LAMPS                                                       M. Ounsworth
Internet-Draft                                                   Entrust
Intended status: Standards Track                                 M. Pala
Expires: 10 December 2022                                      CableLabs
                                                             8 June 2022


              Composite Signatures For Use In Internet PKI
                  draft-ounsworth-pq-composite-sigs-07

Abstract

   The migration to post-quantum cryptography is unique in the history
   of modern digital cryptography in that neither the old outgoing nor
   the new incoming algorithms are fully trusted to protect data for the
   required data lifetimes.  The outgoing algorithms, such as RSA and
   elliptic curve, may fall to quantum cryptanalysis, while the incoming
   post-quantum algorithms face uncertainty about both the underlying
   mathematics as well as hardware and software implementations that
   have not had sufficient maturing time to rule out classical
   cryptanalytic attacks and implementation bugs.

   Cautious implementer may wish to layer cryptographic algorithms such
   that an attacker would need to break all of them in order to
   compromise the data being protected.  For digital signatures, this is
   referred to as "dual", and for encryption key establishment this as
   referred to as "hybrid".  This document, and its companions, defines
   a specific instantiation of the dual and hybrid paradigm called
   "composite" where multiple cryptographic algorithms are combined to
   form a single key, signature, or key encapsulation mechanism (KEM)
   such that they can be treated as a single atomic object at the
   protocol level.

   EDNOTE: the terms "dual" and "hybrid" are currently in flux.  We
   anticipate an Informational draft to normalize terminology, and will
   update this draft accordingly.

   This document defines the structures CompositeSignatureValue, and
   CompositeParams, which are sequences of the respective structure for
   each component algorithm.  The generic composite variant is defined
   which allows arbitrary combinations of signature algorithms to be
   used in the CompositeSignatureValue and CompositeParams structures
   without needing the combination to be pre-registered or pre-agreed.
   The explicit variant is also defined which allows for a set of
   signature algorithm identifier OIDs to be registered together as an
   explicit composite signature algorithm and assigned an OID.





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   This document is intended to be coupled with corresponding documents
   that define the structure and semantics of composite public and
   private keys and encryption [I-D.draft-ounsworth-pq-composite-keys-
   01], however may also be used with non-composite keys, such as when a
   protocol combines multiple certificates into a single cryptographic
   operation.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on 10 December 2022.

Copyright Notice

   Copyright (c) 2022 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
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Changes in version -07  . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Composite Signature Structures  . . . . . . . . . . . . . . .   6
     3.1.  Composite Keys  . . . . . . . . . . . . . . . . . . . . .   6
       3.1.1.  Key Usage Bits  . . . . . . . . . . . . . . . . . . .   6
     3.2.  sa-CompositeSignature . . . . . . . . . . . . . . . . . .   7
     3.3.  CompositeSignatureValue . . . . . . . . . . . . . . . . .   7



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     3.4.  Encoding Rules  . . . . . . . . . . . . . . . . . . . . .   7
   4.  Algorithm Identifiers . . . . . . . . . . . . . . . . . . . .   8
     4.1.  id-alg-composite (Generic Composite Signatures) . . . . .   8
     4.2.  Explicit Composite Signatures . . . . . . . . . . . . . .   9
   5.  Composite Signature Processes . . . . . . . . . . . . . . . .  10
     5.1.  Composite Signature Generation Process  . . . . . . . . .  10
     5.2.  Composite Signature Verification Process  . . . . . . . .  12
   6.  Implementation Considerations . . . . . . . . . . . . . . . .  14
     6.1.  Backwards Compatibility . . . . . . . . . . . . . . . . .  15
       6.1.1.  OR modes  . . . . . . . . . . . . . . . . . . . . . .  15
       6.1.2.  Parallel PKIs . . . . . . . . . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
     8.1.  Policy for Deprecated and Acceptable Algorithms . . . . .  17
     8.2.  OR Modes  . . . . . . . . . . . . . . . . . . . . . . . .  17
       8.2.1.  Subset Signature Generation . . . . . . . . . . . . .  17
       8.2.2.  Subset Signature Verification . . . . . . . . . . . .  18
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Appendix A.  Work in Progress . . . . . . . . . . . . . . . . . .  20
     A.1.  Combiner modes (KofN) . . . . . . . . . . . . . . . . . .  20
   Appendix B.  Creating explicit combinations . . . . . . . . . . .  21
   Appendix C.  Examples . . . . . . . . . . . . . . . . . . . . . .  21
     C.1.  Generic Composite Signature Examples  . . . . . . . . . .  21
     C.2.  Explicit Composite Signature Examples . . . . . . . . . .  21
   Appendix D.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  21
   Appendix E.  Intellectual Property Considerations . . . . . . . .  22
   Appendix F.  Contributors and Acknowledgements  . . . . . . . . .  23
     F.1.  Making contributions  . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Changes in version -07

   *  Merged Generic Composite (Section 4.1) and Explicit Composite
      (Section 4.2) into one document and made them share a wire
      encoding (only differing by the OIDs used).

