set of random “noisy” linear equations in some secretvariables 𝑥 ∈ ℤ𝑛𝑞 , and the task is to recover 𝑥. The noise in the equations is such that standard
algorithms (e.g., Gaussian elimination) are intractable. The LWE problem naturally lends itself tocryptographic applications. For example, if 𝑥 is interpreted as a secret key, then one can encrypta one-bit plaintext value by sampling either an approximately correct linear equation (if theplaintext is zero) or a far-from-correct linear equation (if the plaintext is one). Plausibly, only aparty in possession of 𝑥 can distinguish these two cases. Encryption can then be delegated toanother party by publishing a large collection of noisy linear equations, which can be combined
appropriately by the encrypting party. The result is an asymmetric encryption scheme.
The MLWE problem is similar to the LWE problem. An important difference is that, in MLWE, ℤ𝑛𝑞
is replaced by a certain module 𝑅𝑞𝑘 , which is constructed by taking the 𝑘-fold Cartesian productof a certain polynomial ring 𝑅𝑞 . In particular, the secret in the MLWE problem is an element 𝐱 ofthe module 𝑅𝑞𝑘 . The ring 𝑅𝑞 is discussed in detail in Section 4.3.
The ML-KEM construction. At a high level, the construction of the scheme ML-KEM proceeds in
two steps. First, the ideas discussed previously are used to construct a public-key encryption (PKE)
scheme from the MLWE problem. Second, this PKE scheme is converted into a key-encapsulation
mechanism using the so-called Fujisaki-Okamoto (FO) transform [11, 12]. Due to certain properties of the FO transform, the resulting KEM provides security in a significantly more general
attack model than the PKE scheme. As a result, ML-KEM is believed to satisfy so-called IND-CCA2
security [1, 4, 13, 14].
13

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

The specification of the ML-KEM algorithms in this standard will follow the same pattern. Specifically, this standard will first describe a public-key encryption scheme called K-PKE (in Section 5)
and then use the algorithms of K-PKE as subroutines when describing the algorithms of ML-KEM(in Sections 6 and 7). The cryptographic transformation from K-PKE to ML-KEM is crucial forachieving IND-CCA2 security. The scheme K-PKE is not IND-CCA2-secure and shall not be used asa stand-alone scheme (see Section 3.3).
A notable feature of ML-KEM is the use of the number-theoretic transform (NTT). The NTT̂ linear polynomials.
converts a polynomial 𝑓 ∈ 𝑅𝑞 to an alternative representation as a vector 𝑓 ofWorking with NTT representations enables significantly faster multiplication of polynomials.
Other operations (e.g., addition, rounding, and sampling) can be done in either representation.
ML-KEM satisfies the essential KEM property of correctness. This means that in the absenceof corruption or interference, the process in Figure 1 will result in 𝐾 ′ = 𝐾 with overwhelming
probability. ML-KEM also comes with a proof of asymptotic theoretical security in a certain
heuristic model [4]. Each of the parameter sets of ML-KEM comes with an associated security
strength that was estimated based on current cryptanalysis (see Section 8 for details).
Parameter sets and algorithms. Recall that a KEM consists of algorithms KeyGen, Encaps, andDecaps, along with a collection of parameter sets. In the case of ML-KEM, the three aforementioned algorithms are:
1. ML-KEM.KeyGen (Algorithm 19)
2. ML-KEM.Encaps (Algorithm 20)
3. ML-KEM.Decaps (Algorithm 21)
These algorithms are described and discussed in detail in Section 7.
ML-KEM comes equipped with three parameter sets:• ML-KEM-512 (security category 1)• ML-KEM-768 (security category 3)• ML-KEM-1024 (security category 5)
These parameter sets are described and discussed in detail in Section 8. The security categories
1-5 are defined in SP 800-57, Part 1 [7]. Each parameter set assigns a particular numerical value
to five integer variables: 𝑘, 𝜂1 , 𝜂2 , 𝑑𝑢 , and 𝑑𝑣 . The values of these variables in each parameter
set are given in Table 2 of Section 8. In addition to these five variable parameters, there are also
two constants: 𝑛 = 256 and 𝑞 = 3329.
Decapsulation failures. Provided that all inputs are well-formed and randomness generation is
successful, the key establishment procedure of ML-KEM will never explicitly fail, meaning thatboth ML-KEM.Encaps and ML-KEM.Decaps will each output a 256-bit value. Moreover, if nocorruption or interference is present, the two 256-bit values produced by ML-KEM.Encaps and
ML-KEM.Decaps will be equal with overwhelming probability (i.e., 𝐾 ′ will equal 𝐾 in the processdescribed in Figure 1). The event that 𝐾 ′ ≠ 𝐾 under these conditions is called a decapsulation
14

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

failure. Formally, the decapsulation failure probability is defined to be the probability (conditioned
on no RGB failures) that the process
1. (ek, dk) ← ML-KEM.KeyGen()
2. (𝑐, 𝐾) ← ML-KEM.Encaps(ek)3. 𝐾 ′ ← ML-KEM.Decaps(dk, 𝑐)

(3.1)
(3.2)
(3.3)

results in 𝐾 ′ ≠ 𝐾. The probability is taken over uniformly random seeds 𝑑, 𝑧 (sampled in
ML-KEM.KeyGen) and 𝑚 (sampled in ML-KEM.Encaps) and under the heuristic assumption thathash functions and XOFs behave like uniformly random functions. The decapsulation failure rates
for ML-KEM are listed in Table 1. For details, see Theorem 1 in [8] and the scripts in [15].
Table 1. Decapsulation failure rates for ML-KEM
Parameter set
ML-KEM-512
ML-KEM-768
ML-KEM-1024

Decapsulation failure rate2−138.8
2−164.8
2−174.8

Terminology for keys. A KEM involves three different types of keys: encapsulation keys, decapsulation keys, and shared secret keys. ML-KEM is built on top of the component public-key
encryption scheme K-PKE, which has two additional key types: encryption keys and decryption
keys. In the literature, encapsulation keys and encryption keys are sometimes referred to as“public keys,” while decapsulation keys and decryption keys are sometimes referred to as “private keys.” In order to reduce confusion, this standard will not use the terms “public key” or“private key.” Instead, keys will be referred to only using the more specific terms, i.e., one of“encapsulation key”, “decapsulation key”, “encryption key”, “decryption key”, and “shared secretkey”.

3.3 Requirements for ML-KEM Implementations
This section describes several requirements for cryptographic modules that implement ML-KEM.
Implementation requirements specific to particular algorithms will be described in later sections.
Additional requirements, including requirements for using ML-KEM in specific applications,
are given in SP 800-227 [1]. While conforming implementations must adhere to all of theserequirements, adherence does not guarantee that the result will be secure (see Point 13 in theannouncement).
K-PKE is only a component. The public-key encryption scheme K-PKE described in Section 5 shallnot be used as a stand-alone cryptographic scheme. Instead, the algorithms that comprise K-PKE
may only be used as subroutines in the algorithms of ML-KEM. In particular, the algorithms
K-PKE.KeyGen (Algorithm 13), K-PKE.Encrypt (Algorithm 14), and K-PKE.Decrypt (Algorithm 15)
are not approved for use as a public-key encryption scheme.
15

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Controlled access to internal functions. The key-encapsulation mechanism ML-KEM makes useof internal, “derandomized” functions ML-KEM.KeyGen_internal, ML-KEM.Encaps_internal, and
ML-KEM.Decaps_internal, specified in Section 6. The interfaces for these functions should not
be made available to applications other than for testing purposes. In particular, the sampling ofrandom values required for key generation (as specified in ML-KEM.KeyGen) and encapsulation(as specified in ML-KEM.Encaps) shall be performed by the cryptographic module.
Equivalent implementations. For every algorithm that is specified in this standard, a conforming
implementation may replace the given set of steps with any mathematically equivalent set ofsteps. In other words, the specified algorithm may be replaced with a different procedure that
produces the correct output for every input (where “input” includes the specified input as well
as all parameter values and all randomness).
Approved usage of the shared secret key. If randomness generation is successful, the values
𝐾 and 𝐾 ′ returned by ML-KEM.Encaps and ML-KEM.Decaps, respectively, are always 256-bit
values. Under appropriate conditions (see Sections 3.1 and 3.2, and SP 800-227 [1]), these values
match (i.e., 𝐾 ′ = 𝐾) and can be used directly as a shared secret key for symmetric cryptography.
If further key derivation is needed, the final symmetric keys shall be derived from this 256-bit
shared secret key in an approved manner, as specified in SP 800-108 and SP 800-56C [16, 17].
As discussed in Section 3.2, ML-KEM is an IND-CCA2-secure KEM. However, a combined KEM
that includes ML-KEM as a component might not meet IND-CCA2 security. Implementers should
assess the security of any procedure in which the key derivation methods of SP 800-56C are
applied to ML-KEM in combination with another key establishment procedure. More guidance
regarding combined KEMs is given in SP 800-227 [1].
Randomness generation. Two algorithms in this standard require the generation of randomness
as an internal step: ML-KEM.KeyGen and ML-KEM.Encaps. In pseudocode, this randomness$
generation is denoted by a statement of the form 𝑚 ←− 𝔹32 . A fresh string of random bytes
must be generated for every such invocation. These random bytes shall be generated using anapproved RBG, as prescribed in SP 800-90A, SP 800-90B, and SP 800-90C [18, 19, 20]. Moreover,this RBG shall have a security strength of at least 128 bits for ML-KEM-512, at least 192 bits for
ML-KEM-768, and at least 256 bits for ML-KEM-1024.
Input checking. The algorithms ML-KEM.Encaps and ML-KEM.Decaps require input checking.
Implementers shall ensure that ML-KEM.Encaps and ML-KEM.Decaps are only executed on
inputs that have been checked, as described in Section 7.
Destruction of intermediate values. Data used in intermediate computation steps of KEM
algorithms could be used by an adversary to compromise security. Therefore, implementersshall ensure that intermediate data is destroyed as soon as it is no longer needed. In particular,
for ML-KEM.KeyGen, ML-KEM.Encaps, and ML-KEM.Decaps, only the designated output can beretained in memory after the algorithm terminates. All other data shall be destroyed prior to16

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

the algorithm terminating.
There are two exceptions to this rule:
1. The seed (𝑑, 𝑧) generated in steps 1 and 2 of ML-KEM.KeyGen can be stored for later
expansion using ML-KEM.KeyGen_internal. As this seed can be used to compute thedecapsulation key, it is sensitive data and shall be treated with the same safeguards as adecapsulation key (see SP 800-227 [1]).
2. The matrix 𝐀̂ generated in steps 3-7 of K-PKE.KeyGen (as a subroutine of ML-KEM.KeyGen)
can be stored so that it need not be recomputed in later operations (e.g., ML-KEM.Decaps).
The same matrix 𝐀̂ is also generated in steps 4-8 of K-PKE.Encrypt (as a subroutine of
ML-KEM.Encaps or ML-KEM.Decaps); it can also then be stored. In either case, the matrix
𝐀̂ is data that is easily computed from the public encapsulation key and thus does not
require any special protections.
No floating-point arithmetic. Implementations of ML-KEM shall not use floating-point arithmetic,
as rounding errors in floating-point operations may lead to incorrect results in some cases. In
all pseudocode in this standard in which division is performed (e.g., 𝑥/𝑦) and 𝑦 may not divide𝑥, either ⌊𝑥/𝑦⌋, ⌈𝑥/𝑦⌉, or ⌈𝑥/𝑦⌋ is used. All of these can be computed without floating-point
arithmetic, as ordinary integer division 𝑥/𝑦 computes ⌊𝑥/𝑦⌋, and ⌈𝑥/𝑦⌉ = ⌊(𝑥 + 𝑦 − 1)/𝑦⌋ for
non-negative integers 𝑥 and positive integers 𝑦.

