Post-Quantum Cryptography for Engineers
draft-ietf-pquip-pqc-engineers-05
| Document | Type | Active Internet-Draft (pquip WG) | |
|---|---|---|---|
| Authors | Aritra Banerjee , Tirumaleswar Reddy.K , Dimitrios Schoinianakis , Tim Hollebeek , Mike Ounsworth | ||
| Last updated | 2024-09-19 (Latest revision 2024-09-12) | ||
| Replaces | draft-ar-pquip-pqc-engineers | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | In WG Last Call | |
| Associated WG milestone |
|
||
| Document shepherd | Paul E. Hoffman | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | paul.hoffman@icann.org |
draft-ietf-pquip-pqc-engineers-05
PQUIP A. Banerjee
Internet-Draft T. Reddy
Intended status: Informational D. Schoinianakis
Expires: 16 March 2025 Nokia
T. Hollebeek
DigiCert
M. Ounsworth
Entrust
12 September 2024
Post-Quantum Cryptography for Engineers
draft-ietf-pquip-pqc-engineers-05
Abstract
The presence of a Cryptographically Relevant Quantum Computer (CRQC)
would render state-of-the-art, traditional public-key algorithms
deployed today obsolete, since the assumptions about the
intractability of the mathematical problems for these algorithms that
offer confident levels of security today no longer apply in the
presence of a CRQC. This means there is a requirement to update
protocols and infrastructure to use post-quantum algorithms, which
are public-key algorithms designed to be secure against CRQCs as well
as classical computers. These new public-key algorithms behave
similarly to previous public key algorithms, however the intractable
mathematical problems have been carefully chosen so they are hard for
CRQCs as well as classical computers. This document explains why
engineers need to be aware of and understand post-quantum
cryptography. It emphasizes the potential impact of CRQCs on current
cryptographic systems and the need to transition to post-quantum
algorithms to ensure long-term security. The most important thing to
understand is that this transition is not like previous transitions
from DES to AES or from SHA-1 to SHA-2. While drop-in replacement
may be possible in some cases, others will require protocol re-design
to accommodate significant differences in behavior between the new
post-quantum algorithms and the classical algorithms that they are
replacing.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-pquip-pqc-engineers/.
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Discussion of this document takes place on the pquip 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/.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 16 March 2025.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 6
3. Contributing to This Document . . . . . . . . . . . . . . . . 6
4. Traditional Cryptographic Primitives that Could Be Replaced by
PQC . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Invariants of Post-Quantum Cryptography . . . . . . . . . . . 7
6. NIST PQC Algorithms . . . . . . . . . . . . . . . . . . . . . 7
6.1. NIST candidates selected for standardization . . . . . . 8
6.1.1. PQC Key Encapsulation Mechanisms (KEMs) . . . . . . . 8
6.1.2. PQC Signatures . . . . . . . . . . . . . . . . . . . 8
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6.2. Candidates advancing to the fourth-round for
standardization at NIST . . . . . . . . . . . . . . . . . 8
7. Threat of CRQCs on Cryptography . . . . . . . . . . . . . . . 9
7.1. Symmetric cryptography . . . . . . . . . . . . . . . . . 10
7.2. Asymmetric cryptography . . . . . . . . . . . . . . . . . 11
7.3. Quantum side-channel attacks . . . . . . . . . . . . . . 12
8. Timeline for transition . . . . . . . . . . . . . . . . . . . 12
9. Post-quantum cryptography categories . . . . . . . . . . . . 14
9.1. Lattice-Based Public-Key Cryptography . . . . . . . . . . 14
9.2. Hash-Based Public-Key Cryptography . . . . . . . . . . . 15
9.3. Code-Based Public-Key Cryptography . . . . . . . . . . . 16
10. KEMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. What is a KEM . . . . . . . . . . . . . . . . . . . . . 17
10.1.1. Authenticated Key Exchange (AKE) . . . . . . . . . . 18
10.2. Security properties . . . . . . . . . . . . . . . . . . 22
10.2.1. IND-CCA2 . . . . . . . . . . . . . . . . . . . . . . 22
10.2.2. Binding . . . . . . . . . . . . . . . . . . . . . . 22
10.3. HPKE . . . . . . . . . . . . . . . . . . . . . . . . . . 23
11. PQC Signatures . . . . . . . . . . . . . . . . . . . . . . . 23
11.1. What is a Post-quantum Signature . . . . . . . . . . . . 23
11.2. Security properties . . . . . . . . . . . . . . . . . . 23
11.2.1. EUF-CMA . . . . . . . . . . . . . . . . . . . . . . 23
11.3. Details of FN-DSA, ML-DSA, and SLH-DSA . . . . . . . . . 24
11.4. Details of XMSS and LMS . . . . . . . . . . . . . . . . 25
11.4.1. LMS scheme - key and signature sizes . . . . . . . . 26
11.5. Hash-then-Sign . . . . . . . . . . . . . . . . . . . . . 27
12. Recommendations for Security / Performance Tradeoffs . . . . 28
13. Comparing PQC KEMs/Signatures vs Traditional KEMs
(KEXs)/Signatures . . . . . . . . . . . . . . . . . . . . 30
14. Post-Quantum and Traditional Hybrid Schemes . . . . . . . . . 32
14.1. PQ/T Hybrid Confidentiality . . . . . . . . . . . . . . 33
14.2. PQ/T Hybrid Authentication . . . . . . . . . . . . . . . 33
14.3. Additional Considerations . . . . . . . . . . . . . . . 34
15. Security Considerations . . . . . . . . . . . . . . . . . . . 36
15.1. Cryptanalysis . . . . . . . . . . . . . . . . . . . . . 36
15.2. Cryptographic Agility . . . . . . . . . . . . . . . . . 37
15.3. Hybrid Key Exchange and Signatures: Bridging the Gap
Between Post-Quantum and Traditional Cryptography . . . 37
15.4. Caution: Ciphertext commitment in KEM vs DH . . . . . . 38
16. Further Reading & Resources . . . . . . . . . . . . . . . . . 38
16.1. Reading List . . . . . . . . . . . . . . . . . . . . . . 38
16.2. Developer Resources . . . . . . . . . . . . . . . . . . 38
17. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 38
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 39
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Normative References . . . . . . . . . . . . . . . . . . . . . 39
Informative References . . . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
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1. Introduction
Quantum computing is no longer perceived as a conjecture of
computational sciences and theoretical physics. Considerable
research efforts and enormous corporate and government funding for
the development of practical quantum computing systems are being
invested currently. At the time of writing the document,
Cryptographically Relevant Quantum Computers (CRQCs) that can break
widely used public-key cryptographic algorithms are not yet
available. However, it is worth noting that there is ongoing
research and development in the field of quantum computing, with the
goal of building more powerful and scalable quantum computers. One
common myth is that quantum computers are faster than conventional
CPUs and GPUs in all areas. This is not the case; much as GPUs
outperform general-purpose CPUs only on specific types of problems,
so too will quantum computers have a niche set of problems on which
they excel; unfortunately for cryptographers, integer factorization
and discrete logarithms, the mathematical problems underpinning all
of modern cryptography, happen to fall within the niche that we
expect quantum computers to excel at. As such, as quantum technology
advances, there is the potential for future quantum computers to have
a significant impact on current cryptographic systems. Predicting
the emergence of CRQC is a challenging task, and there is ongoing
uncertainty regarding when they will become practically feasible.
Extensive research has produced several "post-quantum cryptographic
(PQC) algorithms" (sometimes referred to as "quantum-safe"
algorithms) that offer the potential to ensure cryptography's
survival in the quantum computing era. However, transitioning to a
post-quantum infrastructure is not a straightforward task, and there
are numerous challenges to overcome. It requires a combination of
engineering efforts, proactive assessment and evaluation of available
technologies, and a careful approach to product development. This
document aims to provide general guidance to engineers who utilize
public-key cryptography in their software. It covers topics such as
selecting appropriate PQC algorithms, understanding the differences
between PQC Key Encapsulation Mechanisms (KEMs) and traditional
Diffie-Hellman and RSA style key exchange, and provides insights into
expected key sizes and processing time differences between PQC
algorithms and traditional ones. Additionally, it discusses the
potential threat to symmetric cryptography from Cryptographically
Relevant Quantum Computers (CRQCs). It is important to remember that
asymmetric algorithms (also known as public key algorithms) are
largely used for secure communications between organizations or
endpoints that may not have previously interacted, so a significant
amount of coordination between organizations, and within and between
ecosystems needs to be taken into account. Such transitions are some
of the most complicated in the tech industry and will require staged
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migrations in which upgraded agents need to co-exist and communicate
with non-upgraded agents at a scale never before undertaken. It
might be worth mentioning that recently NSA released an article on
Future Quantum-Resistant (QR) Algorithm Requirements for National
Security Systems [CNSA2-0] based on the need to protect against
deployments of CRQCs in the future. Germany's BSI has also released
a PQC migration and recommendations document [BSI-PQC] which largely
aligns with United States NIST and NSA guidance, but does differ on
some of the guidance.
It is crucial for the reader to understand that when the word "PQC"
is mentioned in the document, it means Asymmetric Cryptography (or
Public key Cryptography) and not any algorithms from the Symmetric
side based on stream, block ciphers, hash functions, MACs, etc, which
are far less vulnerable to quantum computers. This document does not
cover such topics as when traditional algorithms might become
vulnerable (for that, see documents such as [QC-DNS] and others). It
also does not cover unrelated technologies like Quantum Key
Distribution or Quantum Key Generation, which use quantum hardware to
exploit quantum effects to protect communications and generate keys,
respectively. Post-quantum cryptography is based on conventional
(i.e., non-quantum) math and software and can be run on any general
purpose computer.
Please note: This document does not go into the deep mathematics or
technical specification of the PQC algorithms, but rather provides an
overview to engineers on the current threat landscape and the
relevant algorithms designed to help prevent those threats. Also,
the cryptographic and algorithmic guidance given in this document
should be taken as non-authoritative if it conflicts with emerging
and evolving guidance from the IRTF's Cryptographic Forum Research
Group (CFRG).
While there is ongoing discussion about whether to use the term
'Post-Quantum' or 'Quantum Ready/Resistant' to describe algorithms
that resist CRQCs, a consensus has not yet been reached. It's
important to clarify that 'Post-Quantum' refers to algorithms
designed to withstand attacks by CRQCs and classical computers alike.
These algorithms are based on mathematically hard cryptographic
problems that neither CRQCs nor classical computers are expected to
break. The term "quantum resistant" or "quantum ready" are used for
algorithms which are synonymous with Post-Quantum termed algorithms
but a final decision has not yet been reached as to the ambiguity of
these terms.
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2. Conventions and Definitions
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.