   *  Removed Composite-OR signature mode.

   *  Added Section 6.1 addressing backwards compatibility and ease of
      migration concerns.

   *  Added CompositeParams := Alg1, Alg2, .. Algn as an input parameter
      to the sig gen and verification processes.

   TODO diff this against the public version and see if there are any
   more changes.



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2.  Introduction

   During the transition to post-quantum cryptography, there will be
   uncertainty as to the strength of cryptographic algorithms; we will
   no longer fully trust traditional cryptography such as RSA, Diffie-
   Hellman, DSA and their elliptic curve variants, but we will also not
   fully trust their post-quantum replacements until they have had
   sufficient scrutiny and time to discover and fix implementation bugs.
   Unlike previous cryptographic algorithm migrations, the choice of
   when to migrate and which algorithms to migrate to, is not so clear.
   Even after the migration period, it may be advantageous for an
   entity's cryptographic identity to be composed of multiple public-key
   algorithms.

   The deployment of composite signatures using post-quantum algorithms
   will face two challenges

   *  Algorithm strength uncertainty: During the transition period, some
      post-quantum signature and encryption algorithms will not be fully
      trusted, while also the trust in legacy public key algorithms will
      start to erode.  A relying party may learn some time after
      deployment that a public key algorithm has become untrustworthy,
      but in the interim, they may not know which algorithm an adversary
      has compromised.

   *  Backwards compatibility: During the transition period, post-
      quantum algorithms will not be supported by all clients.

   This document provides a mechanism to address algorithm strength
   uncertainty concerns by building on [draft-ounsworth-pq-composite-
   keys-00] (NOTE: need kramdown formatting help with this ref) by
   providing formats for encoding multiple signature values into
   existing public signature fields, as well as the process for
   validating a composite signature.  Backwards compatibility is
   addressed via using composite in conjunction with a non-composite
   hybrid mode such as that described in [draft-becker-guthrie-
   noncomposite-hybrid-auth-00] (NOTE: need kramdown formatting help
   with this ref).

   This document is intended for general applicability anywhere that
   digital signatures are used within PKIX and CMS structures.










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

   The following terms are used in this document:

   ALGORITHM: A standardized cryptographic primitive, as well as any
   ASN.1 structures needed for encoding data and metadata needed to use
   the algorithm.  This document is primarily concerned with algorithms
   for producing digital signatures.

   BER: Basic Encoding Rules (BER) as defined in [X.690].

   CLIENT: Any software that is making use of a cryptographic key.  This
   includes a signer, verifier, encrypter, decrypter.

   COMPONENT ALGORITHM: A single basic algorithm which is contained
   within a composite algorithm.

   COMPOSITE ALGORITHM: An algorithm which is a sequence of two or more
   component algorithms, as defined in Section 3.

   DER: Distinguished Encoding Rules as defined in [X.690].

   LEGACY: For the purposes of this document, a legacy algorithm is any
   cryptographic algorithm currently is use which is not believe to be
   resistant to quantum cryptanalysis.

   PKI: Public Key Infrastructure, as defined in [RFC5280].

   POST-QUANTUM ALGORITHM: Any cryptographic algorithm which is believed
   to be resistant to classical and quantum cryptanalysis, such as the
   algorithms being considered for standardization by NIST.

   PUBLIC / PRIVATE KEY: The public and private portion of an asymmetric
   cryptographic key, making no assumptions about which algorithm.

   SIGNATURE: A digital cryptographic signature, making no assumptions
   about which algorithm.








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   STRIPPING ATTACK: An attack in which the attacker is able to
   downgrade the cryptographic object to an attacker-chosen subset of
   original set of component algorithms in such a way that it is not
   detectable by the receiver.  For example, substituting a composite
   public key or signature for a version with fewer components.

3.  Composite Signature Structures

   In order for signatures to be composed of multiple algorithms, we
   define encodings consisting of a sequence of signature primitives
   (aka "component algorithms") such that these structures can be used
   as a drop-in replacement for existing signature fields such as those
   found in PKCS#10 [RFC2986], CMP [RFC4210], X.509 [RFC5280], CMS
   [RFC5652].

