OpenPGP Signature Salt Notation
draft-huigens-openpgp-signature-salt-notation-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Daniel Huigens | ||
| Last updated | 2024-06-26 | ||
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draft-huigens-openpgp-signature-salt-notation-00
Network Working Group D. Huigens, Ed.
Internet-Draft Proton AG
Updates: 4880 (if approved) 26 June 2024
Intended status: Standards Track
Expires: 28 December 2024
OpenPGP Signature Salt Notation
draft-huigens-openpgp-signature-salt-notation-00
Abstract
This document defines the "salt" Notation Name for OpenPGP version 4
signatures. This can be used to salt version 4 signatures in a
backwards-compatible way.
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-signature-salt-notation/. Status
information for this document may be found at
https://datatracker.ietf.org/doc/draft-huigens-openpgp-signature-
salt-notation/.
Discussion of this document takes place on the OpenPGP Working Group
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Source for this draft and an issue tracker can be found at
https://gitlab.com/twisstle/openpgp-signature-salt-notation.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 28 December 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Notation Data Subpacket Type . . . . . . . . . . . . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
8.1. Normative References . . . . . . . . . . . . . . . . . . 4
8.2. Informative References . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
The crypto refresh [crypto-refresh] of the OpenPGP standard [RFC4880]
introduces version 6 signatures, which are salted. This has several
benefits, such as preventing fault attacks against EdDSA signatures.
This document introduces a "salt" Notation Name so that version 4
signatures can benefit from some of the same advantages in a
backwards-compatible way. Note, however, that the notations are not
hashed first in the signature, and thus this does not automatically
benefit from _all_ benefits described in Section 13.2 of
[crypto-refresh]. Therefore, this proposal is not intended to delay
or remove the necessity for deploying version 6 keys and signatures.
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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 [RFC4880] and the OpenPGP crypto refresh
[crypto-refresh]. Any [RFC2119] keyword within this document applies
to compliant applications only.
3. Motivation
Salting signatures can prevent certain attacks, such as fault attacks
against EdDSA [PSSLR17]. In version 6 signatures, a salt was added
(see Section 5.2.3 and Section 13.2 of [crypto-refresh]). For
version 4 signatures, some implementations (originating with Sequoia-
PGP) add a signature notation to add a salt in a backwards-compatible
manner. To comply with [crypto-refresh], implementations have
created implementation-specific notation names under a domain they
control (such as "salt@notations.sequoia-pgp.org"). However, this
adds some overhead for each created signature, compared to a shorter
notation name in the IETF namespace. Additionally, it adds to the
distinguishability of OpenPGP artifacts, unless all implementations
use the same notation name. For implementations that wish to
disguise which implementation created any given artifact, it may be
preferable to use only notations from the IETF namespace.
4. Notation Data Subpacket Type
This document defines a new Notation Data Subpacket Type for use with
OpenPGP, extending Table 7 of [crypto-refresh].
+===============+===========+================+
| Notation Name | Data Type | Allowed Values |
+===============+===========+================+
| salt | binary | random data |
+---------------+-----------+----------------+
Table 1: Signature Salt Notation registration
This notation can be used to store a random salt in version 4
signatures (see section 5.2 of [crypto-refresh]). The length of the
salt MUST match the "V6 signature salt size" value defined for the
hash algorithm as specified in Table 23 of [crypto-refresh]. The
"human-readable" flag MUST NOT be set for this notation name.
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5. Security Considerations
Unlike the salt in version 6 signatures (as defined in Section 5.2.3
of [crypto-refresh]), which is hashed at the start before all other
data, a salt in a Notation Data subpacket is hashed after the data to
be signed (see Section 5.2.3.6 of [crypto-refresh]). Because of
this, the salt notation may not prevent chosen prefix collision
attacks like version 6 signatures do (see Section 13.2 of
[crypto-refresh]). Therefore, this mechanism is not intended to
replace or delay the deployment of version 6 signatures. It should
only be used when the use of version 4 signatures is required (e.g.
for compatibility reasons).
6. IANA Considerations
IANA is requested to add the registration in Table 1 to the "OpenPGP
Signature Notation Data Subpacket Types" registry, with a reference
to this document in the "Reference" column.
7. Acknowledgements
The idea and first implementation of a salt notation came from
Sequoia-PGP.
8. References
8.1. Normative References
[crypto-refresh]
Wouters, P., Huigens, D., Winter, J., and N. Yutaka,
"OpenPGP", January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-
crypto-refresh-13>.
[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>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<https://www.rfc-editor.org/info/rfc4880>.
8.2. Informative References
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[PSSLR17] Poddebniak, D., Somorovsky, J., Schinzel, S., Lochter, M.,
and P. Rösler, "Attacking Deterministic Signature Schemes
using Fault Attacks", October 2017,
<https://eprint.iacr.org/2017/1014>.
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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