This specification describes the OpenPGP Signature Suite created in 2019 for the Linked Data Signatures specification.

This is an experimental specification and is undergoing regular revisions. It is not fit for production deployment.

Introduction

This specification describes the OpenPGP Signature Suite created in 2019 for the Linked Data Signatures [[LD-SIGNATURES]] specification. It uses the RDF Dataset CANONICALIZATION Algorithm [[RDF-DATASET-CANONICALIZATION]] to transform the input document into its canonical form. It uses SHA-256 [[RFC6234]] as the message digest algorithm and the OpenPGP sign detached algorithm defined in OpenPGP [[RFC4880]] as the signature algorithm.

Terminology

The following terms are used to describe concepts involved in the generation and verification of the Linked Data Signature 2019 signature suite.

signature suite
A specified set of cryptographic primitives typically consisting of a canonicalization algorithm, a message digest algorithm, and a signature algorithm that are bundled together by cryptographers for developers for the purposes of safety and convenience.
canonicalization algorithm
An algorithm that takes an input document that has more than one possible representation and always transforms it into a canonical form. This process is sometimes also called CANONICALIZATION.
message digest algorithm
An algorithm that takes an input message and produces a cryptographic output message that is often many orders of magnitude smaller than the input message. These algorithms are often 1) very fast, 2) non-reversible, 3) cause the output to change significantly when even one bit of the input message changes, and 4) make it infeasible to find two different inputs for the same output.
signature algorithm
An algorithm that takes an input message and produces an output value where the receiver of the message can mathematically verify that the message has not been modified in transit and came from someone possessing a particular secret.

The 2019 OpenPGP Signature Suite

The 2019 OpenPGP signature suite MUST be used in conjunction with the signing and verification algorithms in the Linked Data Signatures [[LD-SIGNATURES]] specification. The suite consists of the following algorithms:

Parameter Value Specification
canonicalizationAlgorithm https://w3id.org/security#GCA2015 [[RDF-DATASET-CANONICALIZATION]]
digestAlgorithm https://www.ietf.org/assignments/jwa-parameters#SHA256 [[RFC6234]]
signatureAlgorithm Open PGP Detached Signatures [[RFC4880]]

Modification to Algorithms

This signature suite uses detached Open PGP Signatures as described in [[RFC4880]]. The signature algorithm used is determined by the key type provided, armored OpenPGP keys contain the information necessary to know the specific method, and this flexibility supports integration with exiting software systems that use OpenPGP, GPG or PGP. The steps to construct and verify the digital signature are defined below.

Modifications to Signature Algorithm

The digital signature algorithm defined in Section 11.4: Signature Algorithm takes tbs, a privateKey, and options as inputs and produces a signatureValue as output.

  1. Take tbs, which is the data to be signed, an unlocked private key and options dictionary as input to the signature suite.
  2. Perform the sign detached digital signature algorithm specified in Section 11.4 of [[RFC4880]] and return the result as signatureValue.
  3. The signatureValue will be in one of the two following formats depending on options: -----BEGIN PGP SIGNATURE-----\r\nVersion: OpenPGP.js.... or when passed with compact option: wl4EARMIAAYFAlw6KOAACgkQSnoBzSruDWC....

Modifications to Signature Verification Algorithm

The digital signature algorithm defined in Section 11.4: Signature Verification Algorithm takes the value to be verified, tbv, the public key to the signature algorithm and returns a boolean value.

  1. Take tbv and perform the Sign Detached digital signature verification algorithm specified in Section 11.4 of [[RFC4880]]. If the result is a valid signature, return true, otherwise return false.

Security Considerations

The following section describes security considerations that developers implementing this specification should be aware of in order to create secure software.

TODO: We need to add a complete list of security considerations.
  1. This Signature Suite relies on the security and assumptions made by OpenPGP.js. Or any compatibile OpenPGP library implemented in a different language. We assume specifcally that sign and verify deteched are implemented safely for all supported key types.

Examples

A simple example of an OpenPGP 2019 signature:

{
  "@context": ["http://schema.org/", "https://w3id.org/security/v1"],
  "description": "Hello world!",
  "proof": {
    "type": "OpenPgpSignature2019",
    "created": "2017-10-24T05:33:31Z",
    "creator": "https://example.com/jdoe/keys/1",
    "domain": "example.com",
    "signatureValue": "wl4EARMIAAYFAlw6...KOAACgkQSnoBzSruDWC"
  }
}