3.1.  Composite Keys

   A composite signature MAY be associated with a composite public key
   as defined in [draft-ounsworth-pq-composite-keys-00] (NOTE: need
   kramdown formatting help with this ref), but MAY also be associated
   with multiple public keys from different sources, for example
   multiple X.509 certificates, or multiple cryptographic modules.  In
   the latter case, composite signatures MAY be used as the mechanism
   for carrying multiple signatures in a non-composite authentication
   mechanism such as those described in [draft-becker-guthrie-
   noncomposite-hybrid-auth-00] (NOTE: need kramdown formatting help
   with this ref).

3.1.1.  Key Usage Bits

   For protocols such as X.509 [RFC5280] that specify key usage along
   with the public key, then the composite public key associated with a
   composite signature MUST have a signing-type key usage.

   If the keyUsage extension is present in a Certification Authority
   (CA) certificate that indicates id-composite-key, then any
   combination of the following values MAY be present:

   digitalSignature;
   nonRepudiation;
   keyCertSign; and
   cRLSign.

   If the keyUsage extension is present in an End Entity (EE)
   certificate that indicates id-composite-key, then any combination of
   the following values MAY be present:





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   digitalSignature; and
   nonRepudiation;

3.2.  sa-CompositeSignature

   The ASN.1 algorithm object for a composite signature is:

   sa-CompositeSignature SIGNATURE-ALGORITHM ::= {
       IDENTIFIER identifier
       VALUE CompositeSignatureValue
       PARAMS ANY DEFINED BY ALGORITHM
       PUBLIC-KEYS { pk-Composite }
       SMIME-CAPS { IDENTIFIED BY id-alg-composite } }
   }

   The identifier specifies the type of composite signature and the
   component algorithms.  This document defines a generic composite
   algorithm, identified by id-alg-composite, in Section 4.1, and allows
   for other standards that will define explicit algorithms that specify
   which component algorithms are to be contained within them.

3.3.  CompositeSignatureValue

   The output of the composite signature algorithm is the DER encoding
   of the following structure:

   CompositeSignatureValue ::= SEQUENCE SIZE (2..MAX) OF BIT STRING

   Where each BIT STRING within the SEQUENCE is a signature value
   produced by one of the component keys.  It MUST contain one signature
   value produced by each component algorithm, and in the same order as
   in the associated CompositeParams object.

   A CompositeSignatureValue MUST contain the same number of component
   signatures as the corresponding public and private keys, and the
   order of component signature values MUST correspond to the component
   public keys.

   The choice of SEQUENCE OF BIT STRING, rather than for example a
   single BIT STRING containing the concatenated signature values, is to
   gracefully handle variable-length signature values by taking
   advantage of ASN.1's built-in length fields.

3.4.  Encoding Rules

   Many protocol specifications will require that composite signature
   data structures be represented by an octet string or bit string.




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   When an octet string is required, the DER encoding of the composite
   data structure SHALL be used directly.

   EDNOTE: will this definition include an ASN.1 tag and length byte
   inside the OCTET STRING object?  If so, that's probably an extra
   unnecessary layer.

   When a bit string is required, the octets of the DER encoded
   composite data structure SHALL be used as the bits of the bit string,
   with the most significant bit of the first octet becoming the first
   bit, and so on, ending with the least significant bit of the last
   octet becoming the last bit of the bit string.

   In the interests of simplicity and avoiding compatibility issues,
   implementations that parse these structures MAY accept both BER and
   DER.

4.  Algorithm Identifiers

   This section defines the algorithm identifier for generic composite,
   as well as a framework for defining explicit combinations.  This
   section is not intended to be exhaustive and other authors may define
   others so long as they are compatible with the structures and
   processes defined in this and companion public and private key
   documents.

   Some use-cases desire the flexibility for clients to use any
   combination of supported algorithms, while others desire the rigidity
   of explicitly-specified combinations of algorithms.

4.1.  id-alg-composite (Generic Composite Signatures)

   The id-alg-composite object identifier is used for identifying a
   generic composite signature.  This algorithm allows arbitrary
   combinations of signature algorithms to be used in the
   CompositeSignatureValue and CompositeParams structures without
   needing the combination to be pre-registered or pre-agreed.  This
   identifier MUST be used in sa-CompositeSignature.identifier.

   id-alg-composite OBJECT IDENTIFIER ::= {
       iso(1)  identified-organization(3) dod(6) internet(1) private(4)
       enterprise(1) OpenCA(18227) Algorithms(2) id-alg-composite(1) }

   EDNOTE: this is a temporary OID for the purposes of prototyping.  We
   are requesting IANA to assign a permanent OID, see Section 7.

