OpenPGP Literal Data Metadata Integrity
draft-gallagher-openpgp-literal-metadata-00

openpgp                                                A. Gallagher, Ed.
Internet-Draft                                                PGPKeys.EU
Updates: 4880 (if approved)                               2 January 2024
Intended status: Informational                                          
Expires: 5 July 2024

                OpenPGP Literal Data Metadata Integrity
              draft-gallagher-openpgp-literal-metadata-00

Abstract

   This document specifies a method for ensuring the integrity of file
   metadata when signed using OpenPGP.

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://andrewgdotcom.gitlab.io/draft-gallagher-openpgp-literal-
   metadata.  Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-gallagher-openpgp-literal-
   metadata/.

   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/andrewgdotcom/draft-gallagher-openpgp-literal-
   metadata.

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-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

Gallagher                  Expires 5 July 2024                  [Page 1]
Internet-Draft   OpenPGP Literal Data Metadata Integrity    January 2024

   This Internet-Draft will expire on 5 July 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
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   2
   3.  Literal Data Meta subpacket . . . . . . . . . . . . . . . . .   3
     3.1.  Literal Data Meta (Hashed)  . . . . . . . . . . . . . . .   3
     3.2.  Literal Data Meta (Verbatim)  . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

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.

Gallagher                  Expires 5 July 2024                  [Page 2]
Internet-Draft   OpenPGP Literal Data Metadata Integrity    January 2024

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.

3.1.  Literal Data Meta (Hashed)

   The remainder of the packet contains a 32 octet fixed-length hash
   value.  The hash is computed over the metadata as specified above,
   using SHA-256 [FIPS180].  When an implementation is validating a
   signature containing a Literal Data Meta (Hashed) subpacket in its
   hashed-subpackets area, it MUST re-create the hash from the metadata
   section of the Literal Data packet that is signed over.  If the
   calculated hash value does not match the one in the subpacket, the
   signature MUST be deemed as invalid.

3.2.  Literal Data Meta (Verbatim)

   The remainder of the packet contains a verbatim copy of the metadata
   as specified above.  When an implementation has successfully
   validated a signature containing a Literal Data Meta (Verbatim)
   subpacket in its hashed-subpackets area, the metadata section of the
   Literal Data Packet that is signed over MUST be replaced with the
   copy from the Literal Data Meta subpacket.

Gallagher                  Expires 5 July 2024                  [Page 3]
Internet-Draft   OpenPGP Literal Data Metadata Integrity    January 2024

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:

               +======+===================+===============+
               | Type | Name              | Specification |
               +======+===================+===============+
               | 40   | Literal Data Meta | This document |
               +------+-------------------+---------------+

                  Table 1: Signature Subpacket Registry

6.  References

6.1.  Normative References

   [FIPS180]  National Institute of Standards and Technology, U.S.
              Department of Commerce, "Secure Hash Standard (SHS), FIPS
              180-4", August 2015,
              <http://dx.doi.org/10.6028/NIST.FIPS.180-4>.

   [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/rfc/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/rfc/rfc4880>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

Gallagher                  Expires 5 July 2024                  [Page 4]
Internet-Draft   OpenPGP Literal Data Metadata Integrity    January 2024

6.2.  Informative References

   [LIBREPGP] Koch, W. and R. H. Tse, "LibrePGP Message Format", 2023,
              <https://datatracker.ietf.org/doc/html/draft-koch-
              librepgp>.

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

   Andrew Gallagher (editor)
   PGPKeys.EU
   Email: andrewg@andrewg.com

Gallagher                  Expires 5 July 2024                  [Page 5]