17

FIPS 203

4.

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Auxiliary Algorithms

4.1 Cryptographic Functions
The algorithms specified in this publication require the use of several cryptographic functions.
Each function shall be instantiated by means of an approved hash function or an approved
eXtendable-Output Function (XOF), as prescribed below. The relevant hash functions and XOFs
are described in detail in FIPS 202 [6]. They will be used as follows.
SHA3-256 and SHA3-512 are hash functions with one variable-length input and one fixed-length
output. In this standard, invocations of these functions on an input 𝑀 will be denoted by
SHA3-256(𝑀 ) and SHA3-512(𝑀 ), respectively. The inputs and outputs for both SHA3-256 andSHA3-512 are always byte arrays.
SHAKE128 and SHAKE256 are XOFs with one variable-length input and one variable-length output.
This standard will adhere to the following conventions [6]:• The inputs and outputs for both SHAKE128 and SHAKE256 are always byte arrays.
• When invoking SHAKE128 or SHAKE256, desired output length is always specified in bits.
For example, the expression
𝑟 ∶= SHAKE128(𝑀 , 8 ⋅ 64)(4.1)

implies that 𝑀 is an array of bytes and that 𝑟 is an array of 64 bytes.
The aforementioned functions play several different roles in the algorithms specified in thisstandard and will only be invoked using the wrapper functions defined below. Importantly, thesewrappers will avoid any potential “byte array” versus “bit-length” confusion by only working withbytes and byte array lengths.
Pseudorandom function (PRF). The function PRF takes a parameter 𝜂 ∈ {2, 3}, one 32-byte
input, and one 1-byte input. It produces one (64 ⋅ 𝜂)-byte output. It will be denoted by
PRF ∶ {2, 3} × 𝔹32 × 𝔹 → 𝔹64𝜂 ,(4.2)

PRF𝜂 (𝑠, 𝑏) ∶= SHAKE256(𝑠‖𝑏, 8 ⋅ 64 ⋅ 𝜂) ,(4.3)

and it shall be instantiated aswhere 𝜂 ∈ {2, 3}, 𝑠 ∈ 𝔹32 , and 𝑏 ∈ 𝔹. Note that 𝜂 is only used to specify the desired output
length and not to perform domain separation.
Hash functions. The specification will also make use of three hash functions H, J and G, which
are defined as follows.
The functions H and J each take one variable-length input and produce one 32-byte output. They
will be denoted by H ∶ 𝔹∗ → 𝔹32 and J ∶ 𝔹∗ → 𝔹32 , respectively, and shall be instantiated as
H(𝑠) ∶= SHA3-256(𝑠)and18

J(𝑠) ∶= SHAKE256(𝑠, 8 ⋅ 32)(4.4)

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

where 𝑠 ∈ 𝔹∗ .
The function G takes one variable-length input and produces two 32-byte outputs. It will bedenoted by G ∶ 𝔹∗ → 𝔹32 × 𝔹32 . The two outputs of G will be denoted by (𝑎, 𝑏) ← G(𝑐), where𝑎, 𝑏 ∈ 𝔹32 , 𝑐 ∈ 𝔹∗ , and G(𝑐) = 𝑎‖𝑏. The function G shall be instantiated as
G(𝑐) ∶= SHA3-512(𝑐) .

(4.5)

eXtendable-Output Function (XOF). This standard uses a XOF wrapper defined in terms of theincremental API for SHAKE128 in SP 800-185 [21]. This SHAKE128 API consists of three functions:• ctx ← SHAKE128.Init()
Initializes a XOF “context” ctx.
• ctx ← SHAKE128.Absorb(ctx, str)
Injects data to be used in the “absorbing” phase of SHAKE128 and updates the context
accordingly.
• (ctx, 𝐵) ← SHAKE128.Squeeze(ctx, 8 ⋅ 𝑧)
Extracts 𝑧 output bytes produced during the “squeezing” phase of SHAKE128 and updates
the context accordingly.
While the above functions are constructed using the Keccak-𝑓 permutation rather than the XOF
SHAKE128 directly, they are defined so that a single SHAKE128 call of the form
output ← SHAKE128(str1 ‖ … ‖str𝑚 , 8𝑏1 + … + 8𝑏ℓ )(4.6)

is equivalent to performing Algorithm 2. This equivalence holds whether or not |str𝑖 | and 𝑏𝑗 aremultiples of the SHAKE128 block length.
Algorithm 2 SHAKE128example(str1 , … , str𝑚 , 𝑏1 , … , 𝑏ℓ )Performs a sequence of absorbing operations followed by a sequence of squeezing operations.
Input: byte arrays str1 , … , str𝑚 .
Input: positive integers 𝑏1 , … , 𝑏ℓ .
ℓ
Output: a byte array of length ∑𝑗=1 𝑏𝑗 .
1: ctx ← SHAKE128.Init()▷ initialize context
2: for (𝑖 ← 1; 𝑖 ≤ 𝑚; 𝑖 ++)3:
ctx ← SHAKE128.Absorb(ctx, str𝑖 )▷ absorb byte array str𝑖
4: end for
5: for (𝑗 ← 1; 𝑗 ≤ ℓ; 𝑗 ++)6:(ctx, out𝑗 ) ← SHAKE128.Squeeze(ctx, 8 ⋅ 𝑏𝑗 )▷ squeeze 𝑏𝑗 -many bytes
7: end for
8: output ← out1 ‖ … ‖outℓ▷ return the concatenation of all the results

In this standard, the incremental API for SHAKE128 will only be invoked through a wrapper XOF,
19

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

which is defined as follows.
1. XOF.Init() = SHAKE128.Init().
2. XOF.Absorb(ctx, str) = SHAKE128.Absorb(ctx, str).
3. XOF.Squeeze(ctx, ℓ) = SHAKE128.Squeeze(ctx, 8 ⋅ ℓ).
Note that XOF.Squeeze requires the input length to be specified in bytes. This is consistent withthe convention that all wrapper functions treat inputs and outputs as byte arrays and measure
the lengths of all such arrays in terms of bytes.

4.2 General Algorithms
This section specifies a number of algorithms that will be used as subroutines in ML-KEM.
4.2.1

Conversion and Compression Algorithms

This section specifies several algorithms for converting between bit arrays, byte arrays, and arraysof integers modulo 𝑚. It also specifies a certain operation for compressing integers modulo 𝑞,
and the corresponding decompression operation.
Algorithm 3 BitsToBytes(𝑏)Converts a bit array (of a length that is a multiple of eight) into an array of bytes.
Input: bit array 𝑏 ∈ {0, 1}8⋅ℓ .
Output: byte array 𝐵 ∈ 𝔹ℓ .
1: 𝐵 ← (0, … , 0)
2: for (𝑖 ← 0; 𝑖 < 8ℓ; 𝑖 ++)3:
𝐵 [⌊𝑖/8⌋] ← 𝐵 [⌊𝑖/8⌋] + 𝑏[𝑖] ⋅ 2𝑖 mod 8
4: end for5: return 𝐵
Algorithm 4 BytesToBits(𝐵)Performs the inverse of BitsToBytes, converting a byte array into a bit array.
Input: byte array 𝐵 ∈ 𝔹ℓ .
Output: bit array 𝑏 ∈ {0, 1}8⋅ℓ .
1: 𝐶 ← 𝐵
2: for (𝑖 ← 0; 𝑖 < ℓ; 𝑖 ++)3:for (𝑗 ← 0; 𝑗 < 8; 𝑗 ++)4:𝑏[8𝑖 + 𝑗] ← 𝐶[𝑖] mod 2
5:
𝐶[𝑖] ← ⌊𝐶[𝑖]/2⌋
6:
end for
7: end for
8: return 𝑏▷ copy 𝐵 into array 𝐶 ∈ 𝔹ℓ

20

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Converting between bits and bytes. The algorithms BitsToBytes (Algorithm 3) and BytesToBits(Algorithm 4) convert between bit arrays and byte arrays. The inputs to BitsToBytes and theoutputs of BytesToBits are bit arrays, with each segment of eight bits representing a byte inlittle-endian order. As an example, the 8-bit string 11010001 corresponds to the byte 20 + 21 +23 + 27 = 139.
Compression and decompression. Recall that 𝑞 = 3329, and that the bit length of 𝑞 is 12. For
𝑑 < 12, define
Compress𝑑 ∶ ℤ𝑞 ⟶ ℤ2𝑑
𝑥 ⟼ ⌈(2𝑑 /𝑞) ⋅ 𝑥⌋ mod 2𝑑 .
Decompress𝑑 ∶ ℤ2𝑑 ⟶ ℤ𝑞

(4.7)
(4.8)

𝑦 ⟼ ⌈(𝑞/2𝑑 ) ⋅ 𝑦⌋ .
The input and output types of these functions are integers modulo 𝑚 (see Section 2.4.1). Divisionand rounding in the computation of these functions are performed in the set of rational numbers.
Floating-point computations shall not be used.
The Compress and Decompress algorithms satisfy two important properties. First, decompression
followed by compression preserves the input. That is, Compress𝑑 (Decompress𝑑 (𝑦)) = 𝑦 for
all 𝑦 ∈ ℤ2𝑑 and all 𝑑 < 12. Second, if 𝑑 is large (i.e., close to 12), compression followed bydecompression does not significantly alter the value.
Encoding and decoding. The algorithms ByteEncode (Algorithm 5) and ByteDecode (Algorithm6) will be used for conversion between integers modulo 𝑚 and bytes. The algorithm ByteEncode
converts an array of 𝑛 = 256 integers modulo 𝑚 into a corresponding array of bytes. ByteDecodeperforms the inverse operation, converting an array of bytes into an array of integers modulo 𝑚.
Specifying the modulus 𝑚 is done as described below.
For the following description, it is convenient to view ByteDecode and ByteEncode as converting
between integers and bits. The conversion between bits and bytes is straightforward and doneusing BitsToBytes and BytesToBits. The valid range of values for the parameter 𝑑 is 1 ≤ 𝑑 ≤ 12.
Bit arrays are divided into 𝑑-bit segments. The operations are performed in two different ways,
depending on the value of 𝑑:• For 𝑑 satisfying 1 ≤ 𝑑 ≤ 11, the conversion is one-to-one. ByteDecode𝑑 converts each𝑑-bit segment of its input into one integer modulo 2𝑑 , while ByteEncode𝑑 performs theinverse operation.
• For 𝑑 = 12, ByteDecode12 produces integers modulo 𝑞 as output, while ByteEncode12
receives integers modulo 𝑞 as input. Specifically, ByteDecode12 converts each 12-bit
segment of its input into an integer modulo 212 = 4096 and then reduces the resultmodulo 𝑞. This is no longer a one-to-one operation. Indeed, some 12-bit segments could
correspond to an integer greater than 𝑞 − 1 = 3328 but less than 4096. However, thiscannot occur for arrays produced by ByteEncode12 . These aspects of ByteDecode12 and21

