Adapting Constrained Devices for Post-Quantum Cryptography
draft-reddy-pquip-pqc-hsm-01

PQUIP                                                           T. Reddy
Internet-Draft                                                     Nokia
Intended status: Informational                                   D. Wing
Expires: 24 October 2025                            Cloud Software Group
                                                               B. Salter
                                       UK National Cyber Security Centre
                                                          K. Kwiatkowski
                                                                PQShield
                                                           22 April 2025

       Adapting Constrained Devices for Post-Quantum Cryptography
                      draft-reddy-pquip-pqc-hsm-01

Abstract

   This document offers guidance on incorporating Post-Quantum
   Cryptography (PQC) into resource-constrained devices, including IoT
   devices and lightweight Hardware Security Modules (HSMs), which
   operate under tight limitations on compute power, memory, storage,
   and energy.  It highlights the role of the Root of Trust in enabling
   secure operations, including seed-based key generation to reduce the
   need for persistent storage, efficient approaches to managing
   ephemeral keys, and methods for offloading cryptographic tasks in
   low-resource environments.  Additionally, it examines how PQC affects
   firmware update mechanisms in such constrained systems.

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-reddy-pquip-pqc-hsm/.

   Discussion of this document takes place on the pquip Working Group
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Key Management in Constrained Devices for PQC . . . . . . . .   3
     2.1.  Seed Management . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Seed Storage  . . . . . . . . . . . . . . . . . . . .   4
       2.1.2.  Efficient Key Derivation  . . . . . . . . . . . . . .   5
       2.1.3.  Secure Exporting of Seeds . . . . . . . . . . . . . .   6
   3.  Ephemeral Key Management  . . . . . . . . . . . . . . . . . .   6
   4.  Optimizing Performance in Constrained Devices for PQC Signature
           Algorithms  . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Additional Considerations for PQC Use in Constrained
           Devices . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Post-quantum Firmware Upgrades for Constrained Devices  . . .  10
     6.1.  Post-quantum Firmware Authentication  . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
     7.1.  Side Channel Protection . . . . . . . . . . . . . . . . .  11
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  12
   Informative References  . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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1.  Introduction

   The transition to post-quantum cryptography introduces significant
   challenges for resource-constrained devices such as IoT devices,
   lightweight HSMs, secure elements (e.g., SIMs), and Trusted Execution
   Environments (TEEs).  These devices often operate with limited
   memory, non-volatile storage, processing power, and battery life,
   making the adoption of PQC algorithms which typically involve larger
   key sizes and are more computationally intensive than traditional
   algorithms particularly challenging.  The increased key sizes and
   computational demands of PQC require careful consideration to ensure
   secure and efficient key management within these constrained
   environments.

   This document provides industry guidance and best practices for
   integrating PQC algorithms into constrained devices.  It explores key
   storage strategies, ephemeral key management, and performance
   optimizations specific to resource-limited environments.  One
   approach to mitigating storage constraints is seed-based key
   generation, where only a small seed is stored instead of the full
   private key, as supported by PQC schemes like ML-DSA and SLH-DSA.
   However, this technique increases computational overhead due to the
   need to derive full private keys on demand, a classic computation-
   versus-storage tradeoff.  The document also discusses considerations
   for ephemeral key generation in protocols like TLS and IPsec, along
   with techniques to optimize PQC signature operations to enhance
   performance within constrained crytographic modules.

   This document focuses on the use of PQC algorithms in constrained
   devices, specifically the three algorithms finalized by NIST: ML-DSA,
   ML-KEM, and SLH-DSA.  While other PQC algorithms, such as stateful
   hash-based signatures, also provide post-quantum security, they are
   not covered in this version of the document.  Future revisions may
   expand the scope to include additional PQC algorithms.

2.  Key Management in Constrained Devices for PQC

   The embedded cryptographic components used in constrained devices are
   designed to securely manage cryptographic keys, often under strict
   limitations in memory, storage capacity, and computational resources.
   These limitations are further exhausted by the increased key sizes
   and computational demands of PQC algorithms.

