PQUIP F. Driscoll
Internet-Draft UK National Cyber Security Centre
Intended status: Informational 7 March 2023
Expires: 8 September 2023
Terminology for Post-Quantum Traditional Hybrid Schemes
draft-driscoll-pqt-hybrid-terminology-02
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 be used as a reference and, hopefully, 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
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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. Algorithm Specification . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Informative References . . . . . . . . . . . . . . . . . . . 11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
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 anticipate and prepare to defend against such a
development. Data encrypted today (2023) with an algorithm
vulnerable to a quantum computer could be stored for decryption by a
future attacker with a CRQC. Signing algorithms in products that are
expected to be in use for many years are also at risk if a CRQC is
developed during the operational lifetime of that product.
Preparing for the potential development of a CRQC requires modifying
established (standardised) protocols to use asymmetric algorithms
that are perceived 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
algorithm types. 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. They 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 that only uses post-quantum algorithms.
Examples of solutions that could use both types of algorithm include,
but are not limited to, [I-D.ietf-ipsecme-ikev2-multiple-ke],
[I-D.ietf-tls-hybrid-design], [I-D.ounsworth-pq-composite-sigs], and
[I-D.ietf-lamps-cert-binding-for-multi-auth]. Schemes that combine
post-quantum and traditional algorithms for key establishment or
digital signatures are often called hybrids. For example:
* NIST defines 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];
* ETSI defines 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 [RFC4949],
so using it in the post-quantum context overloads it and risks
misunderstandings. However, this terminology is well-established
amongst the post-quantum cryptography (PQC) community. Therefore, an
attempt to move away from its use for PQC 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 be more general than this, allowing the use of solely
traditional or solely post-quantum algorithms. However, where
relevant, we focus on post-quantum traditional combinations as these
are the motivation for the wider work in the IETF. This document is
intended as a reference terminology guide for other documents to add
clarity and consistency across different protocols, standards, and
organisations. Additionally, this document 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.
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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
produces an output". Examples include RSA, ECDH, CRYSTALS-Kyber and
CRYSTALS-Dilithium. The expression "cryptographic scheme" is used to
refer to a construction that uses a cryptographic algorithm or a
group of cryptographic 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 [RFC8446] is a cryptographic protocol that
includes schemes for key agreement, record layer encryption, and
server authentication.
2. Primitives
This section introduces terminology related to cryptographic
algorithms and 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 attacks using 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 Public
Key Encryption (PKE) algorithms. Component algorithms could be
all traditional, all post-quantum, or a mixture of the two.
*Post-Quantum Traditional (PQ/T) Hybrid Scheme*: A multi-algorithm
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scheme where at least one component algorithm is a post-quantum
algorithm and at least one is a traditional algorithm.
*PQ/T Hybrid Key Encapsulation Mechanism (KEM)*: A multi-algorithm
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 (PKE)*: A multi-algorithm 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 multi-algorithm 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 multi-algorithm scheme 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, which are multi-algorithm
schemes where all component algorithms are Post-Quantum
algorithms.
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.
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For example, in [I-D.ietf-tls-hybrid-design], the client's
keyshare contains two component public keys, one for a post-
quantum algorithm and one for a traditional algorithm.
*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.
*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 to
generate a single shared secret, 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
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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.
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.
It is possible for a PQ/T hybrid protocol to be designed that is
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 algorithm that aims to provide this
property 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 algorithm that aims to provide this
property 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
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hybrid scheme providing hybrid confidentiality or hybrid
authentication. For example, if the post-quantum component algorithm
of a PQ/T hybrid scheme is broken, the scheme will remain secure
against an attacker with a classical computer, but will be vulnerable
to an attacker with a CRQC.
