]> Entrust Limited

2500 Solandt Road – Suite 100 Ottawa, Ontario K2K 3G5 Canada mike.ounsworth@entrust.com

Entrust Limited

2500 Solandt Road – Suite 100 Ottawa, Ontario K2K 3G5 Canada john.gray@entrust.com

OpenCA Labs

New York City, New York United States of America director@openca.org

Bundesdruckerei GmbH

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Cisco Systems

sfluhrer@cisco.com

Security LAMPS X.509 CMS Post-Quantum KEM This document defines combinations of ML-KEM in hybrid with traditional algorithms RSA-OAEP, ECDH, X25519, and X448. These combinations are tailored to meet security best practices and regulatory requirements. Composite ML-KEM is applicable in any application that uses X.509, PKIX, and CMS data structures and protocols that accept ML-KEM, but where the operator wants extra protection against breaks or catastrophic bugs in ML-KEM. For use within CMS, this document is intended to be coupled with the CMS KEMRecipientInfo mechanism in . About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the LAMPS Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Changes in version -05 Interop-affecting changes:

• Fixed a bug in the definition of the Encaps() functions: KEMs, according to both RFC9180 and FIPS 203 should always return (ss, ct), but we had (ct, ss).
• Interoperable composite private key format requires component public keys (because public keys are required for decapsulation).
• Specified that the tradCT and tradPK inputs to the KEM combiner must be the raw values without the OCTET STRING wrapper.
• Adjusted RSA-OAEP section to follow RFC8017 instead of RFC3560. Does not use the RSA-OAEP label.
• Aligning algorithm list with LAMPS WG on-list discussions and draft-openpgp-pqc
  • ML-KEM-768 aligned with P-384 as per Quynh's OpenPGP presentation: https://datatracker.ietf.org/meeting/120/materials/slides-120-openpgp-pqc-with-nist-and-brainpool-curves
  • Removing ML-KEM-512 combinations as per Sophie's recommendation: https://mailarchive.ietf.org/arch/msg/spasm/khasPf3y0_-Lq_0NtJe92unUw6o/
• Specified some options to use HKDF-SHA2, and some to use SHA3 / KMAC to facilitate implementations that do not have easy access to SHA3 outside the ML-KEM module.
• All levels now use 256-bit KDFs, to match ML-KEM's 256-bit shared secret key.
• Tweaks to combiner function, thanks to Quynh and authors of draft-ietf-openpgp-PQC:
  • Removed the counter.
  • Un-twisted tradSS || mlkemSS to mlkemSS || tradSS as you would expect (thanks Quynh for pointing that this is allowed).
  • Simplified to use 256-bit hashes at all security levels (HKDF-SHA256, SHA3-256, KMAC256), which matches ML-KEM's 256 bit shared secret key at all levels.
• Updated prototype OIDs and domain separators to reflect that this version is not compatible with previous version.

Editorial changes:

• Added an Implementation Consideration section explaining why private keys need to contain the public keys.
• Added a security consideration about key reuse.
• Added security considerations about SHA3-vs-HKDF-SHA2 and a warning against generifying this construction to other combinations of ciphers.
• Enhanced the section about how to get this FIPS-certified.
• ASN.1 module fixes (thanks Russ and Carl).
  • Renamed the module from Composite-KEM-2023 -> Composite-MLKEM-2024
  • Simplified the ASN.1 module to make it more compiler-friendly (thanks Carl!) -- should not affect wire encodings.

Still to do in a future version:

• [ ] Wait for NIST SP 800-227 to make sure KEM combiner aligns, and update the section explaining how to get this FIPS-certified.
• [ ] We need PEM samples … hackathon? OQS friends? David @ BC? The right format for samples is probably to follow the hackathon ... a Dilithium or ECDSA trust anchor certificate, a composite KEM end entity certificate, and a CMS EnvelopedData sample encrypted for that composite KEM certificate.
• [ ] Open question: do we need to include the ECDH, X25519, X448, and RSA public keys in the KDF? X-Wing does, but previous versions of this spec do not. In general existing ECC and RSA hardware decrypter implementations might not know their own public key.
• [ ] Other outstanding github issues: https://github.com/lamps-wg/draft-composite-kem/issues

Introduction The advent of quantum computing poses a significant threat to current cryptographic systems. Traditional cryptographic algorithms such as RSA-OAEP, ECDH and their elliptic curve variants are vulnerable to quantum attacks. During the transition to post-quantum cryptography (PQC), there is considerable uncertainty regarding the robustness of both existing and new cryptographic algorithms. While we can no longer fully trust traditional cryptography, we also cannot immediately place complete trust in post-quantum replacements until they have undergone extensive scrutiny and real-world testing to uncover and rectify potential implementation flaws. Unlike previous migrations between cryptographic algorithms, the decision of when to migrate and which algorithms to adopt is far from straightforward. Even after the migration period, it may be advantageous for an entity's cryptographic identity to incorporate multiple public-key algorithms to enhance security. Cautious implementers may opt to combine cryptographic algorithms in such a way that an attacker would need to break all of them simultaneously to compromise the protected data. These mechanisms are referred to as Post-Quantum/Traditional (PQ/T) Hybrids . Certain jurisdictions are already recommending or mandating that PQC lattice schemes be used exclusively within a PQ/T hybrid framework. The use of Composite scheme provides a straightforward implementation of hybrid solutions compatible with (and advocated by) some governments and cybersecurity agencies . In addition, specifically references this specification as a concrete example of hybrid X.509 certificates. A more recent example is , a document co-authored by French Cybersecurity Agency (ANSSI), Federal Office for Information Security (BSI), Netherlands National Communications Security Agency (NLNCSA), and Swedish National Communications Security Authority, Swedish Armed Forces which makes the following statement:

• “In light of the urgent need to stop relying only on quantum-vulnerable public-key cryptography for key establishment, the clear priority should therefore be the migration to post-quantum cryptography in hybrid solutions”

This specification represents the straightforward implementation of the hybrid solutions called for by European cyber security agencies. PQ/T Hybrid cryptography can, in general, provide solutions to two migration problems:

• Algorithm strength uncertainty: During the transition period, some post-quantum signature and encryption algorithms will not be fully trusted, while also the trust in legacy public key algorithms will start to erode. A relying party may learn some time after deployment that a public key algorithm has become untrustworthy, but in the interim, they may not know which algorithm an adversary has compromised.
• Ease-of-migration: During the transition period, systems will require mechanisms that allow for staged migrations from fully classical to fully post-quantum-aware cryptography.

This document defines a specific instantiation of the PQ/T Hybrid paradigm called "composite" where multiple cryptographic algorithms are combined to form a single key encapsulation mechanism (KEM) key and ciphertext such that they can be treated as a single atomic algorithm at the protocol level. Composite algorithms address algorithm strength uncertainty because the composite algorithm remains strong so long as one of its components remains strong. Concrete instantiations of composite ML-KEM algorithms are provided based on ML-KEM, RSA-OAEP and ECDH. Backwards compatibility is not directly covered in this document, but is the subject of . Composite ML-KEM is intended for general applicability anywhere that key establishment or enveloped content encryption is used within PKIX or CMS structures.

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 BCP 14 when, and only when, they appear in all capitals, as shown here. These words may also appear in this document in lower case as plain English words, absent their normative meanings. This document is consistent with all terminology from . In addition, the following terms are used in this document: ALGORITHM: The usage of the term "algorithm" within this document generally refers to any function which has a registered Object Identifier (OID) for use within an ASN.1 AlgorithmIdentifier. This loosely, but not precisely, aligns with the definitions of "cryptographic algorithm" and "cryptographic scheme" given in . COMBINER: A combiner specifies how multiple shared secrets are combined into a single shared secret. DER: Distinguished Encoding Rules as defined in . KEM: A key encapsulation mechanism as defined in . PKI: Public Key Infrastructure, as defined in . SHARED SECRET KEY: A value established between two communicating parties for use as cryptographic key material suitable for direct use by symmetric cryptographic algorithms. This document is concerned with shared secrets established via public key cryptographic operations.

Composite Design Philosophy defines composites as:

• Composite Cryptographic Element: A cryptographic element that incorporates multiple component cryptographic elements of the same type in a multi-algorithm scheme.

Composite keys, as defined here, follow this definition and should be regarded as a single key that performs a single cryptographic operation such as key generation, signing, verifying, encapsulating, or decapsulating -- using its internal sequence of component keys as if they form a single key. This generally means that the complexity of combining algorithms can and should be handled by the cryptographic library or cryptographic module, and the single composite public key, private key, ciphertext and signature can be carried in existing fields in protocols such as PKCS#10 , CMP , X.509 , CMS , and the Trust Anchor Format . In this way, composites achieve "protocol backwards-compatibility" in that they will drop cleanly into any protocol that accepts an analogous single-algorithm cryptographic scheme without requiring any modification of the protocol to handle multiple algorithms.

