Internet-Draft | PQ/T Hybrid Terminology | May 2024 |
Driscoll & Parsons | Expires 10 November 2024 | [Page] |
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.¶
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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 (2024) 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, and that cannot be updated or replaced, 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 cryptographic algorithms. In this document "traditional algorithm" is also used to refer to this class of algorithms.¶
During the transition from traditional to post-quantum algorithms, there may be a requirement for protocols that use both algorithm types. A designer may 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 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], [I-D.ietf-tls-hybrid-design], [I-D.ietf-lamps-pq-composite-kem], 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 [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.¶
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.¶
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, ML-KEM (formerly known as Kyber) and 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.¶
This section introduces terminology related to cryptographic algorithms and to hybrid constructions for cryptographic schemes.¶
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 asymmetric traditional cryptographic algorithms can also be referred to as traditional algorithms for brevity. Traditional algorithms can also be called classical or conventional algorithms.¶
An asymmetric cryptographic algorithm that is intended to be secure against attacks using quantum computers as well as classical computers.¶
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 is designed to provide post-quantum security it will not be compromised.¶
There may be asymmetric cryptographic constructions that are neither post-quantum nor asymmetric traditional algorithms according to the definitions above, but these are out of scope of this document.¶
Each cryptographic algorithm that forms part of a cryptographic scheme.¶
A cryptographic scheme with one component algorithm.¶
A single-algorithm scheme could use either a traditional algorithm or a post-quantum algorithm.¶
A cryptographic scheme that incorporates more than one component algorithm, where the component algorithms have the same cryptographic purpose.¶
For example, a multi-algorithm scheme may include multiple signature algorithms or multiple Public Key Encryption (PKE) algorithms. Component algorithms could be all traditional, all post-quantum, or a mixture of the two.¶
A multi-algorithm scheme where at least one component algorithm is a post-quantum algorithm and at least one is a traditional algorithm.¶
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.¶
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 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 indistiguishability under chosen-ciphertext attacks (IND-CCA security), are required.¶
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.¶
A method that takes two or more component algorithms and combines them to form a PQ/T 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.¶
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.¶
Each cryptographic scheme that makes up a PQ/T hybrid scheme or PQ/T hybrid protocol.¶
Depending on the construction of a PQ/T hybrid scheme or PQ/T hybrid protocol it may or may not be meaningful to define the component schemes as well as the component algorithms. For example, fused hybrids, as defined in [I-D.hale-pquip-hybrid-signature-spectrums], may sufficiently entangle the component algorithms that the component schemes are not relevant.¶
This section introduces terminology related to cryptographic elements and their inclusion in hybrid schemes.¶
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.¶
A cryptographic element of a component algorithm in a multi-algorithm scheme.¶
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.¶
A cryptographic element that incorporates multiple component cryptographic elements of the same type in a multi-algorithm scheme. Note that, at the cryptographic element level, 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.¶
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].¶
This section introduces terminology related to the use of post-quantum and traditional algorithms together in protocols.¶
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 [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 of both types of algorithm, is not a PQ/T hybrid protocol. Protocols that use two or more component algorithms but with different cryptographic functionality, for example a post-quantum KEM and a pre-shared key (PSK) are also not PQ/T hybrid protocols.¶
A PQ/T hybrid protocol that incorporates a PQ/T hybrid 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 [I-D.ietf-tls-hybrid-design].¶
A PQ/T hybrid protocol that incorporates a PQ/T hybrid scheme to achieve 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 authentication could include a single PQ/T hybrid digital signature, with component cryptographic elements being included in a PQ/T hybrid certificate.¶
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.¶
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 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 IKEv2 is enabled by [RFC9370].¶
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 a 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 [I-D.ietf-tls-hybrid-design] describes a PQ/T hybrid protocol with composite key agreement, but with single-algorithm authentication.¶
This section describes some properties that may be desired from or achieved by a PQ/T hybrid scheme or PQ/T hybrid protocol. Properties of PQ/T hybrid schemes are still an active area of research and development, e.g., [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.¶
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.¶
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, [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 single-algorithm X.509 certificates as defined in [RFC5280] only authentication with a single algorithm is achieved.¶
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 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 [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.¶
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.¶
The property that a PQ/T hybrid scheme or 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.¶
The property that a PQ/T hybrid scheme or PQ/T hybrid protocol can be completed successfully provided that both parties support the post-quantum component algorithm, while also using both algorithms if both are supported by both parties.¶
This section introduces terminology related to the use of certificates in hybrid schemes.¶
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.¶
A digital certificate that contains a single public key for a post-quantum digital signature algorithm.¶
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 will be defined in [I-D.ietf-lamps-dilithium-certificates], would be a post-quantum certificate.¶
A certificate chain where all certificate include a public key for a post-quantum algorithm and are signed using a post-quantum digital signature scheme.¶
A certificate chain where all certificates include a public key for a traditional algorithm and are signed using a traditional digital signature scheme.¶
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 is not the only way. An alternative is to use a PQ/T parallel PKI as defined below.¶
A certificate chain containing at least two of the three certificate types defined in this draft (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.¶
Two certificate chains, one a post-quantum certificate chain and one a traditional certificate chain, that are used together in a protocol.¶
A PQ/T parallel PKI might be used achieve hybrid authentication or hybrid interoperability depending on the protocol implementation.¶
Authentication that uses two or more end-entity certificates.¶
For example, multi-certificate authentication may be achieved using a PQ/T parallel PKI.¶
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.¶
This document has no IANA actions.¶
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, Britta Hale, Rebecca Guthrie, Stephen Farrell, Paul Hoffman and Sofía Celi for their contributions. This document is inspired by many others from the IETF and elsewhere.¶