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

ByteEncode12 will be important when considering checking of the ML-KEM encapsulation
key in Section 7.
Algorithm 5 ByteEncode𝑑 (𝐹 )Encodes an array of 𝑑-bit integers into a byte array for 1 ≤ 𝑑 ≤ 12.
𝑑Input: integer array 𝐹 ∈ ℤ256
𝑚 , where 𝑚 = 2 if 𝑑 < 12, and 𝑚 = 𝑞 if 𝑑 = 12.
Output: byte array 𝐵 ∈ 𝔹32𝑑 .
1: for (𝑖 ← 0; 𝑖 < 256; 𝑖 ++)2:
𝑎 ← 𝐹 [𝑖]▷ 𝑎 ∈ ℤ𝑚
3:
for (𝑗 ← 0; 𝑗 < 𝑑; 𝑗 ++)4:𝑏[𝑖 ⋅ 𝑑 + 𝑗] ← 𝑎 mod 2▷ 𝑏 ∈ {0, 1}256⋅𝑑5:𝑎 ← (𝑎 − 𝑏[𝑖 ⋅ 𝑑 + 𝑗])/2▷ note 𝑎 − 𝑏[𝑖 ⋅ 𝑑 + 𝑗] is always even6:
end for
7: end for
8: 𝐵 ← BitsToBytes(𝑏)9: return 𝐵

Algorithm 6 ByteDecode𝑑 (𝐵)Decodes a byte array into an array of 𝑑-bit integers for 1 ≤ 𝑑 ≤ 12.
Input: byte array 𝐵 ∈ 𝔹32𝑑 .
𝑑
Output: integer array 𝐹 ∈ ℤ256
𝑚 , where 𝑚 = 2 if 𝑑 < 12 and 𝑚 = 𝑞 if 𝑑 = 12.
1: 𝑏 ← BytesToBits(𝐵)
2: for (𝑖 ← 0; 𝑖 < 256; 𝑖 ++)𝑑−1
3:
𝐹 [𝑖] ← ∑𝑗←0 𝑏[𝑖 ⋅ 𝑑 + 𝑗] ⋅ 2𝑗 mod 𝑚
4: end for5: return 𝐹
4.2.2

Sampling Algorithms

The algorithms of ML-KEM require two sampling subroutines that are specified in Algorithms 7
and 8. Both of these algorithms can be used to convert a stream of uniformly random bytes intoa sample from some desired distribution. In this standard, these algorithms will be invoked witha stream of pseudorandom bytes as the input. It follows that the output will then be a sample
from a distribution that is computationally indistinguishable from the desired distribution.
Uniform sampling of NTT representations. The algorithm SampleNTT (Algorithm 7) converts
a seed together with two indexing bytes into a polynomial in the NTT domain. If the seed is
uniformly random, the resulting polynomial will be drawn from a distribution that is computationally indistinguishable from the uniform distribution on 𝑇𝑞 . The output of SampleNTT is anarray in ℤ256
that contains the coefficients of the sampled element of 𝑇𝑞 (see Section 2.4.4). See𝑞Appendix B for a note on (optionally) safely bounding the algorithm’s while-loop iterations.
22

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Algorithm 7 SampleNTT(𝐵)
Takes a 32-byte seed and two indices as input and outputs a pseudorandom element of 𝑇𝑞 .
Input: byte array 𝐵 ∈ 𝔹34 .
Output: array 𝑎̂ ∈ ℤ256
𝑞 .
1: ctx ← XOF.Init()
2: ctx ← XOF.Absorb(ctx, 𝐵)
3: 𝑗 ← 0
4: while 𝑗 < 256 do
5:(ctx, 𝐶) ← XOF.Squeeze(ctx, 3)6:
𝑑1 ← 𝐶[0] + 256 ⋅ (𝐶[1] mod 16)7:
𝑑2 ← ⌊𝐶[1]/16⌋ + 16 ⋅ 𝐶[2]8:if 𝑑1 < 𝑞 then9:𝑎[𝑗]
̂ ← 𝑑110:𝑗 ← 𝑗+1
11:
end if12:
if 𝑑2 < 𝑞 and 𝑗 < 256 then13:𝑎[𝑗]
̂ ← 𝑑214:𝑗 ← 𝑗+1
15:
end if
16: end while
17: return 𝑎̂▷ a 32-byte seed along with two indices▷ the coefficients of the NTT of a polynomial▷ input the given byte array into XOF▷ get a fresh 3-byte array 𝐶 from XOF▷ 0 ≤ 𝑑1 < 212▷ 0 ≤ 𝑑2 < 212▷ 𝑎̂ ∈ ℤ256
𝑞

Sampling from the centered binomial distribution. ML-KEM makes use of a special distribution
D𝜂 (𝑅𝑞 ) of polynomials in 𝑅𝑞 with small coefficients. Such polynomials are sometimes referred
to as “errors” or “noise.” The distribution is parameterized by an integer 𝜂 ∈ {2, 3}. To sample apolynomial from D𝜂 (𝑅𝑞 ), each of its coefficients is independently sampled from a certain centered binomial distribution (CBD) on ℤ𝑞 . The algorithm SamplePolyCBD (Algorithm 8) samples
the coefficient array of a polynomial 𝑓 ∈ 𝑅𝑞 according to the distribution D𝜂 (𝑅𝑞 ), provided that
Algorithm 8 SamplePolyCBD𝜂 (𝐵)Takes a seed as input and outputs a pseudorandom sample from the distribution D𝜂 (𝑅𝑞 ).
Input: byte array 𝐵 ∈ 𝔹64𝜂 .
Output: array 𝑓 ∈ ℤ256𝑞 .
1: 𝑏 ← BytesToBits(𝐵)
2: for (𝑖 ← 0; 𝑖 < 256; 𝑖 ++)𝜂−13:𝑥 ← ∑𝑗←0 𝑏[2𝑖𝜂 + 𝑗]

▷ the coefficients of the sampled polynomial▷0≤𝑥≤𝜂𝜂−1𝑦 ← ∑𝑗←0 𝑏[2𝑖𝜂 + 𝜂 + 𝑗]
5:𝑓[𝑖] ← 𝑥 − 𝑦 mod 𝑞
6: end for7: return 𝑓
4:▷0≤𝑦≤𝜂
▷ 0 ≤ 𝑓[𝑖] ≤ 𝜂 or 𝑞 − 𝜂 ≤ 𝑓[𝑖] ≤ 𝑞 − 1

23

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

its input is a stream of uniformly random bytes.

4.3 The Number-Theoretic Transform
The Number-Theoretic Transform (NTT) can be viewed as a specialized, exact version of thediscrete Fourier transform. In the case of ML-KEM, the NTT is used to improve the efficiency ofmultiplication in the ring 𝑅𝑞 . Recall that 𝑅𝑞 is the ring ℤ𝑞 [𝑋]/(𝑋 𝑛 + 1) of polynomials of theform 𝑓 = 𝑓0 + 𝑓1 𝑋 + ⋯ + 𝑓255 𝑋 255 (where 𝑓𝑗 ∈ ℤ𝑞 for all 𝑗), with the ring operations definedby arithmetic modulo 𝑋 𝑛 + 1.
The ring 𝑅𝑞 is isomorphic to another ring 𝑇𝑞 , which is a direct sum of 128 quadratic extensionsof ℤ𝑞 . The NTT is a computationally efficient isomorphism between these two rings. When âpolynomial 𝑓 ∈ 𝑅𝑞 is input, the NTT outputs an element 𝑓 ̂ ∶= NTT(𝑓) of the ring 𝑇𝑞 , where 𝑓 iscalled the “NTT representation” of 𝑓. The isomorphism property implies that̂𝑓 ×𝑅𝑞 𝑔 = NTT−1 (𝑓 ×̂
𝑇𝑞 𝑔),(4.9)

where ×𝑅𝑞 and ×𝑇𝑞 denote multiplication in 𝑅𝑞 and 𝑇𝑞 , respectively. Moreover, since 𝑇𝑞 is a
product of 128 rings that each consist of polynomials of degree at most one, the operation ×𝑇𝑞
is much more efficient than the operation ×𝑅𝑞 . For these reasons, the NTT is considered to be
an integral part of ML-KEM and not merely an optimization.
As the rings 𝑅𝑞 and 𝑇𝑞 have a vector space structure over ℤ𝑞 , the most natural abstract data
type to represent elements from either of these rings is ℤ𝑛𝑞 . For this reason, the choice of data
structure for the inputs and outputs of NTT and NTT−1 are length-𝑛 arrays of integers modulo
𝑞. These arrays are understood to represent elements of 𝑇𝑞 or 𝑅𝑞 , respectively (see Section
2.4.4). Algorithms 9 and 10 describe an efficient means of computing NTT and NTT−1 in place.
However, to clarify the distinction between the algebraic objects before and after the conversion,
the algorithms are written with explicit inputs and outputs. This is consistent with this standard’s
convention that all inputs are passed by copy.
The mathematical structure of the NTT. In ML-KEM, 𝑞 is the prime 3329 = 28 ⋅ 13 + 1, and𝑛 = 256. There are 128 primitive 256-th roots of unity and no primitive 512-th roots of unity inℤ𝑞 . Note that 𝜁 = 17 ∈ ℤ𝑞 is a primitive 256-th root of unity modulo 𝑞. Thus, 𝜁 128 ≡ −1.
Define BitRev7 (𝑖) to be the integer represented by bit-reversing the unsigned 7-bit value that
corresponds to the input integer 𝑖 ∈ {0, … , 127}.
The polynomial 𝑋 256 + 1 factors into 128 polynomials of degree 2 modulo 𝑞 as follows:127

𝑋 256 + 1 = ∏ (𝑋 2 − 𝜁 2BitRev7 (𝑖)+1 ) .

(4.10)

𝑖=0

Therefore, 𝑅𝑞 ∶= ℤ𝑞 [𝑋]/(𝑋 256 + 1) is isomorphic to a direct sum of 128 quadratic extension

24

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

fields of ℤ𝑞 , denoted 𝑇𝑞 . Specifically, this ring has the structure127

𝑇𝑞 ∶= ⨁ ℤ𝑞 [𝑋]/ (𝑋 2 − 𝜁 2BitRev7 (𝑖)+1 ) .

(4.11)

𝑖=0

Thus, the NTT representation 𝑓 ̂ ∈ 𝑇𝑞 of a polynomial 𝑓 ∈ 𝑅𝑞 is the vector that consists of thecorresponding residues of degree at most one:𝑓 ̂ ∶= (𝑓 mod (𝑋 2 − 𝜁 2BitRev7 (0)+1 ), … , 𝑓 mod (𝑋 2 − 𝜁 2BitRev7 (127)+1 )) .

