Internet X.509 Public Key Infrastructure: Algorithm Identifiers for ML-DSA
draft-ietf-lamps-dilithium-certificates-03

LAMPS WG                                                      J. Massimo
Internet-Draft                                             P. Kampanakis
Intended status: Standards Track                                     AWS
Expires: 8 August 2024                                         S. Turner
                                                                   sn3rd
                                                           B. Westerbaan
                                                              Cloudflare
                                                         5 February 2024

Internet X.509 Public Key Infrastructure: Algorithm Identifiers for ML-
                                  DSA
               draft-ietf-lamps-dilithium-certificates-03

Abstract

   Digital signatures are used within X.509 certificates, Certificate
   Revocation Lists (CRLs), and to sign messages.  This document
   describes the conventions for using the Module-Lattice-Based Digital
   Signatures (ML-DSA) in Internet X.509 certificates and certificate
   revocation lists.  The conventions for the associated signatures,
   subject public keys, and private key are also described.

Note

   [EDNOTE: This draft is not expected to be finalized before the NIST
   PQC Project has standardized FIPS 204 Module-Lattice-Based Digital
   Signature Standard.  The current FIPS draft was published August 24,
   2023 for public review.  Final versions are expected by April 2024.
   This specification will use object identifiers for the new algorithms
   that are assigned by NIST, and will use placeholders until these are
   released.]

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on 8 August 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Identifiers . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  ML-DSA Signatures in PKIX . . . . . . . . . . . . . . . . . .   4
   4.  ML-DSA Public Keys in PKIX  . . . . . . . . . . . . . . . . .   5
   5.  Key Usage Bits  . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  ML-DSA Private Keys . . . . . . . . . . . . . . . . . . . . .   8
   7.  ASN.1 Module  . . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  14
   Appendix B.  Security Strengths . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Module-Lattice-Based Digital Signatures (ML-DSA) is a quantum-
   resistant digital signature scheme standardized by the US National
   Institute of Standards and Technology (NIST) PQC project [NIST-PQC].
   This document specifies the use of the ML-DSA algorithm in Public Key
   Infrastructure X.509 (PKIX) certificates and Certificate Revocation
   Lists (CRLs) at three security levels: ML-DSA-44, ML-DSA-65, and ML-
   DSA-87, using object identifiers assigned by NIST.

   This specification includes conventions for the signatureAlgorithm,
   signatureValue, signature, and subjectPublicKeyInfo fields within
   Internet X.509 certificates and CRLs [RFC5280], like [RFC3279] did

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   for classic cryptography and [RFC5480] did for elliptic curve
   cryptography.  It describes the encoding of digital signatures and
   public keys generated with quantum-resistant signature algorithm ML-
   DSA.

1.1.  Requirements Language

   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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Identifiers

   This specification uses placeholders for object identifiers until the
   identifiers for the new algorithms are assigned by NIST.

   The AlgorithmIdentifier type, which is included herein for
   convenience, is defined as follows:

      AlgorithmIdentifier  ::=  SEQUENCE  {
          algorithm   OBJECT IDENTIFIER,
          parameters  ANY DEFINED BY algorithm OPTIONAL
      }

      |  NOTE: The above syntax is from [RFC5280] and matches the
      |  version used therein, i.e., the 1988 ASN.1 syntax.  See
      |  [RFC5912] for ASN.1 copmatible with the 2015 ASN.1 syntax.

   The fields in AlgorithmIdentifier have the following meanings:

   *  algorithm identifies the cryptographic algorithm with an object
      identifier.

   *  parameters, which are optional, are the associated parameters for
      the algorithm identifier in the algorithm field.

   The OIDs are:

      id-ML-DSA-44 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2)
               country(16) us(840) organization(1) gov(101) csor(3)
               nistAlgorithm(4) sigAlgs(3) TBD }

      id-ML-DSA-65 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2)
               country(16) us(840) organization(1) gov(101) csor(3)
               nistAlgorithm(4) sigAlgs(3) TBD }

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      id-ML-DSA-87 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2)
               country(16) us(840) organization(1) gov(101) csor(3)
               nistAlgorithm(4) sigAlgs(3) TBD }

   The contents of the parameters component for each algorithm are
   absent.

