LAMPS WG P. Kampanakis
Internet-Draft Cisco Systems
Intended status: Standards Track Q. Dang
Expires: January 1, 2019 NIST
June 30, 2018

Internet X.509 Public Key Infrastructure: Additional Algorithm Identifiers for RSASSA-PSS and ECDSA using SHAKEs as Hash Functions


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

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Table of Contents

1. Change Log

[ EDNOTE: Remove this section before publication. ]

2. Introduction

This document describes several cryptographic algorithm identifiers for several cryptographic algorithms which use variable length output SHAKE functions introduced in [SHA3] which can be used with the Internet X.509 Certificate and CRL profile [RFC5280].

The SHA-3 family of one-way hash functions is specified in [SHA3]. In the SHA-3 family, two extendable-output functions, called SHAKE128 and SHAKE256 are defined. Four hash functions, 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 lengths, in bits, of the SHAKE hash functions are defined by the d parameter. The corresponding collision and preimage resistance security levels for SHAKE128 and SHAKE256 are respectively min(d/2,128) and min(d,128) and min(d/2,256) and min(d,256) bits.

SHAKEs can be used as the message digest function (to hash the message to be signed) and as the hash function in the mask generating functions in RSASSA-PSS and ECDSA. In this document, we define four new OIDs for RSASSA-PSS and ECDSA when SHAKE128 and SHAKE256 are used as hash functions. The same algorithm identifiers are used for identifying a public key, and identifying a signature.

3. Identifiers

The new identifiers for RSASSA-PSS signatures using SHAKEs are below.


  [ EDNOTE: "TBD" will be specified by NIST later. ]

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

  id-ecdsa-with-shake128 OBJECT IDENTIFIER  ::=  { joint-iso-ccitt(2) 
			country(16) us(840) organization(1) gov(101) csor(3) algorithms(4) 
			id-ecdsa-with-shake(3) TBD }
  id-ecdsa-with-shake256 OBJECT IDENTIFIER  ::=  { joint-iso-ccitt(2) 
			country(16) us(840) organization(1) gov(101) csor(3) algorithms(4) 
			id-ecdsa-with-shake(3) TBD }
  [ EDNOTE: "TBD" will be specified by NIST later. ]

The parameters for these four identifiers above MUST be absent. That is, the identifier SHALL be a SEQUENCE of one component, the OID.

4. Use in PKIX

4.1. Signatures

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

   Certificate  ::=  SEQUENCE  {
      tbsCertificate       TBSCertificate,
      signatureAlgorithm   AlgorithmIdentifier,
      signatureValue       BIT STRING  }

The identifiers defined in Section 3 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 [RFC3280].

Conforming CA implementations MUST specify the algorithms explicitly by using the OIDs specified in Section 3 when encoding RSASSA-PSS and ECDSA with SHAKE signatures, and public keys in certificates and CRLs. Encoding rules for RSASSA-PSS and ECDSA signature values are specified in [RFC4055] and [RFC5480] respectively.

Conforming client implementations that process RSASSA-PSS and ECDSA with SHAKE signatures when processing certificates and CRLs MUST recognize the corresponding OIDs.

4.1.1. RSASSA-PSS Signatures

The RSASSA-PSS algorithm is defined in [RFC8017]. When id-RSASSA-PSS-SHAKE128 or id-RSASSA-PSS-SHAKE256 specified in Section 3 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 in the maskGenAlgorithm 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 maskGenAlgorithm is the MGF1 specified in Section B.2.1 of [RFC8017]. The output length for SHAKE128 or SHAKE256 being used as the hash function in MGF1 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 in the MGF1 will be 223 or 191 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 [RFC8017].

4.1.2. ECDSA Signatures

The Elliptic Curve Digital Signature Algorithm (ECDSA) is defined in [X9.62]. When the id-ecdsa-with-SHAKE128 or id-ecdsa-with-SHAKE256 (specified in Section 3) 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 size of the hash function must be explicitly determined. The output size, 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 in accordance with all the requirements specified in Sections 7.2 and 7.3 of [X9.62] or with all the requirements specified in Section 4.1.3 of [SEC1]. They MAY also generate such signatures in accordance with all the recommendations in [X9.62] or [SEC1] 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.

4.2. Public Keys

Certificates conforming to [RFC5280] 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:

  SubjectPublicKeyInfo  ::=  SEQUENCE  {
       algorithm         AlgorithmIdentifier,
       subjectPublicKey  BIT STRING

The fields in SubjectPublicKeyInfo have the following meanings:

The conventions for RSASSA-PSS and ECDSA public keys algorithm identifiers are as specified in [RFC3279], [RFC4055] and [RFC5480] , but we include them below for convenience.