   The following algorithm parameters MUST be included:




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   CompositeParams ::= SEQUENCE SIZE (2..MAX) OF AlgorithmIdentifier

   The signature's CompositeParams sequence MUST contain the same
   component algorithms listed in the same order as in the associated
   CompositePublicKey.

   The motivation for this variant is primarily for prototyping work
   prior to the standardization of algorithm identifiers for explicit
   combinations of algorithms.  However, the authors envision that this
   variant will remain relevant beyond full standardization for example
   in environments requiring very high levels of crypto agility, for
   example where clients support a large number of algorithms or where a
   large number of keys will be used at a time and it is therefore
   prohibitive to define algorithm identifiers for every combination of
   pairs, triples, quadruples, etc of algorithms.

4.2.  Explicit Composite Signatures

   This variant provides a rigid way of specifying supported
   combinations of algorithms.

   The motivation for this variant is to make it easier to reference and
   enforce specific combinations of algorithms.  The authors envision
   this being useful for client-server negotiated protocols, protocol
   designers who wish to place constraints on allowable algorithm
   combinations in the protocol specification, as well as audited
   environments that wish to prove that only certain combinations will
   be supported by clients.

   Explicit algorithms must define a new signature algorithm which
   consists of:

   *  A new algorithm identifier OID for the explicit algorithm.

   *  The algorithm identifier OID and PUBLIC-KEY type of each component
      algorithm.

   *  Signature parameters either declared ABSENT, or defined with a
      type and encoding.

   See Appendix B for guidance on creating and registering OIDs for
   specific explicit combinations.

   For explicit algorithms, it is not necessary to carry a
   CompositeParams with the list of component algorithms in the
   signature algorithm parameters because clients can infer the expected
   component algorithms from the algorithm identifier.  The PARAMS is
   left optional because some types of component algorithms will require



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   parameters to be carried, such as RSASSA-PSS-params as defined in
   [RFC8017].  Section 3.2 defines PARAMS ANY DEFINED BY ALGORITHM so
   that explicit algorithms may define params as ABSENT, use
   CompositeParams defined in Section 4.1 or use any other encoding that
   is appropriate.

   In this variant, the signature is encoded as defined in Section 3.2,
   however the sa-CompositeSignature.identifier SHALL be an OID which is
   registered to represent a specific combination of component signature
   algorithms.  See Appendix C for examples.

5.  Composite Signature Processes

   This section specifies the processes for generating and verifying
   composite signatures.

   This process addresses algorithm strength uncertainty by providing
   the verifier with parallel signatures from all the component
   signature algorithms; thus forging the composite signature would
   require forging all of the component signatures.

5.1.  Composite Signature Generation Process

   Generation of a composite signature involves applying each component
   algorithm's signature process to the input message according to its
   specification, and then placing each component signature value into
   the CompositeSignatureValue structure defined in Section 3.2.

   The following process is used to generate composite signature values.






















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Input:
     K1, K2, .., Kn     Signing private keys. See note below on
                        composite inputs.

     A1, A2, ... An     Component signature algorithms. See note below on
                        composite inputs.

     M                  Message to be signed, an octet string

Output:
     S                  The signatures, a CompositeSignatureValue

Signature Generation Process:
   1. Generate the n component signatures independently,
      according to their algorithm specifications.

        for i := 1 to n
            Si := Sign( Ki, Ai, M )

   2. Encode each component signature S1, S2, .., Sn into a BIT STRING
      according to its algorithm specification.

        S ::= Sequence { S1, S2, .., Sn }

   3. Output S

   Note on composite inputs: the method of providing the list of
   component keys and algorithms is flexible and beyond the scope of
   this pseudo-code, for example they may be carried in
   CompositePrivateKey and CompositeParams structures.  It is also
   possible to generate a composite signature that combines signatures
   from distinct keys stored in separate software or hardware keystores.
   Variations in the process to accommodate particular private key
   storage mechanisms are considered to be conformant to this document
   so long as it produces the same output as the process sketched above.

   Since recursive composite public keys are disallowed in ~~ Reference
   draft-ounsworth-pq-composite-pubkeys sec-composite-pub-keys ~~, no
   component signature may itself be a composite; ie the signature
   generation process MUST fail if one of the private keys K1, K2, ..,
   Kn is a composite with the OID id-alg-composite.