(4.12)As discussed in Section 2.4.4, the algorithms in this standard represent 𝑓 ̂ as an array of 256
integers modulo 𝑞. Specifically,̂ + 𝑓[2𝑖̂ + 1]𝑋,𝑓 mod (𝑋 2 − 𝜁 2BitRev7 (𝑖)+1 ) = 𝑓[2𝑖](4.13)

for 𝑖 from 0 to 127.
The ML-KEM NTT algorithms. An algorithm for the ML-KEM NTT is described in Algorithm 9. An
algorithm for the inverse operation (NTT−1 ) is described in Algorithm 10. These two algorithms
will be used to transform elements of 𝑅𝑞 to elements of 𝑇𝑞 (using NTT) and vice versa (using
NTT−1 ). In addition, as discussed in Section 2.4.8, these algorithms represent the coordinatewise transformation of structures over those rings. Specifically, they map matrices/vectors withentries in 𝑅𝑞 to matrices/vectors with entries in 𝑇𝑞 (using NTT) and vice versa (using NTT−1 ).
The values 𝜁 BitRev7 (𝑖) mod 𝑞 for 𝑖 = 1, … , 127 used in line 5 of Algorithm 9 and line 5 of Algorithm
10 may be precomputed into an array. This array is given in Appendix A.

25

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Algorithm 9 NTT(𝑓)̂ the given polynomial 𝑓 ∈ 𝑅 .
Computes the NTT representation 𝑓 of𝑞
Input: array 𝑓 ∈ ℤ256
▷ the coefficients of the input polynomial
𝑞 .
256
̂
Output: array 𝑓 ∈ ℤ𝑞 .
▷ the coefficients of the NTT of the input polynomial
1: 𝑓 ̂ ← 𝑓▷ will compute in place on a copy of input array
2: 𝑖 ← 1
3: for (len ← 128; len ≥ 2; len ← len/2)
4:for (start ← 0; start < 256; start ← start + 2 ⋅ len)zeta ← 𝜁 BitRev7 (𝑖) mod 𝑞
5:6:𝑖 ← 𝑖+1
7:
for (𝑗 ← start; 𝑗 < start + len; 𝑗 ++)̂ + len]8:𝑡 ← zeta ⋅ 𝑓[𝑗▷ steps 8-10 done modulo 𝑞̂̂
9:𝑓[𝑗 + len] ← 𝑓[𝑗] − 𝑡̂ ← 𝑓[𝑗]
̂ +𝑡
10:
𝑓[𝑗]
11:
end for12:
end for
13: end for
14: return 𝑓 ̂

Algorithm 10 NTT−1 (𝑓)̂Computes the polynomial 𝑓 ∈ 𝑅𝑞 that corresponds to the given NTT representation 𝑓 ̂ ∈ 𝑇𝑞 .
Input: array 𝑓 ̂ ∈ ℤ256
▷ the coefficients of input NTT representation𝑞 .
256▷ the coefficients of the inverse NTT of the input
Output: array 𝑓 ∈ ℤ𝑞 .
1: 𝑓 ← 𝑓 ̂▷ will compute in place on a copy of input array
2: 𝑖 ← 127
3: for (len ← 2; len ≤ 128; len ← 2 ⋅ len)4:for (start ← 0; start < 256; start ← start + 2 ⋅ len)zeta ← 𝜁 BitRev7 (𝑖) mod 𝑞
5:6:𝑖 ← 𝑖−17:
for (𝑗 ← start; 𝑗 < start + len; 𝑗 ++)8:𝑡 ← 𝑓[𝑗]9:𝑓[𝑗] ← 𝑡 + 𝑓[𝑗 + len]▷ steps 9-10 done modulo 𝑞
10:
𝑓[𝑗 + len] ← zeta ⋅ (𝑓[𝑗 + len] − 𝑡)11:
end for12:
end for
13: end for
14: 𝑓 ← 𝑓 ⋅ 3303 mod 𝑞▷ multiply every entry by 3303 ≡ 128−1 mod 𝑞
15: return 𝑓

26

FIPS 203

4.3.1

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Multiplication in the NTT Domain

The addition and scalar multiplication of elements of 𝑇𝑞 can be done using the corresponding
coordinate-wise arithmetic operations on the coefficient arrays (see Section 2.4.5). This section
describes how to do the remaining ring operation (i.e., multiplication in 𝑇𝑞 ).
Recall from (4.12) that 𝑓 ̂ ∈ 𝑇𝑞 is a vector of 128 degree-one residues modulo quadratic polynomials. Algebraically, multiplication in the ring 𝑇𝑞 consists of independent multiplication in eachof the 128 coordinates with respect to the quadratic modulus of that coordinate. Specifically,̂
the 𝑖-th coordinate in 𝑇𝑞 of the product ℎ̂ = 𝑓 ×
𝑇𝑞 𝑔 ̂ is determined by the calculation
̂ + 𝑓[2𝑖̂ + 1]𝑋)(𝑔[2𝑖]̂̂ + 1]𝑋 = (𝑓[2𝑖]ℎ[2𝑖]+ ℎ[2𝑖̂ + 𝑔[2𝑖̂ + 1]𝑋) mod (𝑋 2 − 𝜁 2BitRev7 (𝑖)+1 ).
(4.14)
Algorithm 11 MultiplyNTTs(𝑓,̂ 𝑔)̂
Computes the product (in the ring 𝑇𝑞 ) of two NTT representations.
and 𝑔 ̂ ∈ ℤ256
▷ the coefficients of two NTT representations
Input: Two arrays 𝑓 ̂ ∈ ℤ256
𝑞
𝑞 .
256
̂
▷ the coefficients of the product of the inputs
Output: An array ℎ ∈ ℤ𝑞 .
1: for (𝑖 ← 0; 𝑖 < 128; 𝑖 ++)̂̂ + 1], 𝑔[2𝑖],̂̂ + 1]) ← BaseCaseMultiply(𝑓[2𝑖],̂̂ + 1], 𝜁 2BitRev7 (𝑖)+1 )2:(ℎ[2𝑖],
ℎ[2𝑖
𝑓[2𝑖
𝑔[2𝑖
3: end for
4: return ℎ̂Thus, one can compute the product of two elements of 𝑇𝑞 using the algorithm MultiplyNTTs (Algorithm 11), which uses BaseCaseMultiply (Algorithm 12) as a subroutine. The values 𝜁 2BitRev7 (𝑖)+1used in Algorithm 11 may be precomputed and stored in an array (see Appendix A). MultiplyNTTs
also enables linear-algebraic arithmetic with matrices and vectors whose entries are in 𝑇𝑞 (seeSection 2.4.7).
Algorithm 12 BaseCaseMultiply(𝑎0 , 𝑎1 , 𝑏0 , 𝑏1 , 𝛾)Computes the product of two degree-one polynomials with respect to a quadratic modulus.
Input: 𝑎0 , 𝑎1 , 𝑏0 , 𝑏1 ∈ ℤ𝑞 .
Input: 𝛾 ∈ ℤ𝑞 .
Output: 𝑐0 , 𝑐1 ∈ ℤ𝑞 .
1: 𝑐0 ← 𝑎0 ⋅ 𝑏0 + 𝑎1 ⋅ 𝑏1 ⋅ 𝛾
2: 𝑐1 ← 𝑎0 ⋅ 𝑏1 + 𝑎1 ⋅ 𝑏0
3: return (𝑐0 , 𝑐1 )▷ the coefficients of 𝑎0 + 𝑎1 𝑋 and 𝑏0 + 𝑏1 𝑋
▷ the modulus is 𝑋 2 − 𝛾▷ the coefficients of the product of the two polynomials▷ steps 1-2 done modulo 𝑞

27

FIPS 203

5.

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

The K-PKE Component Scheme

This section describes the component scheme K-PKE. As discussed in Section 3.3, K-PKE is not
approved for use in a stand-alone fashion. It serves only as a collection of subroutines for use in
the algorithms of the approved scheme ML-KEM, as described in Section 7.
K-PKE consists of three algorithms:
1. Key generation (K-PKE.KeyGen)
2. Encryption (K-PKE.Encrypt)
3. Decryption (K-PKE.Decrypt)
When K-PKE is instantiated as part of ML-KEM, K-PKE inherits the parameter set selected for
ML-KEM. Each parameter set specifies numerical values for each parameter. While 𝑛 is always
256 and 𝑞 is always 3329, the values of the remaining parameters 𝑘, 𝜂1 , 𝜂2 , 𝑑𝑢 , and 𝑑𝑣 vary
among the three parameter sets. Parameters and parameter sets are described in Section 8.
The algorithms in this section do not perform any input checking because they are only invoked as
subroutines of the main ML-KEM algorithms. The algorithms of ML-KEM themselves do perform
input checking as needed.
Each of the algorithms of K-PKE is accompanied by a brief, informal description in text. For
simplicity, this description is written in terms of vectors and matrices whose entries are elementsof 𝑅𝑞 . In the actual algorithm, most of the computations occur in the NTT domain in order toimprove the efficiency of multiplication. The relevant vectors and matrices will then have entries
in 𝑇𝑞 . Linear-algebraic arithmetic with such vectors and matrices (e.g., line 18 of K-PKE.KeyGen)
is performed as described in Sections 2.4.7 and 4.3.1. The encryption and decryption keys of
K-PKE are also stored in the NTT form.

5.1 K-PKE Key Generation
The key generation algorithm K-PKE.KeyGen of K-PKE (Algorithm 13) receives a seed as input andoutputs an encryption key ekPKE and a decryption key dkPKE . As is typically the case for public-key
encryption, the encryption key can be made public, while the decryption key and the randomness
must remain private. Indeed, the encryption key of K-PKE will serve as the encapsulation key of
ML-KEM (see ML-KEM.KeyGen below) and can thus be made public. Meanwhile, the decryption
key and seed of K-PKE.KeyGen must remain private as they can be used to perform decapsulationin ML-KEM.
The matrix 𝐀̂ generated in steps 3-7 of K-PKE.KeyGen can be stored, as specified in Section 3.3.
This allows later operations to use 𝐀̂ directly rather than re-expanding it from the public seed 𝜌.
Informal description. The decryption key of K-PKE.KeyGen is a length-𝑘 vector 𝐬 of elementsof 𝑅𝑞 (i.e., 𝐬 ∈ 𝑅𝑞𝑘 ). Roughly speaking, 𝐬 is a set of secret variables, while the encryption key isa collection of “noisy” linear equations (𝐀, 𝐀𝐬 + 𝐞) in the secret variables 𝐬. The rows of thematrix 𝐀 form the equation coefficients. This matrix is generated pseudorandomly using XOF
with only a seed stored in the encryption key. The secret 𝐬 and the “noise” 𝐞 are sampled from
28