Overview of the Composite ML-KEM Scheme We borrow here the definition of a key encapsulation mechanism (KEM) from , in which a KEM is a cryptographic primitive that consists of three algorithms:

• KeyGen() -> (pk, sk): A probabilistic key generation algorithm, which generates a public key pk and a secret key sk.
• Encap(pk) -> (ss, ct): A probabilistic encapsulation algorithm, which takes as input a public key pk and outputs a ciphertext ct and shared secret ss. Note: this document uses Encap() to conform to , but uses Encaps().
• Decap(sk, ct) -> ss: A decapsulation algorithm, which takes as input a secret key sk and ciphertext ct and outputs a shared secret ss, or in some cases a distinguished error value. Note: this document uses Decap() to conform to , but uses Decaps().

The KEM interface defined above differs from both traditional key transport mechanism (for example for use with KeyTransRecipientInfo defined in ), and key agreement (for example for use with KeyAgreeRecipientInfo defined in ). The KEM interface was chosen as the interface for a composite key establishment because it allows for arbitrary combinations of component algorithm types since both key transport and key agreement mechanisms can be promoted into KEMs as described in and below. This specification uses the Post-Quantum KEM ML-KEM as specified in and . For Traditional KEMs, this document uses the RSA-OAEP algorithm defined in , the Elliptic Curve Diffie-Hellman key agreement schemes ECDH defined in section 5.7.1.2 of , and X25519 / X448 which are defined in . A combiner function is used to combine the two component shared secrets into a single shared secret.

Promotion of RSA-OAEP into a KEM The RSA Optimal Asymmetric Encryption Padding (OAEP), as defined in section 7.1 of is a public key encryption algorithm used to transport key material from a sender to a receiver. It is promoted into a KEM by having the sender generate a random 256 bit secret and encrypt it. Note that, at least at the time of writing, the algorithm RSAOAEPKEM is not defined as a standalone algorithm within PKIX standards and it does not have an assigned algorithm OID, so it cannot be used directly with CMS KEMRecipientInfo ; it is merely a building block for the composite algorithm. Note that the OAEP label L is left to its default value, which is the empty string as per . The shared secret output by the overall Composite ML-KEM already binds a composite domain separator, so there is no need to also utilize the component domain separators. The value of ss_len as well as the RSA-OAEP parameters used within this specification can be found in . Decap(sk, ct) -> ss is accomplished in the analogous way.

Promotion of ECDH into a KEM An elliptic curve Diffie-Hellman key agreement is promoted into a KEM Encap(pk) -> (ss, ct) using a simplified version of the DHKEM definition from ; simplified to remove the context-binding labels since the shared secret output by the overall Composite ML-KEM already binds a composite domain separator, so there is no need to also utilize labels within DHKEM. Note that, at least at the time of writing, the algorithm DHKEM is not defined as a standalone algorithm within PKIX standards and it does not have an assigned algorithm OID, so it cannot be used directly with CMS KEMRecipientInfo ; it is merely a building block for the composite algorithm. Decap(sk, ct) -> ss is accomplished in the analogous way. This construction applies for all variants of elliptic curve Diffie-Hellman used in this specification: ECDH, X25519, and X448. The simplifications from the DHKEM definition in is that since the ciphertext and receiver's public key are included explicitly in the Composite ML-KEM combiner, there is no need to construct the kem_context object, and since a domain separator is included explicitly in the Composite ML-KEM combiner there is no need to perform the labeled steps of ExtractAndExpand().

KEM Combiner The following KEM combiner construction is as follows is used by both Composite-ML-KEM.Encap() and Composite-ML-KEM.Decap() and is split out here for easier analysis.

KEM combiner construction

where:

• KDF(message) represents a key derivation function suitable to the chosen KEMs according to {tab-kem-combiners}. All KDFs produce a 256-bit shared secret key to match ML-KEM.
• mlkemSS is the shared secret key from the ML-KEM component.
• tradSS is the shared secret from the traditional component (elliptic curve or RSA).
• tradCT is the ciphertext from the traditional component (elliptic curve or RSA).
• tradPK is the public key of the traditional component (elliptic curve or RSA).
• Domain is the DER encoded value of the object identifier of the Composite ML-KEM algorithm as listed in .
• || represents concatenation.

Each registered Composite ML-KEM algorithm specifies the choice of KDF andDomain to be used in and below. Given that each Composite ML-KEM algorithm fully specifies the component algorithms, including for example the size of the RSA modulus, all inputs to the KEM combiner are fixed-size and thus do not require length-prefixing. The CompositeKEM.Decap() specified in adds further error handling to protect the KEM combiner from malicious inputs. The construction of the KEM combiner has implications for security, discussed in , and for FIPS-certifiability, discussed in .

Decapsulation failure Provided all inputs are well-formed, the key establishment procedure of ML-KEM will never explicitly fail. Specifically, the ML-KEM.Encaps and ML-KEM.Decaps algorithms from will always output a value with the same data type as a shared secret key, and will never output an error or failure symbol. However, it is possible (though extremely unlikely) that the process will fail in the sense that ML-KEM.Encaps and ML-KEM.Decaps will produce different outputs, even though both of them are behaving honestly and no adversarial interference is present. In this case, the sender and recipient clearly did not succeed in producing a shared secret key. This event is called a decapsulation failure. Estimates for the decapsulation failure probability (or rate) for each of the ML-KEM parameter sets are provided in Table 1 of and reproduced here in . ML-KEM decapsulation failure rates

Parameter set	Decapsulation failure rate
ML-KEM-512	2^(-139)
ML-KEM-768	2^(-164)
ML-KEM-1024	2^(-174)

In the case of ML-KEM decapsulation failure, Composite ML-KEM MUST preserve the same behaviour and return a well-formed output.

Composite ML-KEM Functions

Key Generation To generate a new keypair for Composite schemes, the KeyGen() -> (pk, sk) function is used. The KeyGen() function calls the two key generation functions of the component algorithms for the Composite keypair in no particular order. Multi-process or multi-threaded applications might choose to execute the key generation functions in parallel for better key generation performance. The following process is used to generate composite keypair values:

Composite KeyGen(pk, sk) (pk, sk) Explicit Inputs: None Implicit Input: ML-KEM A placeholder for the specific ML-KEM algorithm and parameter set to use, for example, could be "ML-KEM-65". Trad A placeholder for the specific traditional algorithm and parameter set to use, for example "RSA-OAEP" or "X25519". Output: (pk, sk) The composite keypair. Key Generation Process: 1. Generate componint keys (mlkemPK, mlkemSK) = ML-KEM.KeyGen() (tradPK, tradSK) = Trad.KeyGen() 2. Check for component key gen failure if NOT (mlkemPK, mlkemSK) or NOT (tradPK, tradSK): output "Key generation error" 3. Encode the component keys into composite structures pk = CompositeKEMPublicKey(mlkemPK, tradPK) sk = CompositeKEMPrivateKey(mlkemSK, tradSK) 4. Output the composite keys return (pk, sk) ]]>

The structures CompositeKEMPublicKey and CompositeKEMPrivateKey are described in and respectively and are used here as placeholders since implementations MAY use their own internal key representations in cases where interoperability is not required. In order to ensure fresh keys, the key generation functions MUST be executed for both component algorithms. Compliant parties MUST NOT use or import component keys that are used in other contexts, combinations, or by themselves as keys for standalone algorithm use. Note that in step 2 above, both component key generation processes are invoked, and no indication is given about which one failed. This SHOULD be done in a timing-invariant way to prevent side-channel attackers from learning which component algorithm failed.

Composite-ML-KEM.Encap The Encap(pk) of a Composite ML-KEM algorithm is designed to behave exactly the same as ML-KEM.Encaps(ek) defined in Algorithm 20 in Section 7.2 of . Specifically, Composite-ML-KEM.Encap(pk) produces a 256-bit shared secret key that can be used directly with any symmetric-key cryptographic algorithm. In this way, Composite ML-KEM can be used as a direct drop-in replacement anywhere that ML-KEM is used.

Composite-ML-KEM.Encap(pk) (ss, ct) Explicit Input: pk Composite public key conisting of encryption public keys for each component. Implicit inputs: ML-KEM A placeholder for the specific ML-KEM algorithm and parameter set to use, for example, could be "ML-KEM-768". Trad A placeholder for the specific ML-KEM algorithm and parameter set to use, for example "RSA-OAEP" or "X25519". KDF The KDF specified for the given Composite ML-KEM algorithm. See algorithm specifications below. Domain Domain separator value for binding the signature to the Composite OID. See section on Domain Separators below. Output: ss The shared secret key, a 256-bit key suitable for use with symmetric cryptographic algorithms. ct The ciphertext, a CompositeCiphertextValue. Encap Process: 1. Separate the public keys. (mlkemPK, tradPK) = pk 2. Perform the respective component Encap operations according to their algorithm specifications. (mlkemCT, mlkemSS) = MLKEM.Encaps(mlkemPK) (tradCT, tradSS) = TradKEM.Encap(tradPK) 3. If either ML-KEM.Encaps() or TradKEM.Encap() return an error, then this process must return an error. if NOT (mlkemCT, mlkemSS) or NOT (tradCT, tradSS): output "Encapsulation error" 4. Encode and combine ct = CompositeCiphertextValue(mlkemCT, tradCT) ss = KDF(mlkemSS || tradSS || tradCT || tradPK || Domain) 5. Output shared secret key and ciphertext return (ss, ct) ]]>

The specific values for KDF are defined per Composite ML-KEM algorithm in and the specific values for Domain are defined per Composite ML-KEM algorithm in . CompositeCiphertextValue is defined in .

