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NIST

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General LAMPS WG Digital signatures are used to sign messages, X.509 certificates and CRLs. This document updates the "Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List Profile" (RFC3279) and describes the conventions for using the SHAKE function family in Internet X.509 certificates and revocation lists 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.

Introduction defines cryptographic algorithm identifiers for the Internet X.509 Certificate and Certificate Revocation Lists (CRL) profile . This document updates RFC3279 and defines identifiers for several cryptographic algorithms that use variable length output SHAKE functions introduced in which can be used with . 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 defined as SHAKE(M, d) where the output is a d-bits-long digest of message M. The corresponding collision and second-preimage-resistance strengths for SHAKE128 are min(d/2,128) and min(d,128) bits, respectively (Appendix A.1 ). 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 generation function (MGF) in RSASSA-PSS.

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.

Identifiers This section defines four new object identifiers (OIDs), for RSASSA-PSS and ECDSA with each of SHAKE128 and SHAKE256. The same algorithm identifiers can be 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. Sections 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 (8*ceil((n-1)/8) - 264) or (8*ceil((n-1)/8) - 520) bits, respectively, where n is the RSA modulus size in bits.

Use in PKIX

Signatures Signatures are used in a number of different ASN.1 structures. As shown in the ASN.1 representation from below, in an X.509 certificate, a signature is encoded with an algorithm identifier in the signatureAlgorithm attribute and a signatureValue attribute that contains the actual signature. 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 . The parameters of these signature algorithms are absent as explained in . 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 certificates and CRLs using RSASSA-PSS or ECDSA with SHAKE MUST recognize the corresponding OIDs. Encoding rules for RSASSA-PSS and ECDSA signature values are specified in and , respectively. When using RSASSA-PSS or ECDSA with SHAKEs, the RSA modulus and ECDSA curve order SHOULD be chosen in line with the SHAKE output length. Refer to for more details.

RSASSA-PSS Signatures 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. defines RSASSA-PSS-params that are used to define the algorithms and inputs to the algorithm. This specification does not use parameters because the hash, mask generation algorithm, trailer and salt are embedded in the OID definition. The hash algorithm to hash a message being signed and the hash algorithm used as the mask generation function in RSASSA-PSS MUST be the same: both SHAKE128 or both SHAKE256. The output length of the hash algorithm which hashes the message SHALL be 32 (for SHAKE128) or 64 bytes (for SHAKE256). 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 . 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 . As explained in Step 9 of , the output length of the MGF is emLen - hLen - 1 bytes. emLen is the maximum message length ceil((n-1)/8), where n is the RSA modulus in bits. hLen is 32 and 64-bytes for id-RSASSA-PSS-SHAKE128 and id-RSASSA-PSS-SHAKE256, respectively. Thus when SHAKE is used as the MGF, the SHAKE output length maskLen is (8*emLen - 264) or (8*emLen - 520) bits, respectively. For example, when RSA modulus n is 2048, the output length of SHAKE128 or SHAKE256 as the MGF will be 1784 or 1528-bits when id-RSASSA-PSS-SHAKE128 or id-RSASSA-PSS-SHAKE256 is used, respectively. The RSASSA-PSS saltLength MUST be 32 bytes for id-RSASSA-PSS-SHAKE128 or 64 bytes for id-RSASSA-PSS-SHAKE256. Finally, the trailerField MUST be 1, which represents the trailer field with hexadecimal value 0xBC .

ECDSA Signatures 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 SHOULD generate such signatures with a deterministically 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 those standards. Those standards have not specified SHAKE128 and SHAKE256 as hash algorithm options. However, SHAKE128 and SHAKE256 with output length being 32 and 64 octets, respectively, can be used instead of 256 and 512-bit output hash algorithms such as SHA256 and SHA512.

Public Keys Certificates conforming to can convey a public key for any public key algorithm. The certificate indicates the public key algorithm through an algorithm identifier. This algorithm identifier is an OID and optionally associated parameters. The conventions and encoding for RSASSA-PSS and ECDSA public keys algorithm identifiers are as specified in Sections and , , and . Traditionally, the rsaEncryption object identifier is used to identify RSA public keys. The rsaEncryption object identifier continues to identify the subject public key when the RSA private key owner does not wish to limit the use of the public key exclusively to RSASSA-PSS with SHAKEs. When the RSA private key owner wishes to limit the use of the public key exclusively to RSASSA-PSS with SHAKEs, the AlgorithmIdentifiers for RSASSA-PSS defined in SHOULD be used as the algorithm field in the SubjectPublicKeyInfo sequence . Conforming client implementations that process RSASSA-PSS with SHAKE public keys when processing certificates and CRLs MUST recognize the corresponding OIDs. Conforming CA implementations MUST specify the X.509 public key algorithm explicitly by using the OIDs specified in when encoding ECDSA with SHAKE public keys in certificates and CRLs. Conforming client implementations that process ECDSA with SHAKE public keys when processing certificates and CRLs MUST recognize the corresponding OIDs. The identifier parameters, as explained in , MUST be absent.

IANA Considerations One object identifier for the ASN.1 module in is requested for the SMI Security for PKIX Module Identifiers (1.3.6.1.5.5.7.0) registry:

Decimal	Description	References
TBD	id-mod-pkix1-shakes-2019	RFC 9999

IANA is requested to update the SMI Security for PKIX Algorithms (1.3.6.1.5.5.7.6) registry with four additional entries:

Decimal	Description	References
TBD1	id-RSASSA-PSS-SHAKE128	RFC 9999
TBD2	id-RSASSA-PSS-SHAKE256	RFC 9999
TBD3	id-ecdsa-with-shake128	RFC 9999
TBD4	id-ecdsa-with-shake256	RFC 9999

IANA is also requested to update the Hash Function Textual Names Registry with two additional entries for SHAKE128 and SHAKE256:

Hash Function Name	OID	Reference
shake128	2.16.840.1.101.3.4.2.11	RFC 9999
shake256	2.16.840.1.101.3.4.2.12	RFC 9999

Security Considerations This document updates . The security considerations section of that document applies to this specification as well. NIST has defined appropriate use of the hash functions in terms of the algorithm strengths and expected time frames for secure use in Special Publications (SPs) and . These documents can be used as guides to choose appropriate key sizes for various security scenarios. SHAKE128 with output length of 256-bits offers 128-bits of collision and preimage resistance. Thus, SHAKE128 OIDs in this specification are RECOMMENDED with 2048 (112-bit security) or 3072-bit (128-bit security) RSA modulus or curves with group order of 256-bits (128-bit security). SHAKE256 with 512-bits output length offers 256-bits of collision and preimage resistance. Thus, the SHAKE256 OIDs in this specification are RECOMMENDED with 4096-bit RSA modulus or higher or curves with group order of at least 512 bits such as NIST Curve P-521 (256-bit security). Note that we recommended 4096-bit RSA because we would need 15360-bit modulus for 256-bits of security which is impractical for today's technology.

Acknowledgements We would like to thank Sean Turner, Jim Schaad and Eric Rescorla for their valuable contributions to this document. The authors would like to thank Russ Housley for his guidance and very valuable contributions with the ASN.1 module.

References Normative References National Institute of Standards and Technology (NIST) Informative References Standards for Efficient Cryptography Group American National Standard for Financial Services (ANSI) National Institute of Standards and Technology (NIST) IANA IANA National Institute of Standards and Technology (NIST)

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