Internet DRAFT - draft-vaira-pquip-pqc-use-cases
draft-vaira-pquip-pqc-use-cases
Post-Quantum Use In Protocols A. Vaira
Internet-Draft H. Brockhaus
Intended status: Informational A. Railean
Expires: 5 September 2024 Siemens
J. Gray
M. Ounsworth
Entrust
4 March 2024
Post-quantum cryptography migration use cases
draft-vaira-pquip-pqc-use-cases-01
Abstract
This document is meant to be continuously updated, to incorporate
emerging Post-Quantum Cryptography (PQC) migration use cases, with a
focus on the migration from traditional signature algorithms (e.g.,
RSA, DSA, ECDSA) to PQC signature algorithms (e.g., LMS, XMSS, ML-
DSA, SLH-DSA). This document aims at categorizing real-world
scenarios based on a set of distinctive features. The primary goal
is to facilitate discussions on migration strategies by offering a
systematic taxonomy and a shared understanding among stakeholders.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://avaira77.github.io/pq-ietf-usecase/draft-vaira-pquip-pq-use-
cases.html. Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-vaira-pquip-pqc-use-cases/.
Discussion of this document takes place on the Post-Quantum Use In
Protocols Working Group mailing list (mailto:pqc@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/pqc/. Subscribe
at https://www.ietf.org/mailman/listinfo/pqc/.
Source for this draft and an issue tracker can be found at
https://github.com/avaira77/pq-ietf-usecase.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 4
2.1. Industrial communication protocols . . . . . . . . . . . 4
2.2. Software and Firmware update . . . . . . . . . . . . . . 5
2.3. Trust Anchor deployment . . . . . . . . . . . . . . . . . 6
2.4. CMS (S/MIME) . . . . . . . . . . . . . . . . . . . . . . 7
2.5. Timestamping . . . . . . . . . . . . . . . . . . . . . . 8
2.6. Additional use cases . . . . . . . . . . . . . . . . . . 8
3. Post-quantum Migration Strategies for Signing . . . . . . . . 9
3.1. Multiple Signatures . . . . . . . . . . . . . . . . . . . 9
3.2. Composite Signatures . . . . . . . . . . . . . . . . . . 9
3.3. Employing Stateful Hash-based Signature Schemes . . . . . 10
3.4. Employing Stateless Hash-based Signature Schemes . . . . 10
3.5. Employing Module-Lattice-Based Digital Signature
Schemes . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.6. Cryptographic Agility . . . . . . . . . . . . . . . . . . 11
4. Map of Migration Strategies to Reference Use Cases . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
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7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Post-Quantum Migration Properties . . . . . . . . . 17
A.1. Lifetime . . . . . . . . . . . . . . . . . . . . . . . . 17
A.2. Protocol . . . . . . . . . . . . . . . . . . . . . . . . 18
A.3. Backward compatibility . . . . . . . . . . . . . . . . . 18
Appendix B. Composite Signature individual and organization
position statements . . . . . . . . . . . . . . . . . . . 19
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
How to transition to post-quantum cryptography is a question likely
to stay with us for a considerable period. Within several working
groups at the IETF, a variety of strategies are under discussion,
gradually finding their way into RFCs. The existence of multiple
choices makes it more difficult to select the most suitable approach
for any given use case.
For example:
* An Original Equipment Manufacturer (OEM) must issue its products
today with manufacturer X.509 certificates that might be used at
any time during their lifespan. These certificates will
eventually be utilized to enroll in a domain PKI (Public Key
Infrastructure). The choice of algorithms and the type of hybrid
cryptography to support become critical.
* A PKI must start preparing its CAs today for issuing S/MIME
certificates, necessitating the inclusion of hybrid capabilities.
The question arises: which path should be pursued?
In this document, intended to be a dynamic resource, our main
objective is to compile a list of digital signature use cases and
categorize them based on prominent features. Examples include
distinguishing between long-lived and short-lived scenarios, whether
they include a negotiated protocol, or if backward compatibility is
required.
We also explore the migration strategies that have appeared so far,
proposing the most suitable fit for each of the properties identified
in each use case. Some of these migration strategies make use of
hybrid cryptography, i.e., use both traditional and post-quantum
cryptography.
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The motivation to take into account hybrid cryptography during the
migration phase arises from the requirement of having long-lived
assertions, i.e., digital signatures that require long term
validation, as well as the uncertainty surrounding the longevity of
traditional cryptographic methods and lack of complete trust in
emerging PQC algorithms.
