Internet DRAFT - draft-gallagher-openpgp-literal-metadata
draft-gallagher-openpgp-literal-metadata
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
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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
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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
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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.
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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.
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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>.
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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
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