Internet Engineering Task Force (IETF) F. Driscoll
Request for Comments: 9794 M. Parsons
Category: Informational UK National Cyber Security Centre
ISSN: 2070-1721 B. Hale
Naval Postgraduate School
June 2025
Terminology for Post-Quantum Traditional Hybrid Schemes
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.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9794.
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Table of Contents
1. Introduction
2. Primitives
3. Cryptographic Elements
4. Protocols
5. Properties
6. Certificates
7. Security Considerations
8. IANA Considerations
9. Informative References
Acknowledgments
Authors' Addresses
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). Current
predictions vary on 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 (in 2025) with an algorithm
vulnerable to a quantum computer can 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, and that cannot be updated or
replaced, are also at risk if a CRQC is developed during the
operational lifetime of that product.
Ongoing responses to the potential development of a CRQC include
modifying established (or standardised) protocols to use asymmetric
algorithms that are designed 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 cryptographic algorithms". In
this document, "traditional algorithm" is also used to refer to this
class of algorithms.
At the time of publication, the term "post-quantum" is generally used
to describe cryptographic algorithms that are designed to be secure
against an adversary with access to a CRQC. Post-quantum algorithms
can also be referred to as "quantum-resistant" or "quantum-safe"
algorithms. There are merits to the different terms. For example,
some prefer to use the terms quantum-resistant or quantum-safe to
explicitly indicate that these algorithms are designed to be secure
against quantum computers. Others disagree and prefer to use the
term post-quantum, in case of compromises against such algorithms
that could make the terms quantum-resistant or quantum-safe
misleading. Similarly, some prefer to refer specifically to Shor's
Algorithm or to the mathematical problem that is being used to
prevent attacks. Post-Quantum Cryptography (PQC) is commonly used
amongst the cryptography community, and so it will be used throughout
this document. Similarly, the term "traditional algorithm" will be
used throughout the document as, at the time of publication, it is
widely used in the community, though other terms, including
classical, pre-quantum, or quantum-vulnerable, are preferred by some.
There
To mitigate risks, there may be a requirement for protocols that use
both algorithm types, for example, either during the transition from traditional
to post-
quantum post-quantum algorithms or as a general solution, to mitigate risks. solution. When the risk
of deploying new algorithms is above the accepted threshold for their
use case, a designer may combine a post-quantum algorithm with a
traditional algorithm, with the goal of adding protection against an
attacker with a CRQC to the security properties provided by the
traditional algorithm. They may also 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, [RFC9370], [HYBRID-TLS], [COMPOSITE-KEM], and [RFC9763].
Schemes that combine post-quantum and traditional algorithms for key
establishment or digital signatures are often called "hybrids". For
example:
* The National Institute of Standards and Technology (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].
* The European Telecommunications Standards Institute (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 [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 (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. At the time of publication, hybrid is generally used for
schemes that combine post-quantum and traditional algorithms; it will
be so used throughout this document, though some have alternative
preferences such as double-algorithm or multi-algorithm.
This document provides language for constructions that combine
traditional and post-quantum algorithms. Specific solutions for
enabling the 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, in
order to add clarity and consistency across different protocols,
standards, and organisations. Additionally, this document aims to
reduce misunderstanding misunderstandings about the use of the word "hybrid" as well as
defining and to
define a shared language for different types of post-quantum and
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
produces an output". Examples include RSA, Elliptic Curve Diffie-
Hellman (ECDH), Module-Lattice-Based Key-Encapsulation Mechanism (ML-
KEM) (formerly known as Kyber), and Module-Lattice-Based Digital
Signature Algorithm (ML-DSA) (formerly known as 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
Traditional asymmetric cryptographic algorithm*: algorithm:
An asymmetric cryptographic algorithm based on integer
factorisation, finite field discrete logarithms, elliptic curve
discrete logarithms, or related mathematical problems.
A related mathematical problem is one that can be solved by
solving the integer factorisation, finite field discrete
logarithm, or elliptic curve discrete logarithm problem.
