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Security LAMPS Key Encapsulation Mechanism (KEM) KEMRecipientInfo ML-KEM Kyber Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM) is a quantum-resistant key-encapsulation mechanism (KEM). Three parameter sets for the ML-KEM algorithm are specified by the US National Institute of Standards and Technology (NIST) in FIPS 203. In order of increasing security strength (and decreasing performance), these parameter sets are ML-KEM-512, ML-KEM-768, and ML-KEM-1024. This document specifies the conventions for using ML-KEM with the Cryptographic Message Syntax (CMS) using the KEMRecipientInfo structure defined in "Using Key Encapsulation Mechanism (KEM) Algorithms in the Cryptographic Message Syntax (CMS)" (RFC 9629). About This Document Status information for this document may be found at . Discussion of this document takes place on the Limited Additional Mechanisms for PKIX and SMIME (lamps) Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction The Module Lattice Key Encapsulation Mechanism (ML-KEM) is an IND-CCA2-secure Key Encapsulation Mechanism (KEM) standardized in by the NIST PQC Project . ML-KEM is the name given to the final standardized version and is incompatible with pre-standards versions, often called "Kyber". defines the KEMRecipientInfo structure for the use of KEM algorithms for the CMS enveloped-data content type, the CMS authenticated-data content type, and the CMS authenticated-enveloped-data content type. This document specifies the direct use of ML-KEM in the KEMRecipientInfo structure using each of the three parameter sets from , namely MK-KEM-512, ML-KEM-768, and ML-KEM-1024. It does not address or preclude the use of ML-KEM as part of any hybrid scheme.

Conventions and Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP14 when, and only when, they appear in all capitals, as shown here.

ML-KEM ML-KEM is a lattice-based key encapsulation mechanism using Module Learning with Errors as its underlying primitive, which is a structured lattices variant that offers good performance and relatively small and balanced key and ciphertext sizes. ML-KEM was standardized with three parameter sets: ML-KEM-512, ML-KEM-768, and ML-KEM-1024. The parameters for each of the security levels were chosen to be at least as secure as a generic block cipher of 128, 192, or 256 bits, respectively. provides more information on ML-KEM security levels and sizes. All KEM algorithms provide three functions: KeyGen(), Encapsulate(), and Decapsulate(). The following summarizes these three functions for the ML-KEM algorithm, referencing corresponding functions in :

KeyGen() -> (ek, dk):

Generate the public encapsulation key (ek) and a private decapsulation key (dk). specifies two formats for an ML-KEM private key: a 64-octet seed (d,z) and an (expanded) private decapsulation key (dk). Algorithm 19 (ML-KEM.KeyGen()) from generates the public encapsulation key (ek) and the private decapsulation key (dk). As an alternative, when a seed (d,z) is generated first and then the seed is expanded to get the keys, algorithm 16 (ML-KEM.KeyGen_internal(d,z)) from expands the seed to ek and dk. See for private key encoding considerations.

Encapsulate(ek) -> (c, ss):

Given the recipient's public key (ek), produce both a ciphertext (c) to be passed to the recipient and a shared secret (ss) for use by the originator. Algorithm 20 (ML-KEM.Encaps(ek)) from is the encapsulation function for ML-KEM.

Decapsulate(dk, c) -> ss:

Given the private key (dk) and the ciphertext (c), produce the shared secret (ss) for the recipient. Algorithm 21 (ML-KEM.Decaps(dk,c)) from is the decapsulation function for ML-KEM. If the private key is stored in seed form, ML-KEM.KeyGen_internal(d,z) may be needed as a first step to compute dk. See for consistency considerations if the private key was stored in both seed and expanded formats.

All security levels of ML-KEM use SHA3-256, SHA3-512, SHAKE256, and SHAKE512 internally.

Use of the ML-KEM Algorithm in the CMS The ML-KEM algorithm MAY be employed for one or more recipients in the CMS enveloped-data content type , the CMS authenticated-data content type , or the CMS authenticated-enveloped-data content type . In each case, the KEMRecipientInfo is used with the ML-KEM algorithm to securely transfer the content-encryption key from the originator to the recipient. Processing ML-KEM with KEMRecipientInfo follows the same steps as . To support the ML-KEM algorithm, a CMS originator MUST implement the Encapsulate() function and a CMS recipient MUST implement the Decapsulate() function.