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Algorithm 13 K-PKE.KeyGen(𝑑)Uses randomness to generate an encryption key and a corresponding decryption key.
Input: randomness 𝑑 ∈ 𝔹32 .
Output: encryption key ekPKE ∈ 𝔹384𝑘+32 .
Output: decryption key dkPKE ∈ 𝔹384𝑘 .
1: (𝜌, 𝜎) ← G(𝑑‖𝑘)▷ expand 32+1 bytes to two pseudorandom 32-byte seeds1
2: 𝑁 ← 0
𝑘×𝑘
̂ (ℤ256
3: for (𝑖 ← 0; 𝑖 < 𝑘; 𝑖 ++)▷ generate matrix 𝐀∈
𝑞 )4:for (𝑗 ← 0; 𝑗 < 𝑘; 𝑗 ++)̂ 𝑗] ← SampleNTT(𝜌‖𝑗‖𝑖)
5:𝐀[𝑖,▷ 𝑗 and 𝑖 are bytes 33 and 34 of the input
6:
end for
7: end for𝑘
8: for (𝑖 ← 0; 𝑖 < 𝑘; 𝑖 ++)▷ generate 𝐬 ∈ (ℤ256𝑞 )9:𝐬[𝑖] ← SamplePolyCBD𝜂 (PRF𝜂1 (𝜎, 𝑁 ))▷ 𝐬[𝑖] ∈ ℤ256
sampled from CBD𝑞1
10:
𝑁 ← 𝑁 +1
11: end for𝑘
12: for (𝑖 ← 0; 𝑖 < 𝑘; 𝑖 ++)▷ generate 𝐞 ∈ (ℤ256𝑞 )13:𝐞[𝑖] ← SamplePolyCBD𝜂 (PRF𝜂1 (𝜎, 𝑁 ))▷ 𝐞[𝑖] ∈ ℤ256
sampled from CBD𝑞1
14:
𝑁 ← 𝑁 +1
15: end for
16: 𝐬 ̂ ← NTT(𝐬)▷ run NTT 𝑘 times (once for each coordinate of 𝐬)▷ run NTT 𝑘 times
17: 𝐞̂ ← NTT(𝐞)̂̂
18: 𝐭 ← 𝐀 ∘ 𝐬 ̂ + 𝐞̂
▷ noisy linear system in NTT domain̂̂
19: ekPKE ← ByteEncode12 (𝐭)‖𝜌▷ run ByteEncode12 𝑘 times, then append 𝐀-seed
20: dkPKE ← ByteEncode12 (𝐬)̂▷ run ByteEncode12 𝑘 times
21: return (ekPKE , dkPKE )centered binomial distributions using randomness expanded from another seed 𝜎 via PRF. Once
𝐀, 𝐬, and 𝐞 are generated, the computation of the final part 𝐭 = 𝐀𝐬 + 𝐞 of the encryption key
takes place. The results are appropriately encoded into byte arrays and output.
In K-PKE.KeyGen, the choice of parameter set affects the length of the secret 𝐬 (via the parameter𝑘) and, as a consequence, the sizes of the noise vector 𝐞 and the pseudorandom matrix 𝐀. The
choice of parameter set also affects the noise distribution (via the parameter 𝜂1 ) used to sample
the entries of 𝐬 and 𝐞.

5.2 K-PKE Encryption
The encryption algorithm K-PKE.Encrypt of K-PKE (Algorithm 14) takes an encryption key ekPKE ,a 32-byte plaintext 𝑚, and randomness 𝑟 as input and produces a single output: a ciphertext 𝑐.
1 Byte 33 of the input to G is the module dimension 𝑘 ∈ {2, 3, 4} ⊂ 𝔹. This is included to establish domain separation

between the three parameter sets. For implementations that use the seed in place of the private key, this ensuresthat the expansion will produce an unrelated key if the seed is mistakenly expanded using a parameter set otherthan the originally intended one.

29

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

The matrix 𝐀̂ generated in steps 4-8 of K-PKE.Encrypt can be stored, as specified in Section 3.3.
This allows later operations to use 𝐀̂ directly rather than re-expanding it from the public seed 𝜌.
Algorithm 14 K-PKE.Encrypt(ekPKE , 𝑚, 𝑟)
Uses the encryption key to encrypt a plaintext message using the randomness 𝑟.
Input: encryption key ekPKE ∈ 𝔹384𝑘+32 .
Input: message 𝑚 ∈ 𝔹32 .
Input: randomness 𝑟 ∈ 𝔹32 .
Output: ciphertext 𝑐 ∈ 𝔹32(𝑑𝑢 𝑘+𝑑𝑣 ) .
1: 𝑁 ← 0𝑘
2: 𝐭̂ ← ByteDecode12 (ekPKE [0 ∶ 384𝑘]) ▷ run ByteDecode12 𝑘 times to decode 𝐭̂ ∈ (ℤ256𝑞 )3: 𝜌 ← ekPKE [384𝑘 ∶ 384𝑘 + 32]▷ extract 32-byte seed from ekPKE
𝑘×𝑘++
4: for (𝑖 ← 0; 𝑖 < 𝑘; 𝑖 )▷ re-generate matrix 𝐀̂ ∈ (ℤ256sampled in Alg. 13
𝑞 )5:
for (𝑗 ← 0; 𝑗 < 𝑘; 𝑗 ++)̂ 𝑗] ← SampleNTT(𝜌‖𝑗‖𝑖)
6:𝐀[𝑖,▷ 𝑗 and 𝑖 are bytes 33 and 34 of the input
7:
end for
8: end for𝑘
9: for (𝑖 ← 0; 𝑖 < 𝑘; 𝑖 ++)▷ generate 𝐲 ∈ (ℤ256𝑞 )10:𝐲[𝑖] ← SamplePolyCBD𝜂 (PRF𝜂1 (𝑟, 𝑁 ))▷ 𝐲[𝑖] ∈ ℤ256
sampled from CBD𝑞1
11:𝑁 ← 𝑁 +1
12: end for𝑘
13: for (𝑖 ← 0; 𝑖 < 𝑘; 𝑖 ++)▷ generate 𝐞𝟏 ∈ (ℤ256
𝑞 )
256
14:
𝐞𝟏 [𝑖] ← SamplePolyCBD𝜂 (PRF𝜂2 (𝑟, 𝑁 ))▷ 𝐞𝟏 [𝑖] ∈ ℤ𝑞 sampled from CBD
215:
𝑁 ← 𝑁 +1
16: end for
17: 𝑒2 ← SamplePolyCBD𝜂 (PRF𝜂 (𝑟, 𝑁 ))▷ sample 𝑒2 ∈ ℤ256
from CBD𝑞2
2
18: 𝐲̂ ← NTT(𝐲)▷ run NTT 𝑘 times−1⊺̂
19: 𝐮 ← NTT (𝐀 ∘ 𝐲)
̂ + 𝐞𝟏▷ run NTT−1 𝑘 times
20: 𝜇 ← Decompress1 (ByteDecode1 (𝑚))−1 ̂∘ 𝐲)̂ + 𝑒2 + 𝜇
21: 𝑣 ← NTT (𝐭⊺▷ encode plaintext 𝑚 into polynomial 𝑣
22: 𝑐1 ← ByteEncode𝑑 (Compress𝑑 (𝐮))▷ run ByteEncode𝑑 and Compress𝑑 𝑘 times𝑢𝑢𝑢𝑢
23: 𝑐2 ← ByteEncode𝑑 (Compress𝑑 (𝑣))𝑣𝑣
24: return 𝑐 ← (𝑐1 ‖𝑐2 )Informal description. The algorithm K-PKE.Encrypt begins by extracting the vector 𝐭 and theseed from the encryption key. The seed is then expanded to re-generate the matrix 𝐀 in the samemanner as was done in K-PKE.KeyGen. If 𝐭 and 𝐀 are derived correctly from an encryption key
produced by K-PKE.KeyGen, then they are equal to their corresponding values in K-PKE.KeyGen.
Recall from the description of key generation that the pair (𝐀, 𝐭 = 𝐀𝐬 + 𝐞) can be thought of asa system of noisy linear equations in the secret variables 𝐬. One can generate an additional noisy
linear equation in the same secret variables — without knowing 𝐬 — by picking a random linear
30

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

combination of the noisy equations in the system (𝐀, 𝐭). One can then encode information in the“constant term” (i.e., the entry that is a linear combination of entries of 𝐭) of such a combined
equation. This information can then be deciphered by a party in possession of 𝐬. For example,
one could encode a single bit by deciding whether or not to significantly alter the constant term,
thus making either a nearly correct equation that corresponds to the decrypted bit value of 0 or a
far-from-correct equation that corresponds to the decrypted bit value of 1. In the case of K-PKE,
the constant term is a polynomial with 256 coefficients, so one can encode more information:
one bit in each coefficient.
To this end, K-PKE.Encrypt proceeds by generating a vector 𝐲 ∈ 𝑅𝑞𝑘 and noise terms 𝐞𝟏 ∈ 𝑅𝑞𝑘
and 𝑒2 ∈ 𝑅𝑞 , all of which are sampled from the centered binomial distribution using pseudorandomness expanded via PRF from the input randomness 𝑟. One then computes the “new noisy
equation,” which is (up to some details) (𝐀⊺ 𝐲 + 𝐞1 , 𝐭⊺ 𝐲 + 𝑒2 ). An appropriate encoding 𝜇 of
the input message 𝑚 is then added to the latter term in the pair. Finally, the resulting pair (𝐮, 𝑣)
is compressed, serialized into a byte array, and output as the ciphertext.

5.3 K-PKE Decryption
The decryption algorithm K-PKE.Decrypt of K-PKE (Algorithm 15) takes a decryption key dkPKE
and a ciphertext 𝑐 as input, requires no randomness, and outputs a plaintext 𝑚.
Informal description. The algorithm K-PKE.Decrypt begins by recovering a pair (𝐮′ , 𝑣′ ) from theciphertext 𝑐 (see the description of K-PKE.Encrypt). Here, one can think of 𝐮′ as the coefficientsof the equation and 𝑣′ as the constant term. The decryption key dkPKE contains the vector of
secret variables 𝐬. The decryption algorithm can thus use the decryption key to compute thetrue constant term 𝑣 = 𝐬⊺ 𝐮′ and calculate 𝑣′ − 𝑣. The decryption algorithm ends by decoding
the plaintext message 𝑚 from 𝑣′ − 𝑣 and outputting 𝑚.
Algorithm 15 K-PKE.Decrypt(dkPKE , 𝑐)Uses the decryption key to decrypt a ciphertext.
Input: decryption key dkPKE ∈ 𝔹384𝑘 .
Input: ciphertext 𝑐 ∈ 𝔹32(𝑑𝑢 𝑘+𝑑𝑣 ) .
Output: message 𝑚 ∈ 𝔹32 .
1: 𝑐1 ← 𝑐[0 ∶ 32𝑑𝑢 𝑘]
2: 𝑐2 ← 𝑐[32𝑑𝑢 𝑘 ∶ 32(𝑑𝑢 𝑘 + 𝑑𝑣 )]3: 𝐮′ ← Decompress𝑑 (ByteDecode𝑑 (𝑐1 )) ▷ run Decompress𝑑 and ByteDecode𝑑 𝑘 times𝑢𝑢𝑢𝑢
4: 𝑣′ ← Decompress𝑑 (ByteDecode𝑑 (𝑐2 ))𝑣𝑣
5: 𝐬 ̂ ← ByteDecode12 (dkPKE )▷ run ByteDecode12 𝑘 times−1 ⊺′′6: 𝑤 ← 𝑣 − NTT (𝐬 ̂ ∘ NTT(𝐮 ))▷ run NTT 𝑘 times; run NTT−1 once
7: 𝑚 ← ByteEncode1 (Compress1 (𝑤))
▷ decode plaintext 𝑚 from polynomial 𝑣
8: return 𝑚

31

FIPS 203

6.

