Implement verification for Object Integrity Proofs (fep-8b32)

Gemfile

@@ -96,6 +96,7 @@ gem 'webauthn', '~> 3.0'
 gem 'webpush', github: 'mastodon/webpush', ref: '9631ac63045cfabddacc69fc06e919b4c13eb913'
 
 gem 'json'
+gem 'json-canonicalization', '~> 1.0'
 gem 'json-ld'
 gem 'json-ld-preloaded', '~> 3.2'
 gem 'rdf-normalize', '~> 0.5'

Gemfile.lock

@@ -1001,6 +1001,7 @@ DEPENDENCIES
   irb (~> 1.8)
   jd-paperclip-azure (~> 3.0)
   json
+  json-canonicalization (~> 1.0)
   json-ld
   json-ld-preloaded (~> 3.2)
   json-schema (~> 6.0)

app/lib/activitypub/object_integrity_proof.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+# This is an implementation of https://codeberg.org/fediverse/fep/src/branch/main/fep/8b32/fep-8b32.md
+class ActivityPub::ObjectIntegrityProof
+  include JsonLdHelper
+
+  CONTEXT = 'https://w3id.org/identity/v1'
+  SIGNATURE_CONTEXT = 'https://w3id.org/security/v1'
+
+  def initialize(json)
+    @json = json.with_indifferent_access
+  end
+
+  def verify_actor!(proof_purpose: 'assertionMethod')
+    return unless @json.is_a?(Hash) && @json['proof'].is_a?(Hash)
+
+    proof = @json['proof']
+    return unless proof['type'].present? && proof['verificationMethod'].present? && proof['proofPurpose'].present?
+
+    return if proof_purpose.present? && proof_purpose != proof['proofPurpose']
+
+    return if proof['type'] != 'DataIntegrityProof'
+
+    cryptosuite = proof['cryptosuite']
+    key_uri = proof['verificationMethod']
+
+    return unless cryptosuite == 'eddsa-jcs-2022' && proof['proofValue'].present?
+
+    keypair = Keypair.from_keyid(key_uri)
+    keypair = ActivityPub::FetchRemoteKeyService.new.call(key_uri) if keypair&.public_key.blank?
+    return if keypair.nil? || !keypair.usable? || keypair.type != 'ed25519'
+
+    keypair.actor if ActivityPub::ObjectIntegrityProof.verify_eddsa_jcs_2022(@json, keypair.keypair)
+  rescue OpenSSL::PKey::RSAError
+    false
+  end
+
+  # https://www.w3.org/TR/vc-di-eddsa/#verify-proof-eddsa-jcs-2022
+  def self.verify_eddsa_jcs_2022(document, keypair) # rubocop:disable Naming/VariableNumber
+    unsecured_document = document.without('proof')
+    proof_options = document['proof'].without('proofValue')
+    proof_bytes = Multibase.decode(document['proof']['proofValue'])
+
+    if proof_options['@context'].present?
+      return unless unsecured_document['@context'].is_a?(Array)
+      return unless unsecured_document['@context'][..proof_options['@context'].length] == proof_options['@context']
+    end
+
+    transformed_data = unsecured_document.to_json_c14n
+    proof_config = proof_options.to_json_c14n
+
+    to_be_verified = Digest::SHA256.digest(proof_config) + Digest::SHA256.digest(transformed_data)
+
+    keypair.verify(nil, proof_bytes, to_be_verified)
+  end
+end