Composite-ML-KEM.Decap The Decap(sk, ct) -> ss of a Composite ML-KEM algorithm is designed to behave exactly the same as ML-KEM.Decaps(dk, c) defined in Algorithm 21 in Section 7.3 of . Specifically, Composite-ML-KEM.Decap(sk, ct) produces a 256-bit shared secret key that can be used directly with any symmetric-key cryptographic algorithm. In this way, Composite ML-KEM can be used as a direct drop-in replacement anywhere that ML-KEM is used.

Composite-ML-KEM.Decap(sk, ct) ss Explicit Input: sk Composite private key consisting of decryption private keys for each component. ct The ciphertext, a CompositeCiphertextValue. Implicit inputs: ML-KEM A placeholder for the specific ML-KEM algorithm and parameter set to use, for example, could be "ML-KEM-768". Trad A placeholder for the specific traditional algorithm and parameter set to use, for example "RSA-OAEP" or "X25519". KDF The KDF specified for the given Composite ML-KEM algorithm. See algorithm specifications below. Domain Domain separator value for binding the signature to the Composite OID. See section on Domain Separators below. Output: ss The shared secret key, a 256-bit key suitable for use with symmetric cryptographic algorithms. Decap Process: 1. Separate the private keys and ciphertexts (mlkemSK, tradSK) = sk (mlkemCT, tradCT) = ct 2. Perform the respective component Encap operations according to their algorithm specifications. mlkemSS = MLKEM.Decaps(mlkemSK, mlkemCT) tradSS = TradKEM.Decap(tradSK, tradCT) 3. If either ML-KEM.Decaps() or TradKEM.Decap() return an error, then this process must return an error. if NOT mlkemSS or NOT tradSS: output "Encapsulation error" 4. Combine ss = KDF(mlkemSS || tradSS || tradCT || tradPK || Domain) 5. Output shared secret key return ss ]]>

It is possible to use component private keys stored in separate software or hardware keystores. Variations in the process to accommodate particular private key storage mechanisms are considered to be conformant to this document so long as it produces the same output and error handling as the process sketched above. In order to properly achieve its security properties, the KEM combiner requires that all inputs are fixed-length. Since each Composite ML-KEM algorithm fully specifies its component algorithms, including key sizes, all inputs should be fixed-length in non-error scenarios, however some implementations may need to perform additional checking to handle certain error conditions. In particular, the KEM combiner step should not be performed if either of the component decapsulations returned an error condition indicating malformed inputs. For timing-invariance reasons, it is RECOMMENDED to perform both decapsulation operations and check for errors afterwards to to prevent an attacker from using a timing channel to tell which component failed decapsulation. Also, RSA-based composites MUST ensure that the modulus size (ie the size of tradCT and tradPK) matches that specified for the given Composite ML-KEM algorithm in ; depending on the cryptographic library used, this check may be done by the library or may require an explicit check as part of the CompositeKEM.Decap() routine.

Composite Key Structures In order to form composite public keys and ciphertext values, we define ASN.1-based composite encodings such that these structures can be used as a drop-in replacement for existing public key and ciphertext fields such as those found in PKCS#10 , CMP , X.509 , CMS .

CompositeKEMPublicKey The wire encoding of a Composite ML-KEM public key is: Since RSA and ECDH component public keys are themselves in a DER encoding, the following ASN.1 structures show the internal structure of the various public key types used in this specification: id-raw-key is defined by this document. It signifies that the public key has no ASN.1 wrapping and the raw bits are placed here according to the encoding of the underlying algorithm specification. In some situations and protocols, the key might be wrapped in ASN.1 or may have some other additional decoration or encoding. If so, such wrapping MUST be removed prior to encoding the key itself as a BIT STRING. For use with this document, ML-KEM keys MUST be be the raw BIT STRING representation as specified in and Edwards Curve keys MUST be the raw BIT STRING representation as specified in . Some applications may need to reconstruct the SubjectPublicKeyInfo objects corresponding to each component public key. in provides the necessary mapping between composite and their component algorithms for doing this reconstruction. This also motivates the design choice of SEQUENCE OF BIT STRING instead of SEQUENCE OF OCTET STRING; using BIT STRING allows for easier transcription between CompositeKEMPublicKey and SubjectPublicKeyInfo. When the CompositeKEMPublicKey must be provided in octet string or bit string format, the data structure is encoded as specified in . In order to maintain security properties of the composite, applications that use composite keys MUST always perform fresh key generations of both component keys and MUST NOT reuse existing key material. See for a discussion. The following ASN.1 Information Object Class is defined to allow for compact definitions of each composite algorithm, leading to a smaller overall ASN.1 module. As an example, the public key type pk-MLKEM768-ECDH-P384 can be defined compactly as: The full set of key types defined by this specification can be found in the ASN.1 Module in .

CompositeKEMPrivateKey When a Composite ML-KEM private key is to be exported from a cryptographic module, it uses an analogous definition to the public keys: Each element of the CompositeKEMPrivateKey Sequence is an OCTET STRING according to the encoding of the underlying algorithm specification and will decode into the respective private key structures in an analogous way to the public key structures defined in . This document does not provide helper classes for private keys. The PrivateKey for each component algorithm MUST be in the same order as defined in . Use cases that require an interoperable encoding for composite private keys will often need to place a CompositeKEMPrivateKey inside a OneAsymmetricKey structure defined in , such as when private keys are carried in PKCS #12 , CMP or CRMF . The definition of OneAsymmetricKey is copied here for convenience: When a CompositeKEMPrivateKey is conveyed inside a OneAsymmetricKey structure (version 1 of which is also known as PrivateKeyInfo) , the privateKeyAlgorithm field SHALL be set to the corresponding composite algorithm identifier defined according to and its parameters field MUST be absent. The privateKey field SHALL contain the CompositeKEMPrivateKey, and the publicKey field remains OPTIONAL. If the publicKey field is present, it MUST be a CompositeSignaturePublicKey. Some applications may need to reconstruct the OneAsymmetricKey objects corresponding to each component private key. provides the necessary mapping between composite and their component algorithms for doing this reconstruction. Component keys of a CompositeKEMPrivateKey MUST NOT be used in any other type of key or as a standalone key. For more details on the security considerations around key reuse, see section .

Encoding Rules Many protocol specifications will require that the composite public key and composite private key data structures be represented by an octet string or bit string. When an octet string is required, the DER encoding of the composite data structure SHALL be used directly. When a bit string is required, the octets of the DER encoded composite data structure SHALL be used as the bits of the bit string, with the most significant bit of the first octet becoming the first bit, and so on, ending with the least significant bit of the last octet becoming the last bit of the bit string. In the interests of simplicity and avoiding compatibility issues, implementations that parse these structures MAY accept both BER and DER.

Key Usage Bits When any of the Composite ML-KEM AlgorithmIdentifier appears in the SubjectPublicKeyInfo field of an X.509 certificate , the key usage certificate extension MUST only contain Composite ML-KEM keys MUST NOT be used in a "dual usage" mode because even if the traditional component key supports both signing and encryption, the post-quantum algorithms do not and therefore the overall composite algorithm does not.

Composite ML-KEM Structures

kema-CompositeKEM The ASN.1 algorithm object for a Composite ML-KEM is:

CompositeCiphertextValue The CompositeCipherTextValue is the DER encoding of a SEQUENCE of the ciphertexts from the underlying component algorithms. It is represented in ASN.1 as follows: The order of the component ciphertexts is the same as the order defined in .