   A composite signature MUST produce, and include in the output, a
   signature value for every component key in and include in the output,
   a signature value for every component key in the corresponding
   CompositePublicKey, and they MUST be in the same order; ie in the
   output, S1 MUST correspond to K1, S2 to K2, etc.  The authors
   recognize that there may be valid use cases for "subset signature



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   generation"; see Section 8.2.1 for further discussion of security
   implications, and Section 6.1 for further discussion of backwards
   compatibility implications.

   For security when using a generic composite signature algorithm as
   defined in Section 4.1, the list of component signature algorithms
   A1, A2, .., An, which may be carried in a CompositeParams object,
   SHOULD be included in the signed message M to prevent an attacker
   from substituting a weaker algorithm which is compatible with the
   same public key.  This attack is not unique or new to the composite
   format.

5.2.  Composite Signature Verification Process

   Verification of a composite signature involves applying each
   component algorithm's verification process according to its
   specification.

   In the absence of an application profile specifying otherwise,
   compliant applications MUST output "Valid signature" (true) if and
   only if all component signatures were successfully validated, and
   "Invalid signature" (false) otherwise.

   The following process is used to perform this verification.



























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  Input:
       P1, P2, .., Pn     Public verification keys. See note below on
                          composite inputs.

       M                  Message whose signature is to be verified,
                          an octet string

       S1, S2, .., Sn    Component signature values to be verified.
                         See note below on composite inputs.

       A1, A2, ... An     Component signature algorithms. See note
                          below on composite inputs.

  Output:
      Validity (bool)    "Valid signature" (true) if the composite
                          signature is valid, "Invalid signature"
                          (false) otherwise.

  Signature Verification Procedure::
     1. Check keys, signatures, and algorithms lists for consistency.

        If Error during Desequencing, or the three sequences have
        different numbers of elements, or any of the public keys
        P1, P2, .., Pn or algorithm identifiers A1, A2, .., An are
        composite with the OID id-alg-composite or an explicit composite
        OID then output "Invalid signature" and stop.

     2. Check each component signature individually, according to its
         algorithm specification.
         If any fail, then the entire signature validation fails.

       for i := 1 to n
            if not verify( Pi, M, Si, Ai ), then
              output "Invalid signature"

        if all succeeded, then
          output "Valid signature"

   Note on composite inputs: the method of providing the list of
   component keys, algorithms and signatures is flexible and beyond the
   scope of this pseudo-code, for example they may be carried in
   CompositePublicKey, CompositeParams, and compositesignaturevalue
   structures.  It is also possible to verify a composite signature
   where the component public verification keys belong, for example, to
   separate X.509 certificates or cryptographic modules.  Variations in
   the process to accommodate particular public verification key storage
   mechanisms are considered to be conformant to this document so long
   as it produces the same output as the process sketched above.



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   Since recursive composite public keys are disallowed in ~~ Reference
   draft-ounsworth-pq-composite-keys sec-composite-pub-keys ~~, no
   component signature may be composite; ie the signature verification
   procedure MUST fail if any of the public keys P1, P2, .., Pn or
   algorithm identifiers A1, A2, .., An are composite with the OID id-
   alg-composite.

   Some verification clients may include a policy mechanism for
   specifying acceptable subsets of algorithms.  In these cases,
   implementer MAY, in the interest of performance of compatibility,
   modify the above process to skip one or more signature validations as
   per their local client policy.  See Section 8.2 for a discussion of
   associated risks.

   In the absence of such a policy mechanism that can be easily updated
   to reflect new cryptanalytic breakthroughs, clients MUST perform
   signature verifications in the AND mode defined here.  See
   Section 8.2.1 for further discussion of security implications of
   subset signature verifications, and Section 6.1 for further
   discussion of backwards compatibility implications.

6.  Implementation Considerations

   This section addresses practical issues of how this draft affects
   other protocols and standards.