3.  ML-DSA Signatures in PKIX

   ML-DSA is a digital signature scheme built upon the Fiat-Shamir-with-
   aborts framework [Fiat-Shamir].  The security is based upon the
   hardness of lattice problems over module lattices [Dilithium].  ML-
   DSA provides three parameter sets for the security categories 2, 3
   and 5.

   Signatures are used in a number of different ASN.1 structures.  As
   shown in the ASN.1 representation from [RFC5280] 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.

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

   Signatures are also used in the CRL list ASN.1 representation from
   [RFC5280] below.  In a X.509 CRL, a signature is encoded with an
   algorithm identifier in the signatureAlgorithm attribute and a
   signatureValue attribute that contains the actual signature.

      CertificateList  ::=  SEQUENCE  {
         tbsCertificate       TBSCertList,
         signatureAlgorithm   AlgorithmIdentifier,
         signatureValue       BIT STRING  }

   The identifiers defined in Section 2 can be used as the
   AlgorithmIdentifier in the signatureAlgorithm field in the sequence
   Certificate/CertificateList and the signature field in the sequence
   TBSCertificate/TBSCertList in certificates CRLs, respectively,
   [RFC5280].  The parameters of these signature algorithms are absent,
   as explained in Section 2.

   The signatureValue field contains the corresponding ML-DSA signature
   computed upon the ASN.1 DER encoded tbsCertificate [RFC5280].

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   Conforming Certification Authority (CA) implementations MUST specify
   the algorithms explicitly by using the OIDs specified in Section 2
   when encoding ML-DSA signatures in certificates and CRLs.  Conforming
   client implementations that process certificates and CRLs using ML-
   DSA MUST recognize the corresponding OIDs.  Encoding rules for ML-DSA
   signature values are specified Section 2.

   When the id-ML-DSA identifier appears in the algorithm field as an
   AlgorithmIdentifier, the encoding MUST omit the parameters field.
   That is, the AlgorithmIdentifier SHALL be a SEQUENCE of one
   component, the OID id-ML-DSA.

4.  ML-DSA Public Keys in PKIX

   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:

   *  algorithm is the algorithm identifier and parameters for the
      public key (see above).

   *  subjectPublicKey contains the byte stream of the public key.  The
      algorithms defined in this document always encode the public key
      as TODO.

   The public parameters for ML-DSA are based upon a polynomial ring R_q
   for prime q.  A (k*l) public matrix A is produced, consisting of
   polynomials whose coefficients are sampled uniformly at random from
   the integers modulo q.  This sampling is performed by expanding a
   nonce (rho) using an XOF.

   The ML-DSA public key MUST be encoded using the ASN.1 type
   MLDSAPublicKey:

     MLDSAPublicKey ::= OCTET STRING

   where MLDSAPublicKey is a concatenation of rho and t1.  Here, rho is
   the nonce used to seed the XOF to produce the matrix A, and t1 is a
   vector encoded in 320*k bytes where k is the rank of the vector over
   the polynomial ring R_q.  These parameters MUST be encoded as a
   single OCTET STRING.  The size required to hold a MLDSAPublicKey
   public key element is therefore 32+320*k bytes.

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   The id-ML-DSA identifier defined in Section 2 MUST be used as the
   algorithm field in the SubjectPublicKeyInfo sequence [RFC5280] to
   identify a ML-DSA public key.

   The ML-DSA public key (a concatenation of rho and t1 that is an OCTET
   STRING) is mapped to a subjectPublicKey (a value of type BIT STRING)
   as follows: the most significant bit of the OCTET STRING value
   becomes the most significant bit of the BIT STRING value, and so on;
   the least significant bit of the OCTET STRING becomes the least
   significant bit of the BIT STRING.

   The following is an example of a ML-DSA-44 public key encoded using
   the textual encoding defined in [RFC7468].