Algorithm Identifiers This table summarizes the list of Composite ML-KEM algorithms and lists the OID, two component algorithms, and the KDF to be used within combiner function. Domain separator values are defined below in . EDNOTE: these are prototyping OIDs to be replaced by IANA. <CompKEM>.1 is equal to 2.16.840.1.114027.80.5.2.1

Composite-ML-KEM Algorithm Identifiers Composite ML-KEM key types

Composite ML-KEM Algorithm	OID	First Algorithm	Second Algorithm	KDF
id-MLKEM768-RSA2048	<CompKEM>.21	MLKEM768	RSA-OAEP 2048	HKDF-SHA256/256
id-MLKEM768-RSA3072	<CompKEM>.22	MLKEM768	RSA-OAEP 3072	HKDF-SHA256/256
id-MLKEM768-RSA4096	<CompKEM>.23	MLKEM768	RSA-OAEP 4096	HKDF-SHA256/256
id-MLKEM768-X25519	<CompKEM>.24	MLKEM768	X25519	SHA3-256
id-MLKEM768-ECDH-P384	<CompKEM>.25	MLKEM768	ECDH-P384	HKDF-SHA256/256
id-MLKEM768-ECDH-brainpoolP256r1	<CompKEM>.26	MLKEM768	ECDH-brainpoolp256r1	HKDF-SHA256/256
id-MLKEM1024-ECDH-P384	<CompKEM>.27	MLKEM1024	ECDH-P384	SHA3-256
id-MLKEM1024-ECDH-brainpoolP384r1	<CompKEM>.28	MLKEM1024	ECDH-brainpoolP384r1	SHA3-256
id-MLKEM1024-X448	<CompKEM>.29	MLKEM1024	X448	SHA3-256

For the use of HKDF : a salt is not provided; ie the default salt (all zeroes of length HashLen) will be used. The output length of HKDF is the same as the block size of the underlying hash function; in particular, HKDF-SHA256/256 means HKDF-SHA256 with an output length L of 256 bits (32 octets). Full specifications for the referenced algorithms can be found in .

Domain Separators The KEM combiner defined in section requires a domain separator Domain input. The following table shows the HEX-encoded domain separator for each Composite ML-KEM AlgorithmID; to use it, the value should be HEX-decoded and used in binary form. The domain separator is simply the DER encoding of the composite algorithm OID. Composite ML-KEM fixedInfo Domain Separators

Composite ML-KEM Algorithm	Domain Separator (in Hex encoding)
id-MLKEM768-RSA2048	060B6086480186FA6B50050215
id-MLKEM768-RSA3072	060B6086480186FA6B50050216
id-MLKEM768-RSA4096	060B6086480186FA6B50050217
id-MLKEM768-X25519	060B6086480186FA6B5005021A
id-MLKEM768-ECDH-P384	060B6086480186FA6B50050218
id-MLKEM768-ECDH-brainpoolP256r1	060B6086480186FA6B50050219
id-MLKEM1024-ECDH-P384	060B6086480186FA6B5005021B
id-MLKEM1024-ECDH-brainpoolP384r1	060B6086480186FA6B5005021C
id-MLKEM1024-X448	060B6086480186FA6B5005021D

EDNOTE: these domain separators are based on the prototyping OIDs assigned on the Entrust arc. We will need to ask for IANA early allocation of these OIDs so that we can re-compute the domain separators over the final OIDs.

Rationale for choices

• Pair equivalent levels.
• NIST-P-384 is CNSA approved for all classification levels.
• 521 bit curve not widely used.

A single invocation of SHA3 is known to behave as a dual-PRF, and thus is sufficient for use as a KDF, see , however SHA2 is not us must be wrapped in the HKDF construction. The lower security levels (ie ML-KEM768) are provided with HKDF-SHA2 as the KDF in order to facilitate implementations that do not have easy access to SHA3 outside of the ML-KEM function. Higher security levels (ie ML-KEM1024) are paired with SHA3 for computational efficiency, and the Edwards Curve (X25519 and X448) combinations are paired with SHA3 for compatibility with other similar specifications. While it may seem odd to use 256-bit hash functions at all security levels, this aligns with ML-KEM which produces a 256-bit shared secret key at all security levels. SHA-256 and SHA3-256 both have >= 256 bits of (2nd) pre-image resistance, which is the required property for a KDF to provide 128 bits of security, as allowed in Table 3 of .

RSA-OAEP Parameters Use of RSA-OAEP within id-MLKEM768-RSA2048, id-MLKEM768-RSA3072, and id-MLKEM768-RSA4096 requires additional specification. First, a quick note on the choice of RSA-OAEP as the supported RSA encryption primitive. RSA-KEM is more straightforward to work with, but it has fairly limited adoption and therefore is of limited backwards compatibility value. Also, while RSA-PKCS#1v1.5 is still everywhere, but hard to make secure and no longer FIPS-approved as of the end of 2023 , so it is of limited forwards value. This leaves RSA-OAEP as the remaining choice. The RSA component keys MUST be generated at the 2048-bit and 3072-bit security levels respectively. As with the other Composite ML-KEM algorithms, when id-MLKEM768-RSA2048, id-MLKEM768-RSA3072, or id-MLKEM-RSA4096 is used in an AlgorithmIdentifier, the parameters MUST be absent. The RSA-OAEP SHALL be instantiated with the following hard-coded parameters which are the same for the 2048, 3072 and 4096 bit security levels. RSA-OAEP Parameters

RSAES-OAEP-params	Value
hashAlgorithm	id-sha256
maskGenAlgorithm	mgf1SHA256Identifier
pSourceAlgorithm	pSpecifiedEmpty
ss_len	256 bits

where:

• id-sha256 is defined in .
• mgf1SHA256Identifier is defined in , which refers to the MFG1 function defined in appendix B.2.1.
• pSpecifiedEmpty is defined in to indicate that the empty string is used for the label.

Note: The mask length, according to , is k - hLen - 1, where k is the size of the RSA modulus. Since the choice of hash function and the RSA key size is fixed for each composite algorithm, implementations could choose to pre-compute and hard-code the mask length.

Use in CMS [EDNOTE: The convention in LAMPS is to specify algorithms and their CMS conventions in separate documents. Here we have presented them in the same document, but this section has been written so that it can easily be moved to a standalone document.] Composite ML-KEM algorithms 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 chosen Composite ML-KEM Algorithm to securely transfer the content-encryption key from the originator to the recipient. All recommendations for using Composite ML-KEM in CMS are fully aligned with the use of ML-KEM in CMS .

Underlying Components A compliant implementation MUST support the following algorithm combinations for the KEMRecipientInfo kdf and wrap fields when the corresponding Composite ML-KEM algorithm is listed in the KEMRecipientInfo kem field. The KDFs listed below align with the KDF used internally within the KEM combiner. An implementation MAY also support other key-derivation functions and other key-encryption algorithms within CMS KEMRecipientInfo and SHOULD use algorithms of equivalent strength or greater. Mandatory-to-implement pairings for CMS KDF and WRAP

Composite ML-KEM Algorithm	KDF	Wrap
id-MLKEM768-RSA2048	id-alg-hkdf-with-sha256	id-aes128-wrap
id-MLKEM768-RSA3072	id-alg-hkdf-with-sha256	id-aes128-wrap
id-MLKEM768-RSA4096	id-alg-hkdf-with-sha256	id-aes128-wrap
id-MLKEM768-X25519	id-kmac256	id-aes128-wrap
id-MLKEM768-ECDH-P384	id-alg-hkdf-with-sha256	id-aes256-wrap
id-MLKEM768-ECDH-brainpoolP256r1	id-alg-hkdf-with-sha256	id-aes256-wrap
id-MLKEM1024-ECDH-P384	id-kmac256	id-aes256-wrap
id-MLKEM1024-ECDH-brainpoolP384r1	id-kmac256	id-aes256-wrap
id-MLKEM1024-X448	id-kmac256	id-aes256-wrap

Full specifications for the referenced algorithms can be found either further down in this section, or in . Note that here we differ slightly from the internal KDF used within the KEM combiner in because requires that the KDF listed in the KEMRecipientInfo kdf field must have an interface which accepts KDF(IKM, L, info), so here we need to use KMAC and cannot directly use SHA3. Since we require 256-bits of (2nd) pre-image resistance, we use KMAC256 for the Composite ML-KEM algorithms with internally use SHA3-256, as aligned with Table 3 of .

Use of the HKDF-based Key Derivation Function The HMAC-based Extract-and-Expand Key Derivation Function (HKDF) is defined in . The HKDF function is a composition of the HKDF-Extract and HKDF-Expand functions. HKDF(salt, IKM, info, L) takes the following parameters:

salt:

optional salt value (a non-secret random value). In this document this parameter is unused, that is it is the zero-length string "".

IKM:

input keying material. In this document this is the shared secret outputted from the Encapsulate() or Decapsulate() functions. This corresponds to the IKM KDF input from .

info:

optional context and application specific information. In this document this corresponds to the info KDF input from . This is the ASN.1 DER encoding of CMSORIforKEMOtherInfo.

L:

length of output keying material in octets. This corresponds to the L KDF input from , which is identified in the kekLength value from KEMRecipientInfo. Implementations MUST confirm that this value is consistent with the key size of the key-encryption algorithm.

HKDF may be used with different hash functions, including SHA-256 . The object identifier id-alg-hkdf-with-sha256 is defined in , and specifies the use of HKDF with SHA-256. The parameter field MUST be absent when this algorithm identifier is used to specify the KDF for ML-KEM in KemRecipientInfo.

Use of the KMAC-based Key Derivation Function KMAC256-KDF is a KMAC-based KDF specified for use in CMS in . The definition of KMAC is copied here for convenience. Here, KMAC# indicates the use of either KMAC128-KDF or KMAC256-KDF, although only KMAC256 is used in this specification. KMAC#(K, X, L, S) takes the following parameters:

• K: the input key-derivation key. In this document this is the shared secret outputted from the Encapsulate() or Decapsulate() functions. This corresponds to the IKM KDF input from Section 5 of .