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Main Internal Algorithms

This section specifies three algorithms: ML-KEM.KeyGen_internal, ML-KEM.Encaps_internal,and ML-KEM.Decaps_internal. These three algorithms are all deterministic, meaning that their
output is completely determined by their input. No randomness is sampled inside of thesealgorithms. These three algorithms will be used to construct ML-KEM in Section 7. The algorithms
in this section make use of the parameters 𝑛, 𝑞, 𝑘, 𝑑𝑢 , and 𝑑𝑣 . The subroutines they invoke
additionally make use of the parameters 𝜂1 and 𝜂2 . While 𝑛 is always 256 and 𝑞 is always 3329,
the remaining parameters vary among the possible parameter sets (see Section 8).
The interfaces specified in this section will be used to test ML-KEM implementations throughthe Cryptographic Algorithm Validation Program (CAVP). The key generation function in thissection may also be used to re-expand a key from a seed (see Section 3.3), including whenobtaining assurance of private key possession via regeneration. As prescribed in Section 3.3, theinterfaces specified in this section should not be made available to applications other than for
testing purposes, and the random seeds (as specified in ML-KEM.KeyGen and ML-KEM.Encaps
in Section 7) shall be generated by the cryptographic module.

6.1 Internal Key Generation
The algorithm ML-KEM.KeyGen_internal (Algorithm 16) accepts two random seeds as input, andproduces an encapsulation key and a decapsulation key.
Informal description. The core subroutine of ML-KEM.KeyGen_internal is the key generation
algorithm of K-PKE (Algorithm 13). The encapsulation key is simply the encryption key of K-PKE.
The decapsulation key consists of the decryption key of K-PKE, the encapsulation key, a hashof the encapsulation key, and a random 32-byte value. This random value will be used in the”implicit rejection” mechanism of the internal decapsulation algorithm (Algorithm 18).
Algorithm 16 ML-KEM.KeyGen_internal(𝑑, 𝑧)Uses randomness to generate an encapsulation key and a corresponding decapsulation key.
Input: randomness 𝑑 ∈ 𝔹32 .
Input: randomness 𝑧 ∈ 𝔹32 .
Output: encapsulation key ek ∈ 𝔹384𝑘+32 .
Output: decapsulation key dk ∈ 𝔹768𝑘+96 .
1: (ekPKE , dkPKE ) ← K-PKE.KeyGen(𝑑)2: ek ← ekPKE
3: dk ← (dkPKE ‖ek‖H(ek)‖𝑧)
4: return (ek, dk)▷ run key generation for K-PKE▷ KEM encaps key is just the PKE encryption key▷ KEM decaps key includes PKE decryption key

6.2 Internal Encapsulation
The algorithm ML-KEM.Encaps_internal (Algorithm 17) accepts an encapsulation key and a random byte array as input and outputs a ciphertext and a shared key.
32

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Algorithm 17 ML-KEM.Encaps_internal(ek, 𝑚)
Uses the encapsulation key and randomness to generate a key and an associated ciphertext.
Input: encapsulation key ek ∈ 𝔹384𝑘+32 .
Input: randomness 𝑚 ∈ 𝔹32 .
Output: shared secret key 𝐾 ∈ 𝔹32 .
Output: ciphertext 𝑐 ∈ 𝔹32(𝑑𝑢 𝑘+𝑑𝑣 ) .
1: (𝐾, 𝑟) ← G(𝑚‖H(ek))
2: 𝑐 ← K-PKE.Encrypt(ek, 𝑚, 𝑟)
3: return (𝐾, 𝑐)▷ derive shared secret key 𝐾 and randomness 𝑟▷ encrypt 𝑚 using K-PKE with randomness 𝑟

Informal description. The core subroutine of ML-KEM.Encaps_internal is the encryption algorithm of K-PKE, which is used to encrypt a random value 𝑚 into a ciphertext 𝑐. A copy of theshared secret key 𝐾 and the randomness used during encryption are derived from 𝑚 and theencapsulation key ek via hashing. Specifically, H is applied to ek, and the result is concatenatedwith 𝑚 and then hashed using G. Finally, the algorithm outputs the shared secret key 𝐾 and theciphertext 𝑐.

6.3 Internal Decapsulation
The algorithm ML-KEM.Decaps_internal (Algorithm 18) accepts a decapsulation key and a ciphertext as input, does not use any randomness, and outputs a shared secret key.
Informal description. The algorithm ML-KEM.Decaps_internal begins by parsing out the components of the decapsulation key dk of ML-KEM. These components are an (encryption key,
decryption key) pair for K-PKE, a hash value ℎ, and a random value 𝑧. The decryption key of
K-PKE is then used to decrypt the input ciphertext 𝑐 to get a plaintext 𝑚′ . The decapsulation
algorithm then re-encrypts 𝑚′ and computes a candidate shared secret key 𝐾 ′ in the samemanner as should have been done in encapsulation. Specifically, 𝐾 ′ and the encryption randomness 𝑟′ are computed by hashing 𝑚′ and the encryption key of K-PKE, and a ciphertext 𝑐′ isgenerated by encrypting 𝑚′ using randomness 𝑟′ . Finally, decapsulation checks whether the
resulting ciphertext 𝑐′ matches the provided ciphertext 𝑐. If it does not, the algorithm performs
an “implicit rejection”: the value of 𝐾 ′ is changed to a hash of 𝑐 together with the random value
𝑧 stored in the ML-KEM secret key (see the discussion of decapsulation failures in Section 3.2).
In either case, decapsulation outputs the resulting shared secret key 𝐾 ′ .
The “implicit reject” flag computed in step 9 (by comparing 𝑐 and 𝑐′ ) is a secret piece of intermediate data. As specified in the requirements in Section 3.3, this flag shall be destroyed prior to
ML-KEM.Decaps_internal terminating. In particular, returning the value of the flag as an output
in any form is not permitted.

33

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Algorithm 18 ML-KEM.Decaps_internal(dk, 𝑐)
Uses the decapsulation key to produce a shared secret key from a ciphertext.
Input: decapsulation key dk ∈ 𝔹768𝑘+96 .
Input: ciphertext 𝑐 ∈ 𝔹32(𝑑𝑢 𝑘+𝑑𝑣 ) .
Output: shared secret key 𝐾 ∈ 𝔹32 .
1: dkPKE ← dk[0 ∶ 384𝑘]▷ extract (from KEM decaps key) the PKE decryption key
2: ekPKE ← dk[384𝑘 ∶ 768𝑘 + 32]▷ extract PKE encryption key
3: ℎ ← dk[768𝑘 + 32 ∶ 768𝑘 + 64]▷ extract hash of PKE encryption key
4: 𝑧 ← dk[768𝑘 + 64 ∶ 768𝑘 + 96]▷ extract implicit rejection value
5: 𝑚′ ← K-PKE.Decrypt(dkPKE , 𝑐)
▷ decrypt ciphertext′ ′′6: (𝐾 , 𝑟 ) ← G(𝑚 ‖ℎ)
7: 𝐾̄ ← J(𝑧‖𝑐)8: 𝑐′ ← K-PKE.Encrypt(ekPKE , 𝑚′ , 𝑟′ )▷ re-encrypt using the derived randomness 𝑟′
9: if 𝑐 ≠ 𝑐 ′ then10:
𝐾 ′ ← 𝐾̄▷ if ciphertexts do not match, “implicitly reject”
11: end if
12: return 𝐾 ′34

FIPS 203

7.

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

The ML-KEM Key-Encapsulation Mechanism

This section describes the three main algorithms of the ML-KEM scheme:
1. Key generation (ML-KEM.KeyGen)
2. Encapsulation (ML-KEM.Encaps)
3. Decapsulation (ML-KEM.Decaps)
To instantiate ML-KEM, one must select a parameter set. Each parameter set is associated witha particular trade-off between security and performance. The three possible parameter sets arecalled ML-KEM-512, ML-KEM-768, and ML-KEM-1024 and are described in detail in Table 2 ofSection 8. Each parameter set assigns specific numerical values to the individual parameters 𝑛,
𝑞, 𝑘, 𝜂1 , 𝜂2 , 𝑑𝑢 , and 𝑑𝑣 . While 𝑛 is always 256 and 𝑞 is always 3329, the remaining parameters
vary among the three parameter sets. Implementers shall ensure that the three algorithms of
ML-KEM listed above are only invoked with a valid parameter set, and that this parameter set isselected appropriately for the desired application. Moreover, implementers shall ensure that theparameter set used in any particular invocation of ML-KEM.Encaps or ML-KEM.Decaps matches
the parameter set associated to the provided inputs.

7.1 ML-KEM Key Generation
The key generation algorithm ML-KEM.KeyGen for ML-KEM (Algorithm 19) accepts no input,
generates randomness internally, and produces an encapsulation key and a decapsulation key.
While the encapsulation key can be made public, the decapsulation key shall remain private.
The seed (𝑑, 𝑧) generated in steps 1 and 2 of ML-KEM.KeyGen can be stored for later expansion using ML-KEM.KeyGen_internal (see Section 3.3). As the seed can be used to computethe decapsulation key, it is sensitive data and shall be treated with the same safeguards as adecapsulation key (see SP 800-227 [1]).
Algorithm 19 ML-KEM.KeyGen()
Generates an encapsulation key and a corresponding decapsulation key.
Output: encapsulation key ek ∈ 𝔹384𝑘+32 .
Output: decapsulation key dk ∈ 𝔹768𝑘+96 .
$1: 𝑑 ←▷ 𝑑 is 32 random bytes (see Section 3.3)
− 𝔹32
$
32
2: 𝑧 ←▷ 𝑧 is 32 random bytes (see Section 3.3)
−𝔹
3: if 𝑑 == NULL or 𝑧 == NULL then4:
return ⊥▷ return an error indication if random bit generation failed
5: end if
6: (ek, dk) ← ML-KEM.KeyGen_internal(𝑑, 𝑧)▷ run internal key generation algorithm
7: return (ek, dk)
Secure key establishment depends on the use of key pairs that have been properly generated
via ML-KEM.KeyGen. If the owner of a KEM key pair did not generate the key pair but instead
received it from a trusted third party or other source, the owner may optionally perform certain
35

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

checks on the key pair. While these checks can detect certain corruptions, they do not guaranteethat the key pair was properly generated.
Key pair check. To check a candidate key pair1 (ek, dk), perform the following checks:
1. (Seed consistency) If a seed (𝑑, 𝑧) is available, run ML-KEM.KeyGen_internal(𝑑, 𝑧), andverify that the output is equal to (ek, dk).
2. (Encapsulation key check) Check ek as specified in Section 7.2.
3. (Decapsulation key check) Check dk as specified in Section 7.3.
4. (Pair-wise consistency) Perform the following steps:$

i. Generate an array of 32 random bytes by performing 𝑚 ←− 𝔹32 .
ii. Perform (𝐾, 𝑐) ← ML-KEM.Encaps_internal(ek, 𝑚).
iii. Perform 𝐾 ′ ← ML-KEM.Decaps_internal(dk, 𝑐).
iv. Reject unless 𝐾 == 𝐾 ′ .
It is important to note that this checking process does not guarantee that the key pair is a properly
produced output of ML-KEM.KeyGen.