PQ/T hybrid protocols that offer both confidentiality and
authentication do not necessarily offer both hybrid confidentiality
and hybrid authentication. For example, [I-D.ietf-tls-hybrid-design]
provides hybrid confidentiality but does not address hybrid
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 share support for 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 protocol that aims to achieve both authentication
and confidentiality, PQ/T hybrid interoperability requires that at
least one component authentication algorithm and at least one
component algorithm for confidentiality is 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 without
additional functionality at a protocol level. For PQ/T hybrid
interoperability a scheme needs to work whenever one component
algorithm is supported by both parties, while to achieve PQ/T hybrid
confidentiality all component algorithms need to be used. However,
both properties can be achieved in a PQ/T hybrid protocol by building
in downgrade protection external to the cryptographic schemes. 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 PQ/T
hybrid scheme, so PQ/T hybrid interoperability is achieved.
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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 alone, but it is possible with a PQ/T hybrid protocol
that has appropriate downgrade protection.
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.
*Post-Quantum Certificate*: A digital certificate that contains a
single public key for a post-quantum digital signature algorithm.
*Traditional Certificate*: A digital certificate that contains a
single public key for a traditional digital signature algorithm.
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X.509 certificates as defined in [RFC5280] could be either
traditional or post-quantum certificates depending on the
algorithm in the Subject Public Key Info. For example, a
certificate containing a Dilithium public key, as defined in
[I-D.ietf-lamps-dilithium-certificates], would be a post-quantum
certificate.
*Post-Quantum Certificate Chain*: A certificate chain where each
certificate includes a public key for a post-quantum algorithm and
is signed using a post-quantum digital signature scheme.
*Traditional Certificate Chain*: A certificate chain where all
certificates includes a public key for a traditional algorithm and
is signed using a traditional digital signature scheme.
*PQ/T Hybrid Certificate Chain*: A certificate chain where all
certificates are PQ/T hybrid certificates and each certificate is
signed with 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 chain is one way of achieving hybrid
authentication of the identity of a sender in a protocol, but is not
the only way. An alternative is to incorporate both a post-quantum
certificate chain and a traditional certificate chain in a protocol.
It would be possible to construct a certificate chain containing a
mixture of post-quantum certificates, traditional certificates and
PQ/T hybrid certificates. For example, a post-quantum end-entity
certificate could be signed by a traditional intermediate
certificate, which in turn could be signed by a traditional root.
The security properties of a certificate chain that mixes post-
quantum and traditional algorithms would need to be analysed on a
case-by-case basis.
EDNOTE 3: Do we want a definition of multi-cert authentication or
something similar?
7. Algorithm Specification
This section introduces terminology for specifying the component
algorithms used in PQ/T hybrid schemes or PQ/T hybrid protocols.
*PQ/T Hybrid Scheme Identifier*: A single code point that specifies
all component algorithms used in a PQ/T hybrid scheme.
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8. 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
specification documents.
9. IANA Considerations
This document has no IANA actions.
10. 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,
.
[ETSI_TS103774]
ETSI TS 103 744 V1.1.1, "CYBER; Quantum-safe Hybrid Key
Exchanges", December 2020, .
[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-12, 1 December 2022,
.
[I-D.ietf-lamps-cert-binding-for-multi-auth]
Becker, A., Guthrie, R., and M. J. Jenkins, "Related
Certificates for Use in Multiple Authentications within a
Protocol", Work in Progress, Internet-Draft, draft-ietf-
lamps-cert-binding-for-multi-auth-00, 24 February 2023,
.
[I-D.ietf-lamps-dilithium-certificates]
Massimo, J., Kampanakis, P., Turner, S., and B.
Westerbaan, "Internet X.509 Public Key Infrastructure:
Algorithm Identifiers for Dilithium", Work in Progress,
Internet-Draft, draft-ietf-lamps-dilithium-certificates-
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01, 6 February 2023,
.
[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-06, 27 February 2023,
.
[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-03, 22 October 2022,
.
[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,
.
[NIST_PQC_FAQ]
National Institute of Standards and Technology (NIST),
"Post-Quantum Cryptography FAQs", 5 July 2022,
.
[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,
.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
.
[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,
.
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[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
.
Acknowledgments
TODO
Author's Address
Florence Driscoll
UK National Cyber Security Centre
Email: florence.d@ncsc.gov.uk
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