Where there is little risk of confusion, traditional asymmetric
cryptographic algorithms can also be referred to as "traditional
algorithms" for brevity. Traditional algorithms can also be
called "classical" or "conventional" algorithms.
*Post-quantum
Post-quantum asymmetric cryptographic algorithm*: algorithm:
An asymmetric cryptographic algorithm that is intended to be
secure against attacks using quantum computers as well as
classical computers.
Where there is little risk of confusion, post-quantum asymmetric
cryptographic algorithms can also be referred to as "post-quantum
algorithms" for brevity. Post-quantum algorithms can also be
called "quantum-resistant" or "quantum-safe" algorithms.
As with all cryptography, it always remains the case that attacks,
either quantum or classical, may be found against post-quantum
algorithms. Therefore, it should not be assumed that just because an algorithm
will not be compromised just because it is designed to provide
post-quantum security that it
will not be compromised. cryptography. Should an attack be found against a
post-quantum algorithm, it is commonly still referred to as a
"post-quantum algorithm", as they were designed to protect against
an adversary with access to a CRQC, and the labels are referring
to the designed or desired properties.
There may be asymmetric cryptographic constructions that are neither
post-quantum nor asymmetric traditional algorithms according to the
definitions above. These are out of scope of this document.
*Component
Component asymmetric algorithm*: algorithm:
Each cryptographic algorithm that forms part of a cryptographic
scheme.
An asymmetric component algorithm operates on the input of the
cryptographic operation and produces a cryptographic output that
can be used by itself or jointly to complete the operation. Where
there is little risk of confusion, component asymmetric algorithms
can also be referred to as "component algorithms" for brevity, as
is done in the following definitions.
*Single-algorithm 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*:
Multi-algorithm scheme:
A cryptographic scheme that incorporates more than one component
algorithm, where the component algorithms have the same
cryptographic purpose as each other and as the multi-algorithm
scheme.
For example, a multi-algorithm signature scheme may include
multiple signature algorithms, or a multi-algorithm Public Key
Encryption (PKE) scheme may include multiple PKE algorithms.
Component algorithms could be all traditional, all post-quantum,
or a mixture of the two.
*Post-Quantum
Post-Quantum Traditional (PQ/T) hybrid scheme*: scheme:
A multi-algorithm scheme where at least one component algorithm is
a post-quantum algorithm and at least one is a traditional
algorithm.
Components of a PQ/T hybrid scheme operate on the same input
message and their output is used together to complete the
cryptographic operation either serially or in parallel. The PQ/T
hybrid scheme design is aimed at requiring successful breaking of
all component algorithms to break the PQ/T hybrid scheme's
security properties.
*PQ/T
PQ/T hybrid Key Encapsulation Mechanism (KEM)*: (KEM):
A multi-algorithm KEM 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. The component algorithms could be KEMs
or other key establishment algorithms.
*PQ/T
PQ/T hybrid Public Key Encryption (PKE)*: (PKE):
A multi-algorithm PKE 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. The component algorithms
could be PKE algorithms or other key establishment algorithms.
The standard security property for a PKE scheme is
indistinguishability under chosen-plaintext attack (IND-CPA). (IND-CPA)
[BDPR]. IND-CPA security is not sufficient for secure
communication in the presence of an active attacker. Therefore,
in general, PKE schemes are not appropriate for use on the
Internet, and KEMs, which provide indistinguishability under
chosen-ciphertext attack
(IND-CCA security), (IND-CCA) [BDPR], are required.
*PQ/T
PQ/T hybrid digital signature*: 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.
Note that there are many possible ways of constructing a PQ/T
hybrid digital signature. Examples include parallel signatures,
composite signatures, or nested signatures.
PQ/T hybrid KEMs, PQ/T hybrid PKE, and PQ/T hybrid digital signatures
are all examples of PQ/T hybrid schemes.
*Post-Quantum
Post-Quantum Traditional (PQ/T) hybrid composite scheme*: scheme:
A multi-algorithm scheme where at least one component algorithm is
a post-quantum algorithm and at least one is a traditional
algorithm, and where the resulting composite scheme is exposed as
a singular interface of the same type as the component algorithms.