   ~~~ BEGIN EDNOTE 10~~~

   EDNOTE 10: Possible topics to address:

   *  The size of these certs and cert chains.

   *  In particular, implications for (large) composite keys /
      signatures / certs on the handshake stages of TLS and IKEv2.

   *  If a cert in the chain is a composite cert then does the whole
      chain need to be of composite Certs?

   *  We could also explain that the root CA cert does not have to be of
      the same algorithms.  The root cert SHOULD NOT be transferred in
      the authentication exchange to save transport overhead and thus it
      can be different than the intermediate and leaf certs.

   *  We could talk about overhead (size and processing).

   *  We could also discuss backwards compatibility.

   *  We could include a subsection about implementation considerations.



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   ~~~ END EDNOTE 10~~~

6.1.  Backwards Compatibility

   As noted in the introduction, the post-quantum cryptographic
   migration will face challenges in both ensuring cryptographic
   strength against adversaries of unknown capabilities, as well as
   providing ease of migration.  The composite mechanisms defined in
   this document primarily address cryptographic strength, however this
   section contains notes on how backwards compatibility may be
   obtained.

   The term "ease of migration" is used here to mean that existing
   systems can be gracefully transitioned to the new technology without
   requiring large service disruptions or expensive upgrades.  The term
   "backwards compatibility" is used here to mean something more
   specific; that existing systems as they are deployed today can
   interoperate with the upgraded systems of the future.

   These migration and interoperability concerns need to be thought
   about in the context of various types of protocols that make use of
   X.509 and PKIX with relation to digital signature objects, from
   online negotiated protocols such as TLS 1.3 [RFC8446] and IKEv2
   [RFC7296], to non-negotiated asynchronous protocols such as S/MIME
   signed email [RFC8551], document signing such as in the context of
   the European eIDAS regulations [eIDAS2014], and publicly trusted code
   signing [codeSigningBRsv2.8], as well as myriad other standardized
   and proprietary protocols and applications that leverage CMS
   [RFC5652] signed structures.

6.1.1.  OR modes

   Section 5.1 and Section 5.2 make reference to subset signature
   generation and verification modes to achieve an OR relation between
   component signatures, where senders and / or receivers are permitted
   to ignore some component keys.  Some envisioned uses of this include
   environments where the client encounters a component signature
   algorithm for which it does not posses a compatible implementation
   but wishes to proceed with the signature verification using the
   subset of component signatures for which it does have compatible
   implementations.  Such a mechanism could be designed to provide ease
   of migration by allowing for composite keys to be distributed and
   used before all clients in the environment are fully upgraded, but it
   does not allow for full backwards compatibility since clients would
   at least need to be upgraded from their current state to be able to
   parse the composite structures.





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6.1.2.  Parallel PKIs

   We present the term "Parallel PKI" to refer to the setup where a PKI
   end entity possesses two or more distinct public keys or certificates
   for the same identity (name), but containing keys for different
   cryptographic algorithms.  One could imagine a set of parallel PKIs
   where an existing PKI using legacy algorithms (RSA, ECC) is left
   operational during the post-quantum migration but is shadowed by one
   or more parallel PKIs using pure post quantum algorithms or composite
   algorithms (legacy and post-quantum).

   Equipped with a set of parallel public keys in this way, a client
   would have the flexibility to choose which public key(s) or
   certificate(s) to use in a given signature operation.

   For negotiated protocols, the client could choose which public key(s)
   or certificate(s) to use based on the negotiated algorithms, or could
   combine two of the public keys for example in a non-composite hybrid
   method such as [draft-becker-guthrie-noncomposite-hybrid-auth-00]
   (NOTE: need kramdown formatting help with this ref) or [draft-
   guthrie-ipsecme-ikev2-hybrid-auth-00].  Note that it is possible to
   use the signature algorithms defined in Section 4 as a way to carry
   the multiple signature values generated by one of the non-composite
   public mechanism in protocols where it is easier to support the
   composite signature algorithms than to implement such a mechanism in
   the protocol itself.  There is also nothing precluding a composite
   public key from being one of the components used within a non-
   composite authentication operation; this may lead to greater
   convenience in setting up parallel PKI hierarchies that need to
   service a range of clients implementing different styles of post-
   quantum migration strategies.

   For non-negotiated protocols, the details for obtaining backwards
   compatibility will vary by protocol, but for example in CMS
   [RFC5652], the inclusion of multiple SignerInfo objects is often
   already treated as an OR relationship, so including one for each of
   the signer's parallel PKI public keys would, in many cases, have the
   desired effect of allowing the receiver to choose one they are
   compatible with and ignore the others, thus achieving full backwards