RecipientInfo Conventions When the ML-KEM algorithm is employed for a recipient, the RecipientInfo alternative for that recipient MUST be OtherRecipientInfo using the KEMRecipientInfo structure as defined in . The fields of the KEMRecipientInfo have the following meanings:

• version is the syntax version number; it MUST be 0.

• rid identifies the recipient's certificate or public key.

• kem identifies the KEM algorithm; it MUST contain one of id-alg-ml-kem-512, id-alg-ml-kem-768, or id-alg-ml-kem-1024. These identifiers are reproduced in .

• kemct is the ciphertext produced for this recipient.

• kdf identifies the key-derivation algorithm. Note that the Key Derivation Function (KDF) used for CMS RecipientInfo process MAY be different than the KDF used within the ML-KEM algorithm. Implementations MUST support HKDF with SHA-256 , using the id-alg-hkdf-with-sha256 KDF object identifier . As specified in , the parameter field MUST be absent when this object identifier appears within the ASN.1 type AlgorithmIdentifier. Implementations MAY support other KDFs as well.

• kekLength is the size of the key-encryption key in octets.

• ukm is optional input to the key-derivation function. The secure use of ML-KEM in CMS does not depend on the use of a ukm value, so this document does not place any requirements on this value. See for more information about the ukm parameter.

• wrap identifies a key-encryption algorithm used to encrypt the content-encryption key. Implementations supporting ML-KEM-512 MUST support the AES-Wrap-128 key-encryption algorithm using the id-aes128-wrap key-encryption algorithm object identifier . Implementations supporting ML-KEM-768 or ML-KEM-1024 MUST support the AES-Wrap-256 key-encryption algorithm using the id-aes256-wrap key-encryption algorithm object identifier . Implementations MAY support other key-encryption algorithms as well.

contains an example of establishing a content-encryption key using ML-KEM in the KEMRecipientInfo type.

Underlying Components When ML-KEM is employed in the CMS, the underlying components used within the KEMRecipientInfo structure SHOULD be consistent with a minimum desired security level. Several security levels have been identified in NIST SP 800-57 Part 1 . If underlying components other than those specified in are used, then the following table gives the minimum requirements on the components used with ML-KEM in the KEMRecipientInfo type in order to satisfy the KDF and key wrapping algorithm requirements from : ML-KEM KEMRecipientInfo component security levels

Security Strength	Algorithm	KDF preimage strength	Symmetric key-encryption strength
128-bit	ML-KEM-512	128-bit	128-bit
192-bit	ML-KEM-768	192-bit	192-bit (*)
256-bit	ML-KEM-1024	256-bit	256-bit

(*) In the case of AES Key Wrap, a 256-bit key is typically used because AES-192 is not as commonly deployed.

Use of the HKDF-based Key Derivation Function The HKDF function is a composition of the HKDF-Extract and HKDF-Expand functions. When used with KEMRecipientInfo, the salt parameter is unused, that is it is the zero-length string "". The IKM, info and L parameters correspond to the same KDF inputs from . The info parameter is independently generated by the originator and recipient. Implementations MUST confirm that L is consistent with the key size of the key-encryption algorithm.

Certificate Conventions RFC 5280 specifies the profile for using X.509 Certificates in Internet applications. A recipient static public key is needed for ML-KEM, and the originator obtains that public key from the recipient's certificate. The conventions for carrying ML-KEM public keys are specified in .

SMIME Capabilities Attribute Conventions defines the SMIMECapabilities attribute to announce a partial list of algorithms that an S/MIME implementation can support. When constructing a CMS signed-data content type , a compliant implementation MAY include the SMIMECapabilities attribute that announces support for one or more of the ML-KEM algorithm identifiers. The SMIMECapability SEQUENCE representing the ML-KEM algorithm MUST include one of the ML-KEM object identifiers in the capabilityID field. When one of the ML-KEM object identifiers appears in the capabilityID field, the parameters MUST NOT be present.