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   -----BEGIN PUBLIC KEY-----
   MIIHtDANBgsrBgEEAQKCCwcGBQOCB6EAFyP2dnbMELz5lkTFG7YvOdqVF5km835e
   UHXNaynmIrWHeKMla2gHZTCAwgLWIr9RAh1K11KRvFf+HYMs3ykOrd4xQ2vgLsjP
   jU0bup+rgRK5gvm3Z37rm2jAXDzJNxeM5lEDTklz9BQwOdQGm1rI/MXeMGssnOG4
   a9QvNPERdcG7VpTAEEevuTWmOeiKK3+kNCpoMRZr4WkgeInRyVUZXUvbYKkbm1Yj
   2Mn2ZhgqFW4M1IJwq3LPDCAXZU9R0dTQCt9Ns04HQHNylARZya6TdW+F2k+pfZ3U
   BRGbdI0MbVkOxqgRTfdWLlcXCfPn9/qdbz2K18QKdEuBBtPdZxiwTePDS6XvXjEp
   KZg9bukd7Yxqhg+DPGQp+yQLW4H3lkOkq6mCNW2z86OVnja7xHYL8vAU8xDZ9IeL
   oAIJm37X+qLOMIrT7qJE6zx2k+cJBnxBMSErEmOGOHb+dMvZyn8O77EyOSJS5pyn
   70quzO8k+VieM8jc7ZJan3NUeC7F3Os45uAJzEa1ssKwMx+f4Dtz8GkSjjwCIdFA
   fp6MNzek90lPKqJuxL37NGDB9scH6nLQOKpikaYkBYxD/huNtAe/e9MEHFlD1/Uz
   bSWTN2qQ4UmdV7iJKtIvesDrzx7D6v5EUh9QGdt4D3dNI7NvdyOGrQXwMz8M1EOB
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   pYs8cMoFeHmJpzGUkGlFwE+nGysZOx708p98yDRrQyY/Fw6Qg1xzFle14CTqW86h
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   0H4sdGPvv1AIc6id4VhtrKZgau7JK24c4lqun3WgmZH3+/wyDeyf+Zrb8/DPK3N3
   ZI4R+xkZDzSkuFWLh5om8MykaFT6TNmc5Hc1kO039tgz0vqXpsQHir+EiFPbugEp
   JLoxmoCgdxy9RLvDkSjnDwyI4Hg1djp9nRhpTVjvMdoIw6dSyO11ilEwppm+dNIn
   Gxd6iO7yaE2tJ6UjIc+7tuKoZkzmsO6p4hN8VKhzXcVjnRr6xox4sD5u3esqtJly
   9lsMU0qZnqJU4jbBS3SQloj3m8pl4cdf/j6ZHNiAAnfX6hhUGZDetTEv7MICeSW0
   b77XO5i4udTcYQtLvdo7mmyzG/N8NNR1T9pw89GYJ1maJhC2JyYx3Qwedppk0xac
   BwWednYgQMMNEUKmAGoZxytTHtFfPhGQ4s7gZ6zw2CTiqAEGLfGj6lrwuBgXXAn+
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   pGEsxOfiZRq1CLIRFn2FyczJQ8xFwC7fWio3q91+RGr+hzgBE69HuT8C/Z/pBT/x
   Im+D4lM1l++Xyn1RAoHyfnZy+nQK9i4hZ3eGyvpYfjfNGN5S+cBHY7/pKcr/EFaE
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   xgi58ZRh0oCGv6/4oyNROhwBi/aJYL0RRSYd5ZuARcs/aZ+QhdMOkprdzZSh4wDX
   n3Vw1Rm1GAJEDVEojLoFtpWroe4YdsFHDmkJodZl3PHZIOuwHRr8YsqcHwuXM5nW
   iGZRw+PoJP6gri+ev62U0jW5k7t7v35aF4ccBhwg4AVT2odFnj72hwD6df29LMIa
   Zebj4iagOADfdVJhNUis9YseJwJtmlCJqUAB/6BQsCqeleD9LldTMutN/mq67DSe
   z33nzBeLs0PnhtYiX2z3yjw4s+6/c4Q0o+fikm4t23A1Px8sDi72yplLRl67L6L8
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   kajqEv2JJyWk0rrCi5OzrIfxE9Bl9C4nFWCPsd61qGLmi7u+1MrdN3k6dk6TG/y5
   Qkn9iw7wFyoaq6p6RFtFFPnAJJDiHEMp/ZFfY9Rxquvb4EK6FjAHAtKejzEMte4x
   zXK4C4pBZTy+RTMvu7j0CGVlKRgSyj05LX2c22NY6YB1ZSoe9r8NFeRSOQ0ojHBf
   afbiN3jcuOA/nB1MHq6ll/VpLR9NIAJyC4Xh+isdDK4JIXesSD9iNA5CIVXMUlE4
   x5AXF2HOm4o=
   -----END PUBLIC KEY-----

