]> Entrust Limited

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Security LAMPS Internet-Draft The migration to post-quantum cryptography is unique in the history of modern digital cryptography in that neither the old outgoing nor the new incoming algorithms are fully trusted to protect data for the required data lifetimes. The outgoing algorithms, such as RSA and elliptic curve, may fall to quantum cryptanalysis, while the incoming post-quantum algorithms face uncertainty about both the underlying mathematics as well as hardware and software implementations that have not had sufficient maturing time to rule out classical cryptanalytic attacks and implementation bugs. Cautious implementers may wish to layer cryptographic algorithms such that an attacker would need to break all of them in order to compromise the data being protected using either a Post-Quantum / Traditional Hybrid, Post-Quantum / Post-Quantum Hybrid, or combinations thereof. This document, and its companions, defines a specific instantiation of hybrid paradigm called "composite" where multiple cryptographic algorithms are combined to form a single key, signature, or key encapsulation mechanism (KEM) such that they can be treated as a single atomic object at the protocol level. This document defines the structures CompositeSignatureValue, and CompositeSignatureParams, which are sequences of the respective structure for each component algorithm. The explicit variant is defined which allows for a set of signature algorithm identifier OIDs to be registered together as an explicit composite signature algorithm and assigned an OID. The generic composite variant is also defined which allows arbitrary combinations of signature algorithms to be used in the CompositeSignatureValue and CompositeSignatureParams structures without needing the combination to be pre-registered or pre-agreed. This document is intended to be coupled with corresponding documents that define the structure and semantics of composite public and private keys and encryption , however may also be used with non-composite keys, such as when a protocol combines multiple certificates into a single cryptographic operation.

Changes in version -08 explicit composite combinations defined and ASN.1 module updated Added new OIDs and considerations for the hash-then-sign paradigm for generic composite signatures Removed the OR modes of operation and replaced with K-of-N I-D reference Added considerations for external hashing

Introduction During the transition to post-quantum cryptography, there will be uncertainty as to the strength of cryptographic algorithms; we will no longer fully trust traditional cryptography such as RSA, Diffie-Hellman, DSA and their elliptic curve variants, but we will also not fully trust their post-quantum replacements until they have had sufficient scrutiny and time to discover and fix implementation bugs. Unlike previous cryptographic algorithm migrations, the choice of when to migrate and which algorithms to migrate to, is not so clear. Even after the migration period, it may be advantageous for an entity's cryptographic identity to be composed of multiple public-key algorithms. The deployment of composite signatures using post-quantum algorithms will face two challenges 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. Backwards compatibility: During the transition period, post-quantum algorithms will not be supported by all clients. This document provides a mechanism to address algorithm strength uncertainty concerns by building on by providing formats for encoding multiple signature values into existing public signature fields, as well as the process for validating a composite signature. Backwards compatibility is addressed via using composite in conjunction with a non-composite hybrid mode such as that described in . This document is intended for general applicability anywhere that digital signatures are used within PKIX and CMS structures.

Algorithm Selection Criteria The composite algorithm combinations defined in this document were chosen according to the following guidelines: A single RSA combination is provided (but RSA modulus size not mandated), matched with NIST PQC Level 3 algorithms. Elliptic curve algorithms are provided with combinations on each of the NIST , Brainpool , and Edwards curves. NIST PQC Levels 1 - 3 algorithms are matched with 256-bit curves, while NIST levels 4 - 5 are matched with 384-bit elliptic curves. This provides a balance between matching classical security levels of post-quantum and traditional algorithms, and also selecting elliptic curves which already have wide adoption. NIST level 1 candidates (Falcon512 and Kyber512) are provided, matched with 256-bit elliptic curves, intended for constrained use cases. A single SPHINCS+ combination is provided for use cases that wish to put hash-based signatures into hybrid combination. A generic composite algorithm is provided for implementers who wish to use combinations not listed here, without the overhead of defining new OIDs. Caution should be exercised to avoid issues with compatibility and complex cryptographic policy mechanisms. The authors wish to note that although all the composite structures defined in this and the companion composite keys and composite KEM specifications are defined in such a way as to easily allow 3 or more component algorithms, it was decided to only specify explicit pairs. The generic composite specified in this document allows for an arbitrary number of components. This also does not preclude future specification of explicit combinations with three or more components.