A PQ/T hybrid composite can be referred to as a "PQ/T composite".
Examples
An example of a PQ/T hybrid composites include composite is a single KEM algorithm
comprised of a PQ KEM component and a traditional KEM component,
for which the result presents as a KEM output.
*PQ/T
PQ/T hybrid combiner*: combiner:
A method that takes two or more component algorithms and combines
them to form a PQ/T hybrid scheme.
*PQ/PQ
PQ/PQ hybrid scheme*: scheme:
A multi-algorithm scheme where all components are post-quantum
algorithms.
The definitions for types of PQ/T hybrid schemes can be adapted to
define types of PQ/PQ hybrid schemes, which are multi-algorithm
schemes where all component algorithms are post-quantum
algorithms. These are designed to mitigate risks when the two
post-quantum algorithms are based on different mathematical
problems. Some prefer to refer to these as PQ/PQ multi-algorithm
schemes, and reserve the term "hybrid" for PQ/T hybrids.
In cases where there is little chance of confusion between other
types of hybrid cryptography (e.g., as defined in [RFC4949]) and
where the component algorithms of a multi-algorithm scheme could be
either post-quantum or traditional, it may be appropriate to use the
phrase "hybrid scheme" without PQ/T or PQ/PQ preceding it.
*Component scheme*:
Component scheme:
Each cryptographic scheme that makes up a PQ/T hybrid scheme or
PQ/T hybrid protocol.
3. Cryptographic Elements
This section introduces terminology related to cryptographic elements
and their inclusion in hybrid schemes.
*Cryptographic element*:
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
Component cryptographic element*: element:
A cryptographic element of a component algorithm in a multi-
algorithm scheme.
For example, in [HYBRID-TLS], the client's keyshare contains two
component public keys: one for a post-quantum algorithm and one
for a traditional algorithm.
*Composite
Composite cryptographic element*: element:
A cryptographic element that incorporates multiple component
cryptographic elements of the same type for use in a multi-
algorithm scheme, such that the resulting composite cryptographic
element is exposed as a singular interface of the same type as the
component cryptographic elements.
For example, a composite cryptographic public key is made up of
two component public keys.
*PQ/T
PQ/T hybrid composite cryptographic element*: element:
A cryptographic element that incorporates multiple component
cryptographic elements of the same type for use in a multi-
algorithm scheme, such that the resulting composite cryptographic
element is exposed as a singular interface of the same type as the
component cryptographic elements, where at least one component
cryptographic element is post-quantum and at least one is
traditional.
*Cryptographic
Cryptographic element combiner*: 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 [HYBRID-TLS], 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
PQ/T hybrid protocol*: 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 [HYBRID-TLS], 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
[RFC9370]. 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.
A protocol that can negotiate the use of either a traditional
algorithm or a post-quantum algorithm, but not the use of both
types of algorithm, is not a PQ/T hybrid protocol. Protocols that
use two or more component algorithms but with different
cryptographic functionalities, for example, a post-quantum KEM and
a Pre-Shared Key (PSK), are also not PQ/T hybrid protocols.
*PQ/T
PQ/T hybrid protocol with composite key establishment*: establishment:
A PQ/T hybrid protocol that incorporates a PQ/T hybrid composite
scheme to achieve key establishment, in such a way that the
protocol fields and message flow are the same as those in a
version of the protocol that uses a single-algorithm scheme.
For example, a PQ/T hybrid protocol with composite key
establishment could include a single PQ/T hybrid KEM, such as in
[HYBRID-TLS].
*PQ/T
PQ/T hybrid protocol with composite data authentication*: authentication:
A PQ/T hybrid protocol that incorporates a PQ/T hybrid composite
scheme to achieve data authentication, in such a way that the
protocol fields and message flow are the same as those in a
version of the protocol that uses a single-algorithm scheme.