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   Conforming CA implementations MUST specify the X.509 public key
   algorithm explicitly by using the OIDs specified in Section 2 when
   using ML-DSA public keys in certificates and CRLs.  Conforming client
   implementations that process ML-DSA public keys when processing
   certificates and CRLs MUST recognize the corresponding OIDs.

5.  Key Usage Bits

   The intended application for the key is indicated in the keyUsage
   certificate extension; see Section 4.2.1.3 of [RFC5280].  If the
   keyUsage extension is present in a certificate that indicates id-ML-
   DSA in the SubjectPublicKeyInfo, then the at least one of following
   MUST be present:

     digitalSignature; or
     nonRepudiation; or
     keyCertSign; or
     cRLSign.

   Requirements about the keyUsage extension bits defined in [RFC5280]
   still apply.

6.  ML-DSA Private Keys

   EDNOTE: this section is still under construction as we discuss the
   best way to formulate the private key with the wider working group.

   A ML-DSA private key is encoded as MLDSAPrivateKey in the privateKey
   field as an OCTET STRING.  ML-DSA public keys are optionally
   distributed in the publicKey field of the MLDSAPrivateKey structure.
   This follows the OneAsymmetricKey syntax.

   The ASN.1 encoding for a ML-DSA private key is as follows:

     MLDSAPrivateKey ::= SEQUENCE {
         version                  Version,
         privateKeyAlgorithm      PrivateKeyAlgorithmIdentifier,
         privateKey               OCTET STRING,
         publicKey                [1] MLDSAPublicKey OPTIONAL
     }

   A fully populated ML-DSA private key consists of 6 parameters.  The
   size necessary to hold all private key elements is
   32+32+32+32*[(k+l)*ceiling(log(2*eta+1))+13*k] bytes.  The
   description of k, l, and eta as well as public key and secret key
   sizes for security levels 2, 3, and 5 can be found in Figure 1 of the
   Appendix.

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7.  ASN.1 Module

   This section includes the ASN.1 module for the ML-DSA signature
   algorithm.  This module does not come from any previously existing
   RFC.  This module references [RFC5912].

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   [ EDNOTE: Add ASN.1 here ]

     PKIX1-PQ-Algorithms { iso(1) identified-organization(3) dod(6)
        internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
        id-mod-pkix1-PQ-algorithms(X) }

     DEFINITIONS EXPLICIT TAGS ::=

     BEGIN

     -- EXPORTS ALL;

     IMPORTS

     -- FROM RFC 5912

     PUBLIC-KEY, SIGNATURE-ALGORITHM, DIGEST-ALGORITHM, SMIME-CAPS
     FROM AlgorithmInformation-2009
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-algorithmInformation-02(58) }

     --
     -- Public Key (pk-) Algorithms
     --
     PublicKeys PUBLIC-KEY ::= {
       -- This expands PublicKeys from RFC 5912
       pk-MLDSATBD |
       pk-TBD-TBD,
       ...
     }

     -- The hashAlgorithm is mda-shake256
     -- The XOF seed rho is 32 bytes
     -- The vector t1 is 320*k bytes
     -- These are encoded as a single string

     pk-MLDSA PUBLIC-KEY ::= {
       IDENTIFIER id-MLDSA
       -- KEY no ASN.1 wrapping --
       PARAMS ARE absent
       CERT-KEY-USAGE { nonRepudiation, digitalSignature,
                       keyCertSign, cRLSign }
       --- PRIVATE-KEY no ASN.1 wrapping --
     }

     END

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8.  IANA Considerations

   Extensions in certificates and CRLs are identified using object
   Identifiers (OIDs).  The creation and delegation of these arcs is to
   be determined.