• X: the context, corresponding to the info KDF input from Section 5 of . This is the ASN.1 DER encoding of CMSORIforKEMOtherInfo.

• L: the output length, in bits. This corresponds to the L KDF input from Section 5 of , which is identified in the kekLength value from KEMRecipientInfo. The L KDF input and kekLength values are specified in octets while this L parameter is specified in bits.

• S: the optional customization label. In this document this parameter is unused, that is it is the zero-length string "".

The object identifier for KMAC256-KDF is id-kmac256, as defined in . Since the customization label to KMAC# is not used, the parameter field MUST be absent when id-kmac256 is used as part of an algorithm identifier specifying the KDF to use for Composite ML-KEM in KemRecipientInfo.

RecipientInfo Conventions When Composite ML-KEM 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 MUST have the following values:

• 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 the Composite ML-KEM identifiers listed 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 Composite ML-KEM algorithm or one of its components.

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

• ukm is an optional random input to the key-derivation function. ML-KEM doesn't place any requirements on the ukm contents.

• wrap identifies a key-encryption algorithm used to encrypt the content-encryption key.

Certificate Conventions The conventions specified in this section augment RFC 5280 . The willingness to accept a Composite ML-KEM Algorithm MAY be signaled by the use of the SMIMECapabilities Attribute as specified in Section 2.5.2. of or the SMIMECapabilities certificate extension as specified in . The intended application for the public key MAY be indicated in the key usage certificate extension as specified in Section 4.2.1.3 of . If the keyUsage extension is present in a certificate that conveys a Composite ML-KEM public key, then the key usage extension MUST contain only the following value: The digitalSignature and dataEncipherment values MUST NOT be present. That is, a public key intended to be employed only with a Composite ML-KEM algorithm MUST NOT also be employed for data encryption or for digital signatures. This requirement does not carry any particular security consideration; only the convention that KEM keys be identified with the keyEncipherment key usage.

SMIMECapabilities Attribute Conventions Section 2.5.2 of 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 the RSA-OAEP Algorithm. The SMIMECapability SEQUENCE representing a Composite ML-KEM Algorithm MUST include the appropriate object identifier as per in the capabilityID field.

ASN.1 Module Composite-MLKEM-2024 { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-composite-mlkem-2024(TBDMOD) } DEFINITIONS IMPLICIT TAGS ::= BEGIN EXPORTS ALL; IMPORTS PUBLIC-KEY, AlgorithmIdentifier{}, SMIME-CAPS FROM AlgorithmInformation-2009 -- RFC 5912 [X509ASN1] { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-algorithmInformation-02(58) } KEM-ALGORITHM FROM KEMAlgorithmInformation-2023 { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-kemAlgorithmInformation-2023(109) } SubjectPublicKeyInfo FROM PKIX1Explicit-2009 { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51) } OneAsymmetricKey FROM AsymmetricKeyPackageModuleV1 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) id-mod-asymmetricKeyPkgV1(50) } RSAPublicKey, ECPoint FROM PKIXAlgs-2009 { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-algorithms2008-02(56) } ; -- -- Object Identifiers -- -- Defined in ITU-T X.690 der OBJECT IDENTIFIER ::= {joint-iso-itu-t asn1(1) ber-derived(2) distinguished-encoding(1)} -- Just for testing, to be assigned by IANA id-raw-key OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027) algorithm(80) composite(8) raw(999) 1 } -- -- Composite KEM basic structures -- CompositeKEMPublicKey ::= SEQUENCE SIZE (2) OF BIT STRING CompositeKEMPublicKeyOs ::= OCTET STRING (CONTAINING CompositeKEMPublicKey ENCODED BY der) CompositeKEMPublicKeyBs ::= BIT STRING (CONTAINING CompositeKEMPublicKey ENCODED BY der) CompositeKEMPrivateKey ::= SEQUENCE SIZE (2) OF OCTET STRING CompositeCiphertextValue ::= SEQUENCE SIZE (2) OF OCTET STRING RsaCompositeKemPublicKey ::= SEQUENCE { firstPublicKey BIT STRING (ENCODED BY id-raw-key), secondPublicKey BIT STRING (CONTAINING RSAPublicKey) } EcCompositeKemPublicKey ::= SEQUENCE { firstPublicKey BIT STRING (ENCODED BY id-raw-key), secondPublicKey BIT STRING (CONTAINING ECPoint) } EdCompositeKemPublicKey ::= SEQUENCE { firstPublicKey BIT STRING (ENCODED BY id-raw-key), secondPublicKey BIT STRING (ENCODED BY id-raw-key) } -- -- Information Object Classes -- pk-CompositeKEM {OBJECT IDENTIFIER:id, PublicKeyType} PUBLIC-KEY ::= { IDENTIFIER id KEY PublicKeyType PARAMS ARE absent CERT-KEY-USAGE { keyEncipherment } } kema-CompositeKEM { OBJECT IDENTIFIER:id, PUBLIC-KEY:publicKeyType } KEM-ALGORITHM ::= { IDENTIFIER id VALUE CompositeCiphertextValue PARAMS ARE absent PUBLIC-KEYS { publicKeyType } SMIME-CAPS { IDENTIFIED BY id } } -- -- Composite KEM Algorithms -- -- TODO: OID to be replaced by IANA id-MLKEM768-RSA2048 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027) algorithm(80) explicitcomposite(5) kem(2) 21 } pk-MLKEM768-RSA2048 PUBLIC-KEY ::= pk-CompositeKEM { id-MLKEM768-RSA2048, RsaCompositeKemPublicKey } kema-MLKEM768-RSA2048 KEM-ALGORITHM ::= kema-CompositeKEM{ id-MLKEM768-RSA2048, pk-MLKEM768-RSA2048 } -- TODO: OID to be replaced by IANA id-MLKEM768-RSA3072 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027) algorithm(80) explicitcomposite(5) kem(2) 22 } pk-MLKEM768-RSA3072 PUBLIC-KEY ::= pk-CompositeKEM { id-MLKEM768-RSA3072, RsaCompositeKemPublicKey } kema-MLKEM768-RSA3072 KEM-ALGORITHM ::= kema-CompositeKEM{ id-MLKEM768-RSA3072, pk-MLKEM768-RSA3072 } -- TODO: OID to be replaced by IANA id-MLKEM768-RSA4096 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027) algorithm(80) explicitcomposite(5) kem(2) 23 } pk-MLKEM768-RSA4096 PUBLIC-KEY ::= pk-CompositeKEM { id-MLKEM768-RSA4096, RsaCompositeKemPublicKey } kema-MLKEM768-RSA4096 KEM-ALGORITHM ::= kema-CompositeKEM{ id-MLKEM768-RSA4096, pk-MLKEM768-RSA4096 } -- TODO: OID to be replaced by IANA id-MLKEM768-ECDH-P384 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027) algorithm(80) explicitcomposite(5) kem(2) 25 } pk-MLKEM768-ECDH-P384 PUBLIC-KEY ::= pk-CompositeKEM { id-MLKEM768-ECDH-P384, EcCompositeKemPublicKey } kema-MLKEM768-ECDH-P384 KEM-ALGORITHM ::= kema-CompositeKEM{ id-MLKEM768-ECDH-P384, pk-MLKEM768-ECDH-P384 } -- TODO: OID to be replaced by IANA id-MLKEM768-ECDH-brainpoolP256r1 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027) algorithm(80) explicitcomposite(5) kem(2) 26 } pk-MLKEM768-ECDH-brainpoolP256r1 PUBLIC-KEY ::= pk-CompositeKEM { id-MLKEM768-ECDH-brainpoolP256r1, EcCompositeKemPublicKey } kema-MLKEM768-ECDH-brainpoolP256r1 KEM-ALGORITHM ::= kema-CompositeKEM{ id-MLKEM768-ECDH-brainpoolP256r1, pk-MLKEM768-ECDH-brainpoolP256r1 } -- TODO: OID to be replaced by IANA id-MLKEM768-X25519 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027) algorithm(80) explicitcomposite(5) kem(2) 24 } pk-MLKEM768-X25519 PUBLIC-KEY ::= pk-CompositeKEM { id-MLKEM768-X25519, EdCompositeKemPublicKey } kema-MLKEM768-X25519 KEM-ALGORITHM ::= kema-CompositeKEM{ id-MLKEM768-X25519, pk-MLKEM768-X25519 } -- TODO: OID to be replaced by IANA id-MLKEM1024-ECDH-P384 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027) algorithm(80) explicitcomposite(5) kem(2) 27 } pk-MLKEM1024-ECDH-P384 PUBLIC-KEY ::= pk-CompositeKEM { id-MLKEM1024-ECDH-P384, EcCompositeKemPublicKey } kema-MLKEM1024-ECDH-P384 KEM-ALGORITHM ::= kema-CompositeKEM{ id-MLKEM1024-ECDH-P384, pk-MLKEM1024-ECDH-P384 } -- TODO: OID to be replaced by IANA id-MLKEM1024-ECDH-brainpoolP384r1 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027) algorithm(80) explicitcomposite(5) kem(2) 28 } pk-MLKEM1024-ECDH-brainpoolP384r1 PUBLIC-KEY ::= pk-CompositeKEM{ id-MLKEM1024-ECDH-brainpoolP384r1, EcCompositeKemPublicKey } kema-MLKEM1024-ECDH-brainpoolP384r1 KEM-ALGORITHM ::= kema-CompositeKEM{ id-MLKEM1024-ECDH-brainpoolP384r1, pk-MLKEM1024-ECDH-brainpoolP384r1 } -- TODO: OID to be replaced by IANA id-MLKEM1024-X448 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027) algorithm(80) explicitcomposite(5) kem(2) 29 } pk-MLKEM1024-X448 PUBLIC-KEY ::= pk-CompositeKEM { id-MLKEM1024-X448, EdCompositeKemPublicKey } kema-MLKEM1024-X448 KEM-ALGORITHM ::= kema-CompositeKEM{ id-MLKEM1024-X448, pk-MLKEM1024-X448 } -- -- Expand the S/MIME capabilities set used by CMS [RFC5911] -- -- TODO: this doesn't compile, error: -- "The referenced object in the 'ValueFromObject' -- syntax with the field '&smimeCaps' is invalid or does not exist." -- We need help from an SMIME expert SMimeCaps SMIME-CAPS ::= { kema-MLKEM768-RSA2048.&smimeCaps | kema-MLKEM768-RSA3072.&smimeCaps | kema-MLKEM768-RSA4096.&smimeCaps | kema-MLKEM768-ECDH-P384.&smimeCaps | kema-MLKEM768-ECDH-brainpoolP256r1.&smimeCaps | kema-MLKEM768-X25519.&smimeCaps | kema-MLKEM1024-ECDH-P384.&smimeCaps | kema-MLKEM1024-ECDH-brainpoolP384r1.&smimeCaps | kema-MLKEM1024-X448.&smimeCaps, ... } END ]]>

