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General LAMPS WG Digital signatures are used to sign messages, X.509 certificates and CRLs (Certificate Revocation Lists). This document describes the conventions for using the SHAKE function family in Internet X.509 certificates and CRLs as one-way hash functions with the RSA Probabilistic signature and ECDSA signature algorithms. The conventions for the associated subject public keys are also described.

[ EDNOTE: Remove this section before publication. ] draft-ietf-lamps-pkix-shake-04: Removed paragraph suggesting KMAC to be used in generating k in Deterministric ECDSA. That should be RFC6979-bis. Removed paragraph from Security Considerations that talks about randomness of k because we are using deterministric ECDSA. Various ASN.1 fixes. Text fixes. draft-ietf-lamps-pkix-shake-03: Updates based on suggestions and clarifications by Jim. Added ASN.1. draft-ietf-lamps-pkix-shake-02: Significant reorganization of the sections to simplify the introduction, the new OIDs and their use in PKIX. Added new OIDs for RSASSA-PSS that hardcode hash, salt and MGF, according the WG consensus. Updated Public Key section to use the new RSASSA-PSS OIDs and clarify the algorithm identifier usage. Removed the no longer used SHAKE OIDs from section 3.1. Consolidated subsection for message digest algorithms. Text fixes. draft-ietf-lamps-pkix-shake-01: Changed titles and section names. Removed DSA after WG discussions. Updated shake OID names and parameters, added MGF1 section. Updated RSASSA-PSS section. Added Public key algorithm OIDs. Populated Introduction and IANA sections. draft-ietf-lamps-pkix-shake-00: Initial version

This document describes cryptographic algorithm identifiers for several cryptographic algorithms which use variable length output SHAKE functions introduced in which can be used with the Internet X.509 Certificate and CRL profile . In the SHA-3 family, two extendable-output functions (SHAKEs), SHAKE128 and SHAKE256, are defined. Four other hash function instances, SHA3-224, SHA3-256, SHA3-384, and SHA3-512 are also defined but are out of scope for this document. A SHAKE is a variable length hash function. The output length, in bits, of a SHAKE is defined by the d parameter. The corresponding collision and second preimage resistance strengths for SHAKE128 are min(d/2,128) and min(d,128) bits respectively. And, the corresponding collision and second preimage resistance strengths for SHAKE256 are min(d/2,256) and min(d,256) bits respectively. A SHAKE can be used as the message digest function (to hash the message to be signed) in RSASSA-PSS and ECDSA and as the hash in the mask generating function in RSASSA-PSS. This specification describes the identifiers for SHAKEs to be used in X.509 and their meaning.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

This section defines four new OIDs for RSASSA-PSS and ECDSA when SHAKE128 and SHAKE256 are used. The same algorithm identifiers are used for identifying a public key in RSASSA-PSS. The new identifiers for RSASSA-PSS signatures using SHAKEs are below.

The new algorithm identifiers of ECDSA signatures using SHAKEs are below.

The parameters for the four identifiers above MUST be absent. That is, the identifier SHALL be a SEQUENCE of one component, the OID. and specify the required output length for each use of SHAKE128 or SHAKE256 in RSASSA-PSS and ECDSA. In summary, when hashing messages to be signed, output lengths of SHAKE128 and SHAKE256 are 256 and 512 bits respectively. When the SHAKEs are used as mask generation functions RSASSA-PSS, their output length is (n - 264) or (n - 520) bits respectively, where n is a RSA modulus size in bits.

Signatures can be placed in a number of different ASN.1 structures. The top level structure for an X.509 certificate, to illustrate how signatures are frequently encoded with an algorithm identifier and a location for the signature, is

The identifiers defined in can be used as the AlgorithmIdentifier in the signatureAlgorithm field in the sequence Certificate and the signature field in the sequence tbsCertificate in X.509 . Conforming CA implementations MUST specify the algorithms explicitly by using the OIDs specified in when encoding RSASSA-PSS or ECDSA with SHAKE signatures in certificates and CRLs. Conforming client implementations that process RSASSA-PSS or ECDSA with SHAKE signatures when processing certificates and CRLs MUST recognize the corresponding OIDs. Encoding rules for RSASSA-PSS and ECDSA signature values are specified in and respectively.

The RSASSA-PSS algorithm is defined in . When id-RSASSA-PSS-SHAKE128 or id-RSASSA-PSS-SHAKE256 specified in is used, the encoding MUST omit the parameters field. That is, the AlgorithmIdentifier SHALL be a SEQUENCE of one component, id-RSASSA-PSS-SHAKE128 or id-RSASSA-PSS-SHAKE256. The hash algorithm to hash a message being signed and the hash algorithm as the mask generation function used in RSASSA-PSS MUST be the same, SHAKE128 or SHAKE256 respectively. The output-length of the hash algorithm which hashes the message SHALL be 32 or 64 bytes respectively. The mask generation function takes an octet string of variable length and a desired output length as input, and outputs an octet string of the desired length. In RSASSA-PSS with SHAKES, the SHAKEs MUST be used natively as the MGF function, instead of the MGF1 algorithm that uses the hash function in multiple iterations as specified in Section B.2.1 of . In other words, the MGF is defined as the SHAKE128 or SHAKE256 output of the mgfSeed for id-RSASSA-PSS-SHAKE128 and id-RSASSA-PSS-SHAKE256 respectively. The mgfSeed is the seed from which mask is generated, an octet string . The output length is (n - 264)/8 or (n - 520)/8 bytes respectively, where n is the RSA modulus in bits. For example, when RSA modulus n is 2048, the output length of SHAKE128 or SHAKE256 as the MGF will be 223 or 191-bits when id-RSASSA-PSS-SHAKE128 or id-RSASSA-PSS-SHAKE256 is used respectively. The RSASSA-PSS saltLength MUST be 32 or 64 bytes respectively. Finally, the trailerField MUST be 1, which represents the trailer field with hexadecimal value 0xBC .