For example, a PQ/T hybrid protocol with composite data
authentication could include data authentication through the use
of a PQ/T composite hybrid digital signature, exposed as a single
interface for PQ signature and traditional signature components.
*PQ/T
PQ/T hybrid protocol with composite entity authentication*: authentication:
A PQ/T hybrid protocol that incorporates a PQ/T hybrid composite
scheme to achieve entity authentication, in such a way that the
protocol fields and message flow are the same as those in a
version of the protocol that uses a single-algorithm scheme.
For example, a PQ/T hybrid protocol with composite entity
authentication could include entity authentication through the use
of PQ/T Composite Hybrid certificates.
In a PQ/T hybrid protocol with a composite construction, 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.
*PQ/T
PQ/T hybrid protocol with non-composite key establishment*: establishment:
A PQ/T hybrid protocol that incorporates multiple single-algorithm
schemes to achieve key establishment, 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 a
part of a single-algorithm scheme.
For example, a PQ/T hybrid protocol with non-composite key
establishment could include a traditional key exchange scheme and
a post-quantum KEM. A construction like this for the Internet Key
Exchange Protocol Version 2 (IKEv2) is enabled by [RFC9370].
*PQ/T
PQ/T hybrid protocol with non-composite authentication*: authentication:
A PQ/T hybrid protocol that incorporates multiple single-algorithm
schemes to achieve authentication, 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 a single-
algorithm scheme.
For example, a PQ/T hybrid protocol with non-composite
authentication could use a PQ/T parallel PKI with one traditional
certificate chain and one post-quantum certificate chain.
In a PQ/T hybrid protocol with a non-composite construction, 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 with both
composite and non-composite constructions. For example, a protocol
that offers both confidentiality and authentication could have
composite key agreement and non-composite authentication. Similarly,
it is possible for a PQ/T hybrid protocol to achieve certain
cryptographic outcomes in a non-hybrid manner. For example,
[HYBRID-TLS] describes a PQ/T hybrid protocol with composite key
agreement, but with single-algorithm authentication.
PQ/T hybrid protocols may not specify non-composite aspects, but can
choose to do so for clarity, in particular, if including both
composite and non-composite aspects.
*PQ/T
PQ/T hybrid composite protocol*: protocol:
A PQ/T hybrid protocol that only uses composite constructions can
be referred to as a "PQ/T hybrid composite protocol".
An example of this is a protocol that only provides entity
authentication, and achieves this using PQ/T hybrid composite
entity authentication. Similarly, another example is a protocol
that offers both key establishment and data authentication, and
achieves this using both PQ/T hybrid composite key establishment
and PQ/T hybrid composite data authentication.
*PQ/T
PQ/T hybrid non-composite protocol*: protocol:
A PQ/T hybrid protocol that does not use only composite
constructions can be referred to as a "PQ/T hybrid non-composite
protocol".
For example, a PQ/T hybrid protocol that offers both
confidentiality and authentication and uses composite key
agreement and non-composite authentication would be referred to as
a "PQ/T hybrid non-composite protocol".
5. Properties
This section describes some properties that may be desired from or
achieved by a PQ/T hybrid scheme or a PQ/T hybrid protocol.
Properties of PQ/T hybrid schemes are still an active area of
research and development, e.g., in [BINDELHALE]. This section does
not attempt to be comprehensive, but rather covers a basic set of
properties.
It is not possible for one PQ/T hybrid scheme or PQ/T hybrid protocol
to achieve all of the properties in this section. To understand what
properties are required, a designer or implementer will think about
why they are using a PQ/T hybrid scheme. For example, a scheme that
is designed for implementation security will likely require PQ/T
hybrid confidentiality or PQ/T hybrid authentication, while a scheme
for interoperability will require PQ/T hybrid interoperability.
*PQ/T
PQ/T hybrid confidentiality*: confidentiality:
The property that confidentiality 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.
*PQ/T
PQ/T hybrid authentication*: 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.
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 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, [HYBRID-TLS] provides hybrid
confidentiality but does not address hybrid authentication.