Identifiers All identifiers used to indicate ML-KEM within the CMS are defined in and but reproduced here for convenience:

Security Considerations The Security Considerations sections of and apply to this specification as well. For ongoing discussions of ML-KEM-specific security considerations, refer to . Implementations MUST protect the ML-KEM private key, the key-encryption key, the content-encryption key, message-authentication key, and the content-authenticated-encryption key. Of these keys, all but the private key are ephemeral and MUST be wiped after use. Disclosure of the ML-KEM private key could result in the compromise of all messages protected with that key. Disclosure of the key-encryption key, the content-encryption key, or the content-authenticated-encryption key could result in compromise of the associated encrypted content. Disclosure of the key-encryption key, the message-authentication key, or the content-authenticated-encryption key could allow modification of the associated authenticated content. Additional considerations related to key management may be found in . The generation of private keys relies on random numbers, as does the encapsulation function of ML-KEM. The use of inadequate pseudo-random number generators (PRNGs) to generate these values can result in little or no security. In the case of key generation, a random 32-byte seed is used to deterministically derive the key (with an additional 32 bytes reserved as a rejection value). In the case of encapsulation, a KEM is derived from the underlying ML-KEM public key encryption algorithm by deterministically encrypting a random 32-byte message for the public key. If the random value is weakly-chosen, then an attacker may find it much easier to reproduce the PRNG environment that produced the keys or ciphertext, searching the resulting small set of possibilities for a matching public key or ciphertext value, rather than performing a more complex algorithmic attack against ML-KEM. The generation of quality random numbers is difficult; see Section 3.3 of for some additional information. ML-KEM encapsulation and decapsulation only outputs a shared secret and ciphertext. Implementations MUST NOT use intermediate values directly for any purpose. Implementations SHOULD NOT reveal information about intermediate values or calculations, whether by timing or other "side channels", otherwise an opponent may be able to determine information about the keying data and/or the recipient's private key. Although not all intermediate information may be useful to an opponent, it is preferable to conceal as much information as is practical, unless analysis specifically indicates that the information would not be useful to an opponent. Generally, good cryptographic practice employs a given ML-KEM key pair in only one scheme. This practice avoids the risk that vulnerability in one scheme may compromise the security of the other, and may be essential to maintain provable security. Parties can gain assurance that implementations are correct through formal implementation validation, such as the NIST Cryptographic Module Validation Program (CMVP) .

IANA Considerations For the ASN.1 Module in , IANA is requested to assign an object identifier (OID) for the module identifier (TBD1) with a Description of "id-mod-cms-ml-kem-2024". The OID for the module should be allocated in the "SMI Security for S/MIME Module Identifier" registry (1.2.840.113549.1.9.16.0).

Acknowledgements This document borrows heavily from , , and . Thanks go to the authors of those documents. "Copying always makes things easier and less error prone" - RFC8411. Thanks to Carl Wallace, Jonathan Hammel, and Sean Turner for the detailed review and Carl Wallace and Philippe Cece for interoperability testing for the examples.

References Normative References National Institute of Standards and Technology (U.S.) This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 4.0. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 5751. ITU-T The Cryptographic Message Syntax (CMS) format, and many associated formats, are expressed using ASN.1. The current ASN.1 modules conform to the 1988 version of ASN.1. This document updates those ASN.1 modules to conform to the 2002 version of ASN.1. There are no bits-on-the-wire changes to any of the formats; this is simply a change to the syntax. This document is not an Internet Standards Track specification; it is published for informational purposes. The Cryptographic Message Syntax (CMS) supports key transport and key agreement algorithms. In recent years, cryptographers have been specifying Key Encapsulation Mechanism (KEM) algorithms, including quantum-secure KEM algorithms. This document defines conventions for the use of KEM algorithms by the originator and recipients to encrypt and decrypt CMS content. This document updates RFC 5652. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK] This document describes an additional content type for the Cryptographic Message Syntax (CMS). The authenticated-enveloped-data content type is intended for use with authenticated encryption modes. All of the various key management techniques that are supported in the CMS enveloped-data content type are also supported by the CMS authenticated-enveloped-data content type. [STANDARDS-TRACK] This document specifies a simple Hashed Message Authentication Code (HMAC)-based key derivation function (HKDF), which can be used as a building block in various protocols and applications. The key derivation function (KDF) is intended to support a wide range of applications and requirements, and is conservative in its use of cryptographic hash functions. This document is not an Internet Standards Track specification; it is published for informational purposes. Information Technology Laboratory National Institute of Standards and Technology

US Gaithersburg

RFC 5869 specifies the HMAC-based Extract-and-Expand Key Derivation Function (HKDF) algorithm. This document assigns algorithm identifiers to the HKDF algorithm when used with three common one-way hash functions. This document specifies the conventions for using the Advanced Encryption Standard (AES) algorithm for encryption with the Cryptographic Message Syntax (CMS). [STANDARDS-TRACK] This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] sn3rd AWS AWS Cloudflare The Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM) is a quantum-resistant key-encapsulation mechanism (KEM). This document specifies the conventions for using the ML-KEM in X.509 Public Key Infrastructure. The conventions for the subject public keys and private keys are also specified. Informative References National Institute of Standards and Technology National Institute of Standards and Technology Information Technology Laboratory National Institute of Standards and Technology