   compatibility.

7.  IANA Considerations

   The ASN.1 module OID is TBD.  The id-alg-composite OID is to be
   assigned by IANA.  The authors suggest that IANA assign an OID on the
   id-pkix arc:





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   id-alg-composite OBJECT IDENTIFIER ::= {
       iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) algorithms(6) composite(??) }

8.  Security Considerations

8.1.  Policy for Deprecated and Acceptable Algorithms

   Traditionally, a public key, certificate, or signature contains a
   single cryptographic algorithm.  If and when an algorithm becomes
   deprecated (for example, RSA-512, or SHA1), it is obvious that
   clients performing signature verifications should be updated to fail
   to validate signatures using these algorithms.

   In the composite model this is less obvious since a single public
   key, certificate, or signature may contain a mixture of deprecated
   and non-deprecated algorithms.  Moreover, implementers may decide
   that certain cryptographic algorithms have complementary security
   properties and are acceptable in combination even though neither
   algorithm is acceptable by itself.

   Specifying a modified verification algorithm to handle these
   situations is beyond the scope of this draft, but could be desirable
   as the subject of an application profile document, or to be up to the
   discretion of implementers.

   2. Check policy to see whether A1, A2, ..., An constitutes a valid
      combination of algorithms.

      if not checkPolicy(A1, A2, ..., An), then
        output "Invalid signature"

8.2.  OR Modes

8.2.1.  Subset Signature Generation

   This document defines a composite signature generation process in
   Section 5.1 where the signer MUST produce a signature value with each
   of their component private keys, this providing full protection of
   the content under all available component algorithms.

   The authors recognize that there may be cases where a client may wish
   to generate a composite signature that only uses a subset of the
   available component algorithms, for example to save bandwidth, or
   because a client has been issued a key for which it does not (yet)
   have implementations of all component algorithms.  This could be
   easily encoded by placing a NULL value into the corresponding field
   of the CompositeSignatureValue.  However, this mode was intentionally



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   omitted from this specification as it trivially allows for stripping
   attacks where an attacker replaces a valid component signature value
   with NULL, thus reducing the security of the composite signature to
   the weakest of the available component algorithms.

   Implementer who wish to perform subset signature generations are
   advised to couple it with an out-of-band policy mechanism that limits
   the potential for stripping attacks.  Note that, in an effort to keep
   compliant implementations simple and secure, implementations claiming
   to be compliant with this draft MUST NOT generate subset signatures
   in this way, and MUST reject during verification any subset
   signatures that they encounter.

8.2.2.  Subset Signature Verification

   This document defines a composite signature verification process in
   Section 5.2 where the verifier verifies all component signatures and
   fails if any component fails.  The authors recognize that there will
   be scenarios where the verifier considers a single component
   algorithm -- or subset of component algorithms -- to provide
   sufficient security, and therefore for performance reasons wishes to
   skip the verification of one or more component signatures.

   -- harmonize this with Serge's blurb --

   Implementers who wish to perform subset signature verifications are
   advised to couple it with an out-of-band policy mechanism that can
   control the list of acceptable algorithm combinations, and keep this
   list up to date as new cryptanalytic advances are made.

   Risks:

   *  Failing to update client verification policy in response to
      advances in cryptanalysis

   *  Verifications of a subset of signatures leads to ambiguity in the
      security strength of the signature verification; ie if a message
      carries two signatures, one at 128 bits and the other at 112 bits
      of security and clients are verifying in an OR mode with flexible
      policy, then it becomes difficult to audit the security strength
      used at runtime.

   *  Moreover, verifying multiple algorithms provides security even in
      the event that one of the algorithms has already been broken, but
      knowledge of the break has not been made public yet.

9.  References




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9.1.  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>.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <https://www.rfc-editor.org/info/rfc2986>.

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210,
              DOI 10.17487/RFC4210, September 2005,
              <https://www.rfc-editor.org/info/rfc4210>.

   [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>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

   [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>.

   [RFC8411]  Schaad, J. and R. Andrews, "IANA Registration for the
              Cryptographic Algorithm Object Identifier Range",
              RFC 8411, DOI 10.17487/RFC8411, August 2018,
              <https://www.rfc-editor.org/info/rfc8411>.

   [X.690]    ITU-T, "Information technology - ASN.1 encoding Rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ISO/IEC 8825-1:2015, November 2015.