An additional factor to consider is represented by the requirements
coming from regulatory bodies, which, in several cases will differ
among legislations, in regard to post-quantum algorithms and
acceptable migration strategies. For example
[bsi.quantum-safe.crypto], recommends the incorporation of post-
quantum cryptographic algorithms within hybrid cryptographic schemes,
as a proactive response to the quantum threat. On the contrary,
[CNSA2-0] recommends specific post-quantum cryptographic algorithms
for each use case.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Reference Use Cases
This section is the core of this document. For each use case, we
present a concise overview and highlight the features that can help
to categorize it. This list is not exhaustive, and if you think we
have missed some important use case please consider contributing to
it.
2.1. Industrial communication protocols
Several industrial communication protocols, traditionally do not use
IP network infrastructure, are progressively being updated to make
use of standard IP network infrastructure hence rely on standard
security mechanisms, like for example TLS 1.3 [RFC8446].
The protocol 'Building Automation and Control Networks / Secure
Connect' (BACnet/SC) [ANSI_ASHRAE.Standard.135-2016] is a good
example. BACnet was defined before 1995, when the TCP/IP protocol
suite was expensive and not available for smaller devices common in
building automation. BACnet/SC proposes a new datalink layer option
that makes full use of TLS secured WebSocket connections. This new
BACnet/SC datalink layer option uses a virtual hub-and-spoke topology
where the spokes are WebSocket connections from the nodes to the hub.
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BACnet/SC's implementation adheres to established industry standards
defined in IETF RFCs. Specifically the
[Addendum.bj.to.ANSI_ASHRAE.Standard.135-2016] references to text
encoding of PKIX, PKCS, and CMS structures in [RFC7468], when
defining the format in which operational certificates and signing CA
should be installed onto the target device at configuration time.
The security of the BACnet/SC protocol, as well as of similar
industrial protocols, relies on TLS 1.3 [RFC8446], therefore
implications of post-quantum cryptography have to be considered in
both the TLS handshake and in the X.509 certificates used for the
authentication.
Lifetime: Long-lived.
Protocol: Active Negotiation.
Backward compatibility: Limited.
2.2. Software and Firmware update
Secure firmware updates are crucial for ensuring long-term security
of the device, especially in industrial, and critical infrastructure
fields, where devices can stay active for decades. Such updates
encompass tasks like introducing new trust anchors and upgrading
cryptographic algorithm capabilities. However, upgrading every
device's security capabilities isn't always feasible due to resource,
accessibility, and cost constraints. Some devices may not support
secure firmware updates at all.
Firmware updates are typically authenticated by the Original
Equipment Manufacturer (OEM) by means of a digital signing process.
An update is distributed to target devices, which will validate its
signature against a Trust Anchor (TA). The TA can be an X.509
certificate, a public key, or a hash of a combination of both,
depending on the OEM's security measures.
These devices are typically deployed in highly regulated
environments, in remote or physically constrained locations where
performing upgrades is challenging, or in cases where the cost of
upgrading is prohibitively high. The immutability of these devices
can also be viewed as a security feature, as it restricts potential
attack vectors associated with over-the-air updates. These devices
are designed with a long operational lifespan in mind, often spanning
several decades. Notable examples of such devices encompass:
* Vehicles - scale of deployment or vehicle recall difficulties.
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* Satellites - no 'on-site' service reasonably possible.
* Servers and network devices - air-gapped, locked-down DCs,
geographically distributed.
* Government infrastructure - power grids, nuclear power station
equipment, etc.
* Smart meters - device owned by the utility company, deployed in
private homes.
* Smart cards – used for authenticating to workstations and
buildings, or electronic document signing.
* Security Tokens – such as FIDO2, cheap devices that users will
typically not patch.
Lifetime: Long-lived.
Protocol: Passive Negotiation.
Backward compatibility: Limited.
2.3. Trust Anchor deployment
Trust Anchors, such as X.509 Root CA certificates and raw public
keys, must be made accessible before they can be used for signature
validation. In scenarios like remote software updates, a Trust
Anchor X.509 certificate, for instance, must be installed on a target
device to enable the validation of certificate chains. While
deployment of Trust Anchors may be relatively straightforward for
"corporate IT" and "public web" applications, it can still be a time-
consuming process to ensure that a new Trust Anchor X.509 certificate
is propagated throughout the entire ecosystem. Additionally, when
dealing with post-quantum Trust Anchors, an extra layer of complexity
arises as the desired underlying cryptography may not yet be
supported by the target platform.
There are two common variations of this use case.
* Injection within a factory: in industrial contexts, Trust Anchors
are typically injected into target devices during the
manufacturing phase. To bootstrap a Trust Anchor, the device is
placed in a physically secure environment accessible only to
trusted personnel. This injection can occur during manufacturing
or when a device is being resold. It is important to note that
some devices might not support updating the Trust Anchor in the
field, requiring the return of the device to the OEM for post-
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quantum Trust Anchor injection or, in some cases, it may be even
not supported at all, because, for example, the Trust Anchor is
burnt onto the device at manufacturing time.
