Persistent Symmetric Keys in OpenPGP

Introduction The OpenPGP standard 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 private 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 proposed by the OpenPGP crypto refresh . 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 hash-based message authentication codes (HMAC) as a symmetric counterpart to digital signatures, for long-term storage and archival of attestations of authenticity and certification.

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 . 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 and the OpenPGP crypto refresh . Any keyword within this document applies to compliant applications only.

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 . Symmetric cryptography is also affected by quantum computing but to a lesser extent, which can be countered by using larger keys . While the standardization of quantum-secure asymmetric cryptography in OpenPGP is ongoing , 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 future compromises efficiently by symmetrically re-encrypting the session key, and storing the message symmetrically encrypted for long-term storage and archival.

Reusing and Renaming Packets Rather than introducing new packets for storing persistent symmetric keys, the existing Secret-Key packets are reused for this purpose. To indicate the type of keys, two algorithms (AEAD and HMAC) are registered, whose IDs can be used in the place of public-key algorithm IDs. To accommodate these additions, we propose renaming the Public Key Algorithms registry to Persistent Key Algorithms. Similarly, we reuse the Signature packet for "symmetric signatures". For 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. Nevertheless, given this usage, the naming of these packets may be confusing, so we propose to rename them to "String-to-Key Encrypted Session Key packet" and "Persistent Key Encrypted Session Key packet", instead.

Persistent Symmetric Key Algorithms This document defines two new algorithms for use with OpenPGP, extending the table in section 9.1 of . ID Algorithm Public Key Format Secret Key Format Signature Format PKESK Format 64 AEAD sym. algo, seed hash [] hash seed, key material N/A AEAD algo, IV, length, ciphertext [] 65 HMAC hash algo, seed hash [] hash seed, key material authentication tag [] N/A These algorithm IDs can be used to store symmetric key material in Secret-Key Packets and Secret-Subkey packets (see section 5.5.3 of ). The AEAD algorithm ID can be used to store session keys encrypted using AEAD in PKESK packets (see section 5.1 of ). The HMAC algorithm ID can be used to store HMAC-based signatures in Signature packets (see section 5.2 of ). As the secret key material is required for all cryptographic operations with symmetric keys, implementations SHOULD NOT use these algorithm IDs in Public-Key Packets or Public-Subkey Packets, and SHOULD NOT export Public-Key Packets from Secret-Key Packets holding symmetric key material.

Algorithm-Specific Fields for AEAD keys The public key is this series of values: A one-octet symmetric algorithm identifier (see section 9.3 of ) A 32-octet SHA-256 hash of the seed in the private key material The private key is this series of values: A 32-octet seed value to be hashed for the public key material Symmetric key material of appropriate length for the chosen symmetric algorithm

Algorithm-Specific Fields for HMAC keys The public key is this series of values: A one-octet hash algorithm identifier (see section 9.5 of ) A 32-octet SHA-256 hash of the seed in the private key material The private key is this series of values: A 32-octet seed value to be hashed for the public key material Symmetric key material of the length of the hash output size of the chosen hash algorithm

Algorithm-Specific Fields for AEAD encryption A one-octet AEAD algorithm (see section 9.6 of ) A starting initialization vector of size specified by AEAD mode A one-octet length of the following field A symmetric key encryption of the plaintext value described in section 5.1 of , performed using the selected symmetric-key cipher operating in the given AEAD mode, including the authentication tag.

Algorithm-Specific Fields for HMAC signatures An authentication tag of appropriate length for the hash algorithm Although not required by HMAC, to maintain compatibility with existing signature implementations, HMAC tags are produced from appropriately hashed data, as per section 5.2.4 of .

Security Considerations Security considerations are discussed throughout the document where appropriate.

IANA Considerations

Updates to Public Key Algorithms IANA is requested to rename the "Public Key Algorithms" registry to "Persistent Key Algorithms", and add the entries in to the registry.

Updates to Packet Type Descriptions IANA is requested to modify the "PGP Packet Types/Tags" registry as follows: For Packet Tag 1 ("Public-Key Encrypted Session Key Packet"), change the Packet Type to "Persistent Key Encrypted Session Key Packet". For Packet Tag 3 ("Symmetric-Key Encrypted Session Key Packet"), change the Packet Type to "String-to-Key Encrypted Session Key Packet".

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