   One mitigation of storage limitations is to store only the seed
   rather than the full expanded private key, as the seed is far smaller
   and can derive the expanded private key as necessary.

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2.1.  Seed Management

   The seed generated during the PQC key generation function is highly
   sensitive, as it will be used to compute the private key or
   decapsulation key.  Consequently, seeds must be treated with the same
   level of security as private keys.

   To comply with [ML-KEM], [ML-DSA], [SLH-DSA] and [REC-KEM]
   guidelines:

2.1.1.  Seed Storage

   Seeds must be securely stored within a cryptographic module of the
   device whether hardware or software-based to protect against
   unauthorized access.  Since the seed can be used to deterministically
   derive the private key, it must be safeguarded with the same level of
   protection as the private key itself.  For example, according to
   [ML-DSA] Section 3.6.3, the seed ξ generated during ML-DSA.KeyGen can
   be stored for later use with ML-DSA.KeyGen_internal.

   The choice between storing a seed or an expanded private key involves
   trade-offs between storage efficiency and performance.  Some
   constrained cryptographic modules may store only the seed and derive
   the expanded private key on demand, whereas others may prefer storing
   the full expanded key to reduce computational overhead during key
   usage.

   While vulnerabilities like the "Unbindable Kemmy Schmidt" attack
   [BIND] demonstrate the risks of manipulating expanded private keys in
   environments lacking hardware-backed protections, these attacks
   generally assume an adversary has some level of control over the
   expanded key format.  However, in a hardware-backed protcted
   environment, where private keys are typically protected from such
   manipulation, the primary motivation for storing the seed rather than
   the expanded key is not directly tied to mitigating "Kemmy" attacks.

   The ML-DSA and ML-KEM private key formats, as specified in
   [I-D.ietf-lamps-dilithium-certificates] and
   [I-D.ietf-lamps-kyber-certificates], represent the private key using
   a seed from which the expanded private key is derived.  While these
   formats rely on the seed for key generation, an constrained
   cryptographic module may choose to store the expanded private key to
   avoid the additional computation required for running KeyGen.

   This choice between storing the seed or the expanded private key has
   direct implications on performance, as key derivation incurs
   additional computation.  The impact of this overhead varies depending
   on the algorithm.  For instance, ML-DSA key generation, which

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   primarily involves polynomial operations using the Number Theoretic
   Transform (NTT) and hashing, is computationally efficient compared to
   other post-quantum schemes.  In contrast, SLH-DSA key generation
   requires constructing a Merkle tree and multiple calls to Winternitz
   One-Time Signature (WOTS+) key generation, making it significantly
   slower due to the recursive hash computations involved.  Designers of
   constrained systems must carefully balance storage efficiency and
   computational overhead based on system requirements and operational
   constraints.  While constrained systems employ various key storage
   strategies, the decision to store full private keys or only seeds
   depends on design goals, performance considerations, and standards
   compliance (e.g., PKCS#11).

   A challenge arises when importing an existing private key into a
   system designed to store only seeds.  When a user attempts to import
   an already expanded private key, there is a mismatch between the key
   format used internally (seed-based) and the expanded private key.
   This issue arises because the internal format is designed for
   efficient key storage by deriving the private key from the seed,
   while the expanded private key is already fully computed.  As NIST
   has not defined a single private key format for PQC algorithms, this
   creates a potential gap in interoperability.

   If the seed is not securely stored at the time of key generation, it
   is permanently lost because the process of deriving an expanded key
   from the seed relies on a one-way cryptographic function.  This one-
   way function is designed to ensure that the expanded private key can
   be deterministically derived from the seed, but the reverse
   operation, deriving the original seed from the expanded key is
   computationally infeasible.