IANA Considerations

Object Identifier Allocations EDNOTE to IANA: OIDs will need to be replaced in both the ASN.1 module and in .

Module Registration - SMI Security for PKIX Module Identifier

• Decimal: IANA Assigned - Replace TBDMOD
• Description: Composite-KEM-2023 - id-mod-composite-kems
• References: This Document

Object Identifier Registrations - SMI Security for PKIX Algorithms

• id-raw-key
• Decimal: IANA Assigned
• Description: Designates a public key BIT STRING with no ASN.1 structure.
• References: This Document
• id-MLKEM768-RSA2048
  • Decimal: IANA Assigned
  • Description: id-MLKEM768-RSA2048
  • References: This Document
• id-MLKEM768-RSA3072
  • Decimal: IANA Assigned
  • Description: id-MLKEM768-RSA3072
  • References: This Document
• id-MLKEM768-RSA4096
  • Decimal: IANA Assigned
  • Description: id-MLKEM768-RSA4096
  • References: This Document
• id-MLKEM768-ECDH-P384
  • Decimal: IANA Assigned
  • Description: id-MLKEM768-ECDH-P384
  • References: This Document
• id-MLKEM768-ECDH-brainpoolP256r1
  • Decimal: IANA Assigned
  • Description: id-MLKEM768-ECDH-brainpoolP256r1
  • References: This Document
• id-MLKEM768-X25519
  • Decimal: IANA Assigned
  • Description: id-MLKEM768-X25519
  • References: This Document
• id-MLKEM1024-ECDH-P384
  • Decimal: IANA Assigned
  • Description: id-MLKEM1024-ECDH-P384
  • References: This Document
• id-MLKEM1024-ECDH-brainpoolP384r1
  • Decimal: IANA Assigned
  • Description: id-MLKEM1024-ECDH-brainpoolP384r1
  • References: This Document
• id-MLKEM1024-X448
  • Decimal: IANA Assigned
  • Description: id-MLKEM1024-X448
  • References: This Document

Security Considerations

KEM Combiner Security Analysis EDNOTE: the exact text to put here depends on the outcome of the CFRG KEM Combiners and X-Wing discussion. If CFRG doesn't move fast enough for us, then we may need to leverage this security consideration directly on top of the X-Wing paper . The primary security property of the KEM combiner is that it preserves IND-CCA2 of the overall Composite ML-KEM so long as at least one component is IND-CCA2 X-Wing. Additionally, we also need to consider the case where one of the component algorithms is completely broken; that the private key is known to an attacker, or worse that the public key, private key, and ciphertext are manipulated by the attacker. In this case, we rely on the construction of the KEM combiner to ensure that the value of the other shared secret cannot be leaked or the combined shared secret predicted via manipulation of the broken algorithm. The following sections continue this discussion.

Second pre-image resistance of component KEMs The notion of a second pre-image resistant KEM is defined in being the property that it is computationally difficult to find two different ciphertexts c != c' that will decapsulate to the same shared secret under the same public key. For the purposes of a hybrid KEM combiner, this property means that given two composite ciphertexts (c1, c2) and (c1', c2'), we must obtain a unique overall shared secret so long as either c1 != c1' or c2 != c2' -- i.e. the overall Composite ML-KEM is second pre-image resistant, and therefore secure so, long as one of the component KEMs is. In it is proven that ML-KEM is a second pre-image resistant KEM and therefore the ML-KEM ciphertext can safely be omitted from the KEM combiner. Note that this makes a fundamental assumption on ML-KEM remaining ciphertext second pre-image resistant, and therefore this formulation of KEM combiner does not fully protect against implementation errors in the ML-KEM component -- particularly around the ciphertext check step of the Fujisaki-Okamoto transform -- which could trivially lead to second ciphertext pre-image attacks that break the IND-CCA2 security of the ML-KEM component and of the overall Composite ML-KEM. This could be more fully mitigated by binding the ML-KEM ciphertext in the combiner, but a design decision was made to settle for protection against algorithmic attacks and not implementation attacks against ML-KEM in order to increase performance. However, since neither RSA-OAEP nor ECDH guarantee second pre-image resistance at all, even in a correct implementation, these ciphertexts are bound to the key derivation in order to guarantee that c != c' will yield a unique ciphertext, and thus restoring second pre-image resistance to the overall Composite ML-KEM.

SHA3 vs HKDF-SHA2 In order to achieve the desired security property that the Composite ML-KEM is IND-CCA2 whenever at least one of the component KEMs is, the KDF used in the KEM combiner needs to possess collision and second pre-image resistance with respect to each of its inputs independently; a property sometimes called "dual-PRF" . Collision and second-pre-image resistance protects against compromise of one component algorithm from resulting in the ability to construct multiple different ciphertexts which result in the same shared secret. Pre-image resistance protects against compromise of one component algorithm being used to attack and learn the value of the other shared secret. SHA3 is known to have all of the necessary dual-PRF properties , but SHA2 does not and therefore all SHA2-based constructions MUST use SHA2 within an HMAC construction such as HKDF-SHA2 .

Generifying this construction It should be clear that the security analysis of the presented KEM combiner construction relies heavily on the specific choices of component algorithms and combiner KDF, and this combiner construction SHOULD NOT by applied to any other combination of ciphers without performing the appropriate security analysis.

Key Reuse When using single-algorithm cryptography, the best practice is to always generate fresh keying material for each purpose, for example when renewing a certificate, or obtaining both a TLS and S/MIME certificate for the same device, however in practice key reuse in such scenarios is not always catastrophic to security and therefore often tolerated. With composite keys we have a much stricter security requirement. However this reasoning does not hold in the PQ / Traditional hybrid setting. Within the broader context of PQ / Traditional hybrids, we need to consider new attack surfaces that arise due to the hybrid constructions and did not exist in single-algorithm contexts. One of these is key reuse where the component keys within a hybrid are also used by themselves within a single-algorithm context. For example, it might be tempting for an operator to take already-deployed RSA keys and add an ML-KEM key to them to form a hybrid. Within a hybrid signature context this leads to a class of attacks referred to as "stripping attacks" where one component signature can be extracted and presented as a single-algorithm signature. Hybrid KEMs using a concatenation-style KEM combiner, as is done in this document, do not have the analogous attack surface because even if an attacker is able to extract and decrypt one of the component ciphertexts, this will yield a different shared secret than the overall shared secret derived from the composite, so any subsequent symmetric cryptographic operations will fail. However there is still a risk of key reuse which relates to certificate revocation, as well as general key reuse security issues. Upon receiving a new certificate enrollment request, many certification authorities will check if the requested public key has been previously revoked due to key compromise. Often a CA will perform this check by using the public key hash. Therefore, even if both components of a composite have been previously revoked, the CA may only check the hash of the combined composite key and not find the revocations. Therefore, it is RECOMMENDED to avoid key reuse and always generate fresh component keys for a new composite. It is also RECOMMENDED that CAs performing revocation checks on a composite key should also check both component keys independently.

