GREASE Code Points in OpenPGP

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Table of Contents

1. Introduction

GREASE [RFC8701] is an existing specification to ensure forwards compatibility of assigned code points in TLS. We wish to specify a similar mechanism for OpenPGP. This will be particularly useful in the OpenPGP Interoperability Test Suite [INTEROP], but is not restricted to controlled environments. It is expected that implementations will include PGP-GREASE code points in real-world artifacts on an ongoing basis, to encourage forwards-compatible coding across multiple implementations.¶

2. Conventions and Definitions

The term "OpenPGP Certificate" is used in this document interchangeably with "OpenPGP Transferable Public Key", as defined in Section 10.1 of [RFC9580].¶

For avoidance of confusion with GREASE in TLS, we will use the term "PGP-GREASE" for the equivalent mechanism in OpenPGP.¶

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. Reservation of PGP-GREASE Code Points

We assign the following one-octet code points to PGP-GREASE:¶

These code points will be reserved for PGP-GREASE in the following registries:¶

1. The use of PGP-GREASE code points in the String-to-Key Types and Reason for Revocation registries is not currently specified, however the code points will be reserved for consistency.¶

2. When PGP-GREASE code points from the Signature Subpacket Types registry are used, the critical bit MUST NOT be set.¶

3. All code points allocated in the Symmetric Key Algorithms registry are automatically also allocated in the S2K Usage Octet registry.¶

Due to the smaller number of allocatable code points (63) in the OpenPGP Packet Types registry, most PGP-GREASE values cannot be used as Packet Types. In addition, implementations pre-dating [RFC9580] are not required to ignore unknown packet types. A Marker packet (Section 5.8 of [RFC9580]) SHOULD therefore be used instead. Packet Types 44 and 55 MAY be used if it is known that the receiving implementation supports [RFC9580].¶

Note that [INTEROP] currently uses signature version 23 as a de-facto GREASE code point. This is intentionally not included here, as we prefer code points that are available for reservation across multiple registries.¶

4. Usage of PGP-GREASE Code Points

An implementation MAY insert dummy packets or subpackets into otherwise valid packet sequences. It MAY also include dummy code points in preference lists. The data section of dummy packets and subpackets SHOULD contain only the ten octets "PGP-GREASE", in UTF-8 encoding. All other fields of dummy packets and subpackets SHOULD correspond to those of a real packet or subpacket.¶

A generating implementation SHOULD choose code points deterministically, while ensuring that all PGP-GREASE code points are eventually used.¶

(TODO: how often should we add GREASE?)¶

A receiving implementation MUST gracefully ignore unknown code points when required to by [RFC9580], including PGP-GREASE code points. It MUST NOT treat PGP-GREASE code points any differently from other unknown code points; this includes detecting or indicating the presence of PGP-GREASE code points.¶

PGP-GREASE codepoints MAY be used in either certificates or messages.¶

Since existing legacy implementations have not consistently followed the requirement to ignore unknown code points, a generating implementation is RECOMMENDED to introduce PGP-GREASE code points gradually, over several release cycles. This should ensure that legacy receiving implementations are not overwhelmed by errors in a short period of time.¶

5. Security Considerations

Legacy implementations that do not properly ignore unknown code points will experience failures; this is intentional. None of these failures should have security consequences in themselves, unless other coding errors are present in the legacy implementation. While this should result in increased security in the long term, exposing such bugs may cause problems in the short term.¶

Generating implementations are therefore RECOMMENDED to be cautious in deploying PGP-GREASE initially. In particular, implementations SHOULD NOT generate PGP-GREASE code points in production unless they have been tested first in a controlled environment such as the Interoperability Test Suite [INTEROP].¶

6. IANA Considerations

IANA is requested to allocate the code points listed in Table 1 in each of the registries listed in Table 2, with the note "Reserved (PGP-GREASE)" and a reference pointing to this document.¶

IANA is also requested to allocate code points 44 and 55 in the "OpenPGP Packet Types" registry, with the note "Reserved (PGP-GREASE)" and a reference pointing to this document.¶

Appendix A. Acknowledgments

The author would like to thank Daniel Huigens for inspiring this work.¶

Author's Address