Persistent Symmetric Keys in OpenPGP
draft-ietf-openpgp-persistent-symmetric-keys-02

Network Working Group                                    D. Huigens, Ed.
Internet-Draft                                                 Proton AG
Updates: 9580 (if approved)                              4 November 2025
Intended status: Standards Track                                        
Expires: 8 May 2026

                  Persistent Symmetric Keys in OpenPGP
            draft-ietf-openpgp-persistent-symmetric-keys-02

Abstract

   This document defines a new packet and algorithm for the OpenPGP
   standard (RFC 9580) to support persistent symmetric keys, for message
   encryption using authenticated encryption with additional data (AEAD)
   and for message authentication using AEAD authentication tags.  This
   enables the use of symmetric cryptography for data storage (and other
   contexts that do not require asymmetric cryptography), for improved
   performance, smaller keys, and improved resistance to quantum
   computing.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://twisstle.gitlab.io/openpgp-persistent-symmetric-keys/.
   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-openpgp-persistent-
   symmetric-keys/.

   Discussion of this document takes place on the OpenPGP Working Group
   mailing list (mailto:openpgp@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/openpgp/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/openpgp/.

   Source for this draft and an issue tracker can be found at
   https://gitlab.com/twisstle/openpgp-persistent-symmetric-keys.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Creating and Reusing Packets  . . . . . . . . . . . . . . . .   4
   5.  Persistent Symmetric Key Packet . . . . . . . . . . . . . . .   4
   6.  Extended Transferable Secret Key Grammar  . . . . . . . . . .   5
   7.  Persistent Symmetric Key Algorithm  . . . . . . . . . . . . .   5
     7.1.  Algorithm-Specific Fields for Persistent Symmetric
           Keys  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.2.  Algorithm-Specific Fields for Persistent Symmetric
           Encryption  . . . . . . . . . . . . . . . . . . . . . . .   6
     7.3.  Algorithm-Specific Fields for Persistent Symmetric
           Signatures  . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Updates to Packet Types . . . . . . . . . . . . . . . . .   7
     9.2.  Updates to Public Key Algorithms  . . . . . . . . . . . .   7
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     11.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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1.  Introduction

   The OpenPGP standard [RFC9580] has supported symmetric encryption for
   data packets using session keys since its inception, as well as
   symmetric encryption using password-derived keys.  This document
   extends the use of symmetric cryptography by adding support for
   persistent symmetric keys which can be stored in a transferable
   secret key, and used to symmetrically encrypt session keys, for long-
   term storage and archival of messages.  This document uses
   authenticated encryption with associated data (AEAD) as defined by
   [RFC9580].

   The OpenPGP standard also supports the use of digital signatures for
   authentication and integrity but no similar symmetric mechanism
   exists in the standard.  This document introduces the use of AEAD
   authentication tags as a symmetric counterpart to digital signatures,
   for long-term storage and archival of attestations of authenticity
   and certification.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  Any
   implementation that adheres to the format and methods specified in
   this document is called a compliant application.  Compliant
   applications are a subset of the broader set of OpenPGP applications
   described in [RFC9580].  Any [RFC2119] keyword within this document
   applies to compliant applications only.

3.  Motivation

   When compared to asymmetric cryptography, symmetric cryptography can
   provide improved performance and equivalent security with smaller
   keys.  In contexts that do not require asymmetric cryptography, such
   as secure data storage where the same user encrypts and decrypts
   data, symmetric cryptography can be used to take advantage of these
   benefits.

   Additionally, asymmetric algorithms included in OpenPGP are
   vulnerable to attacks that might become possible on quantum computers
   [Shor].  Symmetric cryptography is also affected by quantum computing
   but to a lesser extent, which can be countered by using larger keys
   [Grover].  While the standardization of quantum-secure asymmetric
   cryptography in OpenPGP is ongoing [PQCinOpenPGP], and will be
   required to secure communications, there is a large body of existing
   messages encrypted with classical algorithms.  Once persistent
   symmetric keys are available, these messages can be protected against

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   future compromises efficiently by symmetrically re-encrypting the
   session key, and storing the message symmetrically encrypted for
   long-term storage and archival.