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. The following terms are used in this document: ALGORITHM: A standardized cryptographic primitive, as well as any ASN.1 structures needed for encoding data and metadata needed to use the algorithm. This document is primarily concerned with algorithms for producing digital signatures. BER: Basic Encoding Rules (BER) as defined in . CLIENT: Any software that is making use of a cryptographic key. This includes a signer, verifier, encrypter, decrypter. COMPONENT ALGORITHM: A single basic algorithm which is contained within a composite algorithm. COMPOSITE ALGORITHM: An algorithm which is a sequence of two or more component algorithms, as defined in . DER: Distinguished Encoding Rules as defined in . LEGACY: For the purposes of this document, a legacy algorithm is any cryptographic algorithm currently is use which is not believe to be resistant to quantum cryptanalysis. PKI: Public Key Infrastructure, as defined in . POST-QUANTUM ALGORITHM: Any cryptographic algorithm which is believed to be resistant to classical and quantum cryptanalysis, such as the algorithms being considered for standardization by NIST. PUBLIC / PRIVATE KEY: The public and private portion of an asymmetric cryptographic key, making no assumptions about which algorithm. SIGNATURE: A digital cryptographic signature, making no assumptions about which algorithm. STRIPPING ATTACK: An attack in which the attacker is able to downgrade the cryptographic object to an attacker-chosen subset of original set of component algorithms in such a way that it is not detectable by the receiver. For example, substituting a composite public key or signature for a version with fewer components.

Composite Signature Structures In order for signatures to be composed of multiple algorithms, we define encodings consisting of a sequence of signature primitives (aka "component algorithms") such that these structures can be used as a drop-in replacement for existing signature fields such as those found in PKCS#10 , CMP , X.509 , CMS .

Composite Keys A composite signature MAY be associated with a composite public key as defined in , but MAY also be associated with multiple public keys from different sources, for example multiple X.509 certificates, or multiple cryptographic modules. In the latter case, composite signatures MAY be used as the mechanism for carrying multiple signatures in a non-composite hybrid authentication mechanism such as those described in .

Key Usage Bits For protocols such as X.509 that specify key usage along with the public key, then the composite public key associated with a composite signature MUST have a signing-type key usage. If the keyUsage extension is present in a Certification Authority (CA) certificate that indicates a composite key, then any combination of the following values MAY be present:

If the keyUsage extension is present in an End Entity (EE) certificate that indicates a composite key, then any combination of the following values MAY be present:

sa-CompositeSignature The ASN.1 algorithm object for a composite signature is:

The following is an explanation how SIGNATURE-ALGORITHM elements are used to create Composite Signatures: SIGNATURE-ALGORITHM element Definition IDENTIFIER The Object ID used to identify the composite Signature Algorithm VALUE The Sequence of BIT STRINGS for each component signature value PARAMS Signature parameters either declared ABSENT, or defined with a type and encoding PUBLIC-KEYS The composite key required to produce the composite signature SMIME_CAPS Not needed for composite

CompositeSignatureValue The output of the composite signature algorithm is the DER encoding of the following structure:

Where each BIT STRING within the SEQUENCE is a signature value produced by one of the component keys. It MUST contain one signature value produced by each component algorithm, and in the same order as in the associated CompositeSignatureParams object. A CompositeSignatureValue MUST contain the same number of component signatures as the corresponding public and private keys, and the order of component signature values MUST correspond to the component public keys. The choice of SEQUENCE OF BIT STRING, rather than for example a single BIT STRING containing the concatenated signature values, is to gracefully handle variable-length signature values by taking advantage of ASN.1's built-in length fields.

CompositeSignatureParameters Composite signature parameters are defined as follows and MAY be used when a composite signature is used with an AlgorithmIdentifier:

The signature's CompositeSignatureParams sequence MUST contain the same component algorithms listed in the same order as in the associated CompositePublicKey. For explicit algorithms, it is not strictly necessary to carry a CompositeSignatureParams with the list of component algorithms in the signature algorithm parameters because clients can infer the expected component algorithms from the algorithm identifier. The PARAMS is left optional because some types of component algorithms will require parameters to be carried, such as RSASSA-PSS-params as defined in . defines PARAMS ANY DEFINED BY ALGORITHM so that explicit algorithms may define params as ABSENT, or use CompositeSignatureParams as defined in ASN.1 module. For generic compsite algorithms, CompositeSignatureParams MUST be included.