* Injection via software and firmware updates: for devices where the
Trust Anchor is not burned onto the device, for example in less
constrained devices and IT equipment, post-quantum Trust Anchors
can be injected through software or firmware update mechanisms.
The deployment of these Trust Anchors may leverage existing update
mechanisms and traditional cryptography to minimize effort.
However, this approach necessitates the distribution of the new
Trust Anchors well in advance of any suspicion that traditional
cryptography may become vulnerable. Given the lead time required
to ensure widespread distribution, the time window where this
mechanism is suitable is further reduced.
Lifetime: Long-lived.
Protocol: Passive Negotiation.
Backward compatibility: Limited.
2.4. CMS (S/MIME)
The Cryptographic Message Syntax (CMS) [RFC5652] establishes a
standard syntax for creating secure messages, incorporating digital
signatures, encryption, and authentication codes. In practical
terms, CMS finds application in scenarios such as secure email
communication, document signing, and PKI-based security services.
Organizations use CMS for secure file transfers and end-to-end
encryption of documents, ensuring confidentiality and integrity. It
is a key component in secure messaging protocols, contributing to the
confidentiality, integrity, and authenticity of communication over
networks. One of the notable features of CMS is flexibility,
allowing the choice of cryptographic algorithms based on specific
security requirements. An important consideration to be made is the
non-uniform adoption and potential challenges in implementing CMS,
particularly in the context of email clients. Varying levels of
maturity and maintenance among email clients will slow down the
adoption of post-quantum algorithms, which will not be uniform across
different clients.
It is worth noting that, similarly to CMS, JOSE and (JSON Object
Signing and Encryption) and COSE (CBOR Object Signing and Encryption)
are data structures used to support signing and encryption of data,
respectively, in JSON and CBOR format. Therefore, several
considerations that are applicable for CMS will extend to JOSE and
COSE as well.
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Lifetime: Short-lived and long-lived.
Protocol: Passive Negotiation.
Backward compatibility: Mandatory.
2.5. Timestamping
A time-stamping service supports assertions of proof that a datum
existed before a particular time, as defined in [RFC3161].
Timestamps, are particularly important in the following scenarios.
* Code and Document Signing: In code and document signing use cases,
timestamps play a critical role in ensuring the ongoing validity
of digital signatures. It is not sufficient to validate the
signature at the time of creation; it must be verifiable even
after the signature certificate has expired. This is particularly
important for long-term archival and verification purposes, where
the historical integrity of the signed code or document needs to
be maintained over time. The timestamp is stored in a CMS data
structure, cf. Section 2.4.
* Non-repudiation: timestamps enhance non-repudiation by preventing
parties from later denying the authenticity or validity of their
digital signatures. Non-repudiation plays a major role in Legal
and regulatory compliance, Intellectual property protection and
Electronic commerce, where the reliability of timestamps is key
for establishing clear timelines with legal and financial
implications.
Lifetime: Long-lived.
Protocol: Passive Negotiation.
Backward compatibility: Optional.
2.6. Additional use cases
Future updates of this document may include use cases which cover
additional aspects, such as FAA airworthiness certifications, medical
records, etc.
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3. Post-quantum Migration Strategies for Signing
People are considering which technological concepts are suitable to
solve the problem of a secure migration from classical cryptography
to quantum computer safe cryptographic algorithms. A variety of
approaches are being discussed. In the following, we would like to
briefly introduce the approaches under discussion and refer to the
respective relevant documents for further details. For a general
introduction, we also refer to [I-D.ietf-pquip-pqc-engineers].
3.1. Multiple Signatures
Several signatures have the approach of defining a second alternative
signature in addition to the primary signature in the protocol or
format, which can be transported in the protocol or format. In
addition to the definition of the alternative signature, the
associated signing algorithm and, if applicable, the associated
public key or a reference to it must also be transferred. For X.509
public key certificates, this is defined in [X.509]. Work is also
underway for other protocols and formats. This approach overlaps
with the protocol and format extension approach described in
Section 3.6.
An alternative approach is to encode a second signature in a second
certificate and bind it to the first one by a reference. For
example, an implementation can decide based on its policy whether
only the first certificate or the second or both certificates should
be used for authentication. Further descriptions of this approach
can be found in A Mechanism for Encoding Differences in Paired
Certificates [I-D.bonnell-lamps-chameleon-certs] and Related
Certificates for Use in Multiple Authentications within a Protocol
[I-D.ietf-lamps-cert-binding-for-multi-auth].