2.1.2.  Efficient Key Derivation

   When storing only the seed in a constrained cryptographic module, it
   is crucial that the device is capable of deriving the private key
   efficiently whenever required.  However, it is important to note that
   constantly re-deriving the private key for every cryptographic
   operation may introduce significant performance overhead.  In
   scenarios where performance is a critical consideration, it may be
   more efficient to store the expanded private key directly instead of
   only the seed.  Higher quality implementations may also retain
   (cache) recently-used or frequently-used private keys to avoid the
   computational overhead and delay of deriving the private key from the
   seed with each request.

   The key derivation process, such as ML-KEM.KeyGen_internal for ML-KEM
   or similar functions for other PQC algorithms, must be implemented in
   a way that can securely operate within the resource constraints of

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   the device.  If using the seed-only model, the derived private key
   should only be temporarily held in memory during the cryptographic
   operation and discarded immediately after use.  However, storing the
   expanded private key may be a more practical solution in time-
   sensitive applications or for devices that frequently perform
   cryptographic operations.

2.1.3.  Secure Exporting of Seeds

   Given the potential for hardware failures or the end-of-life of
   constrained devices, it is essential to plan for backup and recovery
   of the cryptographic seeds.  Constrained devices should support
   secure seed backup mechanisms, ideally leveraging encrypted storage
   and ensuring that the backup data is protected from unauthorized
   access.  In a disaster recovery scenario, the seed should be
   recoverable to enable the re-derivation of the private key, provided
   the proper security measures are in place to prevent unauthorized
   extraction.

   For secure exporting of seeds, PQC encryption algorithms, such as ML-
   KEM, should be used to encrypt the seed before export.  This ensures
   that the seed remains protected even if the export process is
   vulnerable to quantum attacks.  The process for secure export should
   include:

   *  Encrypting the seed using a post-quantum encryption algorithm,
      such as ML-KEM, rather than relying on traditional encryption
      algorithms.

   *  Ensuring the exported seed is accessible only to authorized
      entities.

   *  Enforcing strict access controls and secure transport mechanisms
      to prevent unauthorized access during transfer.

   The seed generation, storage, and usage should remain entirely within
   the cryptographic module.  This minimizes the risk of exposure and
   ensures compliance with established security guidelines.

3.  Ephemeral Key Management

   In protocols like TLS and IPsec, ephemeral keys are used for key
   exchange.  Given the increased size of PQC key material, ephemeral
   key management will have to be optimized for both security and
   performance.

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   For PQC KEMs, ephemeral key-pairs must be generated from an ephemeral
   seed, which needs to be securely stored temporarily and erased after
   use.  This approach ensures that ephemeral key generation is
   deterministic and minimizes storage overhead in constrained devices,
   as only the seed (not the full private key) needs to be stored.  The
   ephemeral seed must be deleted immediately after the key pair is
   generated to prevent potential leakage or misuse.

   Furthermore, once the shared secret is derived, the private key must
   also be deleted.  Since the private key resides in the constrained
   cryptographic module, removing it optimizes memory usage, reducing
   the footprint of PQC key material in constrained HSMs.

   Additionally, ephemeral keys should not be reused across different
   algorithm suites and sessions.  Each ephemeral key-pair must be
   uniquely associated with a specific key exchange instance to prevent
   cryptographic vulnerabilities, such as cross-protocol attacks or
   unintended key reuse.

   Constrained devices implementing PQC ephemeral key management will
   have to:

   *  Generate ephemeral key-pairs on-demand from an ephemeral seed
      stored temporarily within the cryptographic module.

   *  Enforce immediate seed erasure after the key-pair is generated and
      the cryptographic operation is completed.

   *  Delete the private key after the shared secret is derived.

   *  Prevent key reuse across different algorithm suites or sessions.

4.  Optimizing Performance in Constrained Devices for PQC Signature
    Algorithms

   When implementing PQC signature algorithms in constrained
   cryptographic modules, performance optimization becomes a critical
   consideration.  Transmitting the entire message to the cryptographic
   module for signing can lead to significant overhead, especially for
   large payloads.  To address this, implementers can leverage
   techniques that reduce the data transmitted to the cryptographic
   module, thereby improving efficiency and scalability.

