Internet X.509 Public Key Infrastructure: Additional Algorithm Identifiers for RSASSA-PSS and ECDSA Using SHAKEs
Cisco Systems
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NIST
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United States of America
quynh.dang@nist.gov
Security
LAMPS WG
SHAKE in X.509
SHAKEs in PKIX
certificates with SHAKE hashes
Digital signatures are used to sign messages, X.509
certificates, and Certificate Revocation Lists (CRLs). This document updates the
"Algorithms and Identifiers for the Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile" (RFC 3279)
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 Elliptic Curve Digital Signature Algorithm (ECDSA) signature algorithms. The conventions for the
associated subject public keys are also described.
Introduction
defines cryptographic algorithm
identifiers for the "Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile"
. This document updates RFC 3279
and defines identifiers for several cryptographic algorithms that use
variable-length output SHAKE functions introduced in
which can be used with RFC 5280.
In the SHA-3 family, two extendable-output functions (SHAKEs)
are defined: SHAKE128 and SHAKE256. 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 (see Appendix A.1 of
). 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 RSA Probabilistic Signature Scheme (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 MGFs in 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 Certification Authority (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 is 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
MGF
in RSASSA-PSS MUST be the same: both SHAKE128 or both SHAKE256. The
output length of the hash algorithm that hashes the message SHALL be 32 bytes
(for SHAKE128) or 64 bytes (for SHAKE256).
The MGF 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, 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 the 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 bits,
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, nonrandom 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 with optionally associated
parameters. The conventions and encoding for RSASSA-PSS and
ECDSA public key algorithm identifiers are as specified in
Sections and of ,
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
has been assigned in the "SMI Security for PKIX Module Identifier"
(1.3.6.1.5.5.7.0) registry:
Decimal |
Description |
References |

94 |
id-mod-pkix1-shakes-2019 |
RFC 8692 |

IANA has updated the
"SMI Security for PKIX Algorithms"
(1.3.6.1.5.5.7.6) registry with four additional entries:
Decimal |
Description |
References |

30 |
id-RSASSA-PSS-SHAKE128 |
RFC 8692 |

31 |
id-RSASSA-PSS-SHAKE256 |
RFC 8692 |

32 |
id-ecdsa-with-shake128 |
RFC 8692 |

33 |
id-ecdsa-with-shake256 |
RFC 8692 |

IANA has updated 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 8692 |

shake256 |
2.16.840.1.101.3.4.2.12 |
RFC 8692 |

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 a 512-bit 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 a group order of
at least 512 bits, such as the NIST Curve P-521 (256-bit
security). Note that we recommended a 4096-bit RSA because we
would need a 15360-bit modulus for 256 bits of security, which
is impractical for today's technology.
References
Normative References
SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions
National Institute of Standards and Technology
Informative References
SEC 1: Elliptic Curve Cryptography
Standards for Efficient Cryptography Group
Public Key Cryptography for the Financial Services Industry: the
Elliptic Curve Digital Signature Algorithm (ECDSA)
ANSI
Cryptographic Algorithms and Key Sizes for Personal Identity Verification
National Institute of Standards and Technology (NIST)
SMI Security for PKIX Algorithms
IANA
Hash Function Textual Names
IANA
Recommendation for Applications Using Approved Hash Algorithms
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 previously existing RFC. This module references .
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.