Support hs2019 signatures

[fauno]

Following a site from a gotosocial instance throws UnkownAlgorithmError: Don't know how to verify hs2019 signatures.

#24

[fauno]

GoToSocial sets the "algorithm" field in signatures to the value hs2019, which essentially means "derive the algorithm from metadata associated with the keyId". The actual algorithm used for generating signatures is RSA_SHA256, which is in line with other ActivityPub implementations. When validating a GoToSocial HTTP signature, remote servers can safely assume that the signature is generated using sha256.

https://docs.gotosocial.org/en/latest/federation/http_signatures/#outgoing-requests

[fauno]

From node_modules/activitypub-http-signatures/src/index.js the issue is that it should assume hs2019 equals rsa-sha256 (!)

export class Parser {

// [...]

	/**
	 * Construct the signature class for a given algorithm.
	 * Override this method if you want to support additional
	 * algorithms.
	 * @param	{string}	algorithm The algorithm used by the signed request
	 * @param	{object}	options
	 * @param	{Buffer}	options.signature	The signature as a buffer
	 * @param	{string}	options.string	The string that was signed
	 * @param	{string}	options.keyId	The ID of the public key to be used for verification
	 * @returns	{Signature}
	 * @throws	{UnkownAlgorithmError}	If an unknown algorithm was used
	 */
	getSignatureClass(algorithm, { signature, string, keyId }) {
		if(algorithm === 'rsa-sha256') {
			return new Sha256Signature({ signature, string, keyId });
		} else {
			throw new UnkownAlgorithmError(`Don't know how to verify ${algorithm} signatures.`);
		}
	}
}