3.2. Composite Signatures
The goal of composite signatures is to define a signature object to
be used with any protocol or format. It contains two signatures in a
single atomic container that have been generated using two different
cryptographic algorithms. The goal of this approach is to define a
signature format which requires both contained signatures to be
verified. In this way, the security properties of the classical
signature and another signature that is secure when attacked by a
quantum computer are used in the protocol or format without having to
adapt them.
In order for this approach to be applicable in arbitrary protocols
and formats, a composite key must be defined in addition to the
composite signature. According to the definition of composite
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signatures, a composite public is a single atomic container composed
of two public keys. The associated composite private key is a single
atomic private key that is composed of the two private keys which
correspond to the two public keys contained in the composite public
key.
This concept is described in Composite Signatures For Use In Internet
PKI [I-D.ounsworth-pq-composite-sigs] in more detail.
3.3. Employing Stateful Hash-based Signature Schemes
Stateful hash-based signature (HBS) schemes, such as XMSS [RFC8391]
and LMS [RFC8554], including their multi-tree variants, have been the
first post-quantum algorithms to be standardized by NIST in
[NIST.SP.800-208]. Stateful HBS algorithms offer better key and
signature sizes than stateless HBS algorithms, and the underlying
cryptographic building blocks are generally considered well-
understood. However, a critical consideration is the management of
state, which is a fundamental aspect of security. The absence of
standardized operating procedures for state management poses
challenges to the adoption of stateful HBS. This is especially
critical when signing data over extended periods using the same key
pair; i.e., the resulting signatures will be validated with the same
public key over a long period of time. Another aspect worth
considering is that, without solutions for hardware security module
replacements and disaster recovery scenarios, using stateful HBS
might lead to a solution with limited resilience.
3.4. Employing Stateless Hash-based Signature Schemes
[NIST.FIPS.205] specifies the SLH-DSA (SPHINCS+) algorithm. It is a
stateless hash-based signature algorithm and is considered safe
against attacks by quantum computers. The advantage of this
algorithm is that the state problem is resolved as part of the
algorithm. However, the tradeoff is that signature sizes are often
an order of magnitude larger than XMSS or LMS. This may make
deploying these algorithms on constrained devices infeasible.
3.5. Employing Module-Lattice-Based Digital Signature Schemes
[NIST.FIPS.204] specifies the ML-DSA (Dilithium) algorithm, a digital
signature algorithm based on the hardness of lattice problems over
module lattices. It serves as a general purpose post-quantum
signature algorithm, offering smaller key sizes in comparison to SLH-
DSA and fast signature generation. For more details, refer to
[I-D.ietf-pquip-pqc-engineers].
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3.6. Cryptographic Agility
Migration to post-quantum cryptographic algorithms can be regarded as
an instance of the general pattern of _cryptographic agility_, rather
than be viewed as a special, one-off event. A system that approaches
migration this way can undergo multiple transitions without requiring
major architectural changes. Thus, when planning the transition to
post-quantum cryptography, consider that when future cryptanalysis
will trigger a transition to _post-post-quantum_ cryptography, it is
better to be agile than to start from scratch.
Hohm et al. identified circa 30 interpretations of the term
"cryptographic agility" in their literature survey [CAMM], therefore
referring to agility without defining it brings the potential of
being misunderstood. In this document, we encourage readers to
reason about agility by relying on these guiding questions:
1. Can one *select* algorithms based on a specific context?
2. Can one *add* new cryptographic primitives or parameters?
3. Can obsolete crypto be *retired*?
System and protocol designers can adjust the definition for their
particular context, while ensuring that the adjusted definition is
clearly stated, to avoid ambiguities.
Agility in security protocols and message formats, such as IP
Security (IPsec) and Internet Key Exchange (IKE) [RFC6071], Transport
Layer Security (TLS)[RFC8446], Secure/Multipurpose Internet Mail
Extensions (S/MIME)[RFC8551], is understood as the dynamic
referencing of the algorithms to be used - the "select" component
above. A migration strategy that allows the existing and future
cryptographic algorithms to be used simultaneously during a
transition period (the "add" part) is not described in the respective
standards.
Revised versions of standards would be needed to integrate agility
into protocols and formats. This requires effort for standardization
and implementation if a basic functionality, such as multiple
signatures, e.g., in Cryptographic Message Syntax (CMS) [RFC5652], is
not already available. But even in the case of S/MIME and CMS,
profiling is still necessary to describe how the multiple signatures
are to be used specifically for the migration.
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4. Map of Migration Strategies to Reference Use Cases
In this section, we establish a mapping between the reference use
cases and their primary features, as summarized in the table below,
and the digital signature migration strategies identified in the
preceding section.
