OpenPGP Literal Data Metadata Integrity

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Table of Contents

1. Introduction

Due to a design oversight in an early version of PGP, the literal data metadata (file type, file name, and file timestamp) were not covered by any integrity-protection mechanisms. That omission has persisted through subsequent OpenPGP specifications, up to and including [RFC4880]. This document introduces the missing integrity check by adopting and extending the "Literal Data Meta Hash" subpacket from [LIBREPGP], section 5.2.3.33.¶

2. Conventions and Definitions

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. Literal Data Meta subpacket

This subpacket MAY be used to protect the metadata of a Literal Data Packet. It is only useful when located in the hashed-subpackets area of a v4 (or later) signature over a Literal Data Packet, and so it SHOULD NOT appear elsewhere. The packet SHOULD be marked as critical.¶

The metadata is always of the same form as it appears in the Literal Data Packet (type 11, [RFC4880] section 5.9), i.e.:¶

• A one-octet field that describes how the data is formatted.¶

• File name as a string (one-octet length, followed by a file name).¶

• A four-octet number that indicates a date associated with the literal data.¶

The first octet of the subpacket contents indicates the encoding of the subpacket. A value of 0 indicates Hashed encoding, and a value of 1 indicates Verbatim encoding. The remaining octets of the subpacket are encoding-dependent.¶

4. Security Considerations

A signature containing a Literal Data Meta (Verbatim) subpacket can be round-tripped via detached-signature format without loss of integrity. A signature containing a Literal Data Meta (Hashed) subpacket does not have this property, because the metadata cannot be regenerated from the hash and is therefore lost on conversion to a detached signature.¶

It is therefore RECOMMENDED that implementations generate Literal Data Hash subpackets using the Verbatim encoding.¶

5. IANA Considerations

This document requests that the following entry be added to the OpenPGP Signature Subpacket registry:¶

Appendix A. Acknowledgments

The author would like to thank Werner Koch for his earlier work on the Literal Data Meta Hash subpacket.¶

Author's Address