4.  Creating and Reusing Packets

   For storing persistent symmetric keys, we introduce a new packet (see
   Section 5), as handling of persistent symmetric key material requires
   some care.  For example, when extracting a Transferable Public Key
   from a Transferable Secret Key, persistent symmetric keys must be
   ignored.

   For storing session keys encrypted with persistent symmetric keys,
   while a Symmetric-Key Encrypted Session Key packet exists, its
   semantics don't match our requirements, as it's intended to encrypt
   the session key with a user-provided password, and doesn't offer a
   way to store a reference to a persistent key.  Therefore, we reuse
   the Public-Key Encrypted Session Key packet instead, which does offer
   the desired semantics.

   Similarly, we reuse the Signature packet for "symmetric signatures".

   For these use cases, no new packet is required as the handling of
   these packets requires no special care.

   To indicate the type of keys in the new and existing packets, a
   special persistent symmetric algorithm ID value 0 is registered,
   which can be used in the place of a public-key algorithm ID.

5.  Persistent Symmetric Key Packet

   This document defines a new OpenPGP packet, extending table 3 of
   [RFC9580].

      +====+==========+=================================+===========+
      | ID | Critical | Packet Type Description         | Shorthand |
      +====+==========+=================================+===========+
      | 40 | No       | Persistent Symmetric Key Packet | PSK       |
      +----+----------+---------------------------------+-----------+

           Table 1: Persistent Symmetric Key Packet registration

   The Persistent Symmetric Key Packet (Type ID 40) has identical fields
   to the Secret Key Packet (Type ID 5).  However, only version 6 of the
   packet is defined.  Earlier versions of the Secret Key Packet format
   MUST NOT be used with the Persistent Symmetric Key Packet.

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   The Persistent Symmetric Key Packet MUST NOT be used with asymmetric
   algorithms, i.e. any of the public key algorithms defined in table 18
   of [RFC9580].  It may only be used with the persistent symmetric
   algorithm defined below, with special algorithm ID value 0.

   When storing encrypted symmetric key material in a Persistent
   Symmetric Key Packet, AEAD encryption (S2K usage octet 253, see
   section 3.7.2.1 of [RFC9580]) MUST be used, to ensure that the secret
   key material is bound to the fingerprint.  Implementations MUST NOT
   decrypt symmetric key material in a Persistent Symmetric Key Packet
   that was encrypted using a different method.

6.  Extended Transferable Secret Key Grammar

   The Transferable Secret Key grammar defined in section 10.2 of
   [RFC9580] is extended to allow including Persistent Symmetric Key
   Packets.  Persistent Symmetric Keys may be included together with
   asymmetric keys in a single sequence of Transferable Secret Keys.
   However, Persistent Symmetric Keys do not accept subcomponents, such
   as subkeys, User IDs, or direct-key signatures.

7.  Persistent Symmetric Key Algorithm

   This document defines one new algorithm for use with OpenPGP,
   updating table 18 of [RFC9580].

   +==+=========+===============+========+================+============+
   |ID|Algorithm| Public Key    |Secret  | Signature      | PKESK      |
   |  |         | Format        |Key     | Format         | Format     |
   |  |         |               |Format  |                |            |
   +==+=========+===============+========+================+============+
   | 0|AEAD     | sym. algo,    |key     | IV,            | IV,        |
   |  |         | AEAD algo,    |material| authentication | ciphertext |
   |  |         | fingerprint   |        | tag            | [Section   |
   |  |         | seed          |        | [Section 7.3]  | 7.2]       |
   |  |         | [Section      |        |                |            |
   |  |         | 7.1]          |        |                |            |
   +--+---------+---------------+--------+----------------+------------+

          Table 2: Persistent Symmetric Key Algorithm registration

   This algorithm ID can be used to store symmetric key material in a
   Persistent Symmetric Key Packet (see Section 5).  It can also be used
   to store a session key encrypted using AEAD in a PKESK packet (see
   section 5.1 of [RFC9580]).  It can also be used to store an AEAD
   authentication tag in a Signature packet (see section 5.2 of
   [RFC9580]).