   One effective approach involves sending only a message digest to the
   cryptographic module for signing.  By signing the digest of the
   content rather than the entire content, the communication between the
   application and the cryptographic module is minimized, enabling
   better performance.  This method is applicable for any PQC signature

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   algorithm, whether it is ML-DSA, SLH-DSA, or any future signature
   scheme.  For such algorithms, a mechanism is often provided to pre-
   hash or process the message in a way that avoids sending the entire
   raw message for signing.  In particular, algorithms like SLH-DSA
   present challenges due to their construction, which requires multiple
   passes over the message digest during the signing process.  The
   signer does not retain the entire message or its full digest in
   memory at once.  Instead, different parts of the message digest are
   processed sequentially during the signing procedure.  This differs
   from traditional algorithms like RSA or ECDSA, which allow for more
   efficient processing of the message, without requiring multiple
   passes or intermediate processing of the digest.

   A key consideration when deploying ML-DSA in cryptographic module is
   the amount of memory available.  ML-DSA, unlike traditional signature
   schemes such as RSA or ECDSA, requires significant memory during
   signing due to multiple Number Theoretic Transform (NTT) operations,
   matrix expansions, and rejection sampling loops.  These steps involve
   storing large polynomial vectors and intermediate values, making ML-
   DSA more memory-intensive.  If an cryptographic module has sufficient
   memory, this may not be an issue.  However, in constrained
   environments with limited memory, implementing ML-DSA can be
   challenging.  The signer must store and process multiple transformed
   values, leading to increased computational overhead if the
   cryptographic module lacks the necessary memory to manage these
   operations efficiently.

   To address the memory consumption challenge, algorithms like ML-DSA
   offer a form of pre-hash using the mu (message representative) value
   described in Section 6.2 of [ML-DSA].  The mu value provides an
   abstraction for pre-hashing by allowing the hash or message
   representative to be computed outside the cryptographic module.  This
   feature offers additional flexibility by enabling the use of
   different cryptographic modules for the pre-hashing step, reducing
   memory consumption within the cryptographic module.  The pre-computed
   mu value is then supplied to the cryptographic module, eliminating
   the need to transmit the entire message for signing.
   [I-D.ietf-lamps-dilithium-certificates] discusses leveraging
   ExternalMu-ML-DSA, where the pre-hashing step (ExternalMu-ML-
   DSA.Prehash) is performed in a software cryptographic module, and
   only the pre-hashed message (mu) is sent to the hardware
   cryptographic module for signing (ExternalMu-ML-DSA.Sign).  By
   implementing ExternalMu-ML-DSA.Prehash in software and ExternalMu-ML-
   DSA.Sign in an hardware cryptographic module, the cryptographic
   workload is efficiently distributed, making it practical for high-
   volume signing operations even in memory-constrained cryptographic
   modules.

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   The main advantage of this method is that, unlike HashML-DSA, the
   ExternalMu-ML-DSA approach is interoperable with the standard version
   of ML-DSA that does not use pre-hashing.  This means a message can be
   signed using ML-DSA.Sign, and the verifier can independently compute
   mu and use ExternalMu-ML-DSA.Verify for verification -- or vice
   versa.  In both cases, the verifier does not need to know whether the
   signer used internal or external pre-hashing, as the resulting
   signature and verification process remain the same.

5.  Additional Considerations for PQC Use in Constrained Devices

   Key Rotation and Renewal: In constrained devices, managing the
   lifecycle of cryptographic keys including periodic key rotation and
   renewal is critical for maintaining long-term security and supporting
   cryptographic agility.  While constrained devices may rely on
   integrated secure elements or lightweight HSMs for secure key storage
   and operations, the responsibility for orchestrating key rotation
   typically resides in the application layer or external device
   management infrastructure.