The Elliptic Curve Digital Signature Algorithm (ECDSA) is defined in . When the id-ecdsa-with-SHAKE128 or id-ecdsa-with-SHAKE256 (specified in ) algorithm identifier appears, the respective SHAKE function (SHAKE128 or SHAKE256) is used as the hash. The encoding MUST omit the parameters field. That is, the AlgorithmIdentifier SHALL be a SEQUENCE of one component, the OID id-ecdsa-with-SHAKE128 or id-ecdsa-with-SHAKE256. For simplicity and compliance with the ECDSA standard specification, the output length of the hash function must be explicitly determined. The output length, d, for SHAKE128 or SHAKE256 used in ECDSA MUST be 256 or 512 bits respectively. Conforming CA implementations that generate ECDSA with SHAKE signatures in certificates or CRLs MUST generate such signatures with a deterministicly generated, non-random k in accordance with all the requirements specified in . They MAY also generate such signatures in accordance with all other recommendations in or if they have a stated policy that requires conformance to these standards. These standards may have not specified SHAKE128 and SHAKE256 as hash algorithm options. However, SHAKE128 and SHAKE256 with output length being 32 and 64 octets respectively are subtitutions for 256 and 512-bit output hash algorithms such as SHA256 and SHA512 used in the standards.

Certificates conforming to can convey a public key for any public key algorithm. The certificate indicates the algorithm through an algorithm identifier. This algorithm identifier is an OID and optionally associated parameters. In the X.509 certificate, the subjectPublicKeyInfo field has the SubjectPublicKeyInfo type, which has the following ASN.1 syntax:

The fields in SubjectPublicKeyInfo have the following meanings: algorithm is the algorithm identifier and parameters for the public key. subjectPublicKey contains the byte stream of the public key. The algorithms defined in this document always encode the public key as an exact multiple of 8-bits. Conforming CA implementations MUST specify the algorithms explicitly by using the OIDs specified in when encoding RSASSA-PSS or ECDSA with SHAKE public keys in certificates and CRLs. Conforming client implementations that process RSASSA-PSS or ECDSA with SHAKE public key when processing certificates and CRLs MUST recognize the corresponding OIDs. The conventions for RSASSA-PSS and ECDSA public keys algorithm identifiers are as specified in , and , but we include them below for convenience.

defines the following OID for RSA AlgorithmIdentifier in the SubjectPublicKeyInfo with NULL parameters.

Additionally, when the RSA private key owner wishes to limit the use of the public key exclusively to RSASSA-PSS, the AlgorithmIdentifiers for RSASSA-PSS defined in can be used as the algorithm field in the SubjectPublicKeyInfo sequence . The identifier parameters, as explained in section , MUST be absent. Regardless of what public key algorithm identifier is used, the RSA public key, which is composed of a modulus and a public exponent, MUST be encoded using the RSAPublicKey type . The output of this encoding is carried in the certificate subjectPublicKey.

For ECDSA, the public key identifier defined in is

Additionally, the mandatory EC SubjectPublicKey is defined in Section 2.1.1 and its syntax is in Section 2.2 of . We also include them here for convenience: The id-ecPublicKey parameters MUST be absent or present and are defined as

The ECParameters associated with the ECDSA public key in the signer's certificate SHALL apply to the verification of the signature.

[ EDNOTE: Update here only if there are OID allocations by IANA. ] This document has no IANA actions.

The SHAKEs are deterministic functions. Like any other deterministic function, executing multiple times with the same input will produce the same output. Therefore, users should not expect unrelated outputs (with the same or different output lengths) from running a SHAKE function with the same input multiple times. The shorter of any two outputs produced from a SHAKE with the same input is a prefix of the longer one. It is a similar situation as truncating a 512-bit output of SHA-512 by taking its 256 left-most bits. These 256 left-most bits are a prefix of the 512-bit output. Implementations must protect the signer's private key. Compromise of the signer's private key permits masquerade attacks. Implementers should be aware that cryptographic algorithms may become weaker with time. As new cryptanalysis techniques are developed and computing power increases, the work factor or time required to break a particular cryptographic algorithm may decrease. Therefore, cryptographic algorithm implementations should be modular allowing new algorithms to be readily inserted. That is, implementers should be prepared to regularly update the set of algorithms in their implementations.

We would like to thank Sean Turner and Jim Schaad for their valuable contributions to this document.

&RFC2119; &RFC4055; &RFC5280; &RFC5480; &RFC6979; &RFC8017; National Institute of Standards and Technology &RFC3279; Standards for Efficient Cryptography Group American National Standard for Financial Services (ANSI)

This appendix includes the ASN.1 module for SHAKEs in X.509. This module does not come from any existing RFC.