Policy for Deprecated and Acceptable Algorithms Traditionally, a public key or certificate contains a single cryptographic algorithm. If and when an algorithm becomes deprecated (for example, RSA-512, or SHA1), the path to deprecating it and removing it from operational environments is, at least is principle, straightforward. In the composite model this is less obvious since implementers may decide that certain cryptographic algorithms have complementary security properties and are acceptable in combination even though one or both algorithms are deprecated for individual use. As such, a single composite public key or certificate may contain a mixture of deprecated and non-deprecated algorithms. Since composite algorithms are registered independently of their component algorithms, their deprecation can be handled independently from that of their component algorithms. For example a cryptographic policy might continue to allow id-MLKEM512-ECDH-P256 even after ECDH-P256 is deprecated. The Composite ML-KEM design specified in this document, and especially that of the KEM combiner specified in means that the overall Composite ML-KEM algorithm should be considered to have the security strength of the strongest of its component algorithms; ie as long as one component algorithm remains strong, then the overall composite algorithm remains strong.

References Normative References 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. This document supplements RFC 3279. It describes the conventions for using the RSA Probabilistic Signature Scheme (RSASSA-PSS) signature algorithm, the RSA Encryption Scheme - Optimal Asymmetric Encryption Padding (RSAES-OAEP) key transport algorithm and additional one-way hash functions with the Public-Key Cryptography Standards (PKCS) #1 version 1.5 signature algorithm in the Internet X.509 Public Key Infrastructure (PKI). Encoding formats, algorithm identifiers, and parameter formats are specified. [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] This document specifies the syntax and semantics for the Subject Public Key Information field in certificates that support Elliptic Curve Cryptography. This document updates Sections 2.3.5 and 5, and the ASN.1 module of "Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3279. [STANDARDS-TRACK] 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 defines the syntax for private-key information and a content type for it. Private-key information includes a private key for a specified public-key algorithm and a set of attributes. The Cryptographic Message Syntax (CMS), as defined in RFC 5652, can be used to digitally sign, digest, authenticate, or encrypt the asymmetric key format content type. This document obsoletes RFC 5208. [STANDARDS-TRACK] Federal Information Processing Standard, FIPS This document provides recommendations for the implementation of public-key cryptography based on the RSA algorithm, covering cryptographic primitives, encryption schemes, signature schemes with appendix, and ASN.1 syntax for representing keys and for identifying the schemes. This document represents a republication of PKCS #1 v2.2 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series. By publishing this RFC, change control is transferred to the IETF. This document also obsoletes RFC 3447. 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 specifies algorithm identifiers and ASN.1 encoding formats for elliptic curve constructs using the curve25519 and curve448 curves. The signature algorithms covered are Ed25519 and Ed448. The key agreement algorithms covered are X25519 and X448. The encoding for public key, private key, and Edwards-curve Digital Signature Algorithm (EdDSA) structures is provided. When the Curdle Security Working Group was chartered, a range of object identifiers was donated by DigiCert, Inc. for the purpose of registering the Edwards Elliptic Curve key agreement and signature algorithms. This donated set of OIDs allowed for shorter values than would be possible using the existing S/MIME or PKIX arcs. This document describes the donated range and the identifiers that were assigned from that range, transfers control of that range to IANA, and establishes IANA allocation policies for any future assignments within that range. 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. 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. Vigil Security, LLC This document describes the conventions for using the one-way hash functions in the SHA3 family with the Cryptographic Message Syntax (CMS). The SHA3 family can be used as a message digest algorithm, as part of a signature algorithm, as part of a message authentication code, or part of a key derivation function. sn3rd AWS AWS Cloudflare Kyber is a key-encapsulation mechanism (KEM). This document specifies algorithm identifiers and ASN.1 encoding format for Kyber in public key certificates. The encoding for public and private keys are also provided. [EDNOTE: This document is not expected to be finalized before the NIST PQC Project has standardized PQ algorithms. This specification will use object identifiers for the new algorithms that are assigned by NIST, and will use placeholders until these are released.] ITU-T Federal Office for Information Security (BSI) National Institute of Standards and Technology (NIST) National Institute of Standards and Technology (NIST) National Institute of Standards and Technology (NIST) National Institute of Standards and Technology (NIST) National Institute of Standards and Technology (NIST) National Institute of Standards and Technology (NIST) National Institute of Standards and Technology (NIST) 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. Informative References This memo represents a republication of PKCS #10 v1.7 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document, except for the security considerations section, is taken directly from the PKCS #9 v2.0 or the PKCS #10 v1.7 document. This memo provides information for the Internet community. This document describes the Internet X.509 Public Key Infrastructure (PKI) Certificate Management Protocol (CMP). Protocol messages are defined for X.509v3 certificate creation and management. CMP provides on-line interactions between PKI components, including an exchange between a Certification Authority (CA) and a client system. [STANDARDS-TRACK] This document describes the Certificate Request Message Format (CRMF) syntax and semantics. This syntax is used to convey a request for a certificate to a Certification Authority (CA), possibly via a Registration Authority (RA), for the purposes of X.509 certificate production. The request will typically include a public key and the associated registration information. This document does not define a certificate request protocol. [STANDARDS-TRACK] This document defines a certificate extension for inclusion of Secure/Multipurpose Internet Mail Extensions (S/MIME) Capabilities in X.509 public key certificates, as defined by RFC 3280. This certificate extension provides an optional method to indicate the cryptographic capabilities of an entity as a complement to the S/MIME Capabilities signed attribute in S/MIME messages according to RFC 3851. [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 memo proposes several elliptic curve domain parameters over finite prime fields for use in cryptographic applications. The domain parameters are consistent with the relevant international standards, and can be used in X.509 certificates and certificate revocation lists (CRLs), for Internet Key Exchange (IKE), Transport Layer Security (TLS), XML signatures, and all applications or protocols based on the cryptographic message syntax (CMS). This document is not an Internet Standards Track specification; it is published for informational purposes. This document describes a structure for representing trust anchor information. A trust anchor is an authoritative entity represented by a public key and associated data. The public key is used to verify digital signatures, and the associated data is used to constrain the types of information or actions for which the trust anchor is authoritative. The structures defined in this document are intended to satisfy the format-related requirements defined in Trust Anchor Management Requirements. [STANDARDS-TRACK] The RSA-KEM Key Transport Algorithm is a one-pass (store-and-forward) mechanism for transporting keying data to a recipient using the recipient's RSA public key. ("KEM" stands for "key encapsulation mechanism".) This document specifies the conventions for using the RSA-KEM Key Transport Algorithm with the Cryptographic Message Syntax (CMS). The ASN.1 syntax is aligned with an expected forthcoming change to American National Standard (ANS) X9.44. This note describes the fundamental algorithms of Elliptic Curve Cryptography (ECC) as they were defined in some seminal references from 1994 and earlier. These descriptions may be useful for implementing the fundamental algorithms without using any of the specialized methods that were developed in following years. Only elliptic curves defined over fields of characteristic greater than three are in scope; these curves are those used in Suite B. This document is not an Internet Standards Track specification; it is published for informational purposes. PKCS #12 v1.1 describes a transfer syntax for personal identity information, including private keys, certificates, miscellaneous secrets, and extensions. Machines, applications, browsers, Internet kiosks, and so on, that support this standard will allow a user to import, export, and exercise a single set of personal identity information. This standard supports direct transfer of personal information under several privacy and integrity modes. This document represents a republication of PKCS #12 v1.1 from RSA Laboratories' Public Key Cryptography Standard (PKCS) series. By publishing this RFC, change control is transferred to the IETF. This document describes version 2 of the Internet Key Exchange (IKE) protocol. IKE is a component of IPsec used for performing mutual authentication and establishing and maintaining Security Associations (SAs). This document obsoletes RFC 5996, and includes all of the errata for it. It advances IKEv2 to be an Internet Standard. This memo specifies two elliptic curves over prime fields that offer a high level of practical security in cryptographic applications, including Transport Layer Security (TLS). These curves are intended to operate at the ~128-bit and ~224-bit security level, respectively, and are generated deterministically based on a list of required properties. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. 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. This document describes a scheme for hybrid public key encryption (HPKE). This scheme provides a variant of public key encryption of arbitrary-sized plaintexts for a recipient public key. It also includes three authenticated variants, including one that authenticates possession of a pre-shared key and two optional ones that authenticate possession of a key encapsulation mechanism (KEM) private key. HPKE works for any combination of an asymmetric KEM, key derivation function (KDF), and authenticated encryption with additional data (AEAD) encryption function. Some authenticated variants may not be supported by all KEMs. We provide instantiations of the scheme using widely used and efficient primitives, such as Elliptic Curve Diffie-Hellman (ECDH) key agreement, HMAC-based key derivation function (HKDF), and SHA2. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. University of Waterloo Cisco Systems University of Haifa and Amazon Web Services Hybrid key exchange refers to using multiple key exchange algorithms simultaneously and combining the result with the goal of providing security even if all but one of the component algorithms is broken. It is motivated by transition to post-quantum cryptography. This document provides a construction for hybrid key exchange in the Transport Layer Security (TLS) protocol version 1.3. Discussion of this work is encouraged to happen on the TLS IETF mailing list tls@ietf.org or on the GitHub repository which contains the draft: https://github.com/dstebila/draft-ietf-tls-hybrid-design. UK National Cyber Security Centre UK National Cyber Security Centre Naval Postgraduate School One aspect of the transition to post-quantum algorithms in cryptographic protocols is the development of hybrid schemes that incorporate both post-quantum and traditional asymmetric algorithms. This document defines terminology for such schemes. It is intended to be used as a reference and, hopefully, to ensure consistency and clarity across different protocols, standards, and organisations. SandboxAQ Naval Postgraduate School SandboxAQ UK National Cyber Security Centre This document describes classification of design goals and security considerations for hybrid digital signature schemes, including proof composability, non-separability of the component signatures given a hybrid signature, backwards/forwards compatiblity, hybrid generality, and simultaneous verification. Discussion of this work is encouraged to happen on the IETF PQUIP mailing list pqc@ietf.org or on the GitHub repository which contains the draft: https://github.com/dconnolly/draft-ietf-pquip-hybrid- signature-spectrums CryptoNext Security Entrust Limited CryptoNext Security The Module-Lattice-based Key-Encapsulation Mechanism (ML-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 ML-KEM public key. Three parameters sets for the ML-KEM algorithm are specified by NIST in [FIPS203]. 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 KEMRecipientInfo as specified in [RFC9629]. Federal Office for Information Security (BSI) French Cybersecurity Agency (ANSSI) Federal Office for Information Security (BSI) Netherlands National Communications Security Agency (NLNCSA) Swedish National Communications Security Authority, Swedish Armed Forces n.d. n.d. n.d. n.d. National Institute of Standards and Technology (NIST)