+===============+=============+=============+===============+
| Use Case | Lifetime | Protocol | Backward |
| | | | Compatibility |
+===============+=============+=============+===============+
| Industrial | Long-lived | Active | Limited |
| communication | | Negotiation | |
| protocols | | | |
+---------------+-------------+-------------+---------------+
| Software and | Long-lived | Passive | Limited |
| Firmware | | Negotiation | |
| update | | | |
+---------------+-------------+-------------+---------------+
| Trust Anchor | Long-lived | Passive | Limited |
| deployment | | Negotiation | |
+---------------+-------------+-------------+---------------+
| CMS (S/MIME) | Short-lived | Passive | Mandatory |
| | and Long- | Negotiation | |
| | lived | | |
+---------------+-------------+-------------+---------------+
| Timestamping | Long-lived | Passive | Optional |
| | | Negotiation | |
+---------------+-------------+-------------+---------------+
Table 1: Summary of use cases and main features
The map is constructed as a decision tree, which is available at:
https://github.com/avaira77/pq-ietf-usecase/tree/main/flowchart
(https://github.com/avaira77/pq-ietf-usecase/tree/main/flowchart).
5. IANA Considerations
This memo includes no request to IANA.
6. Security Considerations
This document should not affect the security of the Internet.
7. References
7.1. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
7.2. Informative References
[IEEE.802.1AR-2018]
"IEEE Standard for Local and Metropolitan Area Networks -
Secure Device Identity", IEEE,
DOI 10.1109/ieeestd.2018.8423794, ISBN ["9781504450195"],
July 2018, <https://doi.org/10.1109/ieeestd.2018.8423794>.
[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, DOI 10.17487/RFC3161, August
2001, <https://www.rfc-editor.org/rfc/rfc3161>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/rfc/rfc4949>.
[RFC4998] Gondrom, T., Brandner, R., and U. Pordesch, "Evidence
Record Syntax (ERS)", RFC 4998, DOI 10.17487/RFC4998,
August 2007, <https://www.rfc-editor.org/rfc/rfc4998>.
[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/rfc/rfc5280>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/rfc/rfc5652>.
[RFC6421] Nelson, D., Ed., "Crypto-Agility Requirements for Remote
Authentication Dial-In User Service (RADIUS)", RFC 6421,
DOI 10.17487/RFC6421, November 2011,
<https://www.rfc-editor.org/rfc/rfc6421>.
[RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
April 2015, <https://www.rfc-editor.org/rfc/rfc7468>.
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[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/rfc/rfc8446>.
[RFC9019] Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
Firmware Update Architecture for Internet of Things",
RFC 9019, DOI 10.17487/RFC9019, April 2021,
<https://www.rfc-editor.org/rfc/rfc9019>.
[I-D.ietf-pquip-pqc-engineers]
Banerjee, A., Reddy.K, T., Schoinianakis, D., and T.
Hollebeek, "Post-Quantum Cryptography for Engineers", Work
in Progress, Internet-Draft, draft-ietf-pquip-pqc-
engineers-03, 22 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-pquip-
pqc-engineers-03>.
[I-D.ietf-pquip-pqt-hybrid-terminology]
D, F., "Terminology for Post-Quantum Traditional Hybrid
Schemes", Work in Progress, Internet-Draft, draft-ietf-
pquip-pqt-hybrid-terminology-02, 2 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-pquip-
pqt-hybrid-terminology-02>.
[NIST.SP.800-208]
Cooper, D. A., Apon, D. C., Dang, Q. H., Davidson, M. S.,
Dworkin, M. J., Miller, C. A., and NIST, "Recommendation
for Stateful Hash-Based Signature Schemes", NIST Special
Publications (General) 800-208,
DOI 10.6028/NIST.SP.800-208, 29 October 2020,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-208.pdf>.
[NIST.FIPS.186-5]
Moody, D. and National Institute of Standards and
Technology, "Digital Signature Standard (DSS)", NIST
Federal Information Processing Standards
Publications 186-5, DOI 10.6028/NIST.FIPS.186-5, 2023,
<https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.186-5.pdf>.
[RFC8391] Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A.
Mohaisen, "XMSS: eXtended Merkle Signature Scheme",
RFC 8391, DOI 10.17487/RFC8391, May 2018,
<https://www.rfc-editor.org/rfc/rfc8391>.
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[RFC8554] McGrew, D., Curcio, M., and S. Fluhrer, "Leighton-Micali
Hash-Based Signatures", RFC 8554, DOI 10.17487/RFC8554,
April 2019, <https://www.rfc-editor.org/rfc/rfc8554>.