Encoding Rules Many protocol specifications will require that composite signature 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.

Algorithm Identifiers This section defines the algorithm identifiers for explicit combinations. For simplicity and prototyping purposes, the signature algorithm object identifiers specified in this document are the same as the composite key object Identifiers specified in {draft-ounsworth-pq-composite-keys}. A proper implementation should not presume that the object ID of a composite key will be the same as its composite signature algorithm. This section is not intended to be exhaustive and other authors may define others composite signature algorithms so long as they are compatible with the structures and processes defined in this and companion public and private key documents. Some use-cases desire the flexibility for clients to use any combination of supported algorithms, while others desire the rigidity of explicitly-specified combinations of algorithms. The following table summarizes the details for each explicit composite signature algorithms: The OID referenced are TBD for prototyping only, and the following prefix is used for each: replace <CompSig> with the String "2.16.840.1.114027.80.5.1" Therefore <CompSig>.1 is equal to 2.16.840.1.114027.80.5.1.1 replace <SPHINCS> with the String "SPHINCSplusSHA256128sSimple" Signature public key types: Composite Signature AlgorithmID OID First Algorithm Second Algorithm id-Dilithium3-RSA-PSS <CompSig>.14 Dilithium3 SHA256WithRSAPSS id-Dilithium3-RSA-PKCS15-SHA256 <CompSig>.1 Dilithium3 SHA256WithRSAEncryption id-Dilithium3-ECDSA-P256-SHA256 <CompSig>.2 Dilithium3 SHA256withECDSA id-Dilithium3-ECDSA-brainpoolP256r1-SHA256 <CompSig>.3 Dilithium3 SHA256withECDSA id-Dilithium3-Ed25519 <CompSig>.4 Dilithium3 Ed25519 id-Dilithium5-ECDSA-P384-SHA384 <CompSig>.5 Dilithium5 SHA384withECDSA id-Dilithium5-ECDSA-brainpoolP384r1-SHA384 <CompSig>.6 Dilithium5 SHA384withECDSA id-Dilithium5-Ed448 <CompSig>.7 Dilithium5 Ed448 id-Falcon512-ECDSA-P256-SHA256 <CompSig>.8 Falcon512 SHA256withECDSA id-Falcon512-ECDSA-brainpoolP256r1-SHA256 <CompSig>.9 Falcon512 SHA256withECDSA id-Falcon512-Ed25519 <CompSig>.10 Falcon512 Ed25519 id-SPHINCSplusSHA256128sSimple-ECDSA-P256-SHA256 <CompSig>.11 <SPHINCS> SHA256withECDSA id-SPHINCSplusSHA256128sSimple-ECDSA-brainpoolP256r1-SHA256 <CompSig>.12 <SPHINCS> SHA256withECDSA id-SPHINCSplusSHA256128sSimple-Ed25519 <CompSig>.13 <SPHINCS> Ed25519 id-SPHINCSplusSHA256128sSimple-Ed25519 <CompSig>.13 <SPHINCS> Ed25519 id-alg-composite 1.3.6.1.4.1.18227.2.1 Any Any id-alg-composite-sha256 1.3.6.1.4.1.18227.2.1.2 Any Any id-alg-composite-sha512 1.3.6.1.4.1.18227.2.1.4 Any Any The table above contains everything needed to implement the listed explicit composite algorithms. See the ASN.1 module in section for the explicit definitions of the above Composite signature algorithms. Full specifications for the referenced algorithms can be found as follows: Dilithium: ECDSA: Ed25519 / Ed448: Falcon: TBD RSAES-PKCS-v1_5: RSASSA-PSS: SPHICNCSplus: id-alg-composite:

Notes on id-Dilithium3-RSA-PSS Use of RSA-PSS deserves a special explanation. When the id-Dilithium3-RSA-PSS object identifier is used with an AlgorithmIdentifier, the AlgorithmIdentifier.parameters MUST be of type `CompositeSignatureParams as follows:

EDNOTE: We probably should pick concrete crypto for the RSASSA-PSS-params. Once the crypto is fixed, we could omit the parameters entirely and expect implementations to re-constitute the params structures as necessary in order to call into lower-level crypto libraries. TODO: there must be a way to put all this the ASN.1 Module rather than just specifying it as text?