spec/lib/activitypub/object_integrity_proof_spec.rb

@@ -0,0 +1,131 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe ActivityPub::ObjectIntegrityProof do
+  describe '#verify_actor!' do
+    # https://codeberg.org/fediverse/fep/src/branch/main/fep/8b32/fep-8b32.feature#L68
+
+    let(:actor) { Fabricate(:account, username: 'alice', domain: 'server.example.org', uri: 'https://server.example/users/alice', public_key: '') }
+
+    let(:json) do
+      JSON.parse(<<~JSON)
+        {
+            "@context": [
+                "https://www.w3.org/ns/activitystreams",
+                "https://w3id.org/security/data-integrity/v2"
+            ],
+            "id": "https://server.example/activities/1",
+            "type": "Create",
+            "actor": "https://server.example/users/alice",
+            "object": {
+                "id": "https://server.example/objects/1",
+                "type": "Note",
+                "attributedTo": "https://server.example/users/alice",
+                "content": "Hello world",
+                "location": {
+                    "type": "Place",
+                    "longitude": -71.184902,
+                    "latitude": 25.273962
+                }
+            },
+            "proof": {
+                "@context": [
+                    "https://www.w3.org/ns/activitystreams",
+                    "https://w3id.org/security/data-integrity/v2"
+                ],
+                "type": "DataIntegrityProof",
+                "cryptosuite": "eddsa-jcs-2022",
+                "verificationMethod": "https://server.example/users/alice#ed25519-key",
+                "proofPurpose": "assertionMethod",
+                "proofValue": "z42ffGu6AUKPCFcFPiabmUvnGLPJzC7e4DGWC52NUasSSH37UMa9c58tdgVszUcZfytxa4fQ5TYHaJENCxUDe9SdL",
+                "created": "2023-02-24T23:36:38Z"
+            }
+        }
+      JSON
+    end
+
+    before do
+      asn1 = OpenSSL::ASN1::Sequence(
+        [
+          OpenSSL::ASN1::Sequence([OpenSSL::ASN1::ObjectId('ED25519')]),
+          OpenSSL::ASN1::BitString(Multibase.decode_multicodec('z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2')[1]),
+        ]
+      )
+      keypair = OpenSSL::PKey.read(asn1.to_der)
+
+      Fabricate(:keypair, account: actor, uri: 'https://server.example/users/alice#ed25519-key', type: :ed25519, public_key: keypair.public_to_pem)
+    end
+
+    context 'when the signature is correct' do
+      it 'returns the actor' do
+        expect(described_class.new(json).verify_actor!).to eq actor
+      end
+    end
+  end
+
+  describe 'verify_eddsa_jcs_2022' do
+    # https://www.w3.org/TR/vc-di-eddsa/#representation-eddsa-jcs-2022
+
+    let(:keypair) do
+      asn1 = OpenSSL::ASN1::Sequence(
+        [
+          OpenSSL::ASN1::Sequence([OpenSSL::ASN1::ObjectId('ED25519')]),
+          OpenSSL::ASN1::BitString(Multibase.decode_multicodec('z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2')[1]),
+        ]
+      )
+
+      OpenSSL::PKey.read(asn1.to_der)
+    end
+
+    let(:secured_json) do
+      JSON.parse(<<~JSON)
+        {
+          "@context": [
+            "https://www.w3.org/ns/credentials/v2",
+            "https://www.w3.org/ns/credentials/examples/v2"
+          ],
+          "id": "urn:uuid:58172aac-d8ba-11ed-83dd-0b3aef56cc33",
+          "type": [
+            "VerifiableCredential",
+            "AlumniCredential"
+          ],
+          "name": "Alumni Credential",
+          "description": "A minimum viable example of an Alumni Credential.",
+          "issuer": "https://vc.example/issuers/5678",
+          "validFrom": "2023-01-01T00:00:00Z",
+          "credentialSubject": {
+            "id": "did:example:abcdefgh",
+            "alumniOf": "The School of Examples"
+          },
+          "proof": {
+            "type": "DataIntegrityProof",
+            "cryptosuite": "eddsa-jcs-2022",
+            "created": "2023-02-24T23:36:38Z",
+            "verificationMethod": "did:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2#z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2",
+            "proofPurpose": "assertionMethod",
+            "@context": [
+              "https://www.w3.org/ns/credentials/v2",
+              "https://www.w3.org/ns/credentials/examples/v2"
+            ],
+            "proofValue": "z2HnFSSPPBzR36zdDgK8PbEHeXbR56YF24jwMpt3R1eHXQzJDMWS93FCzpvJpwTWd3GAVFuUfjoJdcnTMuVor51aX"
+          }
+        }
+      JSON
+    end
+
+    context 'with a correct signature' do
+      it 'verifies correctly' do
+        expect(described_class.verify_eddsa_jcs_2022(secured_json, keypair)).to be true
+      end
+    end
+
+    context 'with an incorrect signature' do
+      let(:keypair) { OpenSSL::PKey.generate_key('ed25519') }
+
+      it 'does not verify document' do
+        expect(described_class.verify_eddsa_jcs_2022(secured_json, keypair)).to be false
+      end
+    end
+  end
+end