[RFC6071] Frankel, S. and S. Krishnan, "IP Security (IPsec) and
Internet Key Exchange (IKE) Document Roadmap", RFC 6071,
DOI 10.17487/RFC6071, February 2011,
<https://www.rfc-editor.org/rfc/rfc6071>.
[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", RFC 8551, DOI 10.17487/RFC8551,
April 2019, <https://www.rfc-editor.org/rfc/rfc8551>.
[I-D.ounsworth-pq-composite-sigs]
Ounsworth, M., Gray, J., Pala, M., and J. Klaußner,
"Composite Signatures For Use In Internet PKI", Work in
Progress, Internet-Draft, draft-ounsworth-pq-composite-
sigs-12, 11 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ounsworth-pq-
composite-sigs-12>.
[I-D.bonnell-lamps-chameleon-certs]
Bonnell, C., Gray, J., Hook, D., Okubo, T., and M.
Ounsworth, "A Mechanism for Encoding Differences in Paired
Certificates", Work in Progress, Internet-Draft, draft-
bonnell-lamps-chameleon-certs-03, 4 January 2024,
<https://datatracker.ietf.org/doc/html/draft-bonnell-
lamps-chameleon-certs-03>.
[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-03, 29 November 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
cert-binding-for-multi-auth-03>.
[ANSI_ASHRAE.Standard.135-2016]
American National Standards Institute (ANSI), "BACnetTM A
Data Communication Protocol For Building Automation And
Control Network", ANSI Standard 135-2016, 2016,
<https://webstore.ansi.org/standards/ashrae/
ansiashraestandard1352016>.
[Addendum.bj.to.ANSI_ASHRAE.Standard.135-2016]
American National Standards Institute (ANSI), "Addendum bj
to BACnetTM A Data Communication Protocol For Building
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Automation And Control Network", ANSI Addendum bj to
Standard 135-2016, 2016,
<https://www.ashrae.org/File%20Library/
Technical%20Resources/Standards%20and%20Guidelines/
Standards%20Addenda/135_2016_bj_20191118.pdf>.
[bsi.quantum-safe.crypto]
Bundesamt fuer Sicherheit in der Informationstechnik,
"Quantum-safe cryptography – fundamentals, current
developments and recommendations", BSI Recommendations for
action by the BSI, 2021,
<https://www.bsi.bund.de/SharedDocs/Downloads/EN/BSI/
Publications/Brochure/quantum-safe-
cryptography.pdf?__blob=publicationFile&v=4>.
[entrust.composite-pki]
Entrust, "Entrust introduces first commerically available
post quantum ready pki platform", 2024,
<https://www.entrust.com/newsroom/press-releases/2024/
entrust-introduces-first-commercially-available-post-
quantum-ready-pki-platform>.
[CNSA2-0] National Security Agency (NSA), "Announcing the Commercial
National Security Algorithm Suite 2.0", 2022,
<https://media.defense.gov/2022/Sep/07/2003071834/-1/-1/0/
CSA_CNSA_2.0_ALGORITHMS_.PDF>.
[Dilithium.des.team]
CRYSTALS-Dilithium design team, "CRYSTALS-Dilithium design
team web page", 2023,
<https://pq-crystals.org/dilithium/>.
[Google.Sec.Blog]
Google, "Toward Quantum Resilient Security Keys", 2023,
<https://security.googleblog.com/2023/08/toward-quantum-
resilient-security-keys.html?m=1>.
[Hybrid.pqc.sig.hsk]
Ghinea, D., Kaczmarczyck, F., Pullman, J., Cretin, J.,
Koebl, S., Misoczki, R., Picod, J.-M., Invernizzi, L., and
E. Bursztein, "Hybrid Post-quantum Signatures in Hardware
Security Keys", 2023,
<https://doi.org/10.1007/978-3-031-41181-6_26>.
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[NIST.FIPS.204]
National Institute of Standards and Technology (NIST),
"Module-Lattice-Based Digital Signature Standard",
NIST FIPS 204 (Initial Public Draft), 2023,
<https://csrc.nist.gov/pubs/fips/204/ipd>.
[NIST.FIPS.205]
National Institute of Standards and Technology (NIST),
"Stateless Hash-Based Digital Signature Standard",
NIST FIPS 205 (Initial Public Draft), 2023,
<https://csrc.nist.gov/pubs/fips/205/ipd>.
[NIST.SP.800-57.P1R5]
National Institute of Standards and Technology (NIST),
"Recommendation for Key Management: Part 1 – General",
NIST Special Publication 800-57, 2020,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-57pt1r5.pdf>.