Notes on Generic Composite This draft remains backwards compatible with the previous version of composite signatures. Explicit Composite keys are a rigid subset of generic composite keys. In previous versions, the composite signature algorihtm Identifier and the composite key identifier used different Object Identifiers. The generic composite signature algorithm identifier is:

As defined in , a generic composite key uses the following identifier:

See the ASN.1 module in section for the definitions of Generic Composite algorithms

Pre-Hashed Generic Composite Signatures The id-alg-composite-sha256 and id-alg-composite-sha512 object identifiers are used for identifying a pre-hashed generic composite signature that uses SHA-256 or SHA-512. These algorithms allow arbitrary combinations of component signature algorithms to be used in the CompositeSignatureValue and CompositeSignatureParams structures without needing the combination to be pre-registered or standardized. When the id-alg-composite-sha256 and id-alg-composite-sha512 are used, the signature is calculated over the digest of the message rather than the message itself. This mode can be used to generate signatures with any type of key (i.e., classic, post-quantum, or post-post-quantum, etc...) by signing the SHA256 or SHA512 value calculated over the message. This identifier MAY be used in sa-CompositeSignature.identifier.

EDNOTE: this is a temporary OID for the purposes of prototyping. We are requesting IANA to assign a permanent OID, see .

EDNOTE: this is a temporary OID for the purposes of prototyping. We are requesting IANA to assign a permanent OID, see . The following algorithm parameters MUST be included:

The signature's CompositeSignatureParams sequence MUST contain the same component algorithms listed in the same order as in the associated CompositePublicKey.

Composite Signature Processes This section specifies the processes for generating and verifying composite signatures. This process addresses algorithm strength uncertainty by providing the verifier with parallel signatures from all the component signature algorithms; thus forging the composite signature would require forging all of the component signatures.

Composite Signature Generation Process Generation of a composite signature involves applying each component algorithm's signature process to the input message according to its specification, and then placing each component signature value into the CompositeSignatureValue structure defined in . The following process is used to generate composite signature values.

Note on composite inputs: the method of providing the list of component keys and algorithms is flexible and beyond the scope of this pseudo-code, for example they may be carried in CompositePrivateKey and CompositeSignatureParams structures. It is also possible to generate a composite signature that combines signatures from distinct 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 as the process sketched above. Since recursive composite public keys are disallowed in , no component signature may itself be a composite; ie the signature generation process MUST fail if one of the private keys K1, K2, .., Kn is a composite with the OID id-alg-composite or an explicit composite OID. A composite signature MUST produce, and include in the output, a signature value for every component key in and include in the output, a signature value for every component key in the corresponding CompositePublicKey, and they MUST be in the same order; ie in the output, S1 MUST correspond to K1, S2 to K2, etc. For security when using a generic composite signature algorithm, the list of component signature algorithms A1, A2, .., An, which may be carried in a CompositeSignatureParams object, SHOULD be included in the signed message M to prevent an attacker from substituting a weaker algorithm which is compatible with the same public key. This attack is not unique or new to the composite format.

Composite Signature Verification Process Verification of a composite signature involves applying each component algorithm's verification process according to its specification. In the absence of an application profile specifying otherwise, compliant applications MUST output "Valid signature" (true) if and only if all component signatures were successfully validated, and "Invalid signature" (false) otherwise. The following process is used to perform this verification.

Note on composite inputs: the method of providing the list of component keys, algorithms and signatures is flexible and beyond the scope of this pseudo-code, for example they may be carried in CompositePublicKey, CompositeSignatureParams, and compositesignaturevalue structures. It is also possible to verify a composite signature where the component public verification keys belong, for example, to separate X.509 certificates or cryptographic modules. Variations in the process to accommodate particular public verification key storage mechanisms are considered to be conformant to this document so long as it produces the same output as the process sketched above. Since recursive composite public keys are disallowed in , no component signature may be composite; ie the signature verification procedure MUST fail if any of the public keys P1, P2, .., Pn or algorithm identifiers A1, A2, .., An are composite with OID id-alg-composite or an explicit composite OID. Some verification clients may include a policy mechanism for specifying acceptable subsets of algorithms. In these cases, implementer MAY, in the interest of performance of compatibility, modify the above process to skip one or more signature validations as per their local client policy. See for a discussion of implementation and associated risks.