Therefore, if the design in [HYBRID-TLS] is used with single-
algorithm X.509 certificates as defined in [RFC5280], only
authentication with a single algorithm is achieved.
*PQ/T
PQ/T hybrid interoperability*: interoperability:
The property that a PQ/T hybrid scheme or a 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
the approach defined in Section 7.2.2 of [ITU-T-X509-2019]. In
this example, a verifier that has migrated to support post-quantum
algorithms is required to verify only the post-quantum signature,
while a verifier that has not migrated will verify only the
traditional signature.
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 [HYBRID-TLS], 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 it 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.
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.
*PQ/T
PQ/T hybrid backwards compatibility*: compatibility:
The property that a PQ/T hybrid scheme or a PQ/T hybrid protocol
can be completed successfully provided that both parties support
the traditional component algorithm, while also using both
algorithms if both are supported by both parties.
*PQ/T Hybrid Forwards Compatibility*:
PQ/T hybrid forwards compatibility:
The property that a PQ/T hybrid scheme or a PQ/T hybrid protocol
can be completed successfully using a post-quantum component
algorithm provided that both parties support it, while also having
the option to use both post-quantum and traditional algorithms if
both are supported by both parties.
Note that PQ/T hybrid forwards compatibility is a protocol or
scheme property only.
6. Certificates
This section introduces terminology related to the use of
certificates in hybrid schemes.
*PQ/T
PQ/T hybrid certificate*: 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, 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*:
Post-quantum certificate:
A digital certificate that contains a single public key for a
post-quantum digital signature algorithm.
*Traditional certificate*:
Traditional certificate:
A digital certificate that contains a single public key for a
traditional digital signature algorithm.
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 ML-DSA public key, as defined in [ML-DSA], would be a
post-quantum certificate.
*Post-quantum
Post-quantum certificate chain*: chain:
A certificate chain where all certificates include a public key
for a post-quantum algorithm and are signed using a post-quantum
digital signature scheme.
*Traditional
Traditional certificate chain*: chain:
A certificate chain where all certificates include a public key
for a traditional algorithm and are signed using a traditional
digital signature scheme.
*PQ/T
PQ/T hybrid certificate chain*: chain:
A certificate chain where all certificates are PQ/T hybrid
certificates and each certificate is signed with two or more
component algorithms with at least one being a traditional
algorithm and at least one being 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 it is
not the only way. An alternative is to use a PQ/T parallel PKI as
defined below.
*PQ/T
PQ/T mixed certificate chain*: chain:
A certificate chain containing at least two of the three
certificate types defined in this document (PQ/T hybrid
certificates, post-quantum certificates, and traditional
certificates).
For example, a traditional end-entity certificate could be signed
by a post-quantum intermediate certificate, which in turn could be
signed by a post-quantum root certificate. This may be desirable
due to the lifetimes of the certificates, the relative difficulty
of rotating keys, or for efficiency reasons. 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.
*PQ/T
PQ/T parallel PKI*: PKI:
Two certificate chains, one that is a post-quantum certificate
chain and one that is a traditional certificate chain, and that
are used together in a protocol.
A PQ/T parallel PKI might be used to achieve hybrid authentication
or hybrid interoperability depending on the protocol
implementation.
*Multi-certificate authentication*:
Multi-certificate authentication:
Authentication that uses two or more end-entity certificates.
For example, multi-certificate authentication may be achieved
using a PQ/T parallel PKI.
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
specification documents. More general guidance about the security
considerations, timelines, and benefits and drawbacks of the use of
PQ/T hybrids is also out of scope of this document.
8. IANA Considerations
This document has no IANA actions.
9. Informative References
[BDPR] Bellare, M., Desai, A., Pointcheval, D., and P. Rogaway,
"Relations Among Notions of Security for Public-Key
Encryption Schemes", June 2001,
<https://www.cs.ucdavis.edu/~rogaway/papers/
relations.pdf>.
[BINDEL] Bindel, N., Brendel, J., Fischlin, M., Goncalves, B., and
D. Stebila, "Hybrid Key Encapsulation Mechanisms and
Authenticated Key Exchange", Post-Quantum Cryptography,
PQCrypto 2019, Lecture Notes in Computer Science, vol.
