GPG public key schemas are incompatible with both TUF and in-toto specifications

[jku]

Example:

        {
            "hashes": [
                "pgp+SHA2"
            ],
            "keyid": "7b3abb26b97b655ab9296bd15b0bd02e1c768c43",
            "keyval": {
                "private": "",
                "public": {
                    "e": "010001",
                    "n": "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"
                }
            },
            "method": "pgp+rsa-pkcsv1.5",
            "type": "rsa"
        }

and the schemas it uses:

GPG_RSA_PUBKEYVAL_SCHEMA = SCHEMA.Object(
    object_name="GPG_RSA_PUBKEYVAL_SCHEMA", e=SCHEMA.AnyString(), n=HEX_SCHEMA
)
_GPG_RSA_PUBKEY_SCHEMA = SCHEMA.Object(
    object_name="GPG_RSA_PUBKEY_SCHEMA",
    type=SCHEMA.String("rsa"),
    method=SCHEMA.String(GPG_RSA_PUBKEY_METHOD_STRING),
    hashes=SCHEMA.ListOf(SCHEMA.String(GPG_HASH_ALGORITHM_STRING)),
    creation_time=SCHEMA.Optional(UNIX_TIMESTAMP_SCHEMA),
    validity_period=SCHEMA.Optional(SCHEMA.Integer(lo=0)),
    keyid=KEYID_SCHEMA,
    keyval=SCHEMA.Object(
        public=GPG_RSA_PUBKEYVAL_SCHEMA, private=SCHEMA.String("")
    ),
)

This contradicts with the "rsa" type defined in both TUF and in-toto specifications (which define keyval.public as a PEM string). These keys work in in-toto because there's code in in-toto that uses a schema check to figure out which verify function to call, but the metadata is still not spec compliant, and would be impossible for another implementation to use without reverse engineering the reference implementation.

[JustinCappos]

Ouch. Thanks for catching this.

[lukpueh]

Thanks for pointing out the spec incompatibility aspect of this!

#308 already documents the different key metadata formats in tuf and in-toto, and suggests, among other things, to consolidate ['keyval'] values (see e.g. serialized rsa PEM vs. non-serialized rsa numbers).

[lukpueh]

in-toto/specification#33 discusses other spec - implementation inconsistencies of the public key metadata format.

This does not fix the current issue of incompatibility with python-tuf, but I think the correct way is to detach the key and signature format from the specification, and document it as part of an implementation.