ASN.1 Module

Composite-Signatures-2023 { joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027) algorithm(80) id-composite-signatures-2023 (TBD) } DEFINITIONS IMPLICIT TAGS ::= BEGIN EXPORTS ALL; IMPORTS PUBLIC-KEY, SIGNATURE-ALGORITHM, AlgorithmIdentifier{} 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) } 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) } sa-rsaSSA-PSS FROM PKIX1-PSS-OAEP-Algorithms-2009 {iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-rsa-pkalgs-02(54)} pk-Dilithium3-RSA-PSS, id-Dilithium3-RSA-PSS, pk-Dilithium3-RSA-PKCS15-SHA256, id-Dilithium3-RSA-PKCS15-SHA256, pk-Dilithium3-ECDSA-P256-SHA256, id-Dilithium3-ECDSA-P256-SHA256, pk-Dilithium3-ECDSA-brainpoolP256r1, id-Dilithium3-ECDSA-brainpoolP256r1, pk-Dilithium3-Ed25519, id-Dilithium3-Ed25519, pk-Dilithium5-ECDSA-P384, id-Dilithium5-ECDSA-P384, pk-Dilithium5-ECDSA-brainpoolP384r1-SHA384, id-Dilithium5-ECDSA-brainpoolP384r1-SHA384, pk-Dilithium5-Ed448, id-Dilithium5-Ed448, pk-Falcon512-ECDSA-P256-SHA256, id-Falcon512-ECDSA-P256-SHA256, pk-Falcon512-ECDSA-brainpoolP256r1-SHA256, id-Falcon512-ECDSA-brainpoolP256r1-SHA256, pk-Falcon512-Ed25519, id-Falcon512-Ed25519, pk-SPHINCSplusSHA256128sSimple-ECDSA-P256-SHA256, id-SPHINCSplusSHA256128sSimple-ECDSA-P256-SHA256, pk-SPHINCSplusSHA256128sSimple-ECDSA-brainpoolP256r1-SHA256, id-SPHINCSplusSHA256128sSimple-ECDSA-brainpoolP256r1-SHA256, pk-SPHINCSplusSHA256128sSimple-Ed25519, id-SPHINCSplusSHA256128sSimple-Ed25519 FROM CompositeKeys-2023 {iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-composite-keys(TBD)}; -- -- Signature Algorithm -- -- Composite Signature Value is just a sequence of OCTET STRINGS CompositeSignaturePair{FirstSignatureValue, SecondSignatureValue} ::= SEQUENCE { signaturevalue1 FirstSignatureValue, signaturevalue2 SecondSignatureValue } -- An Explicit Compsite Signature is a set of Signatures which -- are composed of OCTET STRINGS ExplicitCompositeSignatureValue ::= CompositeSignaturePair { OCTET STRING,OCTET STRING} ExplicitSignatureParameters{SIGNATURE-ALGORITHM:FirstSignatureAlg, SIGNATURE-ALGORITHM:SecondSignatureAlg} ::= SEQUENCE { signatureAlgorithm1 AlgorithmIdentifier { SIGNATURE-ALGORITHM, {FirstSignatureAlg}}, signatureAlgorithm2 AlgorithmIdentifier { SIGNATURE-ALGORITHM, {SecondSignatureAlg}} } sa-explicitCompositeSignature{OBJECT IDENTIFIER:id, PUBLIC-KEY:publicKeyObject, ExplicitCompositeParams} SIGNATURE-ALGORITHM ::= { IDENTIFIER id VALUE ExplicitCompositeSignatureValue PARAMS TYPE ExplicitCompositeParams ARE required PUBLIC-KEYS {publicKeyObject} } sa-Dilithium3-RSA-PSS-SHA256 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature{id-Dilithium3-RSA-PSS-SHA256, pk-Dilithium3-RSA-PSS-SHA256, ExplicitSignatureParameters{{sa-Dilithium3TBD},{sa-rsaSSA-PSS}} } sa-Dilithium3-RSA-PKCS15-SHA256 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature{ id-Dilithium3-RSA-PKCS15-SHA256, pk-Dilithium3-RSA-PKCS15-SHA256, ExplicitSignatureParameters{{sa-Dilithium3TBD},{sa-rsaWithSHA256}} } sa-Dilithium3-ECDSA-P256-SHA256 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature{id-Dilithium3-ECDSA-P256-SHA256, pk-Dilithium3-ECDSA-P256-SHA256, ExplicitSignatureParameters{{sa-Dilithium3TBD},{sa-ecdsaWithSHA256}} } sa-Dilithium3-ECDSA-brainpoolP256r1-SHA256 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature{id-Dilithium3-ECDSA-brainpoolP256r1-SHA256, pk-Dilithium3-ECDSA-brainpoolP256r1-SHA256, ExplicitSignatureParameters{{sa-Dilithium3TBD},{sa-ecdsaWithSHA256}} } sa-Dilithium3-Ed25519 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature{id-Dilithium3-Ed25519,pk-Dilithium3-Ed25519, ExplicitSignatureParameters{{sa-Dilithium3TBD},{sa-Ed25519}} } sa-Dilithium5-ECDSA-P384-SHA384 