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   Implementations MUST NOT use the symmetric algorithm ID in Public-Key
   Packets, Public-Subkey Packets, Secret-Key Packets or Secret-Subkey
   Packets.

7.1.  Algorithm-Specific Fields for Persistent Symmetric Keys

   The public key material consists of this series of values:

   *  A one-octet symmetric algorithm identifier (see section 9.3 of
      [RFC9580]).

   *  A one-octet AEAD algorithm (see section 9.6 of [RFC9580]).

   *  A 32-octet random seed to randomize the key fingerprint.

   The secret key material consists of this single value:

   *  Symmetric key material of the appropriate length for the given
      symmetric algorithm.

7.2.  Algorithm-Specific Fields for Persistent Symmetric Encryption

   The encrypted session key consists of this series of values:

   *  A initialization vector of the size specified by the AEAD mode of
      the key.

   *  A symmetric key encryption of the plaintext value described in
      section 5.1 of [RFC9580], performed using the symmetric-key cipher
      and AEAD mode of the key, including the authentication tag.

7.3.  Algorithm-Specific Fields for Persistent Symmetric Signatures

   The signature consists of this series of values:

   *  A initialization vector of the size specified by the AEAD mode of
      the key.

   *  An authentication tag of the size specified by the AEAD mode of
      the key, created by encrypting the empty value using the
      symmetric-key cipher and AEAD mode of the key, with as additional
      data the hash digest described in section 5.2.4 of [RFC9580].

   Although not required by AEAD algorithms, to maintain consistency
   with existing signature algorithms, AEAD authentication tags are
   produced from appropriately hashed data, as per section 5.2.4 of
   [RFC9580].

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8.  Security Considerations

   Security considerations are discussed throughout the document where
   appropriate.

9.  IANA Considerations

9.1.  Updates to Packet Types

   IANA is requested to update the "OpenPGP Packet Types" registry with
   the entry in Table 1.

9.2.  Updates to Public Key Algorithms

   IANA is requested to update the "OpenPGP Public Key Algorithms"
   registry with the entry in Table 2.

10.  Acknowledgements

   An initial version of this draft was written by Dan Ristea (Proton
   AG), with guidance from Dr Philipp Jovanovic (University College
   London) and the editor.

   Thanks to feedback and suggestions from Andrew Gallagher, Heiko
   Schäfer, Justus Winter, Falko Strenzke, and Daniel Kahn Gillmor, in
   no particular order.

11.  References

11.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC9580]  Wouters, P., Ed., Huigens, D., Winter, J., and Y. Niibe,
              "OpenPGP", RFC 9580, DOI 10.17487/RFC9580, July 2024,
              <https://www.rfc-editor.org/info/rfc9580>.

11.2.  Informative References

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   [Grover]   Grover, L., "Quantum mechanics helps in searching for a
              needle in a haystack", 1997,
              <https://arxiv.org/abs/quant-ph/9706033>.

   [PQCinOpenPGP]
              Kousidis, S., Strenzke, F., and A. Wussler, "Post-Quantum
              Cryptography in OpenPGP", October 2023,
              <https://datatracker.ietf.org/doc/html/draft-wussler-
              openpgp-pqc-03>.

   [Shor]     Shor, P., "Polynomial-Time Algorithms for Prime
              Factorization and Discrete Logarithms on a Quantum
              Computer", October 1997,
              <http://dx.doi.org/10.1137/S0097539795293172>.

Author's Address

   Daniel Huigens (editor)
   Proton AG
   Route de la Galaise 32
   CH-1228 Plan-les-Ouates
   Switzerland
   Email: d.huigens@protonmail.com

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