   IANA is requested to register the id-mod-pkix1-PQ-algorithms OID for
   the ASN.1 module identifier found in Section 5 in the "SMI Security
   for PKIX Module Identifier" registry.

9.  Security Considerations

   The Security Considerations section of [RFC5280] applies to this
   specification as well.

   The digital signature scheme defined within this document are modeled
   under existentially unforgeable digital signatures with respect to an
   adaptive chosen message attack (EUF-CMA).  For the purpose of
   estimating security strength, it has been assumed that the attacker
   has access to signatures for no more than 2^{64} chosen messages.

   EDNOTE: Discuss implications of not hash-then-sign.  Implications in
   performance too.

   Within the hash-then-sign paradigm, hash functions are used as a
   domain restrictor over the message to be signed.  By pre-hashing, the
   onus of resistance to existential forgeries becomes heavily reliant
   on the collision-resistance of the hash function in use.  As well as
   this security goal, the hash-then-sign paradigm also has the ability
   to improve performance by reducing the size of signed messages.  As a
   corollary, hashing remains mandatory even for short messages and
   assigns a further computational requirement onto the verifier.  This
   makes the performance of hash-then-sign schemes more consistent, but
   not necessarily more efficient.  ML-DSA diverges from the hash-then-
   sign paradigm by hashing the message during the signing procedure (at
   the point in which the challenge polynomial).  However, due to the
   fact that ML-DSA signatures may require the signing procedure to be
   repeated several times for a signature to be produced, ML-DSA
   implementations can make use of pre-hashing the message to prevent
   rehashing with each attempt.

   EDNOTE: Discuss deterministic vs randomized signing and the impact on
   security.

   ML-DSA offers both deterministic and randomized signing.  By default
   ML-DSA signatures are non-deterministic, the private random seed rho'
   is pseudorandomly derived from the signer’s private key, the message,
   and a 256-bit string, rnd - where rnd should be generated by an

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   approved RBG.  In the deterministic version, rng is instead a 256-bit
   constant string.  The source of randomness in the randomized mode has
   been "hedged" against sources of poor entropy, by including the
   signers private key and message into the derivation.  The primary
   purpose of rnd is to facilitate countermeasures to side-channel
   attacks and fault attacks on deterministic signatures.

   EDNOTE: Discuss side-channels for ML-DSA.

   ML-DSA has been designed to provide side-channel resilience by
   eliminating a reliance on Gaussian sampling.  While deliberate design
   decisions such as these can help to deliver a greater ease of secure
   implementation - particularly against side-channel attacks - it does
   not necessarily provide resistance to more powerful attacks such as
   differential power analysis.  Some amount of side-channel leakage has
   been demonstrated in parts of the signing algorithm (specifically the
   bit-unpacking function), from which a demonstration of key recovery
   has been made over a large sample of signatures.  Masking
   countermeasures exist for ML-DSA, but come with a performance
   overhead.

   A fundamental security property also associated with digital
   signatures is non-repudiation.  Non-repudiation refers to the
   assurance that the owner of a signature key pair that was capable of
   generating an existing signature corresponding to certain data cannot
   convincingly deny having signed the data.  The digital signature
   scheme ML-DSA possess three security properties beyond
   unforgeability, that are associated with non-repudiation.  These are
   exclusive ownership, message-bound signatures, and non-resignability.
   These properties are based tightly on the assumed collision
   resistance of the hash function used (in this case SHAKE-256).
   Exclusive ownership is a property in which a signature sigma uniquely
   determines the public key and message for which it is valid.
   Message-bound signatures is the property that a valid signature
   uniquely determines the message for which it is valid, but not
   necessarily the public key.  Non-resignability is the property in
   which one cannot produce a valid signature under another key given a
   signature sigma for some unknown message m.  These properties are not
   provided by classical signature schemes such as DSA or ECDSA, and
   have led to a variety of attacks such as Duplicate-Signature Key
   Selection (DSKS) attacks , and attacks on the protocols for secure
   routing.  A full discussion of these properties in ML-DSA can be
   found at [CDFFJ21].  These properties are dependent, in part, on
   unambiguous public key serialization.  It for this reason the public
   key structure defined in Section 4 is intentionally encoded as a
   single OCTET STRING.