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature{ id-Dilithium5-ECDSA-P384-SHA384, pk-Dilithium5-ECDSA-P384-SHA384, ExplicitSignatureParameters{{sa-Dilithium5TBD},{sa-ecdsaWithSHA384}} } sa-Dilithium5-ECDSA-brainpoolP384r1-SHA384 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature{id-Dilithium5-ECDSA-brainpoolP384r1-SHA384, pk-Dilithium5-ECDSA-brainpoolP384r1-SHA384, ExplicitSignatureParameters{{sa-Dilithium5TBD},{sa-ecdsaWithSHA384}} } sa-Dilithium5-Ed448 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature{id-Dilithium5-Ed448,pk-Dilithium5-Ed448, ExplicitSignatureParameters{{sa-Dilithium5TBD},{sa-ed448}} } sa-Falcon512-ECDSA-P256-SHA256 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature{id-Falcon512-ECDSA-P256-SHA256, pk-Falcon512-ECDSA-P256-SHA256, ExplicitSignatureParameters{{sa-Falcon512TBD},{sa-ecdsaWithSHA256}} } sa-Falcon512-ECDSA-brainpoolP256r1-SHA256 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature{id-Falcon512-ECDSA-brainpoolP256r1-SHA256, pk-Falcon512-ECDSA-brainpoolP256r1-SHA256, ExplicitSignatureParameters{{sa-Falcon512TBD},{sa-ecdsaWithSHA256}} } sa-Falcon512-Ed25519 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature{id-Falcon512-Ed25519,pk-Falcon512-Ed25519, ExplicitSignatureParameters{{sa-Falcon512TBD},{sa-Ed25519}} } sa-SPHINCSplusSHA256128sSimple-ECDSA-P256-SHA256 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature{ id-SPHINCSplusSHA256128sSimple-ECDSA-P256-SHA256, pk-SPHINCSplusSHA256128sSimple-ECDSA-P256-SHA256, ExplicitSignatureParameters{{sa-SPHINCSTBD},{sa-ecdsaWithSHA256}} } sa-SPHINCSplusSHA256128sSimple-ECDSA-brainpoolP256r1-SHA256 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature { id-SPHINCSplusSHA256128sSimple-ECDSA-brainpoolP256r1-SHA256, pk-SPHINCSplusSHA256128sSimple-ECDSA-brainpoolP256r1-SHA256, ExplicitSignatureParameters{{sa-SPHINCSTBD}, {sa-ecdsaWithSHA256}} } sa-SPHINCSplusSHA256128sSimple-Ed25519 SIGNATURE-ALGORITHM ::= sa-explicitCompositeSignature{id-SPHINCSplusSHA256128sSimple-Ed25519, pk-SPHINCSplusSHA256128sSimple-Ed25519, ExplicitSignatureParameters{{sa-SPHINCSTBD},{sa-Ed25519}} } -- Generic Composite definition -- -- Object Identifiers -- id-alg-composite OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) dod(6) internet(1) private(4) enterprise(1) opencA(18227) algorithms(2) id-alg-composite(1) } id-alg-composite-sha256 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) dod(6) internet(1) private(4) enterprise(1) openCA(18227) algorithms(2) id-alg-composite(1) id-alg-composite-sha256(2) } id-alg-composite-sha512 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) dod(6) internet(1) private(4) enterprise(1) openCA(18227) algorithms(2) id-alg-composite(1) id-alg-composite-sha512(4) } sa-CompositeSignature SIGNATURE-ALGORITHM ::= { IDENTIFIER id-alg-composite VALUE CompositeSignatureValue PARAMS TYPE CompositeSignatureParams ARE required PUBLIC-KEYS { pk-Composite } } sa-CompositeSignature-SHA256 SIGNATURE-ALGORITHM ::= { IDENTIFIER id-alg-composite-sha256 VALUE CompositeSignatureValue PARAMS TYPE CompositeSignatureParams ARE required PUBLIC-KEYS { pk-Composite } } sa-CompositeSignature-SHA512 SIGNATURE-ALGORITHM ::= { IDENTIFIER id-alg-composite-sha512 VALUE CompositeSignatureValue PARAMS TYPE CompositeSignatureParams ARE required PUBLIC-KEYS { pk-Composite } } CompositeSignatureValue ::= SEQUENCE SIZE (2..MAX) OF BIT STRING CompositeSignatureParams ::= SEQUENCE SIZE (2..MAX) OF AlgorithmIdentifier{SIGNATURE-ALGORITHM, {SignatureAlgSet}} END ]]>