3. Contributing to This Document
The guide was inspired by a thread in September 2022 on the
pqc@ietf.org (mailto:pqc@ietf.org) mailing list. The document is
being collaborated on through a GitHub repository
(https://github.com/tireddy2/pqc-for-engineers).
The editors actively encourage contributions to this document.
Please consider writing a section on a topic that you think is
missing. Short of that, writing a paragraph or two on an issue you
found when writing code that uses PQC would make this document more
useful to other coders. Opening issues that suggest new material is
fine too, but relying on others to write the first draft of such
material is much less likely to happen than if you take a stab at it
yourself.
4. Traditional Cryptographic Primitives that Could Be Replaced by PQC
Any asymmetric cryptographic algorithm based on integer
factorization, finite field discrete logarithms or elliptic curve
discrete logarithms will be vulnerable to attacks using Shor's
Algorithm on a sufficiently large general-purpose quantum computer,
known as a CRQC. This document focuses on the principal functions of
asymmetric cryptography:
* Key Agreement and Key Transport: Key Agreement schemes, typically
referred to as Diffie-Hellman (DH) or Elliptic Curve Diffie-
Hellman (ECDH), as well as Key Transport, typically using RSA
Encryption, are used to establish a shared cryptographic key for
secure communication. They are one of the mechanisms that can be
replaced by PQC, as this is based on public key cryptography and
is therefore vulnerable to the Shor's algorithm. A CRQC can
employ Shor's algorithm to efficiently find the prime factors of a
large public key (in case of RSA), which in turn can be exploited
to derive the private key. In the case of Diffie-Hellman, a CRQC
has the potential to calculate the exponent or discrete logarithm
of the (short or long-term) Diffie-Hellman public key. This, in
turn, would reveal the precise secret required to derive the
session key.
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* Digital Signatures: Digital Signature schemes are used to
authenticate the identity of a sender, detect unauthorized
modifications to data and underpin trust in a system. Similar to
Key Agreement, signatures also depend on a public-private key pair
based on the same mathematics as for Key Agreement and Key
Transport, and hence a break in public key cryptography will also
affect traditional digital signatures, hence the importance of
developing post-quantum digital signatures.
5. Invariants of Post-Quantum Cryptography
In the context of PQC, symmetric-key cryptographic algorithms are
generally not directly impacted by quantum computing advancements.
Symmetric-key cryptography, which includes keyed primitives such as
block ciphers (e.g., AES) and message authentication mechanisms
(e.g., HMAC-SHA2), rely on secret keys shared between the sender and
receiver. Symmetric cryptography also includes hash functions (e.g.,
SHA-256) that are used for secure message digesting without any
shared key material. HMAC is a specific construction that utilizes a
cryptographic hash function (such as SHA-2) and a secret key shared
between the sender and receiver to produce a message authentication
code.
CRQCs, in theory, do not offer substantial advantages in breaking
symmetric-key algorithms compared to classical computers, meaning
that current symmetric algorithms can continue to be used with
potentially straightforward increases to key size to stay ahead of
quantum-boosted brute-forcing attacks (see Section 7.1 for more
details).
6. NIST PQC Algorithms
In 2016, the National Institute of Standards and Technology (NIST)
started a process to solicit, evaluate, and standardize one or more
quantum-resistant public-key cryptographic algorithms, as seen here
(https://csrc.nist.gov/projects/post-quantum-cryptography). The
first set of algorithms for standardization
(https://csrc.nist.gov/publications/detail/nistir/8413/final) were
selected in July 2022.
NIST announced as well that they will be opening a fourth round
(https://csrc.nist.gov/csrc/media/Projects/post-quantum-
cryptography/documents/round-4/guidelines-for-submitting-tweaks-
fourth-round.pdf) to standardize an alternative KEM, and a call
(https://csrc.nist.gov/csrc/media/Projects/pqc-dig-sig/documents/
call-for-proposals-dig-sig-sept-2022.pdf) for new candidates for a
post-quantum signature algorithm.
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These algorithms are not a drop-in replacement for classical
asymmetric cryptographic algorithms. For instance, RSA [RSA] and ECC
[RFC6090] can be used as both a key encapsulation method (KEM) and as
a signature scheme, whereas there is currently no post-quantum
algorithm that can perform both functions. When upgrading protocols,
it is important to replace the existing use of classical algorithms
with either a PQC KEM or a PQC signature method, depending on how the
classical algorithm was previously being used. Additionally, KEMs,
as described in Section 10, present a different API than either key
agreement or key transport primitives. As a result, they may require
protocol-level or application-level changes in order to be
incorporated.
6.1. NIST candidates selected for standardization
6.1.1. PQC Key Encapsulation Mechanisms (KEMs)
* ML-KEM (https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.203.ipd.pdf): Module-Lattice-based Key-Encapsulation
Mechanism Standard (FIPS-203).
6.1.2. PQC Signatures
* ML-DSA (https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.204.ipd.pdf): Module-Lattice-Based Digital Signature
Standard (FIPS-204).
* SLH-DSA (https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.205.ipd.pdf): Stateless Hash-Based Digital Signature
(FIPS-205). Standard (FIPS-205).
* FN-DSA (https://falcon-sign.info/): FN-DSA is a lattice signature
scheme (Section 9.1 and Section 11.3).
6.2. Candidates advancing to the fourth-round for standardization at
NIST
The fourth-round of the NIST process focuses only on KEMs. The goal
of that round is to select an alternative algorithm that is based on
different hard problem than ML-KEM. The candidates still advancing
for standardization are:
* Classic McEliece (https://classic.mceliece.org/): Based on the
hardness of syndrome decoding of Goppa codes. Goppa codes are a
class of error-correcting codes that can correct a certain number
of errors in a transmitted message. The decoding problem involves
recovering the original message from the received noisy codeword.
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* BIKE (https://bikesuite.org/): Based on the the hardness of
syndrome decoding of QC-MDPC codes. Quasi-Cyclic Moderate Density
Parity Check (QC-MDPC) code are a class of error correcting codes
that leverages bit flipping technique to efficiently correct
errors.
* HQC (http://pqc-hqc.org/) : Based on the hardness of syndrome
decoding of Quasi-cyclic concatenated Reed Muller Reed Solomon
(RMRS) codes in the Hamming metric. Reed Muller (RM) codes are a
class of block error correcting codes used especially in wireless
and deep space communications. Reed Solomon (RS) are a class of
block error correcting codes that are used to detect and correct
multiple bit errors.
* SIKE (https://sike.org/) (Broken): Supersingular Isogeny Key
Encapsulation (SIKE) is a specific realization of the SIDH
(Supersingular Isogeny Diffie-Hellman) protocol. Recently, a
mathematical attack (https://eprint.iacr.org/2022/975.pdf) based
on the "glue-and-split" theorem from 1997 from Ernst Kani was
found against the underlying chosen starting curve and torsion
information. In practical terms, this attack allows for the
efficient recovery of the private key. NIST announced that SIKE
was no longer under consideration, but the authors of SIKE had
asked for it to remain in the list so that people are aware that
it is broken. While SIKE is broken, Isogenies in general remain
an active area of cryptographic research due to their very
attractive bandwidth usage, and we may yet see more cryptographic
primitives in the future from this research area.
7. Threat of CRQCs on Cryptography
Post-quantum cryptography or quantum-safe cryptography refers to
cryptographic algorithms that are secure against cryptographic
attacks from both CRQCs and classic computers.
When considering the security risks associated with the ability of a
quantum computer to attack traditional cryptography, it is important
to distinguish between the impact on symmetric algorithms and public-
key ones. Dr. Peter Shor and Dr. Lov Grover developed two algorithms
that changed the way the world thinks of security under the presence
of a CRQC.
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It is also worth some discussion of the term "quantum adversary".
Quantum computers are, by their nature, hybrids of classical and
quantum computational units. For example, Shor's algorithm consists
of a combination of quantum and classical computational steps. Thus,
the term "quantum adversary" should be thought of as 'quantum-
enhanced adversary,' meaning they have access to both classical and
quantum computational techniques.
Despite the fact that large-scale quantum computers do not yet exist
to experiment on, the theoretical properties of quantum computation
are very well understood. This allows us to reason today about the
upper limits of quantum-enhanced computation, and indeed to design
cryptographic algorithms that are resistant to any conceivable for of
quantum cryptanalysis.
7.1. Symmetric cryptography
For unstructured data such as symmetric encrypted data or
cryptographic hashes, although CRQCs can search for specific
solutions across all possible input combinations (e.g., Grover's
Algorithm), no quantum algorithm is known to break the underlying
security properties of these classes of algorithms.
Grover's algorithm is a quantum search algorithm that provides a
theoretical quadratic speedup for searching an unstructured database,
compared to classical search algorithms. If we consider the mapping
of hash values to their corresponding hash inputs (also known as pre-
image), or of ciphertext blocks to the corresponding plaintext
blocks, as an unstructured database, then Grover’s algorithm
theoretically requires doubling the key sizes of the symmetric
algorithms that are currently deployed today to counter the quadratic
speedup and maintain current security level. This is because
Grover’s algorithm reduces the amount of operations to break 128-bit
symmetric cryptography to 2^{64} quantum operations, which might
sound computationally feasible. However, 2^{64} operations performed
in parallel are feasible for modern classical computers, but 2^{64}
quantum operations performed serially in a quantum computer are not.
Grover's algorithm is highly non-parallelizable and even if one
deploys 2^c computational units in parallel to brute-force a key
using Grover's algorithm, it will complete in time proportional to
2^{(128−c)/2}, or, put simply, using 256 quantum computers will only
reduce runtime by a factor of 16, 1024 quantum computers will only
reduce runtime by a factor of 32 and so forth (see [NIST] and
[Cloudflare]). Therefore, while Grover's attack suggests that we
should double the sizes of symmetric keys, the current consensus
among experts is that the current key sizes remain secure in
practice.
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How can someone be sure that an improved algorithm won’t outperform
Grover's algorithm at some point in time? Christof Zalka has shown
that Grover's algorithm (and in particular its non-parallel nature)
achieves the best possible complexity for unstructured search
[Grover-search].
Finally, in their evaluation criteria for PQC, NIST is assessing the
security levels of proposed post-quantum algorithms by comparing them
against the equivalent classical and quantum security of AES-128,
192, and 256. This indicates that NIST is confident in the stable
security properties of AES, even in the presence of both classical
and quantum attacks. As a result, 128-bit algorithms can be
considered quantum-safe for the foreseeable future.
7.2. Asymmetric cryptography
“Shor’s algorithm” on the other side, efficiently solves the integer
factorization problem (and the related discrete logarithm problem),
which offer the foundations of the vast majority of public-key
cryptography that the world uses today. This implies that, if a CRQC
is developed, today’s public-key cryptography algorithms (e.g., RSA,
Diffie-Hellman and Elliptic Curve Cryptography, as well as less
commonly-used variants such as ElGamal and Schnorr signatures) and
protocols would need to be replaced by algorithms and protocols that
can offer cryptanalytic resistance against CRQCs. Note that Shor’s
algorithm cannot run solely on a classic computer, it needs a CRQC.
For example, to provide some context, one would need 20 million noisy
qubits to break RSA-2048 in 8 hours [RSAShor][RSA8HRS] or 4099 stable
(or logical) qubits to break it [RSA10SC].
For structured data such as public keys and signatures, instead,
CRQCs can fully solve the underlying hard problems used in classic
cryptography (see Shor's Algorithm). Because an increase of the size
of the key-pair would not provide a secure solution short of RSA keys
that are many gigabytes in size [PQRSA], a complete replacement of
the algorithm is needed. Therefore, post-quantum public-key
cryptography must rely on problems that are different from the ones
used in classic public-key cryptography (i.e., the integer
factorization problem, the finite-field discrete logarithm problem,
and the elliptic-curve discrete logarithm problem).
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7.3. Quantum side-channel attacks
The field of cryptographic side-channel attacks potentially stands to
gain a boost in attacker power once cryptanalytic techniques can be
enhanced with quantum computation techniques. While a full
discussion of quantum side-channel techniques is beyond the scope of
this document, implementers of cryptographic hardware should be aware
that current best-practices for side-channel resistance may not be
sufficient against quantum adversaries.
8. Timeline for transition
The timeline, and driving motivation for transition differs slightly
between data confidentiality (e.g., encryption) and data
authentication (e.g., signature) use-cases.
For data confidentiality, we are concerned with the so-called
"Harvest Now, Decrypt Later" attack where a malicious actor with
adequate resources can launch an attack to store sensitive encrypted
data today that can be decrypted once a CRQC is available. This
implies that, every day, sensitive encrypted data is susceptible to
the attack by not implementing quantum-safe strategies, as it
corresponds to data being deciphered in the future.
For authentication, it is often the case that signatures have a very
short lifetime between signing and verifying -- such as during a TLS
handshake -- but some authentication use-cases do require long
lifetimes, such as signing firmware or software that will be active
for decades, signing legal documents, or signing certificates that
will be embedded into hardware devices such as smartcards. And even
for short-lived signatures use cases, the infrastructure often relies
on long-lived root keys which can be difficult to update or replace
on in-field devices.
+------------------------+----------------------------+
| | |
| y | x |
+------------------------+----------+-----------------+
| | <--------------->
| z | Security gap
+-----------------------------------+
Figure 1: Mosca model
These challenges are illustrated nicely by the so-called Mosca model
discussed in [Threat-Report]. In the Figure 1, "x" denotes the time
that our systems and data need to remain secure, "y" the number of
years to fully migrate to a PQC infrastructure and "z" the time until
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a CRQC that can break current cryptography is available. The model
assumes either that encrypted data can be intercepted and stored
before the migration is completed in "y" years, or that signatures
will still be relied upon for "x" years after their creation. This
data remains vulnerable for the complete "x" years of their lifetime,
thus the sum "x+y" gives us an estimate of the full timeframe that
data remain insecure. The model essentially asks how are we
preparing our IT systems during those "y" years (or in other words,
how can one minimize those "y" years) to minimize the transition
phase to a PQC infrastructure and hence minimize the risks of data
being exposed in the future.
Finally, other factors that could accelerate the introduction of a
CRQC should not be under-estimated, like for example faster-than-
expected advances in quantum computing and more efficient versions of
Shor’s algorithm requiring fewer qubits. Innovation often comes in
waves, so it is to the industry’s benefit to remain vigilant and
prepare as early as possible. Bear in mind also that while we track
advances from public research institutions such as universities and
companies that publish their results, there is also a great deal of
large-budget quantum research being conducted privately by various
national interests. Therefore, the true state of quantum computer
advancement is likely several years ahead of the publicly available
research.
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Organizations should also consider carefully and honestly what their
migration timeline "y" actually is. If you think only of the time
between receiving a patch from your technology vendor, and rolling
that patch out, then "y" might seem as short as a few weeks.
However, this represents the minority of migration cases; more often
a PQC migration will involve at least some amount of hardware
replacement. For example performance-sensitive applications will
need CPUs with PQC hardware acceleration. Security-sensitive
applications will need PQC TPMs, TEEs, Secure Enclaves, and other
cryptographic co-processors. Smartcard applications will require
replacement of the cards and also of the readers which can come in
many form-factors: tap-for-entry door and turnstile readers, PIN pad
machines, laptops with built-in smartcard readers, and many others.
Included in "y" is not only the deployment time, but also preparation
time: integration, testing, auditing and re-certification of
cryptographic environments. Consider also upstream effects that
contribute to "y", including lead-times for your vendors to produce
PQC-ready products, which may itself include auditing and
certification delays, time for regulating bodies to adopt PQC
policies, time for auditors to become familiar with the new
requirements, etc. If you measure the full migration time "y" from
when your vendors begin implementing PQC functionality, to when you
switch off your last non-PQC-capable device, then "y" can be quite
long; likely measured in years or decades for most moderately-sized
organizations.
9. Post-quantum cryptography categories
The current set of problems used in post-quantum cryptography can be
currently grouped into three different categories: lattice-based,
hash-based and code-based.
9.1. Lattice-Based Public-Key Cryptography
Lattice-based public-key cryptography leverages the simple
construction of lattices (i.e., a regular collection of points in a
Euclidean space that are evenly spaced) to create 'trapdoor'
problems. These problems are efficient to compute if you possess the
secret information but challenging to compute otherwise. Examples of
such problems include the Shortest Vector, Closest Vector, Shortest
Integer Solution, Learning with Errors, Module Learning with Errors,
and Learning with Rounding problems. All of these problems feature
strong proofs for worst-to-average case reduction, effectively
relating the hardness of the average case to the worst case.
The possibility to implement public-key schemes on lattices is tied
to the characteristics of the vector basis used for the lattice. In
particular, solving any of the mentioned problems can be easy when
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using "reduced" or "good" bases (i.e., as short as possible and as
orthogonal as possible), while it becomes computationally infeasible
when using "bad" bases (i.e., long vectors not orthogonal). Although
the problem might seem trivial, it is computationally hard when
considering many dimensions, or when the underlying field is not
simple numbers, but high-order polynomials. Therefore, a typical
approach is to use "bad" basis for public keys and "good" basis for
private keys. The public keys ("bad" basis) let you easily verify
signatures by checking, for example, that a vector is the closest or
smallest, but do not let you solve the problem (i.e., finding the
vector) that would yield the private key. Conversely, private keys
(i.e., the "good" basis) can be used for generating the signatures
(e.g., finding the specific vector).
Lattice-based schemes usually have good performances and average size
public keys and signatures (average within the PQC primitives at
least, they are still several orders of magnitude larger than RSA or
ECC signatures), making them the best available candidates for
general-purpose use such as replacing the use of RSA in PKIX
certificates.
Examples of such class of algorithms include ML-KEM, FN-DSA and ML-
DSA.
It is noteworthy that lattice-based encryption schemes require a
rounding step during decryption which has a non-zero probability of
"rounding the wrong way" and leading to a decryption failure, meaning
that valid encryptions are decrypted incorrectly; as such, an
attacker could significantly reduce the security of lattice-based
schemes that have a relatively high failure rate. However, for most
of the NIST Post-Quantum Proposals, the number of required oracle
queries to force a decryption failure is above practical limits, as
has been shown in [LattFail1]. More recent works have improved upon
the results in [LattFail1], showing that the cost of searching for
additional failing ciphertexts after one or more have already been
found, can be sped up dramatically [LattFail2]. Nevertheless, at
this point in time (July 2023), the PQC candidates by NIST are
considered secure under these attacks and we suggest constant
monitoring as cryptanalysis research is ongoing.
9.2. Hash-Based Public-Key Cryptography
Hash based PKC has been around since the 1970s, when it was developed
by Lamport and Merkle. It is used to create digital signature
algorithms and its security is mathematically based on the security
of the selected cryptographic hash function. Many variants of hash-
based signatures (HBS) have been developed since the 70s including
the recent XMSS [RFC8391], HSS/LMS [RFC8554] or BPQS schemes. Unlike
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digital signature techniques, most hash-based signature schemes are
stateful, which means that signing necessitates the update and
careful tracking of the secret key. Producing multiple signatures
using the same secret key state results in loss of security and
ultimately signature forgery attacks against that key.
Stateful hash-based signatures with long service lifetimes require
additional operational complexity compared with other signature
types. For example, consider a 20-year root key; there is an
expectation that 20 years is longer than the expected lifetime of the
hardware that key is stored on, and therefore the key will need to be
migrated to new hardware at some point. Disaster-recovery scenarios
where the primary node fail without warning can be similarly tricky.
This requires careful operational and compliance consideration to
ensure that no private key state can be re-used across the migration
or disaster recovery event. One approach for avoiding these issues
is to only use stateful HBS for short-term use cases that do not
require horizontal scaling, for example signing a batch of firmware
images and then retiring the signing key.
The SLH-DSA algorithm on the other hand leverages the HORST (Hash to
Obtain Random Subset with Trees) technique and remains the only hash
based signature scheme that is stateless, thus avoiding all the
complexities with state management.
SLH-DSA is an advancement on SPHINCS which reduces the signature
sizes in SPHINCS and makes it more compact. SLH-DSA was recently
standardized by NIST.
9.3. Code-Based Public-Key Cryptography
This area of cryptography started in the 1970s and 80s based on the
seminal work of McEliece and Niederreiter which focuses on the study
of cryptosystems based on error-correcting codes. Some popular error
correcting codes include the Goppa codes (used in McEliece
cryptosystems), encoding and decoding syndrome codes used in Hamming
Quasi-Cyclic (HQC) or Quasi-cyclic Moderate density parity check (QC-
MDPC) codes.
Examples include all the NIST Round 4 (unbroken) finalists: Classic
McEliece, HQC, BIKE.
10. KEMs
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10.1. What is a KEM
A Key Encapsulation Mechanism (KEM) is a cryptographic technique used
for securely exchanging symmetric key material between two parties
over an insecure channel. It is commonly used in hybrid encryption
schemes, where a combination of asymmetric (public key) and symmetric
encryption is employed. The KEM encapsulation results in a fixed-
length symmetric key that can be used with a symmetric algorithm,
typically a block cipher, in one of two ways: (1) Derive a Data
Encryption Key (DEK) to encrypt the data (2) Derive a Key Encryption
Key (KEK) used to wrap a DEK. These techniques are often referred to
as "hybrid public key encryption (HPKE)" [RFC9180] mechanism.
The term "encapsulation" is chosen intentionally to indicate that KEM
algorithms behave differently at the API level than the Key Agreement
or Key Encipherment / Key Transport mechanisms that we are accustomed
to using today. Key Agreement schemes imply that both parties
contribute a public / private keypair to the exchange, while Key
Encipherment / Key Transport schemes imply that the symmetric key
material is chosen by one party and "encrypted" or "wrapped" for the
other party. KEMs, on the other hand, behave according to the
following API:
KEM relies on the following primitives [PQCAPI]:
* def kemKeyGen() -> (pk, sk)
* def kemEncaps(pk) -> (ss, ct)
* def kemDecaps(ct, sk) -> ss
where pk is public key, sk is secret key, ct is the ciphertext
representing an encapsulated key, and ss is shared secret. The
following figure illustrates a sample flow of KEM based key exchange:
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+---------+ +---------+
| Client | | Server |
+---------+ +---------+
+----------------------+ | |
| pk, sk = kemKeyGen() |-| |
+----------------------+ | |
| |
| pk |
|---------->|
| | +-----------------------+
| |-| ss, ct = kemEncaps(pk)|
| | +-----------------------+
| |
| ct |
|<----------|
+------------------------+ | |
| ss = kemDecaps(ct, sk) |-| |
+------------------------+ | |
| |
Figure 2: KEM based Key Exchange
10.1.1. Authenticated Key Exchange (AKE)
Authenticated Key Exchange with KEMs where both parties contribute a
KEM public key to the overall session key is interactive as described
in [I-D.draft-ietf-lake-edhoc-22]. However, single-sided KEM, such
as when one peer has a KEM key in a certificate and the other peer
wants to encrypt for it (as in S/MIME or OpenPGP email), can be
achieved using non-interactive HPKE [RFC9180]. The following figure
illustrates the Diffie-Hellman (DH) Key exchange:
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+---------+ +---------+
| Client | | Server |
+---------+ +---------+
+-----------------------+ | |
| Long-term client key: | | |
| sk1, pk1 |-| |
+-----------------------+ | |
| |
| pk1 |
|---------->|
| | +------------------------+
| |-| Long-term server key: |
| | | sk2, pk2 |
| | | ss = KeyEx(pk1, sk2) |
| | +------------------------+
| |
| pk2|
|<----------|
+-------------------------+ | |
| ss = KeyEx(pk2, sk1) | | |
| encryptContent(ss) |-| |
+-------------------------+ | |
| encrypted |
| content |
|---------->|
| | +------------------------+
| | | decryptContent(ss) |
| | +------------------------+
Figure 3: Diffie-Hellman based Authenticated Key Exchange
What's important to note about the sample flow above is that the
shared secret ss is derived using key material from both the Client
and the Server, which classifies it as an Authenticated Key Exchange
(AKE). There is another property of a key exchange, called Non-
Interactive Key Exchange (NIKE) which refers to whether the sender
can compute the shared secret ss and encrypting content without
requiring active interaction -- ie an exchange of network messages --
with the recipient. Figure 3 shows a Diffie-Hellman key exchange
which is an AKE, since both parties are using long-term keys which
can have established trust for example via certificates, but it is
not a NIKE since the client needs to wait for the network interaction
to receive the receiver's public key pk2 before it can compute the
shared secret ss and begin content encryption. However, a DH key
exchange can be an AKE and a NIKE at the same time if the receiver's
public key is known to the sender in advance, and many Internet
Protocols rely on this property of DH-based key exchanges.
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+---------+ +---------+
| Client | | Server |
+---------+ +---------+
+-----------------------+ | |
| Long-term client key: | | |
| sk1, pk1 |-| |
| Long-term server key: | | |
| pk2 | | |
| ss = KeyEx(pk2, sk1) | | |
| encryptContent(ss) |-| |
+-----------------------+ | |
| |
| pk1, |
| encrypted |
| content |
|---------->|
| | +------------------------+
| |-| Long-term server key: |
| | | sk2, pk2 |
| | | ss = KeyEx(pk1, sk2) |
| | | decryptContent(ss) |
| | +------------------------+
Figure 4: Diffie-Hellman based Authenticated Key Exchange and
Non-Interactive Key Exchange simultaneously
The complication with KEMs is that a KEM Encaps() is non-
deterministic; it involves randomness chosen by the sender of that
KEM. Therefore, in order to perform an AKE, the client must wait for
the server to generate the needed randomness and perform Encaps()
against the client key, which necessarily requires a network round-
trip. Therefore a KEM-based protocol can either be an AKE or a NIKE,
but cannot be both at the same time. Consequently, certain Internet
protocols will necessitate redesign to accommodate this distinction,
either by introducing extra network round-trips or by making trade-
offs in security properties.
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+---------+ +---------+
| Client | | Server |
+---------+ +---------+
+------------------------+ | |
| pk1, sk1 = kemKeyGen() |-| |
+------------------------+ | |
| |
|pk1 |
|---------->|
| | +--------------------------+
| |-| ss1, ct1 = kemEncaps(pk1)|
| | | pk2, sk2 = kemKeyGen() |
| | +--------------------------+
| |
| ct1,pk2|
|<----------|
+------------------------+ | |
| ss1 = kemDecaps(ct1, sk1)|-| |
| ss2, ct2 = kemEncaps(pk2)| |
| ss = Combiner(ss1, ss2)| | |
+------------------------+ | |
| |
|ct2 |
|---------->|
| | +--------------------------+
| |-| ss2 = kemDecaps(ct2, sk2)|
| | | ss = Combiner(ss1, ss2) |
| | +--------------------------+
Figure 5: KEM based Authenticated Key Exchange
Here, Combiner(ss1, ss2), often referred to as a KEM Combiner is a
cryptographic construction that takes in two shared secrets and
returns a single combined shared secret. The simplest combiner is
concatenation ss1 || ss2, but combiners can vary in complexity
depending on the cryptographic properties required. For example if
the combination should preserve IND-CCA2 of either input even if the
other is chosen maliciously, then a more complex construct is
required. Another consideration for combiner design is so-called
"binding properties" introduced in [KEEPINGUP] which may require the
ciphertexts and recipient public keys to be included in the combiner.
KEM combiner security analysis becomes more complicated in hybrid
settings where the two KEMs represent different algorithms, for
example one is ML-KEM and the other is ECDHE. For a more thorough
discussion of KEM combiners, see [KEEPINGUP],
[I-D.draft-ounsworth-cfrg-kem-combiners-04], and
[I-D.draft-connolly-cfrg-xwing-kem-02].
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10.2. Security properties
10.2.1. IND-CCA2
IND-CCA2 : IND-CCA2 (INDistinguishability under adaptive Chosen-
Ciphertext Attack) is an advanced security notion for encryption
schemes. It ensures the confidentiality of the plaintext and
resistance against chosen-ciphertext attacks. An appropriate
definition of IND-CCA2 security for KEMs can be found in [CS01] and
[BHK09]. ML-KEM [ML-KEM] and Classic McEliece provide IND-CCA2
security.
Understanding IND-CCA2 security is essential for individuals involved
in designing or implementing cryptographic systems and protocols in
order to evaluate the strength of the algorithm, assess its
suitability for specific use cases, and ensure that data
confidentiality and security requirements are met. Understanding
IND-CCA2 security is generally not necessary for developers migrating
to using an IETF-vetted key establishment method (KEM) within a given
protocol or flow. IND-CCA2 is considered the highest bar that a
public key encryption mechanism can meet, and therefore is suitable
for all uses. IETF specification authors should include all security
concerns in the 'Security Considerations' section of the relevant RFC
and not rely on implementers being deep experts in cryptographic
theory.
10.2.2. Binding
KEMs also have an orthogonal set of properties to consider when
designing protocols around them: binding [KEEPINGUP]. This can be
"ciphertext binding", "public key binding", "context binding", or any
other property that is important to not be substituted between KEM
invocations. In general, a KEM is considered to bind a certain value
if substitution of that value by an attacker will necessarily result
in a different shared secret being derived. As an example, if an
attacker can construct two different ciphertexts which will
decapsulate to the same shared secret; or can construct a ciphertext
which will decapsulate to the same shared secret under two different
public keys, or can substitute whole KEM exchanges from one session
into another, then the construction is not ciphertext binding, public
key binding, or context binding respectively. Similarly, protocol
designers may wish to bind protocol state information such as a
transaction ID or nonce so that attempts to replay ciphertexts from
one session inside a different session will be blocked at the
cryptographic level because the server derives a different shared
secret and is thus is unable to decrypt the content.
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The solution to binding is generally achieved at the protocol design
level: do not use the KEM output shared secret directly. Even though
modern KEMs such as ML-KEM produce full-entropy shared secrets, it is
still advisable for binding reasons to pass it through a key
derivation function (KDF) and also include all values that you wish
to bind; then finally you will have a shared secret that is safe to
use at the protocol level.
10.3. HPKE
Modern cryptography has long used the notion of "hybrid encryption"
where an asymmetric algorithm is used to establish a key, and then a
symmetric algorithm is used for bulk content encryption.
HPKE (Hybrid Public Key Encryption) [RFC9180] is a specific
instantiation of this which works with a combination of KEMs, KDFs
and AEAD schemes (Authenticated Encryption with Additional Data).
HPKE includes three authenticated variants, including one that
authenticates possession of a pre-shared key and two optional ones
that authenticate possession of a key encapsulation mechanism (KEM)
private key. HPKE can be extended to support hybrid post-quantum KEM
[I-D.westerbaan-cfrg-hpke-xyber768d00-02]. ML-KEM does not support
the static-ephemeral key exchange that allows HPKE based on DH based
KEMs and its optional authenticated modes as discussed in Section 1.2
of [I-D.westerbaan-cfrg-hpke-xyber768d00-02] and section 1.5 of
[I-D.draft-connolly-cfrg-xwing-kem-02].
11. PQC Signatures
11.1. What is a Post-quantum Signature
Any digital signature scheme that provides a construction defining
security under post-quantum setting falls under this category of PQ
signatures.
11.2. Security properties
11.2.1. EUF-CMA
EUF-CMA (Existential Unforgeability under Chosen Message Attack)
[GMR88] is a security notion for digital signature schemes. It
guarantees that an adversary, even with access to a signing oracle,
cannot forge a valid signature for an arbitrary message. EUF-CMA
provides strong protection against forgery attacks, ensuring the
integrity and authenticity of digital signatures by preventing
unauthorized modifications or fraudulent signatures. ML-DSA, FN-DSA
and SLH-DSA provide EUF-CMA security.
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Understanding EUF-CMA security is essential for individuals involved
in designing or implementing cryptographic systems in order to ensure
the security, reliability, and trustworthiness of digital signature
schemes. It allows for informed decision-making, vulnerability
analysis, compliance with standards, and designing systems that
provide strong protection against forgery attacks. Understanding
EUF-CMA security is generally not necessary for developers migrating
to using an IETF-vetted post-quantum cryptography (PQC) signature
scheme within a given protocol or flow. EUF-CMA is considered the
highest bar that a public key signature algorithm can meet, and
therefore is suitable for all uses. IETF specification authors
should include all security concerns in the 'Security Considerations'
section of the relevant RFC and should not assume that implementers
are deep experts in cryptographic theory
11.3. Details of FN-DSA, ML-DSA, and SLH-DSA
ML-DSA [ML-DSA] is a digital signature algorithm (part of the
CRYSTALS suite) based on the hardness of lattice problems over module
lattices (i.e., the Module Learning with Errors problem (MLWE)). The
design of the algorithm is based on the "Fiat-Shamir with Aborts"
[Lyu09] framework introduced by Lyubashevsky, that leverages
rejection sampling to render lattice based FS schemes compact and
secure. ML-DSA uses uniformly-distributed random number sampling
over small integers for computing coefficients in error vectors,
which makes the scheme easier to implement compared with FN-DSA
[FN-DSA] which uses Guassian-distributed numbers.
ML-DSA offers both deterministic and randomized signing and is
instantiated with 3 parameter sets providing different security
levels. Security properties of ML-DSA are discussed in Section 9 of
[I-D.ietf-lamps-dilithium-certificates].
FN-DSA [FN-DSA] is based on the GPV hash-and-sign lattice-based
signature framework introduced by Gentry, Peikert and Vaikuntanathan,
which is a framework that requires a certain class of lattices and a
trapdoor sampler technique.
The main design principle of FN-DSA is compactness, i.e. it was
designed in a way that achieves minimal total memory bandwidth
requirement (the sum of the signature size plus the public key size).
This is possible due to the compactness of NTRU lattices. FN-DSA
also offers very efficient signing and verification procedures. The
main potential downsides of FN-DSA refer to the non-triviality of its
algorithms and the need for floating point arithmetic support in
order to support Gaussian-distributed random number sampling where
the other lattice schemes use the less efficient but easier to
support uniformly-distributed random number sampling.
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Implementers of FN-DSA need to be aware that FN-DSA signing is highly
susceptible to side-channel attacks, unless constant-time 64-bit
floating-point operations are used. This requirement is extremely
platform-dependent, as noted in NIST's report.
The performance characteristics of ML-DSA and FN-DSA may differ based
on the specific implementation and hardware platform. Generally, ML-
DSA is known for its relatively fast signature generation, while FN-
DSA can provide more efficient signature verification. The choice
may depend on whether the application requires more frequent
signature generation or signature verification (See [LIBOQS]). For
further clarity on the sizes and security levels, please refer to the
tables in sections Section 12 and Section 13.
SLH-DSA [SLH-DSA] utilizes the concept of stateless hash-based
signatures, where each signature is unique and unrelated to any
previous signature (as discussed in Section 9.2). This property
eliminates the need for maintaining state information during the
signing process. SLH-DSA was designed to sign up to 2^64 messages
and it offers three security levels. The parameters for each of the
security levels were chosen to provide 128 bits of security, 192 bits
of security, and 256 bits of security. SLH-DSA offers smaller public
key sizes, larger signature sizes, slower signature generation, and
slower verification when compared to ML-DSA and FN-DSA. SLH-DSA does
not introduce a new hardness assumption beyond those inherent to the
underlying hash functions. It builds upon established foundations in
cryptography, making it a reliable and robust digital signature
scheme for a post-quantum world. The advantages and disadvantages of
SLH-DSA over other signature algorithms is discussed in Section 3.1
of [I-D.draft-ietf-cose-sphincs-plus].
11.4. Details of XMSS and LMS
The eXtended Merkle Signature Scheme (XMSS) [RFC8391] and
Hierarchical Signature Scheme (HSS) / Leighton-Micali Signature (LMS)
[RFC8554] are stateful hash-based signature schemes, where the secret
key changes over time. In both schemes, reusing a secret key state
compromises cryptographic security guarantees.
Multi-Tree XMSS and LMS can be used for signing a potentially large
but fixed number of messages and the number of signing operations
depends upon the size of the tree. XMSS and LMS provide
cryptographic digital signatures without relying on the conjectured
hardness of mathematical problems, instead leveraging the properties
of cryptographic hash functions. XMSS and Hierarchical Signature
System (HSS) use a hierarchical approach with a Merkle tree at each
level of the hierarchy. [RFC8391] describes both single-tree and
multi-tree variants of XMSS, while [RFC8554] describes the Leighton-
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Micali One-Time Signature (LM-OTS) system as well as the LMS and HSS
N-time signature systems. Comparison of XMSS and LMS is discussed in
Section 10 of [RFC8554].
The number of tree layers in XMSS^MT provides a trade-off between
signature size on the one side and key generation and signing speed
on the other side. Increasing the number of layers reduces key
generation time exponentially and signing time linearly at the cost
of increasing the signature size linearly.
Due to the complexities described above, the XMSS and LMS are not a
suitable replacement for classical signature schemes like RSA or
ECDSA. Applications that expect a long lifetime of a signature, like
firmware update or secure boot, are typical use cases where those
schemes can be successfully applied.
11.4.1. LMS scheme - key and signature sizes
The LMS scheme is characterized by four distinct parameter sets -
underlying hash function (SHA2-256 or SHAKE-256), the length of the
digest (24 or 32 bytes), LMS tree height - parameter that controls a
maximal number of signatures that the private key can produce
(possible values are 5,10,15,20,25) and the width of the Winternitz
coefficients (see [RFC8554], section 4.1) that can be used to trade-
off signing time for signature size (possible values are 1,2,4,8).
Parameters can be mixed, providing 80 possible parametrizations of
the scheme.
The public (PK) and private (SK) key size depends on the length of
the digest (M). The signature size depends on the Winternitz
parameter (W), the LMS tree height (H), and the length of the digest.
The tables below provides key and signature sizes for
parameterization with the digest size M=32 of the scheme.
+====+====+===+======+======+======+======+======+
| PK | SK | W | H=5 | H=10 | H=15 | H=20 | H=25 |
+====+====+===+======+======+======+======+======+
| 56 | 52 | 1 | 8684 | 8844 | 9004 | 9164 | 9324 |
+----+----+---+------+------+------+------+------+
| 56 | 52 | 2 | 4460 | 4620 | 4780 | 4940 | 5100 |
+----+----+---+------+------+------+------+------+
| 56 | 52 | 4 | 2348 | 2508 | 2668 | 2828 | 2988 |
+----+----+---+------+------+------+------+------+
| 56 | 52 | 8 | 1292 | 1452 | 1612 | 1772 | 1932 |
+----+----+---+------+------+------+------+------+
Table 1
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11.5. Hash-then-Sign
Within the hash-then-sign paradigm, the message is hashed before
signing it. By pre-hashing, the onus of resistance to existential
forgeries becomes heavily reliant on the collision-resistance of the
hash function in use. The hash-then-sign paradigm has the ability to
improve application performance by reducing the size of signed
messages that need to be transmitted between application and
cryptographic module, and making the signature size predictable and
manageable. As a corollary, hashing remains mandatory even for short
messages and assigns a further computational requirement onto the
verifier. This makes the performance of hash-then-sign schemes more
consistent, but not necessarily more efficient. Using a hash
function to produce a fixed-size digest of a message ensures that the
signature is compatible with a wide range of systems and protocols,
regardless of the specific message size or format. Crucially for
hardware security modules, Hash-then-Sign also significantly reduces
the amount of data that needs to be transmitted and processed by a
hardware security module. Consider scenarios such as a networked HSM
located in a different data center from the calling application or a
smart card connected over a USB interface. In these cases, streaming
a message that is megabytes or gigabytes long can result in notable
network latency, on-device signing delays, or even depletion of
available on-device memory.
Note that the vast majority of Internet protocols that sign large
messages already perform some level form of content hashing at the
protocol level, so this tends to be more of a concern with
proprietary cryptographic protocols, and protocols from non-IETF
standards bodies. Protocols like TLS 1.3 and DNSSEC use the Hash-
then-Sign paradigm. In TLS 1.3 [RFC8446] CertificateVerify message,
the content that is covered under the signature includes the
transcript hash output (Section 4.4.1 of [RFC8446]), while DNSSEC
[RFC4033] uses it to provide origin authentication and integrity
assurance services for DNS data. Similarly, the Cryptographic
Message Syntax (CMS) [RFC5652] includes a mandatory message digest
step before invoking the signature algorithm.
In the case of ML-DSA, it internally incorporates the necessary hash
operations as part of its signing algorithm. ML-DSA directly takes
the original message, applies a hash function internally, and then
uses the resulting hash value for the signature generation process.
In case of SLH-DSA, it internally performs randomized message
compression using a keyed hash function that can process arbitrary
length messages. In case of FN-DSA, a hash function is used as part
of the signature process, it uses the SHAKE-256 hash function to
derive a digest of the message being signed. Therefore, ML-DSA, FN-
DSA, and SLH-DSA offer enhanced security over the traditional Hash-
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then-Sign paradigm because by incorporating dynamic key material into
the message digest, a pre-computed hash collision on the message to
be signed no longer yields a signature forgery. Applications
requiring the performance and bandwidth benefits of Hash-then-Sign
may still pre-hash at the protocol level prior to invoking ML-DSA,
FN-DSA, or SLH-DSA, but protocol designers should be aware that doing
so re-introduces the weakness that hash collisions directly yield
signature forgeries. Signing the full un-digested message is
strongly preferred where applications can tolerate it.
12. Recommendations for Security / Performance Tradeoffs
The table below denotes the 5 security levels provided by NIST
required for PQC algorithms. Neither NIST nor the IETF make any
specific recommendations about which security level to use. In
general, protocols will include algorithm choices at multiple levels
so that users can choose the level appropriate to their policies and
data classification, similar to how organizations today choose which
size of RSA key to use. The security levels are defined as requiring
computational resources comparable to or greater than an attack on
AES (128, 192 and 256) and SHA2/SHA3 algorithms, i.e., exhaustive key
recovery for AES and optimal collision search for SHA2/SHA3. This
information is a re-print of information provided in the NIST PQC
project {NIST} as of time of writing (July 2023).
+=============+=====================+===========================+
| PQ Security | AES/SHA(2/3) | PQC Algorithm |
| Level | hardness | |
+=============+=====================+===========================+
| 1 | AES-128 (exhaustive | ML-KEM-512, FN-DSA-512, |
| | key recovery) | SLH-DSA-SHA2/SHAKE-128f/s |
+-------------+---------------------+---------------------------+
| 2 | SHA-256/SHA3-256 | ML-DSA-44 |
| | (collision search) | |
+-------------+---------------------+---------------------------+
| 3 | AES-192 (exhaustive | ML-KEM-768, ML-DSA-65, |
| | key recovery) | SLH-DSA-SHA2/SHAKE-192f/s |
+-------------+---------------------+---------------------------+
| 4 | SHA-384/SHA3-384 | No algorithm tested at |
| | (collision search) | this level |
+-------------+---------------------+---------------------------+
| 5 | AES-256 (exhaustive | ML-KEM-1024, FN-DSA-1024, |
| | key recovery) | ML-DSA-87, SLH-DSA-SHA2/ |
| | | SHAKE-256f/s |
+-------------+---------------------+---------------------------+
Table 2
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Please note the SLH-DSA-x-yf/s "f/s" in the above table denotes
whether its the SLH-DSA uses SHAKE or SHA-2 as an underlying hash
function "x" and whether it is fast (f) version or small (s) version
for "y" bit AES security level. Refer to
[I-D.ietf-lamps-cms-sphincs-plus-02] for further details on SLH-DSA
algorithms.
The following table discusses the signature size differences for
similar SLH-DSA algorithm security levels with the "simple" version
but for different categories i.e., (f) for fast verification and (s)
for compactness/smaller. Both SHA-256 and SHAKE-256 parameterization
output the same signature sizes, so both have been included.
+==========+===========================+========+=======+===========+
| PQ | Algorithm | Public |Private| Signature |
| Security | | key |key | size (in |
| Level | | size |size | bytes) |
| | | (in |(in | |
| | | bytes) |bytes) | |
+==========+===========================+========+=======+===========+
| 1 | SLH-DSA-{SHA2,SHAKE}-128f | 32 |64 | 17088 |
+----------+---------------------------+--------+-------+-----------+
| 1 | SLH-DSA-{SHA2,SHAKE}-128s | 32 |64 | 7856 |
+----------+---------------------------+--------+-------+-----------+
| 3 | SLH-DSA-{SHA2,SHAKE}-192f | 48 |96 | 35664 |
+----------+---------------------------+--------+-------+-----------+
| 3 | SLH-DSA-{SHA2,SHAKE}-192s | 48 |96 | 16224 |
+----------+---------------------------+--------+-------+-----------+
| 5 | SLH-DSA-{SHA2,SHAKE}-256f | 64 |128 | 49856 |
+----------+---------------------------+--------+-------+-----------+
| 5 | SLH-DSA-{SHA2,SHAKE}-256s | 64 |128 | 29792 |
+----------+---------------------------+--------+-------+-----------+
Table 3
The following table discusses the impact of performance on different
security levels in terms of private key sizes, public key sizes and
ciphertext/signature sizes.
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+==========+=============+============+============+================+
| PQ | Algorithm | Public key | Private | Ciphertext/ |
| Security | | size (in | key size | Signature |
| Level | | bytes) | (in bytes) | size (in |
| | | | | bytes) |
+==========+=============+============+============+================+
| 1 | ML-KEM-512 | 800 | 1632 | 768 |
+----------+-------------+------------+------------+----------------+
| 1 | FN-DSA-512 | 897 | 1281 | 666 |
+----------+-------------+------------+------------+----------------+
| 2 | ML-DSA-44 | 1312 | 2528 | 2420 |
+----------+-------------+------------+------------+----------------+
| 3 | ML-KEM-768 | 1184 | 2400 | 1088 |
+----------+-------------+------------+------------+----------------+
| 3 | ML-DSA-65 | 1952 | 4000 | 3309 |
+----------+-------------+------------+------------+----------------+
| 5 | FN-DSA-1024 | 1793 | 2305 | 1280 |
+----------+-------------+------------+------------+----------------+
| 5 | ML-KEM-1024 | 1568 | 3168 | 1588 |
+----------+-------------+------------+------------+----------------+
| 5 | ML-DSA-87 | 2592 | 4864 | 4627 |
+----------+-------------+------------+------------+----------------+
Table 4
13. Comparing PQC KEMs/Signatures vs Traditional KEMs (KEXs)/Signatures
In this section, we provide two tables for comparison of different
KEMs and Signatures respectively, in the traditional and post-quantum
scenarios. These tables will focus on the secret key sizes, public
key sizes, and ciphertext/signature sizes for the PQC algorithms and
their traditional counterparts of similar security levels.
The first table compares traditional vs. PQC KEMs in terms of
security, public, private key sizes, and ciphertext sizes.
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+=============+=====================+========+=========+============+
| PQ Security | Algorithm | Public | Private | Ciphertext |
| Level | | key | key | size (in |
| | | size | size | bytes) |
| | | (in | (in | |
| | | bytes) | bytes) | |
+=============+=====================+========+=========+============+
| Traditional | P256_HKDF_SHA-256 | 65 | 32 | 65 |
+-------------+---------------------+--------+---------+------------+
| Traditional | P521_HKDF_SHA-512 | 133 | 66 | 133 |
+-------------+---------------------+--------+---------+------------+
| Traditional | X25519_HKDF_SHA-256 | 32 | 32 | 32 |
+-------------+---------------------+--------+---------+------------+
| 1 | ML-KEM-512 | 800 | 1632 | 768 |
+-------------+---------------------+--------+---------+------------+
| 3 | ML-KEM-768 | 1184 | 2400 | 1088 |
+-------------+---------------------+--------+---------+------------+
| 5 | ML-KEM-1024 | 1568 | 3168 | 1568 |
+-------------+---------------------+--------+---------+------------+
Table 5
The next table compares traditional vs. PQC Signature schemes in
terms of security, public, private key sizes, and signature sizes.
+=============+=============+============+============+===========+
| PQ Security | Algorithm | Public key | Private | Signature |
| Level | | size (in | key size | size (in |
| | | bytes) | (in bytes) | bytes) |
+=============+=============+============+============+===========+
| Traditional | RSA2048 | 256 | 256 | 256 |
+-------------+-------------+------------+------------+-----------+
| Traditional | P256 | 64 | 32 | 64 |
+-------------+-------------+------------+------------+-----------+
| 1 | FN-DSA-512 | 897 | 1281 | 666 |
+-------------+-------------+------------+------------+-----------+
| 2 | ML-DSA-44 | 1312 | 2528 | 768 |
+-------------+-------------+------------+------------+-----------+
| 3 | ML-DSA-65 | 1952 | 4000 | 3293 |
+-------------+-------------+------------+------------+-----------+
| 5 | FN-DSA-1024 | 1793 | 2305 | 1280 |
+-------------+-------------+------------+------------+-----------+
| 5 | ML-DSA-87 | 2592 | 4864 | 4627 |
+-------------+-------------+------------+------------+-----------+
Table 6
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As one can clearly observe from the above tables, leveraging a PQC
KEM/Signature significantly increases the key sizes and the
ciphertext/signature sizes compared to traditional
KEM(KEX)/Signatures. But the PQC algorithms do provide the
additional security level in case there is an attack from a CRQC,
whereas schemes based on prime factorization or discrete logarithm
problems (finite field or elliptic curves) would provide no level of
security at all against such attacks.
These increased key and signatures sizes could introduce problems in
protocols. As an example, IKEv2 uses UDP as the transport for its
messages. One challenge with integrating PQC key exchange into the
initial IKEv2 exchange is that IKE fragmentation cannot be utilized.
To address this issue, [RFC9242] introduces a solution by defining a
new exchange called the 'Intermediate Exchange' which can be
fragmented using the IKE fragmentation mechanism. [RFC9370] then
uses this Intermediate Exchange to carry out the PQC key exchange
after the initial IKEv2 exchange and before the IKE_AUTH exchange.
Another example from [SP-1800-38C] section 6.3.3 shows that increased
key and signature sizes cause protocol key exchange messages to span
more network packets, therefore it results in a higher total loss
probability per packet. In lossy network conditions this may
increase the latency of the key exchange.
14. Post-Quantum and Traditional Hybrid Schemes
The migration to PQC is unique in the history of modern digital
cryptography in that neither the traditional algorithms nor the post-
quantum algorithms are fully trusted to protect data for the required
lifetimes. The traditional algorithms, such as RSA and elliptic
curve, will fall to quantum cryptanalysis, while the post-quantum
algorithms face uncertainty about the underlying mathematics,
compliance issues, unknown vulnerabilities, and hardware and software
implementations that have not had sufficient maturing time to rule
out classical cryptanalytic attacks and implementation bugs.
During the transition from traditional to post-quantum algorithms,
there may be a desire or a requirement for protocols that use both
algorithm types. [I-D.ietf-pquip-pqt-hybrid-terminology] defines the
terminology for the Post-Quantum and Traditional Hybrid Schemes.
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14.1. PQ/T Hybrid Confidentiality
The PQ/T Hybrid Confidentiality property can be used to protect from
a "Harvest Now, Decrypt Later" attack described in Section 8, which
refers to an attacker collecting encrypted data now and waiting for
quantum computers to become powerful enough to break the encryption
later. Two types of hybrid key agreement schemes are discussed
below:
1. Concatenate hybrid key agreement scheme: The final shared secret
that will be used as an input of the key derivation function is
the result of the concatenation of the secrets established with
each key agreement scheme. 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.
The hybrid-aware client and server establish a hybrid secret by
concatenating the two shared secrets, which is used as the shared
secret in the existing TLS 1.3 key schedule.
2. Cascade hybrid key agreement scheme: The final shared secret is
computed by applying as many iterations of the key derivation
function as the number of key agreement schemes composing the
hybrid key agreement scheme. For example, [RFC9370] extends the
Internet Key Exchange Protocol Version 2 (IKEv2) to allow one or
more PQC algorithms in addition to the traditional algorithm to
derive the final IKE SA keys using the cascade method as
explained in Section 2.2.2 of [RFC9370].
Various instantiations of these two types of hybrid key agreement
schemes have been explored. One must be careful when selecting which
hybrid scheme to use. The chosen schemes at IETF are IND-CCA2
robust, that is IND-CCA2 security is guaranteed for the scheme as
long as at least one of the component algorithms is IND-CCA2 secure.
14.2. PQ/T Hybrid Authentication
The PQ/T Hybrid Authentication property can be utilized in scenarios
where an on-path attacker possesses network devices equipped with
CRQCs, capable of breaking traditional authentication protocols, or
where an attacker can attack long-lived authenticated data such as CA
certificates or signed software images. This property ensures
authentication through a PQ/T hybrid scheme or a PQ/T hybrid
protocol, as long as at least one component algorithm remains secure
to provide the intended security level. For instance, a PQ/T hybrid
certificate can be employed 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
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[I-D.ounsworth-pq-composite-keys]; separate certificates could be
used for individual component algorithms
[I-D.ietf-lamps-cert-binding-for-multi-auth].
The frequency and duration of system upgrades and the time when CRQCs
will become widely available need to be weighed in to determine
whether and when to support the PQ/T Hybrid Authentication property.
14.3. Additional Considerations
It is also possible to use more than two algorithms together in a
hybrid scheme, with various methods for combining them. For post-
quantum transition purposes, the combination of a post-quantum
algorithm with a classical algorithm is the most straightforward.
The use of multiple post-quantum algorithms with different
mathematical bases has also been considered. Combining algorithms in
a way that requires both to be used together ensures stronger
security, while combinations that do not require both will sacrifice
security but offer other benefits like backwards compatibility and
crypto agility. Including a traditional key alongside a post-quantum
key often has minimal bandwidth impact.
When combining keys in an "and" mode, it may make more sense to
consider them to be a single composite key, instead of two keys.
This generally requires fewer changes to various components of PKI
ecosystems, many of which are not prepared to deal with two keys or
dual signatures. To those protocol- or application-layer parsers, a
"composite" algorithm composed of two "component" algorithms is
simply a new algorithm, and support for adding new algorithms
generally already exists. Treating multiple "component" keys as a
single "composite" key also has security advantages such as
preventing cross-protocol reuse of the individual component keys and
guarantees about revoking or retiring all component keys together at
the same time, especially if the composite is treated as a single
object all the way down into the cryptographic module. All that
needs to be done is to standardize the formats of how the two keys
from the two algorithms are combined into a single data structure,
and how the two resulting signatures or KEMs are combined into a
single signature or KEM. The answer can be as simple as
concatenation, if the lengths are fixed or easily determined. At
time of writing, security research is ongoing as to the security
properties of concatenation-based composite signatures and KEMs vs
more sophisticated signature and KEM combiners, and in which protocol
contexts those simpler combiners are sufficient.
One last consideration is the pairs of algorithms that can be
combined. A recent trends in protocols is to only allow a small
number of "known good" configurations that make sense, often referred
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to in cryptography as a "ciphersuite", instead of allowing arbitrary
combinations of individual configuration choices that may interact in
dangerous ways. The current consensus is that the same approach
should be followed for combining cryptographic algorithms, and that
"known good" pairs should be explicitly listed ("explicit
composite"), instead of just allowing arbitrary combinations of any
two crypto algorithms ("generic composite").
The same considerations apply when using multiple certificates to
transport a pair of related keys for the same subject. Exactly how
two certificates should be managed in order to avoid some of the
pitfalls mentioned above is still an active area of investigation.
Using two certificates keeps the certificate tooling simple and
straightforward, but in the end simply moves the problems with
requiring that both certs are intended to be used as a pair, must
produce two signatures which must be carried separately, and both
must validate, to the certificate management layer, where addressing
these concerns in a robust way can be difficult.
An important security note when using particularly hybrid signature
keys, but also to a lesser extent hybrid KEM keys, is key re-use. In
traditional cryptography, problems can occur with so-called "cross-
protocol attacks" when the same key can be used for multiple
protocols; for example signing TLS handshakes and signing S/MIME
emails. While it is not best-practice to re-use keys within the same
protocol, for example using the same key for multiple S/MIME
certificates for the same user, it is not generally catastrophic for
security. However, key re-use becomes a large security problem
within hybrids. Consider an {RSA, ML-DSA} hybrid key where the RSA
key also appears within a single-algorithm certificate. In this
case, an attacker could perform a "stripping attack" where they take
some piece of data signed with the {RSA, ML-DSA} key, remove the ML-
DSA signature and present the data as if it was intended for the RSA
only certificate. This leads to a set of security definitions called
"non-separability properties", which refers to how well the signature
scheme resists various complexities of downgrade / stripping attacks
[I-D.draft-ietf-pquip-hybrid-signature-spectrums]. Therefore,
implementers must either reuse the entire hybrid key as a whole, or
perform fresh keygens of all component keys per usage, and must not
take an existing key and reuse it as a component of a hybrid.
At least one scheme has been proposed that allows the pair of
certificates to exist as a single certificate when being issued and
managed, but dynamically split into individual certificates when
needed (https://datatracker.ietf.org/doc/draft-bonnell-lamps-
chameleon-certs/).
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Another potential application of hybrids bears mentioning, even
though it is not directly PQC-related. That is using hybrids to
navigate inter-jurisdictional cryptographic connections. Traditional
cryptography is already fragmented by jurisdiction, consider that
while most jurisdictions support Elliptic Curve Diffie-Hellman, those
in the United States will prefer the NIST curves while those in
Germany will prefer the brainpool curves. China, Russia, and other
jurisdictions have their own national cryptography standards. This
situation of fragmented global cryptography standards is unlikely to
improve with PQC. If "and" mode hybrids become standardized for the
reasons mentioned above, then one could imagine leveraging them to
create "ciphersuites" in which a single cryptographic operation
simultaneously satisfies the cryptographic requirements of both
endpoints.
Many of these points are still being actively explored and discussed,
and the consensus may change over time.
15. Security Considerations
15.1. Cryptanalysis
Classical cryptanalysis exploits weaknesses in algorithm design,
mathematical vulnerabilities, or implementation flaws, that are
exploitable with classical (i.e., non-quantum) hardware whereas
quantum cryptanalysis harnesses the power of CRQCs to solve specific
mathematical problems more efficiently. Another form of quantum
cryptanalysis is 'quantum side-channel' attacks. In such attacks, a
device under threat is directly connected to a quantum computer,
which then injects entangled or superimposed data streams to exploit
hardware that lacks protection against quantum side-channels. Both
pose threats to the security of cryptographic algorithms, including
those used in PQC. Developing and adopting new cryptographic
algorithms resilient against these threats is crucial for ensuring
long-term security in the face of advancing cryptanalysis techniques.
Recent attacks on the side-channel implementations using deep
learning based power analysis have also shown that one needs to be
cautious while implementing the required PQC algorithms in hardware.
Two of the most recent works include: one attack on ML-KEM
[KyberSide] and one attack on Saber [SaberSide]. Evolving threat
landscape points to the fact that lattice based cryptography is
indeed more vulnerable to side-channel attacks as in [SideCh],
[LatticeSide]. Consequently, there were some mitigation techniques
for side channel attacks that have been proposed as in [Mitigate1],
[Mitigate2], and [Mitigate3].
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15.2. Cryptographic Agility
Cryptographic agility is relevant for both classical and quantum
cryptanalysis as it enables organizations to adapt to emerging
threats, adopt stronger algorithms, comply with standards, and plan
for long-term security in the face of evolving cryptanalytic
techniques and the advent of CRQCs.
Several PQC schemes are available that need to be tested;
cryptography experts around the world are pushing for the best
possible solutions, and the first standards that will ease the
introduction of PQC are being prepared. It is of paramount
importance and a call for imminent action for organizations, bodies,
and enterprises to start evaluating their cryptographic agility,
assess the complexity of implementing PQC into their products,
processes, and systems, and develop a migration plan that achieves
their security goals to the best possible extent.
An important and often overlooked step in achieving cryptographic
agility is maintaining a cryptographic inventory. Modern software
stacks incorporate cryptography in numerous places, making it
challenging to identify all instances. Therefore, cryptographic
agility and inventory management take two major forms: First,
application developers responsible for software maintenance should
actively search for instances of hard-coded cryptographic algorithms
within applications. When possible, they should design the choice of
algorithm to be dynamic, based on application configuration. Second,
administrators, policy officers, and compliance teams should take
note of any instances where an application exposes cryptographic
configurations. These instances should be managed either through
organization-wide written cryptographic policies or automated
cryptographic policy systems.
Numerous commercial solutions are available for both detecting hard-
coded cryptographic algorithms in source code and compiled binaries,
as well as providing cryptographic policy management control planes
for enterprise and production environments.
15.3. Hybrid Key Exchange and Signatures: Bridging the Gap Between
Post-Quantum and Traditional Cryptography
Post-quantum algorithms selected for standardization are relatively
new and they they have not been subject to the same depth of study as
traditional algorithms. PQC implementations will also be new and
therefore more likely to contain implementation bugs than the battle-
tested crypto implementations that we rely on today. In addition,
certain deployments may need to retain traditional algorithms due to
regulatory constraints, for example FIPS [SP-800-56C] or PCI
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compliance. Hybrid key exchange enables potential security against
"Harvest Now, Decrypt Later" attack and hybrid signatures provide for
time to react in the case of the announcement of a devastating attack
against any one algorithm, while not fully abandoning traditional
cryptosystems.
15.4. Caution: Ciphertext commitment in KEM vs DH
The ciphertext generated by a KEM is not necessarily inherently
linked to the shared secret it produces. In contrast, in some other
cryptographic schemes like Diffie-Hellman, a change in the public key
results in a change in the derived shared secret.
16. Further Reading & Resources
16.1. Reading List
(A reading list. Serious Cryptography (https://nostarch.com/
seriouscrypto). Pointers to PQC sites with good explanations. List
of reasonable Wikipedia pages.)
16.2. Developer Resources
* Open Quantum Safe (https://openquantumsafe.org/) and corresponding
github (https://github.com/open-quantum-safe)
* PQUIP WG list of PQC-related protocol work within the IETF
(https://github.com/ietf-wg-pquip/state-of-protocols-and-pqc)
17. Contributors
The authors would like to acknowledge that this content is assembled
from countless hours of discussion and countless megabytes of email
discussions. We have tried to reference as much source material as
possible, and apologize to anyone whose work was inadvertently
missed.
In particular, the authors would like to acknowledge the
contributions to this document by the following individuals:
Kris Kwiatkowski
PQShield, LTD
United Kingdom.
kris@amongbytes.com
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Acknowledgements
This document leverages text from https://github.com/paulehoffman/
post-quantum-for-engineers/blob/main/pqc-for-engineers.md. Thanks to
Dan Wing, Florence D, Thom Wiggers, Sophia Grundner-Culemann, Panos
Kampanakis, Ben S3, Sofia Celi, Melchior Aelmans, Falko Strenzke,
Deirdre Connolly, and Daniel Van Geest for the discussion, review and
comments.
References
Normative References
[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>.
[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>.
[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>.
[RFC9242] Smyslov, V., "Intermediate Exchange in the Internet Key
Exchange Protocol Version 2 (IKEv2)", RFC 9242,
DOI 10.17487/RFC9242, May 2022,
<https://www.rfc-editor.org/rfc/rfc9242>.
[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/rfc/rfc9370>.
Informative References
[BHK09] "Subtleties in the Definition of IND-CCA: When and How
Should Challenge-Decryption be Disallowed?",
<https://eprint.iacr.org/2009/418>.
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[BSI-PQC] "Quantum-safe cryptography – fundamentals, current
developments and recommendations", May 2022,
<https://www.bsi.bund.de/SharedDocs/Downloads/EN/BSI/
Publications/Brochure/quantum-safe-
cryptography.html?nn=916626>.
[Cloudflare]
"NIST’s pleasant post-quantum surprise",
<https://blog.cloudflare.com/nist-post-quantum-surprise/>.
[CNSA2-0] "Announcing the Commercial National Security Algorithm
Suite 2.0", <https://media.defense.gov/2022/
Sep/07/2003071834/-1/-1/0/CSA_CNSA_2.0_ALGORITHMS_.PDF>.
[CS01] "Design and Analysis of Practical Public-Key Encryption
Schemes Secure against Adaptive Chosen Ciphertext Attack",
<https://eprint.iacr.org/2001/108>.
[FN-DSA] "Fast Fourier lattice-based compact signatures over NTRU",
<https://falcon-sign.info/>.
[GMR88] "A digital signature scheme secure against adaptive
chosen-message attacks.",
<https://people.csail.mit.edu/silvio/
Selected%20Scientific%20Papers/Digital%20Signatures/
A_Digital_Signature_Scheme_Secure_Against_Adaptive_Chosen-
Message_Attack.pdf>.
[Grover-search]
"C. Zalka, “Grover’s quantum searching algorithm is
optimal,” Physical Review A, vol. 60, pp. 2746-2751,
1999.".
[I-D.draft-connolly-cfrg-xwing-kem-02]
Connolly, D., Schwabe, P., and B. Westerbaan, "X-Wing:
general-purpose hybrid post-quantum KEM", Work in
Progress, Internet-Draft, draft-connolly-cfrg-xwing-kem-
02, 26 March 2024, <https://datatracker.ietf.org/doc/html/
draft-connolly-cfrg-xwing-kem-02>.
[I-D.draft-ietf-cose-sphincs-plus]
Prorock, M., Steele, O., Misoczki, R., Osborne, M., and C.
Cloostermans, "SLH-DSA for JOSE and COSE", Work in
Progress, Internet-Draft, draft-ietf-cose-sphincs-plus-04,
8 July 2024, <https://datatracker.ietf.org/doc/html/draft-
ietf-cose-sphincs-plus-04>.
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[I-D.draft-ietf-lake-edhoc-22]
Selander, G., Mattsson, J. P., and F. Palombini,
"Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
Progress, Internet-Draft, draft-ietf-lake-edhoc-22, 25
August 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-lake-edhoc-22>.
[I-D.draft-ietf-pquip-hybrid-signature-spectrums]
Bindel, N., Hale, B., Connolly, D., and F. D, "Hybrid
signature spectrums", Work in Progress, Internet-Draft,
draft-ietf-pquip-hybrid-signature-spectrums-00, 24 May
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
pquip-hybrid-signature-spectrums-00>.
[I-D.draft-ounsworth-cfrg-kem-combiners-04]
Ounsworth, M., Wussler, A., and S. Kousidis, "Combiner
function for hybrid key encapsulation mechanisms (Hybrid
KEMs)", Work in Progress, Internet-Draft, draft-ounsworth-
cfrg-kem-combiners-04, 8 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ounsworth-
cfrg-kem-combiners-04>.
[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-05, 29 April 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
cert-binding-for-multi-auth-05>.
[I-D.ietf-lamps-cms-sphincs-plus-02]
Housley, R., Fluhrer, S., Kampanakis, P., and B.
Westerbaan, "Use of the SPHINCS+ Signature Algorithm in
the Cryptographic Message Syntax (CMS)", Work in Progress,
Internet-Draft, draft-ietf-lamps-cms-sphincs-plus-02, 17
May 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-lamps-cms-sphincs-plus-02>.
[I-D.ietf-lamps-dilithium-certificates]
Massimo, J., Kampanakis, P., Turner, S., and B.
Westerbaan, "Internet X.509 Public Key Infrastructure:
Algorithm Identifiers for ML-DSA", Work in Progress,
Internet-Draft, draft-ietf-lamps-dilithium-certificates-
04, 22 July 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-lamps-dilithium-certificates-04>.
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[I-D.ietf-pquip-pqt-hybrid-terminology]
D, F., P, M., and B. Hale, "Terminology for Post-Quantum
Traditional Hybrid Schemes", Work in Progress, Internet-
Draft, draft-ietf-pquip-pqt-hybrid-terminology-04, 10
September 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-pquip-pqt-hybrid-terminology-04>.
[I-D.ietf-tls-hybrid-design]
Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key
exchange in TLS 1.3", Work in Progress, Internet-Draft,
draft-ietf-tls-hybrid-design-10, 5 April 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
hybrid-design-10>.
[I-D.ounsworth-pq-composite-keys]
Ounsworth, M., Gray, J., Pala, M., and J. Klaußner,
"Composite Public and Private Keys For Use In Internet
PKI", Work in Progress, Internet-Draft, draft-ounsworth-
pq-composite-keys-05, 29 May 2023,
<https://datatracker.ietf.org/doc/html/draft-ounsworth-pq-
composite-keys-05>.
[I-D.westerbaan-cfrg-hpke-xyber768d00-02]
Westerbaan, B. and C. A. Wood, "X25519Kyber768Draft00
hybrid post-quantum KEM for HPKE", Work in Progress,
Internet-Draft, draft-westerbaan-cfrg-hpke-xyber768d00-02,
4 May 2023, <https://datatracker.ietf.org/doc/html/draft-
westerbaan-cfrg-hpke-xyber768d00-02>.
[KEEPINGUP]
"Keeping Up with the KEMs: Stronger Security Notions for
KEMs and automated analysis of KEM-based protocols", n.d.,
<https://eprint.iacr.org/2023/1933>.
[KYBER2018]
"CRYSTALS - Kyber: A CCA-Secure Module-Lattice-Based KEM",
n.d., <https://ieeexplore.ieee.org/document/8406610>.
[KyberSide]
"A Side-Channel Attack on a Hardware Implementation of
CRYSTALS-Kyber", <https://eprint.iacr.org/2022/1452>.
[LattFail1]
"Decryption Failure Attacks on IND-CCA Secure Lattice-
Based Schemes", <https://link.springer.com/
chapter/10.1007/978-3-030-17259-6_19#chapter-info>.
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[LattFail2]
"(One) Failure Is Not an Option: Bootstrapping the Search
for Failures in Lattice-Based Encryption Schemes.",
<https://link.springer.com/
chapter/10.1007/978-3-030-45727-3_1>.
[LatticeSide]
"Generic Side-channel attacks on CCA-secure lattice-based
PKE and KEM schemes", <https://eprint.iacr.org/2019/948>.
[LIBOQS] "LibOQS - Open Quantum Safe",
<https://github.com/open-quantum-safe/liboqs>.
[Lyu09] "V. Lyubashevsky, “Fiat-Shamir With Aborts: Applications
to Lattice and Factoring-Based Signatures“, ASIACRYPT
2009", <https://www.iacr.org/archive/
asiacrypt2009/59120596/59120596.pdf>.
[Mitigate1]
"POLKA: Towards Leakage-Resistant Post-Quantum CCA-Secure
Public Key Encryption",
<https://eprint.iacr.org/2022/873>.
[Mitigate2]
"Leakage-Resilient Certificate-Based Authenticated Key
Exchange Protocol",
<https://ieeexplore.ieee.org/document/9855226>.
[Mitigate3]
"Post-Quantum Authenticated Encryption against Chosen-
Ciphertext Side-Channel Attacks",
<https://eprint.iacr.org/2022/916>.
[ML-DSA] "FIPS-204: Module-Lattice-Based Digital Signature
Standard", <https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.204.ipd.pdf>.
[ML-KEM] "FIPS-203: Module-Lattice-based Key-Encapsulation
Mechanism Standard",
<https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.203.ipd.pdf>.
[NIST] "Post-Quantum Cryptography Standardization",
<https://csrc.nist.gov/projects/post-quantum-cryptography/
post-quantum-cryptography-standardization>.
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[PQCAPI] "PQC - API notes",
<https://csrc.nist.gov/CSRC/media/Projects/Post-Quantum-
Cryptography/documents/example-files/api-notes.pdf>.
[PQRSA] "Post-quantum RSA", April 2017,
<https://cr.yp.to/papers/pqrsa-20170419.pdf>.
[QC-DNS] "Quantum Computing and the DNS",
<https://www.icann.org/octo-031-en.pdf>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/rfc/rfc4033>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/rfc/rfc5652>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090,
DOI 10.17487/RFC6090, February 2011,
<https://www.rfc-editor.org/rfc/rfc6090>.
[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>.
[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/rfc/rfc9180>.
[RSA] "A Method for Obtaining Digital Signatures and Public-Key
Cryptosystems+",
<https://dl.acm.org/doi/pdf/10.1145/359340.359342>.
[RSA10SC] "Breaking RSA Encryption - an Update on the State-of-the-
Art", <https://www.quintessencelabs.com/blog/breaking-rsa-
encryption-update-state-art>.
[RSA8HRS] "How to factor 2048 bit RSA integers in 8 hours using 20
million noisy qubits", <https://arxiv.org/abs/1905.09749>.
[RSAShor] "Circuit for Shor’s algorithm using 2n+3 qubits",
<https://arxiv.org/pdf/quant-ph/0205095.pdf>.
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[SaberSide]
"A side-channel attack on a masked and shuffled software
implementation of Saber",
<https://link.springer.com/article/10.1007/
s13389-023-00315-3>.
[SideCh] "Side-Channel Attacks on Lattice-Based KEMs Are Not
Prevented by Higher-Order Masking",
<https://eprint.iacr.org/2022/919>.
[SLH-DSA] "FIPS-205: Stateless Hash-Based Digital Signature
Standard", <https://doi.org/10.6028/NIST.FIPS.205>.
[SP-1800-38C]
"Migration to Post-Quantum Cryptography Quantum Readiness:
Quantum-Resistant Cryptography Technology Interoperability
and Performance Report",
<https://www.nccoe.nist.gov/sites/default/files/2023-12/
pqc-migration-nist-sp-1800-38c-preliminary-draft.pdf>.
[SP-800-56C]
"Recommendation for Key-Derivation Methods in Key-
Establishment Schemes",
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-56Cr2.pdf>.
[Threat-Report]
"Quantum Threat Timeline Report 2020",
<https://globalriskinstitute.org/publications/quantum-
threat-timeline-report-2020/>.
Authors' Addresses
Aritra Banerjee
Nokia
Munich
Germany
Email: aritra.banerjee@nokia.com
Tirumaleswar Reddy
Nokia
Bangalore
Karnataka
India
Email: kondtir@gmail.com
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Dimitrios Schoinianakis
Nokia
Athens
Greece
Email: dimitrios.schoinianakis@nokia-bell-labs.com
Timothy Hollebeek
DigiCert
Pittsburgh,
United States of America
Email: tim.hollebeek@digicert.com
Mike Ounsworth
Entrust Limited
2500 Solandt Road – Suite 100
Ottawa, Ontario K2K 3G5
Canada
Email: mike.ounsworth@entrust.com
Banerjee, et al. Expires 16 March 2025 [Page 46]