[X.509] International Telecommunications Union, "Information
technology – Open Systems Interconnection – The Directory:
Public-key and attribute certificate frameworks",
ITU-T Recommendation X.509, 2019.
[CAMM] Hohm, J., Heinemann, A., and A. Wiesmaier, "Towards a
maturity model for crypto-agility assessment", 2022,
<http://arxiv.org/abs/2202.07645>.
Appendix A. Post-Quantum Migration Properties
This section aims to establish a collection of characteristics for
categorizing the use cases outlined in Section 2. The objective is
to enhance the document's utility by providing a framework for
classifying use cases not explicitly addressed here. For instance,
implementors can categorize their own use case and subsequently
identify a similar one in this document based on shared properties/
classification.
A.1. Lifetime
This classification distinguishes between short-lived and long-lived
use cases. However, in practical terms, this distinction is
challenging due to the nature of each use case's lifespan, which can
be on a spectrum.
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1. Short-lived: In this context, a short-lived use case is
characterized by a duration of less than 5 years. This timeframe
aligns with common organizational practices, where hardware, for
example servers in a data center, is typically replaced within a
5-year cycle.
2. Long-lived: In the context of this document, a long-lived use
case spans more than 10 years. While there isn't a specific
rationale for this timeframe, it is noteworthy that cryptographic
recommendations, for example [NIST.SP.800-57.P1R5], often provide
guidance for a duration of up to ten years from the time of their
publication.
A.2. Protocol
Cryptographic protocols can be divided in Active Negotiation (real-
time cryptography), Passive Negotiation (asynchronous cryptography),
and Non Agile (no graceful migration).
1. Active Negotiation: Protocols with existing mechanisms for real-
time cryptographic negotiation such as TLS and IKE already
contain mechanisms for upgraded clients to downgrade the
cryptography in a given session in order to communicate with a
legacy peer. These protocols provide the easiest migration path
as these mechanisms should be used to bridge across traditional
and post-quantum cryptography.
2. Passive Negotiation: Protocols with existing mechanisms for non-
real-time or asynchronous cryptographic negotiation. For example
a PKI end entity who publishes multiple encryption certificates
for themselves, each containing a public key for a different
algorithm, or code signing object carrying multiple signatures on
different algorithms.
3. Non-agile: Non-agile or flag day implies no graceful migration is
possible; the community decides that as of a certain date legacy
clients will no longer be able to interoperate with upgraded
clients.
A.3. Backward compatibility
The following scenarios may arise:
1. Optional: Backward compatibility isn't needed, either because
post-quantum migration is unnecessary or already addressed within
a specific protocol.
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2. Limited: Backward compatibility is necessary for a defined
period, such as during a migration time window.
3. Mandatory: Backward compatibility is essential throughout the use
case's entire lifespan due to the absence of identifiable
migration strategies.
Appendix B. Composite Signature individual and organization position
statements
1. BSI - The document [bsi.quantum-safe.crypto] recommends requiring
that PQC lattice schemes only be used within a PQ/T hybrid. More
specifically this document includes the following recommendation:
"Therefore, quantum computer-resistant methods should not be used
alone - at least in a transitional period - but only in hybrid
mode, i.e. in combination with a classical method. For this
purpose, protocols must be modified or supplemented accordingly.
In addition, public key infrastructures, for example, must also
be adapted"
Also Stavros Kousidis from BSI says: "from a strategic point of
view we don’t want to replace our current RSA algorithm with
standalone Dilithium since: If Dilithium does not withstand
cryptanalysis in the future then all our efforts are for nothing.
With a composite signature Dilithium+ECDSA in AND-mode we can buy
ourselves some time in case the Dilithium security guarantees do
not withstand future cryptanalysis."
2. Google: according to [Google.Sec.Blog]: "Relying on a hybrid
signature is critical as the security of Dilithium and other
recently standardized quantum resistant algorithms haven’t yet
stood the test of time and recent attacks on Rainbow (another
quantum resilient algorithm) demonstrate the need for caution.
This cautiousness is particularly warranted for security keys as
most can’t be upgraded – although we are working toward it for
OpenSK. The hybrid approach is also used in other post-quantum
efforts like Chrome’s support for TLS".
3. Entrust: During the transition to post-quantum cryptography,
there will be uncertainty as to the strength of cryptographic
algorithms; we will no longer fully trust traditional
cryptography such as RSA, Diffie-Hellman, DSA and their elliptic
curve variants, but we will also not fully trust their post-
quantum replacements until they have had sufficient scrutiny and
time to discover and fix implementation bugs. Unlike previous
cryptographic algorithm migrations, the choice of when to migrate
and which algorithms to migrate to, is not so clear. Even after
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the migration period, it may be advantageous for an entity's
cryptographic identity to be composed of multiple public-key
algorithms. In 2024 Entrust added support for composite
signatures in PKI infrastructure ([entrust.composite-pki]):
"With this launch, the company’s cloud-based PKI as a Service
offering now can provide both composite and pure quantum-safe
certificate authority hierarchies, enabling customers to test or
implement quantum-safe scenarios and infrastructure."
4. Robert Hulshof: "The rationale behind combined keys is that I can
see an important use-case for very sensitive data (government,
financial or other high value data) to combine multiple (PQ) key
algorithms, and that this migration to PQ is a good time to start
supporting that by default in the crypto libraries. Trying to
estimate the probability that a NIST standardized Crypto
algorithm gets broken in the next 5-10 years is very difficult.
However I assume that everybody agrees that this probability is
definitely not zero. Personally I would put that probability
somewhere in the range of 0.1% – 1%. If I were the government/
bank etc. I would not like to have a 1% risk that all my secrets
get exposed. Adding one or two more PQ algorithms would reduce
that probability to 1 in 5 million or 1 in a Billion would be
much more acceptable."
5. MTG - Falko Strenzke: "Without hybrid signatures, a decision to
move away from traditional signatures to Dilithium (or other non-
hash-based signatures) has a certain risk to make things worse
and I think many decision makers will not be ready to take the
responsibility for it until the quantum computer threat becomes
imminent. If composite signature is not standardised, non-
composite hybrids would be left. This implies protocol changes
which will:
1. need more discussion,
2. need more changes to existing applications,
3. and thus be more bug prone.
4. Not having hybrid signatures at all will likely cause many
decision makers to
5. use hash-based schemes where possible / affordable
6. and elsewhere stick to traditional schemes as long as
possible, thus effectively delaying the migration to PQC."
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6. Transmute - Orie Steele: "There are use cases for long lived
verifiable credentials, and attribute cert like stuff we work on
in supply chain, with DHS / CBP."
7. CRYSTALS-Dilithium design team states in [Dilithium.des.team]
that: “For users who are interested in using Dilithium, we
recommend the following: Use Dilithium in a so-called hybrid mode
in combination with an established "pre-quantum" signature
scheme.”
8. Hybrid Post-Quantum Signatures in Hardware Security Keys: the
paper [Hybrid.pqc.sig.hsk] describes a hybrid signature scheme.
Below an excerpt from it: “A hybrid signature scheme combines a
classical signature algorithm with a post-quantum secure
signature algorithm. Before discussing the design of our hybrid
scheme, we explain why such an approach is relevant instead of
simply replacing classically secure schemes with post-quantum
secure schemes. We present the assumptions below:
1. Cryptographically-Relevant Quantum Computers (i.e. with
enough qubits to break ECDSA) are not available yet.
2. Classical signature algorithms withstands attacks from
classical computers.
3. The post-quantum secure signature algorithm might be
breakable by classical computers due to design or
implementation bugs. If any of these assumptions fails,
using a hybrid approach instead of replacing classical
schemes with post-quantum schemes indeed does not add any
security. We believe that all of these assumptions are
currently correct. The third assumption is motivated by a
newly discovered attack against Rainbow, one of the NIST
standardization finalists. We can now discuss the informal
requirements a hybrid scheme H should satisfy:
4. If a quantum computer becomes available, and hence H’s
underlying classical scheme is broken, H should maintain the
security of its underlying post-quantum scheme.
5. If a classical attack for H’s underlying post-quantum secure
scheme is discovered, H should maintain the security of its
underlying classical scheme."
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Acknowledgements
This draft would not be possible without the support of a great
number of contributors. We thank Stavros Kousidis, Robert Hulshof,
Falko Strenzke and Orie Steele for allowing us to use their
statements regarding composite signatures.
Authors' Addresses
Antonio Vaira
Siemens
Werner-von-Siemens-Strasse 1
80333 Munich
Germany
Email: antonio.vaira@siemens.com
URI: https://www.siemens.com
Hendrik Brockhaus
Siemens
Werner-von-Siemens-Strasse 1
80333 Munich
Germany
Email: hendrik.brockhaus@siemens.com
URI: https://www.siemens.com
Alexander Railean
Siemens
Werner-von-Siemens-Strasse 1
80333 Munich
Germany
Email: alexander.railean@siemens.com
URI: https://www.siemens.com
John Gray
Entrust
1187 Park Place
Minneapolis, MN 55379
United States of America
Email: john.gray@entrust.com
URI: https://www.entrust.com
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Mike Ounsworth
Entrust
1187 Park Place
Minneapolis, MN 55379
United States of America
Email: mike.ounsworth@entrust.com
URI: https://www.entrust.com
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