IANA Considerations This document registers the following in the SMI "Security for PKIX Algorithms (1.3.6.1.5.5.7.6)" registry:

Plus the ASN.1 Module OID for Composite-Signatures-2023.

Security Considerations

Policy for Deprecated and Acceptable Algorithms Traditionally, a public key, certificate, or signature contains a single cryptographic algorithm. If and when an algorithm becomes deprecated (for example, RSA-512, or SHA1), then clients performing signatures or verifications should be updated to adhere to appropriate policies. In the composite model this is less obvious since a single public key, certificate, or signature may contain a mixture of deprecated and non-deprecated algorithms. Moreover, implementers may decide that certain cryptographic algorithms have complementary security properties and are acceptable in combination even though neither algorithm is acceptable by itself. Specifying a modified verification algorithm to handle these situations is beyond the scope of this draft, but could be desirable as the subject of an application profile document, or to be up to the discretion of implementers.

K-of-N Validation Mode This document defines a composite signature verification process in where the verifier verifies all component signatures and fails if any component fails. The authors recognize that there will be scenarios where the verifier considers a single component algorithm -- or subset of component algorithms -- to provide sufficient security, and therefore for performance, or other practical reasons, wishes to skip the verification of one or more component signatures. For example, it might be the case where the signer (whether it be a CA or an end entity) needs to be able to specify how its signatures are to be interpreted and verified. Another use-case is where one of the composite algorithms' implementation might not be stable yet, possibly causing signature validation or generation issues across different providers or vendors. A detailed description of a K of N validation mode that address all these use-cases is described in .

&RFC2119; &RFC2986; &RFC4210; &RFC5280; &RFC5480; &RFC5639; &RFC5652; &RFC6090; &RFC7748; &RFC8174; &RFC8410; &RFC8411; &I-D.draft-ounsworth-pq-composite-keys-04; &I-D.draft-massimo-lamps-pq-sig-certificates-00; &I-D.draft-ietf-lamps-dilithium-certificates-01; &I-D.draft-ietf-lamps-cms-sphincs-plus-01; ITU-T &RFC3279; &RFC7296; &RFC8446; &RFC8551; &RFC8017; &I-D.draft-ounsworth-pq-composite-kem-00; &I-D.draft-becker-guthrie-noncomposite-hybrid-auth-00; &I-D.draft-guthrie-ipsecme-ikev2-hybrid-auth-00; &I-D.draft-truskovsky-lamps-pq-hybrid-x509-01; &I-D.draft-pala-klaussner-composite-kofn-00;

Work in Progress

Combiner modes (KofN) For content commitment use-cases, such as legally-binding non-repudiation, the signer (whether it be a CA or an end entity) needs to be able to specify how its signature is to be interpreted and verified. For now we have removed combiner modes (AND, OR, KofN) from this draft, but we are still discussing how to incorporate this for the cases where it is needed (maybe a X.509 v3 extension, or a signature algorithm param).