   Although the underlying cryptographic module may offer primitives to
   securely generate new key pairs, store fresh seeds, or delete
   obsolete keys, these capabilities must be integrated into the
   device’s broader key management framework.  This process is
   especially important in the context of PQC, where evolving research
   may lead to changes in recommended algorithms, parameters, and key
   management practices.

   The security of PQC schemes continues to evolve, with potential risks
   arising from advances in post-quantum algorithms, cryptanalytic or
   implementation vulnerabilities.  As a result, constrained devices
   should be designed to support flexible and updatable key management
   policies.  This includes the ability to:

   *  Rotate keys periodically to provide forward-secrecy,

   *  Update algorithm choices or key sizes based on emerging security
      guidance,

   *  Reconfigure cryptographic profile of the device via firmware
      updates.

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6.  Post-quantum Firmware Upgrades for Constrained Devices

   Constrained devices deployed in the field require periodic firmware
   upgrades to patch security vulnerabilities, introduce new
   cryptographic algorithms, and improve overall functionality.
   However, the firmware upgrade process itself can become a critical
   attack vector if not designed to be post-quantum.  If an adversary
   compromises the update mechanism, they could introduce malicious
   firmware, undermining all other security properties of the
   cryptographic modules.  Therefore, ensuring a post-quantum firmware
   upgrade process is critical for the security of deployed constrained
   devices.

   CRQCs pose an additional risk by breaking traditional digital
   signatures (e.g., RSA, ECDSA) used to authenticate firmware updates.
   If firmware verification relies on traditional signature algorithms,
   attackers could generate forged signatures in the future and
   distribute malicious updates.

6.1.  Post-quantum Firmware Authentication

   To ensure the integrity and authenticity of firmware updates,
   constrained devices will have to adopt PQC digital signature schemes
   for code signing.  Recommended post-quantum algorithms include:

   *  SLH-DSA (Stateless Hash-Based Digital Signature Algorithm): SLH-
      DSA does not introduce any new hardness assumptions beyond those
      inherent to its underlying hash functions.  It builds upon
      established foundations in cryptography, making it a reliable and
      robust digital signature scheme for a post-quantum world.  While
      attacks on lattice-based schemes like ML-DSA can compromise their
      security, SLH-DSA will remain unaffected by these attacks due to
      its distinct mathematical foundations.  This ensures the ongoing
      security of systems and protocols that use SLH-DSA for digital
      signatures.  Given that the first vulnerabilities in PQC
      algorithms are more likely to arise from implementation flaws
      rather than fundamental mathematical weaknesses, SLH-DSA is still
      susceptible to attacks if not properly implemented.

   *  HSS-LMS (Hierarchical Signature System - Leighton-Micali
      Signature): A hash-based signature scheme, providing long-term
      security and efficient key management for firmware authentication
      (see [REC-SHS]).

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   *  XMSS (eXtended Merkle Signature Scheme): Another stateful hash-
      based signature scheme similar to HSS-LMS [RFC8391].  XMSS
      signatures are slightly shorter than HSS-LMS signatures for
      equivalent security.  However, HSS-LMS provides performance
      advantages and HSS-LMS is considered simpler (see Section 10 of
      [RFC8554]).

   Firmware images can be signed using one of these post-quantum
   algorithms before being distributed to constraied devices.
   [I-D.wiggers-hbs-state] discusses various strategies for a correct
   state and backup management for stateful hash-based signatures.

   Firmware images often have a long lifetime, requiring cryptographic
   algorithms that provide strong security assurances over extended
   periods.  ML-DSA is not included in this list because it is a
   lattice-based signature scheme, making it susceptible to potential
   advances in quantum and classical attacks on structured lattices.
   The long-term security of ML-DSA depends on the continued hardness of
   lattice-based problems, which remain an active area of research.  In
   contrast, SLH-DSA, HSS-LMS, and XMSS are based on well-studied hash
   functions, ensuring their security does not rely on unproven
   assumptions about lattice hardness.  Given this uncertainty, use of a
   hash-based signature such as SLH-DSA may be preferable to ML-DSA for
   firmware authentication, where cryptographic stability over a long
   lifetime is a critical requirement.