4.2.1. RSASSA-PSS Public Keys

[RFC3279] defines the following OID for RSA AlgorithmIdentifier in the SubjectPublicKeyInfo with NULL parameters.

  rsaEncryption OBJECT IDENTIFIER ::=  { pkcs-1 1}

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 Section 3 can be used as the algorithm field in the SubjectPublicKeyInfo sequence [RFC3280]. The identifier parameters, as explained in section Section 3, 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 [RFC4055]. The output of this encoding is carried in the certificate subjectPublicKey.

  RSAPublicKey ::= SEQUENCE {
        modulus INTEGER, -- n
        publicExponent INTEGER  -- e

4.2.2. ECDSA Public Keys

For ECDSA, when id-ecdsa-with-shake128 or id-ecdsa-with-shake256 is used as the AlgorithmIdentifier in the algorithm field of SubjectPublicKeyInfo, the parameters, as explained in section Section 3, MUST be absent.

Additionally, the mandatory EC SubjectPublicKey is defined in Section 2.1.1 and its syntax is in Section 2.2 of [RFC5480]. We also include them here for convenience:

  id-ecPublicKey OBJECT IDENTIFIER ::= {
       iso(1) member-body(2) us(840) ansi-X9-62(10045) keyType(2) 1 }

The id-ecPublicKey parameters MUST be present and are defined as

  ECParameters ::= CHOICE {
      namedCurve         OBJECT IDENTIFIER
      -- implicitCurve   NULL
      -- specifiedCurve  SpecifiedECDomain 

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

5. IANA Considerations

This document uses several new registries [ EDNOTE: Update here. ]

6. Security Considerations

The SHAKEs are deterministic functions. Like any other deterministic functions, executing each function with the same input multiple times will produce the same output. Therefore, users should not expect unrelated outputs (with the same or different output lengths) from excuting a SHAKE function with the same input multiple times.

Implementations must protect the signer's private key. Compromise of the signer's private key permits masquerade.

Implementations must randomly generate one-time values, such as the k value when generating a ECDSA signature. In addition, the generation of public/private key pairs relies on random numbers. The use of inadequate pseudo-random number generators (PRNGs) to generate such cryptographic values can result in little or no security. The generation of quality random numbers is difficult. [RFC4086] offers important guidance in this area, and [SP800-90A] series provide acceptable PRNGs.

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.

7. Acknowledgements

We would like to thank Sean Turner for his valuable contributions to this document.

8. References

8.1. Normative References

[RFC3280] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, DOI 10.17487/RFC3280, April 2002.
[RFC4055] Schaad, J., Kaliski, B. and R. Housley, "Additional Algorithms and Identifiers for RSA Cryptography for use in the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 4055, DOI 10.17487/RFC4055, June 2005.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R. and T. Polk, "Elliptic Curve Cryptography Subject Public Key Information", RFC 5480, DOI 10.17487/RFC5480, March 2009.
[RFC8017] Moriarty, K., Kaliski, B., Jonsson, J. and A. Rusch, "PKCS #1: RSA Cryptography Specifications Version 2.2", RFC 8017, DOI 10.17487/RFC8017, November 2016.
[SHA3] National Institute of Standards and Technology, "SHA-3 Standard - Permutation-Based Hash and Extendable-Output Functions FIPS PUB 202", August 2015.

8.2. Informative References

[RFC3279] Bassham, L., Polk, W. and R. Housley, "Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3279, DOI 10.17487/RFC3279, April 2002.
[RFC4086] Eastlake 3rd, D., Schiller, J. and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005.
[SEC1] Standards for Efficient Cryptography Group, "SEC 1: Elliptic Curve Cryptography", May 2009.
[SP800-90A] National Institute of Standards and Technology, "Recommendation for Random Number Generation Using Deterministic Random Bit Generators. NIST SP 800-90A", June 2015.
[X9.62] American National Standard for Financial Services (ANSI), "X9.62-2005 Public Key Cryptography for the Financial Services Industry: The Elliptic Curve Digital Signature Standard (ECDSA)", November 2005.

Appendix A. ASN.1 module

[ EDNOTE: More here. ]

Authors' Addresses

Panos Kampanakis Cisco Systems EMail:
Quynh Dang NIST 100 Bureau Drive, Stop 8930 Gaithersburg, MD 20899-8930 USA EMail: