Terminology for Post-Quantum Traditional Hybrid Schemes
draft-driscoll-pqt-hybrid-terminology-01

TBD                                                          F. Driscoll
Internet-Draft                         UK National Cyber Security Centre
Intended status: Informational                           20 October 2022
Expires: 23 April 2023

        Terminology for Post-Quantum Traditional Hybrid Schemes
                draft-driscoll-pqt-hybrid-terminology-01

Abstract

   One aspect of the transition to post-quantum algorithms in
   cryptographic protocols is the development of hybrid schemes that
   incorporate both post-quantum and traditional asymmetric algorithms.
   This document defines terminology for such schemes.  It is intended
   to ensure consistency and clarity across different protocols,
   standards, and organisations.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-driscoll-pqt-hybrid-
   terminology/.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Primitives  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Cryptographic Elements  . . . . . . . . . . . . . . . . . . .   5
   4.  Protocols . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Functionality . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Certificates  . . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The mathematical problems of integer factorisation and discrete
   logarithms over finite fields or elliptic curves underpin most of the
   asymmetric algorithms used for key establishment and digital
   signatures on the internet.  These problems, and hence the algorithms
   based on them, will be vulnerable to attacks using Shor's Algorithm
   on a sufficiently large general-purpose quantum computer, known as a
   Cryptographically Relevant Quantum Computer (CRQC).  It is difficult
   to predict when, or if, such a device will exist.  However, it is
   necessary to defend against this possibility.  Data encrypted today
   with an algorithm vulnerable to a quantum computer could be stored
   for decryption by a future attacker with a CRQC.  Signing algorithms
   that are expected to be in use for many years are also at risk if a
   CRQC is developed during the operational lifetime of the algorithm.

   Preparing for the potential development of a CRQC requires modifying
   standardised protocols to use asymmetric algorithms that are believed
   to be secure against quantum computers as well as today's classical
   computers.  These algorithms are called post-quantum, while
   algorithms based on integer factorisation, finite-field discrete
   logarithms or elliptic-curve discrete logarithms are called
   traditional algorithms.

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   During the transition from traditional to post-quantum algorithms
   there may be a desire or a requirement for protocols that use both
   types of algorithm.  Most post-quantum algorithms are less well
   studied than traditional asymmetric algorithms, so a designer may
   choose to combine a post-quantum algorithm with a traditional
   algorithm to add protection against an attacker with a CRQC to the
   security properties provided by the traditional algorithm.  A
   designer may also choose to implement a post-quantum algorithm
   alongside a traditional algorithm for ease of migration from an
   ecosystem where only traditional algorithms are implemented and used,
   to one which uses post-quantum algorithms.  Work on solutions that
   could use both types of algorithm includes
   [I-D.ietf-ipsecme-ikev2-multiple-ke], [I-D.ietf-tls-hybrid-design],
   [I-D.ounsworth-pq-composite-sigs],
   [I-D.becker-guthrie-noncomposite-hybrid-auth].  Schemes that combine
   post-quantum and traditional algorithms for key establishment or
   digital signatures are often called hybrids.  For example, NIST
   define hybrid key establishment to be a "scheme that is a combination
   of two or more components that are themselves cryptographic key-
   establishment schemes"[NIST_PQC_FAQ] and ETSI define hybrid key
   exchanges to be "constructions that combine a traditional key
   exchange...with a post-quantum key exchange...into a single key
   exchange"[ETSI_TS103774].  The word hybrid is also used in
   cryptography to describe encryption schemes that combine asymmetric
   and symmetric algorithms [RFC9180], so using it in the post-quantum
   context overloads it and risks misunderstandings.  However, this
   terminology is well-established amongst the post-quantum cryptography
   community so an attempt to move away from its use could lead to
   multiple definitions for the same concept, resulting in confusion and
   lack of clarity.

   This document provides language for constructions that combine
   traditional and post-quantum algorithms.  Specific solutions for
   enabling use of multiple asymmetric algorithms in cryptographic
   schemes may in fact be more general than this, allowing the use of
   solely traditional, or solely post-quantum algorithms.  However,
   where relevant, we focus on combinations of post-quantum and
   traditional algorithms as these are the motivation for the wider work
   in the IETF.  It is intended as a terminology guide for other
   documents to add clarity and consistency across different protocols,
   standards, and organisations.  Additionally, it aims to reduce
   misunderstanding about use of the word "hybrid" as well as defining a
   shared language for different types of post-quantum/traditional
   hybrid constructions.

   In this document, a "cryptographic algorithm" is defined, as in
   [NIST_SP_800-152], to be a "well-defined computational procedure that
   takes variable inputs, often including a cryptographic key, and

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   produces an output".  Examples include RSA, ECDH, CRYSTALS-Kyber and
   CRYSTALS-Dilithium.  The expression "cryptographic scheme" is used to
   mean a construction that uses an algorithm or a group of algorithms
   to achieve a particular cryptographic outcome, e.g. key agreement.  A
   cryptographic scheme may be made up of a number of functions.  For
   example, a Key Encapsulation Mechanism (KEM) is a cryptographic
   scheme consisting of three functions: Key Generation, Encapsulation
   and Decapsulation.  A cryptographic protocol incorporates one or more
   cryptographic schemes.  For example, TLS is a cryptographic protocol
   which includes schemes for key agreement, record layer encryption,
   and server authentication.

2.  Primitives

   This section introduces terminology related to cryptographic
   algorithms, as well as to hybrid constructions for cryptographic
   schemes.

   *Traditional Algorithm*:  An asymmetric cryptographic algorithm based
      on integer factorisation, finite field discrete logarithms or
      elliptic curve discrete logarithms.

   *Post-Quantum Algorithm*:  An asymmetric cryptographic algorithm that
      is believed to be secure against quantum computers as well as
      classical computers.

   *Component Algorithm*:  Each cryptographic algorithm that forms part
      of a cryptographic scheme.

   *Single-Algorithm Scheme*:  A cryptographic scheme with one component
      algorithm.

      A single-algorithm scheme could use either a traditional algorithm
      or a post-quantum algorithm.

   *Multi-Algorithm Scheme*:  A cryptographic scheme with more than one
      component algorithm.

      In a multi-algorithm scheme all component algorithms are of the
      same type, e.g. all are signature algorithms or all are PKE
      algorithms.

   *Post-Quantum/Traditional (PQ/T) Hybrid Scheme*:  A cryptographic
      scheme made up of two or more component algorithms where at least
      one is a post-quantum algorithm and at least one is a traditional
      algorithm.

   *PQ/T Hybrid Key Encapsulation Mechanism*:  A Key Encapsulation

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      Mechanism (KEM) made up of two or more component KEM algorithms
      where at least one is a post-quantum algorithm and at least one is
      a traditional algorithm.

   *PQ/T Hybrid Public Key Encryption*:  A Public Key Encryption (PKE)
      scheme made up of two or more component PKE algorithms where at
      least one is a post-quantum algorithm and at least one is a
      traditional algorithm.

   *PQ/T Hybrid Digital Signature*:  A digital signature scheme made up
      of two or more component digital signature algorithms where at
      least one is a post-quantum algorithm and at least one is a
      traditional algorithm.

      PQ/T hybrid KEMs, PQ/T hybrid PKE, and PQ/T hybrid digital
      signatures are all examples of PQ/T hybrid schemes.

   *PQ/T Hybrid Combiner*:  A method that takes two or more component
      algorithms and combines them to form a PQ/T hybrid scheme.

   *PQ/PQ Hybrid Scheme*:  A cryptographic scheme made up of two or more
      component algorithms where all components are post-quantum
      algorithms.

      The definitions for types of PQ/T hybrid schemes can adapted to
      define types of PQ/PQ hybrid schemes in the natural way.

3.  Cryptographic Elements

   This section introduces terminology related to cryptographic elements
   and their inclusion in hybrid schemes.

   *Cryptographic Element*:  Any data type (private or public) that
      contains an input or output value for a cryptographic algorithm or
      for a function making up a cryptographic algorithm.

      Types of cryptographic elements include public keys, private keys,
      plaintexts, ciphertexts, shared secrets, and signature values.

   *Component Cryptographic Element*:  A cryptographic element of a
      component algorithm in a multi-algorithm scheme.

   *Composite Cryptographic Element*:  A cryptographic element that
      incorporates multiple component cryptographic elements of the same
      type in a multi-algorithm scheme.

      For example, a composite cryptographic public key is made up of
      two component public keys.

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   *Cryptographic Element Combiner*:  A method that takes two or more
      component cryptographic elements of the same type and combines
      them to form a composite cryptographic element.

      A cryptographic element combiner could be concatenation, such as
      where two component public keys are concatenated to form a
      composite public key as in [I-D.ietf-tls-hybrid-design], or
      something more involved such as the dualPRF defined in [BINDEL].

4.  Protocols

   This section introduces terminology related to the use of post-
   quantum and traditional algorithms together in protocols.

   *PQ/T Hybrid Protocol*:  A protocol that uses two or more component
      algorithms providing the same cryptographic functionality, where
      at least one is a post-quantum algorithm and at least one is a
      traditional algorithm.

      For example, a PQ/T hybrid protocol providing confidentiality
      could use a PQ/T hybrid KEM such as in
      [I-D.ietf-tls-hybrid-design], or it could combine the output of a
      post-quantum KEM and a traditional KEM at the protocol level, such
      as in [I-D.ietf-ipsecme-ikev2-multiple-ke].  Similarly, a PQ/T
      hybrid protocol providing authentication could use a PQ/T hybrid
      digital signature scheme, or it could include both post-quantum
      and traditional single-algorithm digital signature schemes.

   *Composite PQ/T Hybrid Protocol*:  A protocol that incorporates one
      or more PQ/T hybrid schemes in such a way that the protocol fields
      and message flow are the same as those in a version of the
      protocol that uses single-algorithm schemes.

      In a composite PQ/T hybrid protocol, changes are primarily made to
      the formats of the cryptographic elements, while the protocol
      fields and message flow remain largely unchanged.  In
      implementations most changes are likely to be made to the
      cryptographic libraries, with minimal changes to the protocol
      libraries.

   *Non-composite PQ/T Hybrid Protocol*:  A protocol that incorporates
      multiple single-algorithm schemes of the same type, where at least
      one uses a post-quantum algorithm and at least one uses a
      traditional algorithm, in such a way that the formats of the
      component cryptographic elements are the same as when they are
      used as part of single-algorithm schemes.

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      In a non-composite PQ/T hybrid protocol, changes are primarily
      made to the protocol fields, the message flow, or both, while
      changes to cryptographic elements are minimised.  In
      implementations, most changes are likely to be made to the
      protocol libraries, with minimal changes to the cryptographic
      libraries.

   NOTE: A PQ/T hybrid protocol could be neither entirely composite nor
   entirely non-composite.  For example, in a protocol that offers both
   confidentiality and authentication, the key establishment could be
   done in a composite manner while the authentication is done in a non-
   composite manner.

5.  Functionality

   This section describes properties that may be desired from or
   achieved by a PQ/T hybrid scheme or PQ/T hybrid protocol.

   *PQ/T Hybrid Confidentiality*:  The property that confidentiality is
      achieved by a PQ/T hybrid scheme or PQ/T hybrid protocol as long
      as at least one component encryption algorithm remains secure.

   *PQ/T Hybrid Authentication*:  The property that authentication is
      achieved by a PQ/T hybrid scheme or a PQ/T hybrid protocol as long
      as at least one component authentication algorithm remains secure.

   EDNOTE 1: It may be useful to distinguish between source
   authentication (i.e. authentication of the sender of a particular
   message) and identity authentication (i.e. authentication of the
   identity of the sender).

   The security properties of a PQ/T hybrid scheme or protocol depend on
   the security of its component algorithms, the choice of PQ/T hybrid
   combiner and the capability of an attacker.  Changes to the security
   of a component algorithm can impact the security properties of a PQ/T
   hybrid scheme providing hybrid confidentiality or hybrid
   authentication.  For example, if a post-quantum component algorithm
   is broken, the PQ/T hybrid scheme is likely to continue to achieve
   confidentiality against a classical attacker, but will be vulnerable
   to a quantum attacker.

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   Note that PQ/T hybrid protocols that offer both confidentiality and
   authentication do not necessarily offer both PQ/T hybrid
   confidentiality and PQ/T hybrid authentication.  For example,
   [I-D.ietf-tls-hybrid-design] provides PQ/T hybrid confidentiality but
   does not address authentication.  Therefore, if the design in
   [I-D.ietf-tls-hybrid-design] is used with X.509 certificates as
   defined in [RFC5280] only authentication with a single algorithm is
   achieved.

   *PQ/T Hybrid Interoperability*:  The property that a PQ/T hybrid
      scheme or PQ/T hybrid protocol can be completed successfully
      provided that both parties support at least one component
      algorithm.

      For example, a PQ/T hybrid digital signature might achieve hybrid
      interoperability if the signature can be verified by either
      verifying the traditional or the post-quantum component, such as
      in the OR modes described in [I-D.ounsworth-pq-composite-sigs].

      In the case of a PQ/T hybrid protocol which aims to achieve both
      authentication and confidentiality then at least one component
      algorithm for each type of scheme must be supported by both
      parties.

      It is not possible for a PQ/T hybrid scheme to achieve both PQ/T
      hybrid interoperability and PQ/T hybrid confidentiality.  For PQ/T
      hybrid interoperability the scheme needs to work with any one of
      the component algorithms, while to achieve PQ/T hybrid
      confidentiality all component algorithms need to be used.
      However, it is possible for a PQ/T hybrid protocol to achieve PQ/T
      hybrid interoperability and PQ/T hybrid confidentiality by
      building in downgrade protection at the protocol level.  For
      example in [I-D.ietf-tls-hybrid-design] the client uses the TLS
      supported groups extension to advertise support for a PQ/T hybrid
      scheme and the server can select this group if it supports the
      scheme.  This is protected using TLS's existing downgrade
      protection, so achieves PQ/T hybrid confidentiality, but the
      connection can still be made if either the client or server does
      not support the scheme, so PQ/T hybrid interoperability is
      achieved.

      The same is true for PQ/T hybrid interoperability and PQ/T hybrid
      authentication.  It is not possible to achieve both with a PQ/T
      hybrid scheme, but it is possible with a PQ/T hybrid protocol that
      has appropriate downgrade protection.

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   EDNOTE 2: Other properties may be desired from a PQ/T Hybrid scheme
   e.g. backwards compatibility, crypt agility.  Should these be defined
   here?

6.  Certificates

   This section introduces terminology related to the use of
   certificates in hybrid schemes.

   *PQ/T Hybrid Certificate*:  A digital certificate that contains
      public keys for two or more component algorithms where at least
      one is a traditional algorithm, and at least one is a post-quantum
      algorithm.

      A PQ/T hybrid certificate could be used to facilitate a PQ/T
      hybrid authentication protocol.  However, a PQ/T hybrid
      authentication protocol does not need to use a PQ/T hybrid
      certificate; separate certificates could be used for individual
      component algorithms.

      The component public keys in a PQ/T hybrid certificate could be
      included as a composite public key or as individual component
      public keys.

      The use of a PQ/T hybrid certificate does not necessarily achieve
      hybrid authentication of the identity of the sender; this is
      determined by properties of the chain of trust.  For example, an
      end-entity certificate that contains a composite public key as
      defined in [I-D.ounsworth-pq-composite-keys] but which is signed
      using a single-algorithm digital signature scheme could be used to
      provide hybrid authentication of the source of a message, but
      would not achieve hybrid authentication of the identity of the
      sender.