7.2 ML-KEM Encapsulation
The encapsulation algorithm ML-KEM.Encaps of ML-KEM (Algorithm 20) accepts an encapsulation key as input, generates randomness internally, and outputs a ciphertext and a shared key.
This algorithm requires input checking, as specified below.
Encapsulation key check. To check a candidate encapsulation key ek, perform the following:
1. (Type check) If ek is not an array of bytes of length 384𝑘 + 32 for the value of 𝑘 specified
by the relevant parameter set, then input checking failed.
2. (Modulus check) Perform the computation
test ← ByteEncode12 (ByteDecode12 (ek[0 ∶ 384𝑘]))(7.1)(see Section 4.2.1). If test ≠ ek[0 ∶ 384𝑘], then input checking failed. This check ensuresthat the integers encoded in the public key are in the valid range [0, 𝑞 − 1].
If both checks pass, then ML-KEM.Encaps can be run with input ek ∶= ek. It is important tonote that this checking process does not guarantee that ek is a properly produced output of
ML-KEM.KeyGen.
ML-KEM.Encaps shall not be run with an encapsulation key that has not been checked as above.
However, checking of the encapsulation key need not be performed by the encapsulating party,
1 In discussions of input checking, the “low overline” in the notation indicates that the input has not yet beenchecked (e.g., ek has not yet been checked, while ek has passed the check).

36

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

nor with every execution of ML-KEM.Encaps. Instead, assurance that these checks have been
performed can be acquired through other means (see SP 800-227 [1]).
Algorithm 20 ML-KEM.Encaps(ek)Uses the encapsulation key to generate a shared secret key and an associated ciphertext.
Checked input: encapsulation key ek ∈ 𝔹384𝑘+32 .
Output: shared secret key 𝐾 ∈ 𝔹32 .
Output: ciphertext 𝑐 ∈ 𝔹32(𝑑𝑢 𝑘+𝑑𝑣 ) .
$1: 𝑚 ←▷ 𝑚 is 32 random bytes (see Section 3.3)
− 𝔹32
2: if 𝑚 == NULL then3:
return ⊥▷ return an error indication if random bit generation failed
4: end if
5: (𝐾, 𝑐) ← ML-KEM.Encaps_internal(ek, 𝑚)▷ run internal encapsulation algorithm
6: return (𝐾, 𝑐)

7.3 ML-KEM Decapsulation
The decapsulation algorithm ML-KEM.Decaps of ML-KEM (Algorithm 21) accepts a decapsulation
key and an ML-KEM ciphertext as input, does not use any randomness, and outputs a sharedsecret. This algorithm requires input checking, as specified below.
Decapsulation input check. To check a candidate decapsulation key dk and ciphertext 𝑐, performthe following checks:
1. (Ciphertext type check) If 𝑐 is not a byte array of length 32(𝑑𝑢 𝑘 + 𝑑𝑣 ) for the values of 𝑑𝑢 ,𝑑𝑣 , and 𝑘 specified by the relevant parameter set, then input checking has failed.
2. (Decapsulation key type check) If dk is not a byte array of length 768𝑘 + 96 for the value of𝑘 specified by the relevant parameter set, then input checking has failed.
3. (Hash check) Perform the computation
test ← H(dk[384𝑘 ∶ 768𝑘 + 32])) .

(7.2)

If test ≠ dk[768𝑘 + 32 ∶ 768𝑘 + 64], then input checking has failed.
If all of the above checks pass, then ML-KEM.Decaps can be run with inputs dk ∶= dk and 𝑐 ∶= 𝑐. It
is important to note that this checking process does not guarantee that dk is a properly produced
output of ML-KEM.KeyGen, nor that 𝑐 is a properly produced output of ML-KEM.Encaps.
ML-KEM.Decaps shall not be run with a decapsulation key or a ciphertext unless both havebeen checked. However, checking of the decapsulation key need not be performed by the
decapsulating party, nor with every execution of ML-KEM.Decaps. Instead, assurance that thischeck has been performed can be acquired through other means (see SP 800-227 [1]). Ciphertext
checking shall be performed with every execution of ML-KEM.Decaps.

37

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Algorithm 21 ML-KEM.Decaps(dk, 𝑐)
Uses the decapsulation key to produce a shared secret key from a ciphertext.
Checked input: decapsulation key dk ∈ 𝔹768𝑘+96 .
Checked input: ciphertext 𝑐 ∈ 𝔹32(𝑑𝑢 𝑘+𝑑𝑣 ) .
Output: shared secret key 𝐾 ∈ 𝔹32 .
1: 𝐾 ′ ← ML-KEM.Decaps_internal(dk, 𝑐)
2: return 𝐾 ′38▷ run internal decapsulation algorithm

FIPS 203

8.

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Parameter Sets

ML-KEM is equipped with three parameter sets, each of the which comprises five individual
parameters: 𝑘, 𝜂1 , 𝜂2 , 𝑑𝑢 , and 𝑑𝑣 . There are also two constants: 𝑛 = 256 and 𝑞 = 3329. The
following is a brief and informal description of the roles played by the variable parameters in thealgorithms of K-PKE and ML-KEM. See Section 5 for details.
• The parameter 𝑘 determines the dimensions of the matrix 𝐀̂ that appears in K-PKE.KeyGen
and K-PKE.Encrypt. It also determines the dimensions of vectors 𝐬 and 𝐞 in K-PKE.KeyGen
and the dimensions of vectors 𝐲 and 𝐞1 in K-PKE.Encrypt.
• The parameter 𝜂1 is required to specify the distribution for generating the vectors 𝐬 and 𝐞in K-PKE.KeyGen and the vector 𝐲 in K-PKE.Encrypt.
• The parameter 𝜂2 is required to specify the distribution for generating the vectors 𝐞1 and𝑒2 in K-PKE.Encrypt.
• The parameters 𝑑𝑢 and 𝑑𝑣 serve as parameters and inputs for the functions Compress,
Decompress, ByteEncode, and ByteDecode in K-PKE.Encrypt and K-PKE.Decrypt.
This standard approves the parameter sets given in Table 2. Each parameter set is associatedwith a required security strength for randomness generation (see Section 3.3). The sizes of the
ML-KEM keys and ciphertexts for each parameter set are summarized in Table 3.
Table 2. Approved parameter sets for ML-KEM

ML-KEM-512
ML-KEM-768
ML-KEM-1024

𝑛
256
256
256

𝑞
𝑘
3329 23329 33329 4

𝜂1
3
2
2

𝜂2
2
2
2

𝑑𝑢
10
10
11

𝑑𝑣
4
4
5

required RBG strength (bits)
128
192
256

Table 3. Sizes (in bytes) of keys and ciphertexts of ML-KEM

ML-KEM-512
ML-KEM-768
ML-KEM-1024

encapsulation key
800
1184
1568

decapsulation key
1632
2400
3168

ciphertext
768
1088
1568

shared secret key
32
32
32

A parameter set name can also be said to denote a (parameter-free) KEM. Specifically, ML-KEM-𝑥
can be used to denote the parameter-free KEM that results from instantiating the scheme
ML-KEM with the parameter set ML-KEM-𝑥.
The three parameter sets included in Table 2 were designed to meet certain security strengthcategories defined by NIST in its original Call for Proposals [4, 22]. These security strengthcategories are explained further in SP 800-57, Part 1 [7].
Using this approach, security strength is not described by a single number, such as “128 bits of
security.” Instead, each ML-KEM parameter set is claimed to be at least as secure as a generic
39

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

block cipher with a prescribed key size or a generic hash function with a prescribed output
length. More precisely, it is claimed that the computational resources needed to break ML-KEM
are greater than or equal to the computational resources needed to break the block cipher orhash function when those computational resources are estimated using any realistic model ofcomputation. Different models of computation can be more or less realistic and, accordingly,
lead to more or less accurate estimates of security strength. Some commonly studied models
are discussed in [23].
Concretely, ML-KEM-512 is claimed to be in security category 1, ML-KEM-768 is claimed to be
in security category 3, and ML-KEM-1024 is claimed to be in security category 5. For additional
discussion of the security strength of MLWE-based cryptosystems, see [4].
Selecting an appropriate parameter set. When initially establishing cryptographic protections
for data, the strongest possible parameter set should be used. This has a number of advantages,
including reducing the likelihood of costly transitions to higher-security parameter sets in the
future. At the same time, it should be noted that some parameter sets might have adverse
performance effects for the relevant application (e.g., the algorithm may be unacceptably slow,
or objects such as keys or ciphertexts may be unacceptably large).
NIST recommends using ML-KEM-768 as the default parameter set, as it provides a large security
margin at a reasonable performance cost. In cases where this is impractical or even higher
security is required, other parameter sets may be used.

40

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

References[1] National Institute of Standards and Technology (2024) Recommendations for keyencapsulation mechanisms, (National Institute of Standards and Technology, Gaithersburg, MD), NIST Special Publication (SP) 800-227. [Forthcoming; will be available athttps://csrc.nist.gov/publications].
[2] Barker EB, Chen L, Roginsky AL, Vassilev A, Davis R (2018) Recommendation for pair-wise
key-establishment schemes using discrete logarithm cryptography (National Institute ofStandards and Technology, Gaithersburg, MD), NIST Special Publication (SP) 800-56A Revision 3. https://doi.org/10.6028/NIST.SP.800-56Ar3.
[3] Barker EB, Chen L, Roginsky AL, Vassilev A, Davis R, Simon S (2019) Recommendation forpair-wise key-establishment using integer factorization cryptography (National Instituteof Standards and Technology, Gaithersburg, MD), NIST Special Publication (SP) 800-56B
Revision 2. https://doi.org/10.6028/NIST.SP.800-56Br2.
[4] Avanzi R, Bos J, Ducas L, Kiltz E, Lepoint T, Lyubashevsky V, Schanck JM, Schwabe P, Seiler G,
Stehlé D (2020) CRYSTALS-Kyber algorithm specifications and supporting documentation,
Third-round submission to the NIST’s post-quantum cryptography standardization process.
Available at https://csrc.nist.gov/Projects/post-quantum-cryptography/post-quantum-cry
ptography-standardization/round-3-submissions.
[5] National Institute of Standards and Technology (2015) Secure hash standard (SHS), (U.S.
Department of Commerce, Washington, DC), Federal Information Processing Standards
Publication (FIPS) 180-4. https://doi.org/10.6028/NIST.FIPS.180-4.
[6] National Institute of Standards and Technology (2015) SHA-3 standard: Permutation-basedhash and extendable-output functions, (U.S. Department of Commerce, Washington, DC),
Federal Information Processing Standards Publication (FIPS) 202. https://doi.org/10.6028/
NIST.FIPS.202.
[7] Barker EB (2020) Recommendation for key management: Part 1 - General, (National Instituteof Standards and Technology, Gaithersburg, MD), NIST Special Publication (SP) 800-57 Part
1, Rev. 5 [or as amended]. https://doi.org/10.6028/NIST.SP.800-57pt1r5.
[8] Bos J, Ducas L, Kiltz E, Lepoint T, Lyubashevsky V, Schanck JM, Schwabe P, Seiler G, Stehlé
D (2018) CRYSTALS-Kyber: A CCA-secure module-lattice-based KEM. 2018 IEEE European
Symposium on Security and Privacy (EuroS&P), pp 353–367. https://doi.org/10.1109/Euro
SP.2018.00032.
[9] Langlois A, Stehlé D (2015) Worst-case to average-case reductions for module lattices.
Designs, Codes and Cryptography 75(3):565–599. https://doi.org/10.1007/s10623-014-9
938-4.
[10] Regev O (2005) On lattices, learning with errors, random linear codes, and cryptography.
Proceedings of the Thirty-Seventh Annual ACM Symposium on Theory of Computing STOC
’05 (Association for Computing Machinery, New York, NY, USA), pp 84––93. https://doi.org/
10.1145/1060590.1060603.