10.  References

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10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
              DOI 10.17487/RFC5912, June 2010,
              <https://www.rfc-editor.org/info/rfc5912>.

   [RFC7468]  Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
              PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
              April 2015, <https://www.rfc-editor.org/info/rfc7468>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2.  Informative References

   [CDFFJ21]  Cremers, Cas., Düzlü, S., Fiedler, R., Fischlin, M., and
              C. Janson, "BUFFing signature schemes beyond
              unforgeability and the case of post-quantum signatures",
              In Proceedings of the 42nd IEEE Symposium on Security and
              Privacy, 2021, <https://eprint.iacr.org/2020/1525.pdf>.

   [Dilithium]
              Bai, S., Ducas, L., Lepoint, T., Lyubashevsky, V.,
              Schwabe, P., Seiler, G., and D. Stehlé, "CRYSTALS-
              Dilithium Algorithm Specifications and Supporting
              Documentation", 2021, <https://pq-
              crystals.org/dilithium/data/dilithium-specification-
              round3-20210208.pdf>.

   [Fiat-Shamir]
              Lyubashevsky, V., "Fiat-Shamir with aborts: Applications
              to lattice and factoring-based signatures", International
              Conference on the Theory and Application of Cryptology and
              Information Security, 2009, <https://www.iacr.org/archive/
              asiacrypt2009/59120596/59120596.pdf>.

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   [FIPS204]  Raimondo, G. M. and L. E. Locascio, "FIPS 204 (Initial
              Public Draft): Module-Lattice-Based Digital Signature
              Standard", National Institute of Standards and Technology,
              2023, <https://doi.org/10.6028/NIST.FIPS.204.ipd>.

   [NIST-PQC] National Institute of Standards and Technology (NIST),
              "Post-Quantum Cryptography Project", 20 December 2016,
              <https://csrc.nist.gov/Projects/post-quantum-
              cryptography>.

   [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, <https://www.rfc-editor.org/info/rfc3279>.

   [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,
              <https://www.rfc-editor.org/info/rfc5480>.

Appendix A.  Acknowledgements

   We would like to thank ... for their insightful comments.

Appendix B.  Security Strengths

   Instead of defining the strength of a quantum algorithm in a
   traditional manner using precise estimates of the number of bits of
   security, NIST has instead elected to define a collection of broad
   security strength categories.  Each category is defined by a
   comparatively easy-to-analyze reference primitive that cover a range
   of security strengths offered by existing NIST standards in symmetric
   cryptography, which NIST expects to offer significant resistance to
   quantum cryptanalysis.  These categories describe any attack that
   breaks the relevant security definition that must require
   computational resources comparable to or greater than those required
   for: Level 1 - key search on a block cipher with a 128-bit key (e.g.,
   AES128), Level 2 - collision search on a 256-bit hash function (e.g.,
   SHA256/ SHA3-256), Level 3 - key search on a block cipher with a
   192-bit key (e.g., AES192), Level 4 - collision search on a 384-bit
   hash function (e.g.  SHA384/ SHA3-384), Level 5 - key search on a
   block cipher with a 256-bit key (e.g., AES 256).

   The parameter sets defined for NIST security levels 2, 3 and 5 are
   listed in the Figure 1, along with the resulting signature size,
   public key, and private key sizes in bytes.

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   |=======+=======+=====+========+========+========|
   | Level | (k,l) | eta |  Sig.  | Public | Private|
   |       |       |     |  (B)   | Key(B) | Key(B) |
   |=======+=======+=====+========+========+========|
   |   2   | (4,4) |  2  |  2420  |  1312  |  2528  |
   |   3   | (6,5) |  4  |  3293  |  1952  |  4000  |
   |   5   | (8,7) |  2  |  4595  |  2592  |  4864  |
   |=======+=======+=====+========+========+========|

                                  Figure 1

Authors' Addresses

   Jake Massimo
   AWS
   United States of America
   Email: jakemas@amazon.com

   Panos Kampanakis
   AWS
   United States of America
   Email: kpanos@amazon.com

   Sean Turner
   sn3rd
   Email: sean@sn3rd.com

   Bas Westerbaan
   Cloudflare
   Email: bas@westerbaan.name

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