US Gaithersburg

Cisco Systems National Institute of Standards and Technology Ericsson Quantinuum Arqit Quantum Inc NIST standardized ML-KEM as FIPS 203 in August 2024. This document discusses how to use ML-KEM and how to use it within protocols - that is, what problem it solves, and what you need to do to use it securely. The RSA Key Encapsulation Mechanism (RSA-KEM) algorithm is a one-pass (store-and-forward) cryptographic mechanism for an originator to securely send keying material to a recipient using the recipient's RSA public key. The RSA-KEM algorithm is specified in Clause 11.5 of ISO/IEC: 18033-2:2006. This document specifies the conventions for using the RSA-KEM algorithm as a standalone KEM algorithm and the conventions for using the RSA-KEM algorithm with the Cryptographic Message Syntax (CMS) using KEMRecipientInfo as specified in RFC 9629. This document obsoletes RFC 5990. Amazon Web Services Amazon Web Services NIST recently standardized ML-KEM, a new key encapsulation mechanism, which can be used for quantum-resistant key establishment. This draft specifies how to use ML-KEM as an additional key exchange in IKEv2 along with traditional key exchanges. This Post-Quantum Traditional Hybrid Key Encapsulation Mechanism approach allows for negotiating IKE and Child SA keys which are safe against cryptanalytically-relevant quantum computers and theoretical weaknesses in ML-KEM.

ASN.1 Module This appendix includes the ASN.1 module for ML-KEM. This module imports objects from , , , .

Parameter Set Security and Sizes Instead of defining the strength of a quantum algorithm in a traditional manner using the imprecise notion of bits of security, NIST has defined security levels by picking a reference scheme, which NIST expects to offer notable levels of resistance to both quantum and classical attack. To wit, a KEM algorithm that achieves NIST PQC security must require computational resources to break IND-CCA2 security comparable or greater than that required for key search on AES-128, AES-192, and AES-256 for Levels 1, 3, and 5, respectively. Levels 2 and 4 use collision search for SHA-256 and SHA-384 as reference. ML-KEM parameter sets, NIST Security Level, and sizes in bytes

Parameter Set	Level	Encap. Key Size	Decap. Key Size	Ciphertext Size	Shared Secret Size
ML-KEM-512	1	800	1632	768	32
ML-KEM-768	3	1184	2400	1088	32
ML-KEM-1024	5	1568	3168	1568	32

ML-KEM CMS Authenticated-Enveloped-Data Example This example shows the establishment of an AES-128 content-encryption key using:

• ML-KEM-512;
• KEMRecipientInfo key derivation using HKDF with SHA-256; and
• KEMRecipientInfo key wrap using AES-128-KEYWRAP.

In real-world use, the originator would encrypt the content- encryption key in a manner that would allow decryption with their own private key as well as the recipient's private key. This is omitted in an attempt to simplify the example.

Originator CMS Processing Alice obtains Bob's ML-KEM-512 public key: Bob's ML-KEM-512 public key has the following key identifier: Alice generates a shared secret and ciphertext using Bob's ML-KEM-512 public key: Shared secret: Ciphertext: Alice encodes the CMSORIforKEMOtherInfo: Alice derives the key-encryption key from the shared secret and CMSORIforKEMOtherInfo using HKDF with SHA-256: Alice randomly generates a 128-bit content-encryption key: Alice uses AES-128-KEYWRAP to encrypt the content-encryption key with the key-encryption key: Alice encrypts the padded content using AES-128-GCM with the content-encryption key and encodes the AuthEnvelopedData (using KEMRecipientInfo) and ContentInfo, and then sends the result to Bob. The Base64-encoded result is: This result decodes to:

Recipient CMS Processing Bob's ML-KEM-512 private key: Bob decapsulates the ciphertext in the KEMRecipientInfo to get the ML-KEM-512 shared secret, encodes the CMSORIforKEMOtherInfo, derives the key-encryption key from the shared secret and the DER-encoded CMSORIforKEMOtherInfo using HKDF with SHA-256, uses AES-128-KEYWRAP to decrypt the content-encryption key with the key-encryption key, and decrypts the encrypted contents with the content-encryption key, revealing the plaintext content: