diff --git a/draft-ietf-pquip-pqc-engineers.md b/draft-ietf-pquip-pqc-engineers.md index 7836432..ee5cf19 100644 --- a/draft-ietf-pquip-pqc-engineers.md +++ b/draft-ietf-pquip-pqc-engineers.md @@ -3,7 +3,7 @@ title: "Post-Quantum Cryptography for Engineers" abbrev: "PQC for Engineers" category: info -docname: draft-ietf-pquip-pqc-engineers-04 +docname: draft-ietf-pquip-pqc-engineers-latest submissiontype: IETF number: date: @@ -51,6 +51,16 @@ author: city: Pittsburgh country: USA email: "tim.hollebeek@digicert.com" + - + ins: M. Ounsworth + name: Mike Ounsworth + org: Entrust Limited + abbrev: Entrust + street: 2500 Solandt Road – Suite 100 + city: Ottawa, Ontario + country: Canada + code: K2K 3G5 + email: mike.ounsworth@entrust.com normative: @@ -85,6 +95,10 @@ informative: title: "FIPS-204: Module-Lattice-Based Digital Signature Standard" target: https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.204.ipd.pdf date: false + SLH-DSA: + title: "FIPS-205: Stateless Hash-Based Digital Signature Standard" + target: https://doi.org/10.6028/NIST.FIPS.205 + date: false FN-DSA: title: "Fast Fourier lattice-based compact signatures over NTRU" target: https://falcon-sign.info/ @@ -185,6 +199,14 @@ informative: title: "Migration to Post-Quantum Cryptography Quantum Readiness: Quantum-Resistant Cryptography Technology Interoperability and Performance Report" target: https://www.nccoe.nist.gov/sites/default/files/2023-12/pqc-migration-nist-sp-1800-38c-preliminary-draft.pdf date: false + KYBER2018: + title: "CRYSTALS - Kyber: A CCA-Secure Module-Lattice-Based KEM" + target: https://ieeexplore.ieee.org/document/8406610 + KEEPINGUP: + title: "Keeping Up with the KEMs: Stronger Security Notions for KEMs and automated analysis of KEM-based protocols" + target: https://eprint.iacr.org/2023/1933 + + --- abstract @@ -196,9 +218,9 @@ The presence of a Cryptographically Relevant Quantum Computer (CRQC) would rende 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 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 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. +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 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. 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. +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). @@ -225,7 +247,9 @@ Any asymmetric cryptographic algorithm based on integer factorization, finite fi # 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 (see {{symmetric}} for more details). + 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 {{symmetric}} for more details). # NIST PQC Algorithms @@ -263,12 +287,16 @@ Post-quantum cryptography or quantum-safe cryptography refers to cryptographic a 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. -## Symmetric cryptography {#symmetric} +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. -Grover's algorithm is a quantum search algorithm that provides a theoretical quadratic speedup for searching an unstructured database, compared to classical 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 achieve quantum resistance. 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. +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. + +## Symmetric cryptography {#symmetric} 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. + 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. @@ -279,8 +307,11 @@ Finally, in their evaluation criteria for PQC, NIST is assessing the security le 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 in 10 seconds {{RSA10SC}}. -For structured data such as public-key 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). +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). +## 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. # Timeline for transition {#timeline} @@ -288,13 +319,13 @@ The timeline, and driving motivation for transition differs slightly between dat 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. +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 | +| y | x | +------------------------+----------+-----------------+ | | <---------------> | z | Security gap @@ -307,6 +338,8 @@ These challenges are illustrated nicely by the so-called Mosca model discussed i 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. +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. + # 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. @@ -327,7 +360,9 @@ It is noteworthy that lattice-based encryption schemes require a rounding step d 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 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. -The SPHINCS 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. +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. @@ -337,17 +372,18 @@ This area of cryptography started in the 1970s and 80s based on the seminal work Examples include all the NIST Round 4 (unbroken) finalists: Classic McEliece, HQC, BIKE. - # KEMs {#KEMs} ## 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 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 the DEK. 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: +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) -> (ct, ss) +* 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: @@ -357,7 +393,7 @@ where pk is public key, sk is secret key, ct is the ciphertext representing an e | Client | | Server | +---------+ +---------+ +----------------------+ | | - | sk, pk = kemKeyGen() |-| | + | pk, sk = kemKeyGen() |-| | +----------------------+ | | | | | pk | @@ -384,39 +420,71 @@ Authenticated Key Exchange with KEMs where both parties contribute a KEM public +---------+ +---------+ | Client | | Server | +---------+ +---------+ - +----------------------+ | | - | sk1, pk1 = KeyGen() |-| | - +----------------------+ | | - | | - | pk1 | - |---------->| - | | +------------------------+ - | |-| sk2, pk2 = KeyGen() | - | | | ss = KeyEx(pk1, sk2) | - | | +------------------------+ - | | - | pk2| - |<----------| -+------------------------+ | | -| ss = KeyEx(pk2, sk1) |-| | -+------------------------+ | | - | | + +-----------------------+ | | + | 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) | + | | +------------------------+ ~~~~~ {: #tab-dh-ake title="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). It's also worth noting that in an Ephemeral-Static Diffie-Hellman (DH) scenario, where `sk2` and `pk2` represent long-term keys. such as those contained in an email encryption certificate, the client can compute `ss = KeyEx(pk2, sk1)` without waiting for a response from the Server. This characteristic transforms it into a non-interactive and authenticated key exchange method. Many Internet protocols rely on this aspect of DH. When using Key Encapsulation Mechanisms (KEMs) as the underlying primitive, a flow may be non-interactive or authenticated, but not both. 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. +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. {{tab-dh-ake}} 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. -Post-Quantum KEMs are inherently interactive Key Exchange (KE) protocols because they involve back-and-forth communication to negotiate and establish a shared secret key. This is unlike Diffie-Hellman (DH) Key Exchange (KEX) or RSA Key Transport, which provide the non-interactive key exchange (NIKE) property. NIKE is a cryptographic primitive that enables two parties who know each other's public keys to agree on a symmetric shared key without requiring any real-time interaction. Consider encrypted email, where the content needs to be encrypted and sent even if the receiving device containing the decryption keys (e.g., a phone or laptop) is currently offline. +~~~~~ aasvg + +---------+ +---------+ + | 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) | + | | +------------------------+ +~~~~~ +{: #tab-dh-ake-nike title="Diffie-Hellman based Authenticated Key Exchange and Non-Interactive Key Exchange simultaneously"} -Another important property of Diffie-Hellman is that in addition to being a NIKE, it is also an Authenticated Key Exchange (AKE), meaning that since both parties needed to involve their asymmetric keypair, both parties have proof-of-identity of the other party. In order to achieve an AKE with KEM primitives, two full KEM exchanges need to be performed, and their results combined to form a single shared secret. +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. ~~~~~ aasvg +---------+ +---------+ | Client | | Server | +---------+ +---------+ - +----------------------+ | | - | sk1, pk1 = kemKeyGen() |-| | - +----------------------+ | | ++------------------------+ | | +| sk1, pk1 = kemKeyGen() |-| | ++------------------------+ | | | | |pk1 | |---------->| @@ -442,17 +510,28 @@ Another important property of Diffie-Hellman is that in addition to being a NIKE ~~~~~ {: #tab-kem-ake title="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. Sometimes combiners require both the shared secrets and ciphertexts as input and can act on more than two KEMs; `Combiner(ss1, ct1, ss2, ct2, ..)`. For a more thorough discussion of KEM combiners, see {{?I-D.draft-ounsworth-cfrg-kem-combiners-04}}. +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}}. + +## Security properties + +### 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, resistance against chosen-ciphertext attacks, and prevents the adversary from forging valid ciphertexts (given access to the public key). An appropriate definition of IND-CCA2 security for KEMs can be found in [CS01] and [BHK09]. ML-KEM [ML-KEM] and Classic McEliece provides IND-CCA2 security. -## Security property +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. -* 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, resistance against chosen-ciphertext attacks, and prevents the adversary from forging valid ciphertexts (given access to the public key). An appropriate definition of IND-CCA2 security for KEMs can be found in [CS01] and [BHK09]. ML-KEM [ML-KEM] and Classic McEliece provides IND-CCA2 security. +### 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. + +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. -Understanding IND-CCA2 security is essential for individuals involved in designing or implementing cryptographic systems and protocols 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. 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. ## HPKE {#hpke} -HPKE (Hybrid Public Key Encryption) {{?RFC9180}} deals with a variant of KEM which is essentially a PKE of arbitrary sized plaintexts for a recipient public key. It 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}}. +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}}. # PQC Signatures @@ -460,19 +539,21 @@ HPKE (Hybrid Public Key Encryption) {{?RFC9180}} deals with a variant of KEM whi Any digital signature scheme that provides a construction defining security under post-quantum setting falls under this category of PQ signatures. -## Security property +## Security properties -* 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. +### EUF-CMA -Understanding EUF-CMA security is essential for individual involved in designing or implementing cryptographic systems 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. 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 +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. + +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 ## Details of FN-DSA, ML-DSA, and SLH-DSA {#sig-scheme} -ML-DSA [ML-DSA] is a digital signature algorithm (part of the CRYSTALS suite) based on the hardness 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 uniform distribution over small integers for computing coefficients in error vectors, which makes the scheme easier to implement. +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 class of lattices and a trapdoor sampler technique. +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. @@ -480,7 +561,7 @@ Implementers of FN-DSA need to be aware that FN-DSA signing is highly susceptibl 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 {{RecSecurity}} and {{Comparisons}}. -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 {{hash-based}}). 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 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}}. +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 {{hash-based}}). 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}}. ## Details of XMSS and LMS @@ -491,7 +572,7 @@ Multi-Tree XMSS and LMS can be used for signing a potentially large but fixed nu 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 succesfully applied. +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. ### 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. @@ -507,16 +588,16 @@ The public (PK) and private (SK) key size depends on the length of the digest (M ## 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 performance by reducing the size of signed messages, 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. +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. -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. +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-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. # Recommendations for Security / Performance Tradeoffs {#RecSecurity} -The table below denotes the 5 security levels provided by NIST required for PQC algorithms. Users can leverage the appropriate algorithm based on the security level required for their use case. 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). +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 Level | AES/SHA(2/3) hardness | PQC Algorithm | | ----------------- | ----------------------------------------------- | ---------------------------------------------------------- | @@ -528,7 +609,7 @@ The table below denotes the 5 security levels provided by NIST required for PQC 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 parametrisation output the same signature sizes, so both have been included. +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 Security Level | Algorithm | Public key size (in bytes) | Private key size (in bytes) | Signature size (in bytes) | | ------------------ | --------------------------------- | --------------------------- | --------------------------- | ------------------------------------ | @@ -596,11 +677,11 @@ The PQ/T Hybrid Confidentiality property can be used to protect from a "Harvest 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 and will be discussed further. 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. +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. ## 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. 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 {{?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 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 {{?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. @@ -608,15 +689,17 @@ The frequency and duration of system upgrades and the time when CRQCs will becom It is also possible to use more than two algorithms together in a hybrid scheme, and there are multiple possible ways those algorithms can be combined. For the purposes of a post-quantum transition, the simple combination of a post-quantum algorithm with a single classical algorithm is the most straightforward, but the use of multiple post-quantum algorithms with different hard math problems has also been considered. When combining algorithms, it is possible to require that both algorithms be used together (the so-called "and" mode) or that only one does (the "or" mode), or even some more complicated scheme. Schemes that do not require both algorithms to validate only have the strength of the weakest algorithm, and therefore offer little or no security benefit but may offer backwards compatibility, crypto agility, or ease-of-migration benefits. Care should be taken when designing "or" mode hybrids to ensure that the larger PQ keys are not required to be transmitted to and processed by legacy clients that will not use them; this was the major drawback of the failed proposal {{?I-D.draft-truskovsky-lamps-pq-hybrid-x509}}. This combination of properties makes optionally including post-quantum keys without requiring their use to be generally unattractive in most use cases. On the other hand, including a classical key -- particularly an elliptic curve key -- alongside a lattice key is generally considered to be negligible in terms of the extra bandwidth usage. -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 (August 2023) 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. +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, 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"). +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 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 SHOULD either reuse the entire hybrid key as a whole, or perform fresh keygens of all component keys per usage, and SHOULD 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/). -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 standardised 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. +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. @@ -625,21 +708,22 @@ Many of these points are still being actively explored and discussed, and the co ## 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}}. ## 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 hardcoded 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. +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 hardcoded cryptographic algorithms in source code and compiled binaries, as well as providing cryptographic policy management control planes for enterprise and production environments. +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. ## 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 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. +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 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. ## Caution: Ciphertext commitment in KEM vs DH @@ -670,14 +754,6 @@ In particular, the authors would like to acknowledge the contributions to this d kris@amongbytes.com - Mike Ounsworth - - Entrust - - Canada - - mike.ounsworth@entrust.com - # Acknowledgements {:numbered="false"}