   TODO 1: Terminology for certificate chains and PKI.

   TODO 2: Terminology for algorithm specification.

7.  Security Considerations

   This document defines security-relevant terminology to be used in
   documents specifying PQ/T hybrid protocols and schemes.  However, the
   document itself does not have a security impact on internet
   protocols.  The security considerations for each PQ/T hybrid protocol
   are specific to that protocol and should be discussed in the relevant
   documents.

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8.  IANA Considerations

   This document has no IANA actions.

9.  Informative References

   [BINDEL]   Bindel, N., Brendel, J., Fischlin, M., Goncalves, B., and
              D. Stebila, "Hybrid Key Encapsulation Mechanisms and
              Authenticated Key Exchange", Post-Quantum Cryptography
              pp.206-226, DOI 10.1007/978-3-030-25510-7_12, July 2019,
              <https://doi.org/10.1007/978-3-030-25510-7_12>.

   [ETSI_TS103774]
              ETSI TS 103 744 V1.1.1, "CYBER; Quantum-safe Hybrid Key
              Exchanges", December 2020, <https://www.etsi.org/deliver/
              etsi_ts/103700_103799/103744/01.01.01_60/
              ts_103744v010101p.pdf>.

   [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.ietf-ipsecme-ikev2-multiple-ke]
              Tjhai, C., Tomlinson, M., Bartlett, G., Fluhrer, S., Van
              Geest, D., Garcia-Morchon, O., and V. Smyslov, "Multiple
              Key Exchanges in IKEv2", Work in Progress, Internet-Draft,
              draft-ietf-ipsecme-ikev2-multiple-ke-07, 6 October 2022,
              <https://www.ietf.org/archive/id/draft-ietf-ipsecme-ikev2-
              multiple-ke-07.txt>.

   [I-D.ietf-tls-hybrid-design]
              Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key
              exchange in TLS 1.3", Work in Progress, Internet-Draft,
              draft-ietf-tls-hybrid-design-05, 28 August 2022,
              <https://www.ietf.org/archive/id/draft-ietf-tls-hybrid-
              design-05.txt>.

   [I-D.ounsworth-pq-composite-keys]
              Ounsworth, M., Pala, M., and J. Klaußner, "Composite
              Public and Private Keys For Use In Internet PKI", Work in
              Progress, Internet-Draft, draft-ounsworth-pq-composite-
              keys-02, 8 June 2022, <https://www.ietf.org/archive/id/
              draft-ounsworth-pq-composite-keys-02.txt>.

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   [I-D.ounsworth-pq-composite-sigs]
              Ounsworth, M. and M. Pala, "Composite Signatures For Use
              In Internet PKI", Work in Progress, Internet-Draft, draft-
              ounsworth-pq-composite-sigs-07, 8 June 2022,
              <https://www.ietf.org/archive/id/draft-ounsworth-pq-
              composite-sigs-07.txt>.

   [NIST_PQC_FAQ]
              National Institute of Standards and Technology (NIST),
              "Post-Quantum Cryptography FAQs", 5 July 2022,
              <https://csrc.nist.gov/Projects/post-quantum-cryptography/
              faqs>.

   [NIST_SP_800-152]
              Barker, E. B., Smid, M., Branstad, D., and National
              Institute of Standards and Technology (NIST), "NIST SP
              800-152 A Profile for U. S. Federal Cryptographic Key
              Management Systems", October 2015,
              <https://doi.org/10.6028/NIST.SP.800-152>.

   [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>.

   [RFC9180]  Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
              Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
              February 2022, <https://www.rfc-editor.org/info/rfc9180>.

Acknowledgments

   TODO acknowledge

Author's Address

   Florence Driscoll
   UK National Cyber Security Centre
   Email: florence.d@ncsc.gov.uk

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