Samples TODO

Component Algorithm Reference This section provides references to the full specification of the algorithms used in the composite constructions. Component Encryption Algorithms used in Composite Constructions

Component Signature Algorithm ID	OID	Specification
id-ML-KEM-768	2.16.840.1.101.3.4.4.2
id-ML-KEM-1024	2.16.840.1.101.3.4.4.3
id-X25519	1.3.101.110
id-X448	1.3.101.111
id-ecDH	1.3.132.1.12
id-RSAES-OAEP	1.2.840.113549.1.1.7

Elliptic Curves used in Composite Constructions

Elliptic CurveID	OID	Specification
secp256r1	1.2.840.10045.3.1.7
secp384r1	1.3.132.0.34
brainpoolP256r1	1.3.36.3.3.2.8.1.1.7
brainpoolP384r1	1.3.36.3.3.2.8.1.1.11

Hash algorithms used in Composite Constructions

HashID	OID	Specification
id-sha256	2.16.840.1.101.3.4.2.1
id-sha512	2.16.840.1..101.3.4.2.3
id-alg-hkdf-with-sha256	1.2.840.113549.1.9.16.3.28
id-sha3-256	2.16.840.1.101.3.4.2.8
id-KMAC128	2.16.840.1.101.3.4.2.21

Fixed Component Algorithm Identifiers The following sections list explicitly the DER encoded AlgorithmIdentifier that MUST be used when reconstructing SubjectPublicKeyInfo objects for each component public key, which may be required for example if cryptographic library requires the public key in this form in order to process each component algorithm. The public key BIT STRING should be taken directly from the respective component of the CompositeKEMPublicKey. ML-KEM-768 ML-KEM-1024 ASN.1: RSA-OAEP - all sizes ECDH NIST-P-384 ECDH Brainpool-256 ECDH Brainpool-384 X25519 X448

Implementation Considerations

FIPS Certification TODO -- update this once NIST SP 800-227 is published. EDNOTE At time of writing, it is unclear that the KEM combiner presented in this document would pass a FIPS certification. The SHA3 instantiations should pass under SP 800-56Cr2 Option 1, but the HKDF-SHA2 combinations are technically not allowed under SP 800-56Cr2 even though Option 2 allows HMAC-based constructions, but unfortunately only HKDF-Extract is FIPS-allowed, not HKDF-Expand. The authors have been in contact with NIST to ensure compatibility either by NIST making SP 800-227 more flexible, or by changing this specification to only use HKDF-Extract. One of the primary design goals of this specification is for the overall composite algorithm to be able to be considered FIPS-approved even when one of the component algorithms is not. Implementers seeking FIPS certification of a Composite ML-KEM algorithm where only one of the component algorithms has been FIPS-validated or FIPS-approved should credit the FIPS-validated component algorithm with full security strength, the non-FIPS-validated component algorithm with zero security, and the overall composite should be considered full strength and thus FIPS-approved. The authors wish to note that this gives composite algorithms great future utility both for future cryptographic migrations as well as bridging across jurisdictions; for example defining composite algorithms which combine FIPS cryptography with cryptography from a different national standards body.

FIPS certification of Combiner Function TODO: update this to NIST SP 800-227, once it is published. One of the primary NIST documents which is relevant for certification of a composite algorithm is NIST SP.800-56Cr2 by using the allowed "hybrid" shared secret of the form Z' = Z || T. Compliance is achieved in the following way: SP.800-56Cr2 section 4 "One-Step Key Derivation" requires a counter which begins at the 4-byte value 0x00000001. However, the counter is allowed to be omitted when the hash function is executed only once, as specified on page 159 of the FIPS 140-3 Implementation Guidance . The HKDF-SHA2 options can be certified under SP.800-56Cr2 One-Step Key Derivation Option 1: H(x) = hash(x). The SHA3 options can be certified under SP.800-56Cr2 One-Step Key Derivation Option 2: H(x) = HMAC-hash(salt, x) with the salt omitted.

Backwards Compatibility As noted in the introduction, the post-quantum cryptographic migration will face challenges in both ensuring cryptographic strength against adversaries of unknown capabilities, as well as providing ease of migration. The composite mechanisms defined in this document primarily address cryptographic strength, however this section contains notes on how backwards compatibility may be obtained. The term "ease of migration" is used here to mean that existing systems can be gracefully transitioned to the new technology without requiring large service disruptions or expensive upgrades. The term "backwards compatibility" is used here to mean something more specific; that existing systems as they are deployed today can inter-operate with the upgraded systems of the future. These migration and interoperability concerns need to be thought about in the context of various types of protocols that make use of X.509 and PKIX with relation to key establishment and content encryption, from online negotiated protocols such as TLS 1.3 and IKEv2 , to non-negotiated asynchronous protocols such as S/MIME signed email , as well as myriad other standardized and proprietary protocols and applications that leverage CMS encrypted structures.

Decapsulation Requires the Public Key ML-KEM always requires the public key in order to perform various steps of the Fujisaki-Okamoto decapsulation , and for this reason the private key encoding specified in FIPS 203 includes the public key. Therefore it is not required to carry it in the OneAsymmetricKey.publicKey field, which remains optional, but is strictly speaking redundant since an ML-KEM public key can be parsed from an ML-KEM private key, and thus populating the OneAsymmetricKey.publicKey field would mean that two copies of the public key data are transmitted. With regard to the traditional algorithms, RSA or Elliptic Curve, in order to achieve the public-key binding property the KEM combiner used to form the Composite ML-KEM, the combiner requires the traditional public key as input to the KDF that derives the output shared secret. Therefore it is required to carry the public key within the respective OneAsymmetricKey.publicKey as per the private key encoding given in . Implementers who choose to use a different private key encoding than the one specified in this document MUST consider how to provide the component public keys to the decapsulate routine. While some implementations might contain routines to computationally derive the public key from the private key, it is not guaranteed that all implementations will support this; for this reason the interoperable composite private key format given in this document in requires the public key of the traditional component to be included.

Intellectual Property Considerations The following IPR Disclosure relates to this draft: https://datatracker.ietf.org/ipr/3588/ EDNOTE TODO: Check with Max Pala whether this IPR actually applies to this draft.

Contributors and Acknowledgments This document incorporates contributions and comments from a large group of experts. The Editors would especially like to acknowledge the expertise and tireless dedication of the following people, who attended many long meetings and generated millions of bytes of electronic mail and VOIP traffic over the past year in pursuit of this document: Serge Mister (Entrust), Ali Noman (Entrust), Peter C. (UK NCSC), Sophie Schmieg (Google), Deirdre Connolly (SandboxAQ), Falko Strenzke (MTG AG), Dan van Geest (Crypto Next), Piotr Popis (Enigma), and Douglas Stebila (University of Waterloo). We are grateful to all, including any contributors who may have been inadvertently omitted from this list. This document borrows text from similar documents, including those referenced below. Thanks go to the authors of those documents. "Copying always makes things easier and less error prone" - .