7.  Security Considerations

   The security considerations for key management in constrained devices
   for PQC focus on the secure storage and handling of cryptographic
   seeds, which are used to derive private keys.  Seeds must be
   protected with the same security measures as private keys, and key
   derivation should be efficient and secure within resource-constrained
   cryptographic module.  Secure export and backup mechanisms for seeds
   are essential to ensure recovery in case of hardware failure, but
   these processes must be encrypted and protected from unauthorized
   access.

7.1.  Side Channel Protection

   Side-channel attacks exploit physical leaks during cryptographic
   operations, such as timing information, power consumption,
   electromagnetic emissions, or other physical characteristics, to
   extract sensitive data like private keys or seeds.  Given the
   sensitivity of the seed and private key in PQC key generation, it is
   critical to consider side-channel protection in cryptographic module
   design.  While side-channel attacks remain an active research topic,
   their significance in secure hardware design cannot be understated.

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   Cryptographic modules must incorporate strong countermeasures against
   side-channel vulnerabilities to prevent attackers from gaining
   insights into secret data during cryptographic operations.

Acknowledgements

   Thanks to Jean-Pierre Fiset, Richard Kettlewell, Mike Ounsworth, and
   Aritra Banerjee for the detailed review.

Informative References

   [BIND]     "Unbindable Kemmy Schmid", n.d.,
              <https://eprint.iacr.org/2024/523.pdf>.

   [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-
              07, 2 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
              dilithium-certificates-07>.

   [I-D.ietf-lamps-kyber-certificates]
              Turner, S., Kampanakis, P., Massimo, J., and B.
              Westerbaan, "Internet X.509 Public Key Infrastructure -
              Algorithm Identifiers for the Module-Lattice-Based Key-
              Encapsulation Mechanism (ML-KEM)", Work in Progress,
              Internet-Draft, draft-ietf-lamps-kyber-certificates-10, 16
              April 2025, <https://datatracker.ietf.org/doc/html/draft-
              ietf-lamps-kyber-certificates-10>.

   [I-D.wiggers-hbs-state]
              Wiggers, T., Bashiri, K., Kölbl, S., Goodman, J., and S.
              Kousidis, "Hash-based Signatures: State and Backup
              Management", Work in Progress, Internet-Draft, draft-
              wiggers-hbs-state-02, 1 April 2025,
              <https://datatracker.ietf.org/doc/html/draft-wiggers-hbs-
              state-02>.

   [ML-DSA]   "FIPS-204: Module-Lattice-Based Digital Signature
              Standard", <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.204.pdf>.

   [ML-KEM]   "FIPS-203: Module-Lattice-based Key-Encapsulation
              Mechanism Standard",
              <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.203.pdf>.

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   [REC-KEM]  "Recommendations for Key-Encapsulation Mechanisms", n.d.,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-227.ipd.pdf>.

   [REC-SHS]  "Recommendation for Stateful Hash-Based Signature Scheme",
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-208.pdf>.

   [RFC8391]  Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A.
              Mohaisen, "XMSS: eXtended Merkle Signature Scheme",
              RFC 8391, DOI 10.17487/RFC8391, May 2018,
              <https://www.rfc-editor.org/rfc/rfc8391>.

   [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>.

   [SLH-DSA]  "FIPS-205: Stateless Hash-Based Digital Signature
              Standard", <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.205.pdf>.

Authors' Addresses

   Tirumaleswar Reddy
   Nokia
   Bangalore
   Karnataka
   India
   Email: kondtir@gmail.com

   Dan Wing
   Cloud Software Group Holdings, Inc.
   United States of America
   Email: danwing@gmail.com

   Ben Salter
   UK National Cyber Security Centre
   Email: ben.s3@ncsc.gov.uk

   Kris Kwiatkowski
   PQShield
   Email: kris@amongbytes.com

Reddy, et al.            Expires 24 October 2025               [Page 13]