41

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM[11] Fujisaki E, Okamoto T (2013) Secure integration of asymmetric and symmetric encryption
schemes. Journal of Cryptology 26:80–101. https://doi.org/10.1007/s00145-011-9114-1.
[12] Hofheinz D, Hövelmanns K, Kiltz E (2017) A modular analysis of the Fujisaki-Okamoto transformation. Theory of Cryptography, eds Kalai Y, Reyzin L (Springer International Publishing,Cham), pp 341–371. https://doi.org/10.1007/978-3-319-70500-2_12.
[13] Katz J, Lindell Y (2020) Introduction to Modern Cryptography (Chapman & Hall/CRC), 3rd Ed.
[14] Almeida JB, Olmos SA, Barbosa M, Barthe G, Dupressoir F, Grégoire B, Laporte V, Léchenet JC,
Low C, Oliveira T, Pacheco H, Quaresma M, Schwabe P, Strub PY (2024) Formally verifying Kyber episode V: Machine-checked IND-CCA security and correctness of ML-KEM in EasyCrypt,
Cryptology ePrint Archive, Paper 2024/843. Available at https://eprint.iacr.org/2024/843.
[15] Ducas L, Schanck J (2021) Security estimation scripts for Kyber and Dilithium, Github repository. Available at https://github.com/pq-crystals/security-estimates.
[16] Chen L (2022) Recommendation for key derivation using pseudorandom functions, (National
Institute of Standards and Technology, Gaithersburg, MD), NIST Special Publication (SP)
800-108r1-upd1, Includes updates as of February 2, 2024. https://doi.org/10.6028/NIST.SP.
800-108r1-upd1.
[17] Barker EB, Chen L, Davis R (2020) Recommendation for key-derivation methods in keyestablishment schemes (National Institute of Standards and Technology, Gaithersburg, MD),NIST Special Publication (SP) 800-56C Revision 2. https://doi.org/10.6028/NIST.SP.800-56C
r2.
[18] Barker EB, Kelsey JM (2015) Recommendation for random number generation using deterministic random bit generators, (National Institute of Standards and Technology, Gaithersburg, MD), NIST Special Publication (SP) 800-90A, Rev. 1. https://doi.org/10.6028/NIST.SP.
800-90Ar1.
[19] Sönmez Turan M, Barker EB, Kelsey JM, McKay KA, Baish ML, Boyle M (2018) Recommendation for the entropy sources used for random bit generation, (National Instituteof Standards and Technology, Gaithersburg, MD), NIST Special Publication (SP) 800-90B.
https://doi.org/10.6028/NIST.SP.800-90B.
[20] Barker E, Kelsey J, McKay K, Roginsky A, Turan MS (2024) Recommendation for random bit
generator (RBG) constructions, (National Institute of Standards and Technology, Gaithersburg, MD), NIST Special Publication (SP) 800-90C 4pd. https://doi.org/10.6028/NIST.SP.80
0-90C.4pd.
[21] Kelsey J, Chang S, Perlner R (2016) SHA-3 Derived Functions: cSHAKE, KMAC, TupleHash
and ParallelHash, (National Institute of Standards and Technology, Gaithersburg, MD), NIST
Special Publication (SP) 800-185 [or as amended]. https://doi.org/10.6028/NIST.SP.800-1
85.
[22] National Institute of Standards and Technology (2016) Submission requirements and evaluation criteria for the post-quantum cryptography standardization process. Available athttps://csrc.nist.gov/CSRC/media/Projects/Post-Quantum-Cryptography/documents/call-f
or-proposals-final-dec-2016.pdf.
42

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM[23] Alagic G, Apon D, Cooper D, Dang Q, Dang T, Kelsey J, Lichtinger J, Liu YK, Miller C, Moody
D, Peralta R, Perlner R, Robinson A, Smith-Tone D (2022) Status report on the third roundof the NIST post-quantum cryptography standardization process (National Institute ofStandards and Technology, Gaithersburg, MD), NIST Interagency or Internal Report (IR)8413. https://doi.org/10.6028/NIST.IR.8413-upd1.
[24] CRYSTALS-Kyber submission team (2023) “Discussion about Kyber’s tweaked FO transform”,
PQC-Forum Post. Available at https://groups.google.com/a/list.nist.gov/g/pqc-forum/c/W
FRDl8DqYQ4.
[25] CRYSTALS-Kyber submission team (2023) “Kyber decisions, part 2: FO transform”, PQCForum Post. Available at https://groups.google.com/a/list.nist.gov/g/pqc-forum/c/C0D3W
1KoINY/m/99kIvydoAwAJ.

43

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Appendix A — Precomputed Values for the NTT
The following 128 numbers are the values of 𝜁 BitRev7 (𝑖) mod 𝑞 for 𝑖 ∈ {0, … , 127}. These numbers
are used in Algorithms 9 and 10.
{1

1729

2580

3289

2642

630

1897

848

1062

1919

193

797

2786

3260

569

1746

296

2447

1339

1476

3046

56

2240

1333

1426

2094

535

2882

2393

2879

1974

821

289

331

3253

1756

1197

2304

2277

2055

650

1977

2513

632

2865

33

1320

1915

2319

1435

807

452

1438

2868

1534

2402

2647

2617

1481

648

2474

3110

1227

910

17

2761

583

2649

1637

723

2288

1100

1409

2662

3281

233

756

2156

3015

3050

1703

1651

2789

1789

1847

952

1461

2687

939

2308

2437

2388

733

2337

268

641

1584

2298

2037

3220

375

2549

2090

1645

1063

319

2773

757

2099

561

2466

2594

2804

1092

403

1026

1143

2150

2775

886

1722

1212

1874

1029

2110

2935

885

2154 }

44

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

When implementing Algorithm 11, the values 𝜁 2BitRev7 (𝑖)+1 mod 𝑞 need to be computed. The
following array contains these values for 𝑖 ∈ {0, … , 127}:
{ 17

-17

2761

-2761

583

-583

2649

-2649

1637

-1637

723

-723

2288

-2288

1100

-1100

1409

-1409

2662

-2662

3281

-3281

233

-233

756

-756

2156

-2156

3015

-3015

3050

-3050

1703

-1703

1651

-1651

2789

-2789

1789

-1789

1847

-1847

952

-952

1461

-1461

2687

-2687

939

-939

2308

-2308

2437

-2437

2388

-2388

733

-733

2337

-2337

268

-268

641

-641

1584

-1584

2298

-2298

2037

-2037

3220

-3220

375

-375

2549

-2549

2090

-2090

1645

-1645

1063

-1063

319

-319

2773

-2773

757

-757

2099

-2099

561

-561

2466

-2466

2594

-2594

2804

-2804

1092

-1092

403

-403

1026

-1026

1143

-1143

2150

-2150

2775

-2775

886

-886

1722

-1722

1212

-1212

1874

-1874

1029

-1029

2110

-2110

2935

-2935

885

-885

2154

-2154 }

45

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Appendix B — SampleNTT Loop Bounds
In SampleNTT (Algorithm 7), the algorithm repeatedly generates byte arrays from the XOF tocreate an element of 𝑇𝑞 . If a generated byte array value is out of bounds for a coefficient of 𝑇𝑞 ,the algorithm tries again until all 256 coefficients are created. On average, this while loop will
resolve within a reasonable number of iterations. However, there may be cases in which thegenerated byte arrays are consistently out of bounds and the algorithm may run for a higher
number of iterations.
Implementations should not bound this loop, if at all possible. An incorrect limit will causeinteroperability errors, and the chances for SampleNTT to iterate longer become exponentiallyrare. If an implementation does bound the number of iterations of SampleNTT, it shall not use alimit lower than those presented in Table 4. The calculated probability of SampleNTT exceeding
the limit is included and calculated under standard assumptions about the output distributionsof XOFs and hash functions.
Table 4. While-loop limits and probabilities of occurrence for SampleNTT
Number of iterations

Probability of reaching limit

280

2−261

If a limit is used and the number of iterations exceeds the limit, then the algorithm shall destroy
all intermediate results. If a return value or exception is produced, it shall be the same value forany execution in which the maximum number of iterations is exceeded.

46

FIPS 203

MODULE-LATTICE-BASED KEY-ENCAPSULATION MECHANISM

Appendix C — Differences From the CRYSTALS-Kyber Submission
This appendix lists the differences between CRYSTALS-KYBER (as described in [4]) and the ML-KEM
scheme (specified in this document) that result in differing input-output behavior of the mainalgorithms (i.e., KeyGen, Encaps, Decaps). Since a conforming implementation need only matchthe input-output behavior of these three algorithms (see “Implementations” and Section 3.3
below), the list does not include any of the numerous differences in how the main algorithms actually produce outputs from inputs (e.g., via different computational steps or different subroutines),
nor any differences in presentation between this standard and [4].

C.1 Differences Between CRYSTALS-Kyber and FIPS 203 Initial Public Draft• In the third-round specification [4], the shared secret key was treated as a variable-length
value whose length depended on how it would be used in the relevant application. In thisspecification, the length of the shared secret key is fixed to 256 bits. It can be used directly
in applications as a symmetric key, or symmetric keys can be derived from it, as specified
in Section 3.3.
• The ML-KEM.Encaps and ML-KEM.Decaps algorithms in this specification use a different
variant of the Fujisaki-Okamoto transform (see [24, 25]) than the third-round specification [4]. Specifically, ML-KEM.Encaps no longer includes a hash of the ciphertext in thederivation of the shared secret, and ML-KEM.Decaps has been adjusted to match thischange.
• In the third-round specification [4], the initial randomness 𝑚 in the ML-KEM.Encaps algorithm was first hashed before being used. Specifically, between lines 1 and 2 in Algorithm
20, there was an additional step that performed the operation 𝑚 ← 𝐻(𝑚). The purposeof this step was to safeguard against the use of flawed randomness generation processes.
As this standard requires the use of NIST-approved randomness generation, this step isunnecessary and is not performed in ML-KEM.
• This specification includes explicit input checking steps that were not part of the third-round
specification [4]. For example, ML-KEM.Encaps requires that the byte array containing theencapsulation key correctly decodes to an array of integers modulo 𝑞 without any modular
reductions.

C.2 Changes From FIPS 203 Initial Public Draft
The differences between CRYSTALS-KYBER and ML-KEM as described in Appendix C were includedin the initial public draft (ipd) of FIPS 203, which was posted on August 24, 2023. Based oncomments submitted on the draft ML-KEM, domain separation was added to K-PKE.KeyGen toprevent the misuse of keys generated to target one security level from being used for a different
security level when saving a key as a seed.
Additionally, FIPS 203 ipd had inadvertently swapped the indices of matrix 𝐀̂ in K-PKE.KeyGen and
K-PKE.Encrypt. This was changed back in the final version of ML-KEM to match CRYSTALS-KYBER.
47