9.2.  Informative References








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   [Bindel2017]
              Bindel, N., Herath, U., McKague, M., and D. Stebila,
              "Transitioning to a quantum-resistant public key
              infrastructure", 2017, <https://link.springer.com/
              chapter/10.1007/978-3-319-59879-6_22>.

   [I-D.becker-guthrie-noncomposite-hybrid-auth]
              Becker, A., Guthrie, R., and M. J. Jenkins, "Non-Composite
              Hybrid Authentication in PKIX and Applications to Internet
              Protocols", Work in Progress, Internet-Draft, draft-
              becker-guthrie-noncomposite-hybrid-auth-00, 22 March 2022,
              <https://www.ietf.org/archive/id/draft-becker-guthrie-
              noncomposite-hybrid-auth-00.txt>.

   [I-D.ounsworth-pq-composite-keys]
              Ounsworth, M. and M. Pala, "Composite Public and Private
              Keys For Use In Internet PKI", Work in Progress, Internet-
              Draft, draft-ounsworth-pq-composite-keys-00, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-ounsworth-pq-
              composite-keys-00.txt>.

   [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>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

Appendix A.  Work in Progress

A.1.  Combiner modes (KofN)

   For content commitment use-cases, such as legally-binding non-
   repudiation, the signer (whether it be a CA or an end entity) needs
   to be able to specify how its signature is to be interpreted and
   verified.

   For now we have removed combiner modes (AND, OR, KofN) from this
   draft, but we are still discussing how to incorporate this for the
   cases where it is needed (maybe a X.509 v3 extension, or a signature
   algorithm param).






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Appendix B.  Creating explicit combinations

   The following ASN.1 Information Objects may be useful in defining and
   parsing explicit pairs of signature algorithms.

   ... TODO ... copy & adapt from the keys draft.

Appendix C.  Examples

C.1.  Generic Composite Signature Examples

   TODO

C.2.  Explicit Composite Signature Examples

   TODO

Appendix D.  ASN.1 Module

   <CODE STARTS>

   Composite-Signatures-2019
     { TBD }

   DEFINITIONS IMPLICIT TAGS ::= BEGIN

   EXPORTS ALL;

   IMPORTS
     PUBLIC-KEY, SIGNATURE-ALGORITHM
       FROM AlgorithmInformation-2009  -- RFC 5912 [X509ASN1]
         { iso(1) identified-organization(3) dod(6) internet(1)
           security(5) mechanisms(5) pkix(7) id-mod(0)
           id-mod-algorithmInformation-02(58) }

     SubjectPublicKeyInfo
       FROM PKIX1Explicit-2009
         { iso(1) identified-organization(3) dod(6) internet(1)
           security(5) mechanisms(5) pkix(7) id-mod(0)
           id-mod-pkix1-explicit-02(51) }

     OneAsymmetricKey
       FROM AsymmetricKeyPackageModuleV1
         { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
           pkcs-9(9) smime(16) modules(0)
           id-mod-asymmetricKeyPkgV1(50) } ;

   --



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   -- Object Identifiers
   --

   id-alg-composite OBJECT IDENTIFIER ::= { TBD }

   --
   -- Public Key
   --

   pk-Composite PUBLIC-KEY ::= {
       IDENTIFIER id-alg-composite
       KEY CompositePublicKey
       PARAMS ARE absent
       PRIVATE-KEY CompositePrivateKey
   }

   CompositePublicKey ::= SEQUENCE SIZE (2..MAX) OF SubjectPublicKeyInfo

   CompositePrivateKey ::= SEQUENCE SIZE (2..MAX) OF OneAsymmetricKey

   --
   -- Signature Algorithm
   --

   sa-CompositeSignature SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-alg-composite
       VALUE CompositeSignatureValue
       PARAMS TYPE CompositeParams ARE required
       PUBLIC-KEYS { pk-Composite }
       SMIME-CAPS { IDENTIFIED BY id-alg-composite } }

   CompositeParams ::= SEQUENCE SIZE (2..MAX) OF AlgorithmIdentifier

   CompositeSignatureValue ::= SEQUENCE SIZE (2..MAX) OF BIT STRING

   END

   <CODE ENDS>

Appendix E.  Intellectual Property Considerations

   The following IPR Disclosure relates to this draft:

   https://datatracker.ietf.org/ipr/3588/







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Appendix F.  Contributors and Acknowledgements

   This document incorporates contributions and comments from a large
   group of experts.  The Editors would especially like to acknowledge
   the expertise and tireless dedication of the following people, who
   attended many long meetings and generated millions of bytes of
   electronic mail and VOIP traffic over the past year in pursuit of
   this document:

   John Gray (Entrust), Serge Mister (Entrust), Scott Fluhrer (Cisco
   Systems), Panos Kampanakis (Cisco Systems), Daniel Van Geest (ISARA),
   Tim Hollebeek (Digicert), and Francois Rousseau.

   We are grateful to all, including any contributors who may have been
   inadvertently omitted from this list.

   This document borrows text from similar documents, including those
   referenced below.  Thanks go to the authors of those documents.
   "Copying always makes things easier and less error prone" -
   [RFC8411].

F.1.  Making contributions

   Additional contributions to this draft are welcome.  Please see the
   working copy of this draft at, as well as open issues at:

   https://github.com/EntrustCorporation/draft-ounsworth-composite-sigs

Authors' Addresses

   Mike Ounsworth
   Entrust Limited
   2500 Solandt Road -- Suite 100
   Ottawa, Ontario  K2K 3G5
   Canada
   Email: mike.ounsworth@entrust.com


   Massimiliano Pala
   CableLabs
   Email: director@openca.org










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