11505, 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>.
[BINDELHALE]
Bindel, N. and B. Hale, "A Note on Hybrid Signature
Schemes", Cryptology ePrint Archive, Paper 2023/423, 23
July 2023, <https://eprint.iacr.org/2023/423.pdf>.
[COMPOSITE-KEM]
Ounsworth, M., Gray, J., Pala, M., Klaussner, J., and S.
Fluhrer, "Composite ML-KEM for use in X.509 Public Key
Infrastructure and CMS", Work in Progress, Internet-Draft,
draft-ietf-lamps-pq-composite-kem-06, 18 March 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
pq-composite-kem-06>.
[ETSI_TS103774]
European Telecommunications Standards Institute (ETSI),
"CYBER; Quantum-safe Hybrid Key Exchanges", ETSI TS 103
744 v1.1.1, December 2020, <https://www.etsi.org/deliver/
etsi_ts/103700_103799/103744/01.01.01_60/
ts_103744v010101p.pdf>.
[HYBRID-TLS]
Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key
exchange in TLS 1.3", Work in Progress, Internet-Draft,
draft-ietf-tls-hybrid-design-12, 14 January 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
hybrid-design-12>.
[ITU-T-X509-2019]
ITU-T, "Information Technology - Open Systems
Interconnection - The Directory: Public-key and attribute
certificate frameworks", ITU-T Recommendation X.509,
October 2019,
<https://www.itu.int/rec/T-REC-X.509-201910-I>.
[ML-DSA] Massimo, J., Kampanakis, P., Turner, S., and B. E.
Westerbaan, "Internet X.509 Public Key Infrastructure -
Algorithm Identifiers for Module-Lattice-Based Digital
Signature Algorithm (ML-DSA)", Work in Progress, Internet-
Draft, draft-ietf-lamps-dilithium-certificates-11, 22 May
2025, <https://datatracker.ietf.org/doc/html/draft-ietf-
lamps-dilithium-certificates-11>.
[NIST_PQC_FAQ]
NIST, "Post-Quantum Cryptography (PQC) FAQs", 31 January
2025, <https://csrc.nist.gov/Projects/post-quantum-
cryptography/faqs>.
[NIST_SP_800-152]
Barker, E., Smid, M., and D. Branstad, "A Profile for U.
S. Federal Cryptographic Key Management Systems", NIST
SP 800-152, DOI 10.6028/NIST.SP.800-15, October 2015,
<https://doi.org/10.6028/NIST.SP.800-152>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
[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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[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>.
[RFC9370] Tjhai, CJ., Tomlinson, M., Bartlett, G., Fluhrer, S., Van
Geest, D., Garcia-Morchon, O., and V. Smyslov, "Multiple
Key Exchanges in the Internet Key Exchange Protocol
Version 2 (IKEv2)", RFC 9370, DOI 10.17487/RFC9370, May
2023, <https://www.rfc-editor.org/info/rfc9370>.
[RFC9763] Becker, A., Guthrie, R., and M. Jenkins, "Related
Certificates for Use in Multiple Authentications within a
Protocol", RFC 9763, DOI 10.17487/RFC9763, June 2025,
<https://www.rfc-editor.org/info/rfc9763>.
Acknowledgments
This document is the product of numerous fruitful discussions in the
IETF PQUIP group. Thank you in particular to Mike Ounsworth, John
Gray, Tim Hollebeek, Wang Guilin, Rebecca Guthrie, Stephen Farrell,
Paul Hoffman, and Sofía Celi for their contributions. This document
is inspired by many others from the IETF and elsewhere.
Authors' Addresses
Florence Driscoll
UK National Cyber Security Centre
Email: florence.d@ncsc.gov.uk
Michael Parsons
UK National Cyber Security Centre
Email: michael.p1@ncsc.gov.uk
Britta Hale
Naval Postgraduate School
Email: britta.hale@nps.edu