Samples

Generic Composite Signature Examples TODO

Explicit Composite Signature Examples TODO

Implementation Considerations This section addresses practical issues of how this draft affects other protocols and standards. ~~~ BEGIN EDNOTE 10~~~ EDNOTE 10: Possible topics to address: The size of these certs and cert chains. In particular, implications for (large) composite keys / signatures / certs on the handshake stages of TLS and IKEv2. If a cert in the chain is a composite cert then does the whole chain need to be of composite Certs? We could also explain that the root CA cert does not have to be of the same algorithms. The root cert SHOULD NOT be transferred in the authentication exchange to save transport overhead and thus it can be different than the intermediate and leaf certs. We could talk about overhead (size and processing). We could also discuss backwards compatibility. We could include a subsection about implementation considerations. ~~~ END EDNOTE 10~~~

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 interoperate 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 digital signature objects, from online negotiated protocols such as TLS 1.3 and IKEv2 , to non-negotiated asynchronous protocols such as S/MIME signed email , document signing such as in the context of the European eIDAS regulations [eIDAS2014], and publicly trusted code signing [codeSigningBRsv2.8], as well as myriad other standardized and proprietary protocols and applications that leverage CMS signed structures.

Composite K-of-N (OR modes) and make reference to subset signature generation and verification modes to achieve an OR relation between component signatures, where senders and / or receivers are permitted to ignore some component keys. Some envisioned uses of this include environments where the client encounters a component signature algorithm for which it does not posses a compatible implementation but wishes to proceed with the signature verification using the subset of component signatures for which it does have compatible implementations. Such a mechanism could be designed to provide ease of migration by allowing for composite keys to be distributed and used before all clients in the environment are fully upgraded. It is important to notice that although Composite Crypto does not allow for full backwards compatibility (since clients would at least need to be upgraded from their current state to be able to parse the composite structures), upgrading today's systems to be able to handle composite keys and signatures will also provide the tool for the next generation of migration. In other words, the work to support Composite Crypto will also provide crypto agility options for subsequent crypto-migration in a truly crypto-agile fashion (i.e., entities will always able to process composite signatures independent from the individual cryptographic algorithms that are supported today). Please refer to for more information.

Parallel PKIs We present the term "Parallel PKI" to refer to the setup where a PKI end entity possesses two or more distinct public keys or certificates for the same identity (name), but containing keys for different cryptographic algorithms. One could imagine a set of parallel PKIs where an existing PKI using legacy algorithms (RSA, ECC) is left operational during the post-quantum migration but is shadowed by one or more parallel PKIs using pure post quantum algorithms or composite algorithms (legacy and post-quantum). Equipped with a set of parallel public keys in this way, a client would have the flexibility to choose which public key(s) or certificate(s) to use in a given signature operation. For negotiated protocols, the client could choose which public key(s) or certificate(s) to use based on the negotiated algorithms, or could combine two of the public keys for example in a non-composite hybrid method such as or . Note that it is possible to use the signature algorithms defined in as a way to carry the multiple signature values generated by one of the non-composite public mechanism in protocols where it is easier to support the composite signature algorithms than to implement such a mechanism in the protocol itself. There is also nothing precluding a composite public key from being one of the components used within a non-composite authentication operation; this may lead to greater convenience in setting up parallel PKI hierarchies that need to service a range of clients implementing different styles of post-quantum migration strategies. For non-negotiated protocols, the details for obtaining backwards compatibility will vary by protocol, but for example in CMS , the inclusion of multiple SignerInfo objects is often already treated as an OR relationship, so including one for each of the signer's parallel PKI public keys would, in many cases, have the desired effect of allowing the receiver to choose one they are compatible with and ignore the others, thus achieving full backwards compatibility.

Hybrid Extensions (Keys and Signatures) The use of Composite Crypto provides the possibility to process multiple algorithms without changing the logic of applications, but updating the cryptographic libraries: one-time change across the whole system. However, when it is not possible to upgrade the crypto engines/libraries, it is possible to leverage X.509 extensions to encode the additional keys and signatures. When the custom extensions are not marked critical, although this approach provides the most backward-compatible approach where clients can simply ignore the post-quantum (or extra) keys and signatures, it also requires all applications to be updated for correctly processing multiple algorithms together. Please refer to for more information.

Intellectual Property Considerations The following IPR Disclosure relates to this draft: https://datatracker.ietf.org/ipr/3588/

Contributors and Acknowledgements 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), Scott Fluhrer (Cisco Systems), Panos Kampanakis (Cisco Systems), Daniel Van Geest (ISARA), Tim Hollebeek (Digicert), and Francois Rousseau. 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" - .

Making contributions Additional contributions to this draft are welcome. Please see the working copy of this draft at, as well as open issues at: https://github.com/EntrustCorporation/draft-ounsworth-composite-sigs