Compute Web Bot Auth keyid via RFC 7638 thumbprint instead of jwt-eddsa's kid

lib/linzer/jws.rb

@@ -3,6 +3,8 @@
 require "jwt"
 require "jwt/eddsa"
 require "ed25519"
+require "digest"
+require "base64"
 
 module Linzer
   # JSON Web Signature (JWS) compatible key support.
@@ -101,6 +103,40 @@ def verify(signature, data)
         algo.verify(data: data, signature: signature, verification_key: verify_key)
       end
 
+      # Computes the RFC 7638 JWK SHA-256 Thumbprint for this key's public
+      # material.
+      #
+      # This is computed directly from the exported JWK rather than
+      # delegating to the underlying jwt-eddsa gem's own thumbprint/kid
+      # generation: jwt-eddsa (<= 0.9.0) has a bug where its OKP JWK class
+      # computes that value over the wrong members (an RSA-shaped {kty, n,
+      # x} instead of the RFC 8037-correct {crv, kty, x}), which silently
+      # produces a keyid that a spec-compliant verifier will reject.
+      #
+      # @return [String] base64url-encoded (no padding) SHA-256 thumbprint
+      # @raise [Error] if this key's JWK "kty" is not supported
+      #
+      # @see https://www.rfc-editor.org/rfc/rfc7638 RFC 7638 - JSON Web Key (JWK) Thumbprint
+      # @see https://www.rfc-editor.org/rfc/rfc8037 RFC 8037 - EdDSA for JWS/JWK
+      def jwk_thumbprint
+        # XXX: drop this method custom implementation and just
+        # return material.key_digest
+        # once https://github.com/jwt/ruby-jwt-eddsa/pull/26 is resolved
+        #
+        exported = material.export
+
+        members =
+          case exported[:kty]
+          when "OKP"
+            {crv: exported[:crv], kty: exported[:kty], x: exported[:x]}
+          else
+            raise Error, "Unsupported JWK kty for thumbprint: #{exported[:kty]}"
+          end
+
+        digest = Digest::SHA256.digest(JWT::JSON.generate(members))
+        Base64.urlsafe_encode64(digest, padding: false)
+      end
+
       private
 
       # @return [Boolean] true if this key can verify signatures

lib/linzer/signature/profile/web_bot_auth.rb

@@ -139,7 +139,7 @@ def set_params!(key, components, params)
           #
           params[:expires] ||= Time.now.to_i + 3600
           params[:tag]     ||= "web-bot-auth"
-          params[:keyid]   ||= key.material.key_digest
+          params[:keyid]   ||= key.jwk_thumbprint
         end
 
         # Injects and signs the Signature-Agent header.

spec/jws_spec.rb

@@ -39,6 +39,28 @@
   end
 end
 
+RSpec.describe Linzer::JWS::Key do
+  describe "#jwk_thumbprint" do
+    # Test vector from RFC 8037 Appendix A.3 (JWK Thumbprint Canonicalization):
+    # https://www.rfc-editor.org/rfc/rfc8037#appendix-A.3
+    #
+    # Computed directly here (not delegated to jwt-eddsa's own key_digest)
+    # because jwt-eddsa (<= 0.9.0) computes that value over the wrong JWK
+    # members for OKP keys; see Linzer::JWS::Key#jwk_thumbprint docs.
+    let(:jwk) { {kty: "OKP", crv: "Ed25519", x: "11qYAYKxCrfVS_7TyWQHOg7hcvPapiMlrwIaaPcHURo"} }
+    let(:key) { Linzer::JWS.jwk_import(jwk) }
+
+    it "matches the RFC 7638 JWK thumbprint of the crv/kty/x members" do
+      expect(key.jwk_thumbprint).to eq("kPrK_qmxVWaYVA9wwBF6Iuo3vVzz7TxHCTwXBygrS4k")
+    end
+
+    it "raises for unsupported JWK key types" do
+      rsa_key = Linzer::JWS.jwk_import(JWT::JWK.new(OpenSSL::PKey::RSA.new(2048)))
+      expect { rsa_key.jwk_thumbprint }.to raise_error(Linzer::Error, /Unsupported JWK kty/)
+    end
+  end
+end
+
 RSpec.describe Linzer::Signer do
   context "with JWS EdDSA algorithm" do
     let(:request) do

spec/signature/profile/web_bot_auth_spec.rb

@@ -67,7 +67,7 @@
         apply_profile
 
         expect(params[:tag]).to     eq("web-bot-auth")
-        expect(params[:keyid]).to   eq(key.material.key_digest)
+        expect(params[:keyid]).to   eq(key.jwk_thumbprint)
         expect(params[:expires]).to eq(Time.now.to_i + 3600)
       end
 

spec/web_bot_auth_spec.rb

@@ -23,7 +23,7 @@
       expect(signature.parameters).to          include("created")
       expect(signature.parameters).to          include("expires" => be > Time.now.utc.to_i)
       expect(signature.parameters["tag"]).to   eq("web-bot-auth")
-      expect(signature.parameters["keyid"]).to eq(key.material.key_digest)
+      expect(signature.parameters["keyid"]).to eq(key.jwk_thumbprint)
       expect(signature.parameters).to          have_key("nonce")
       expect(signature.metadata).to            include('"@authority"').or include('"@target-uri"')
       expect(headers["signature-agent"]).to    eq("my-sig=\"https://example.com/someagent\"")