Parse signCount and expose at AuthenticatorData#sign_count

lib/webauthn/authenticator_data.rb

@@ -10,6 +10,8 @@ class AuthenticatorData
     FLAGS_LENGTH = 1
     SIGN_COUNT_LENGTH = 4
 
+    SIGN_COUNT_POSITION = RP_ID_HASH_LENGTH + FLAGS_LENGTH
+
     USER_PRESENT_FLAG_POSITION = 0
     ATTESTED_CREDENTIAL_DATA_INCLUDED_FLAG_POSITION = 6
 
@@ -46,6 +48,10 @@ def credential
       attested_credential_data.credential
     end
 
+    def sign_count
+      @sign_count ||= data_at(SIGN_COUNT_POSITION, SIGN_COUNT_LENGTH).unpack1('L>')
+    end
+
     def attested_credential_data
       @attested_credential_data ||=
         AttestedCredentialData.new(data_at(attested_credential_data_position))

spec/support/fake_authenticator.rb

@@ -2,9 +2,10 @@
 
 class FakeAuthenticator
   class Base
-    def initialize(challenge: fake_challenge, rp_id: "localhost", context: {})
+    def initialize(challenge: fake_challenge, rp_id: "localhost", sign_count: 0, context: {})
       @challenge = challenge
       @rp_id = rp_id
+      @sign_count = sign_count
       @context = context
     end
 
@@ -24,14 +25,14 @@ def credential_id
       @credential_id ||= SecureRandom.random_bytes(16)
     end
 
-    private
-
-    attr_reader :challenge, :context, :rp_id
-
     def rp_id_hash
       OpenSSL::Digest::SHA256.digest(rp_id)
     end
 
+    private
+
+    attr_reader :challenge, :context, :rp_id
+
     def raw_flags
       ["#{user_present_bit}00000#{attested_credential_data_present_bit}0"].pack("b*")
     end
@@ -49,7 +50,7 @@ def attested_credential_data
     end
 
     def raw_sign_count
-      "0000"
+      [@sign_count].pack('L>')
     end
 
     def user_present_bit

spec/webauthn/authenticator_data_spec.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+RSpec.describe WebAuthn::AuthenticatorData do
+  let(:authenticator) do
+    FakeAuthenticator::Base.new(rp_id: rp_id, sign_count: sign_count, context: { user_present: user_presence })
+  end
+
+  let(:rp_id) { "localhost" }
+  let(:sign_count) { 42 }
+  let(:user_presence) { true }
+
+  let(:authenticator_data) { described_class.new(authenticator.authenticator_data) }
+
+  describe "#rp_id_hash" do
+    subject { authenticator_data.rp_id_hash }
+    it { is_expected.to eq(authenticator.rp_id_hash) }
+  end
+
+  describe "#sign_count" do
+    subject { authenticator_data.sign_count }
+    it { is_expected.to eq(42) }
+  end
+
+  describe "#user_present?" do
+    subject { authenticator_data.user_present? }
+    context "when UP flag is set" do
+      let(:user_presence) { true }
+      it { is_expected.to be_truthy }
+    end
+
+    context "when UP flag is not set" do
+      let(:user_presence) { false }
+      it { is_expected.to be_falsy }
+    end
+  end
+end