feat: add ACT+ECT over MCP demo with LangGraph agent

End-to-end PoC demonstrating Agent Context Token authorization and
Execution Context Token accountability over MCP tool calls, using a
LangGraph agent with ES256-signed JWT tokens and DAG verification.

demo/act-ect-mcp/tests/test_http_sig.py

@@ -0,0 +1,84 @@
+"""RFC-9421-shaped HTTP signature round-trip and tamper-detection."""
+
+from __future__ import annotations
+
+import pytest
+
+from act.errors import ACTSignatureError
+
+from poc.http_sig import sign_request, verify_request
+
+
+def _sign_verify_ok(identities, body: bytes):
+    agent = identities["agent"]
+    target = "http://127.0.0.1:8765/mcp"
+    signed = sign_request(
+        method="POST",
+        target_uri=target,
+        body=body,
+        wimse_ect="ect.placeholder.compact",
+        wimse_aud=identities["mcp-server"].name,
+        keyid=agent.kid,
+        private_key=agent.private_key,
+    )
+    parsed = verify_request(
+        method="POST",
+        target_uri=target,
+        body=body,
+        wimse_ect_header="ect.placeholder.compact",
+        content_digest_header=signed.content_digest,
+        signature_input_header=signed.signature_input,
+        signature_header=signed.signature,
+        expected_audience=identities["mcp-server"].name,
+        public_key=agent.public_key,
+    )
+    return signed, parsed
+
+
+def test_signature_round_trips(identities):
+    signed, parsed = _sign_verify_ok(identities, body=b'{"method":"tools/call"}')
+    assert parsed.keyid == identities["agent"].kid
+    assert parsed.wimse_aud == "mcp-server"
+    assert parsed.alg == "ecdsa-p256-sha256"
+
+
+def test_signature_fails_on_tampered_body(identities):
+    agent = identities["agent"]
+    signed, _ = _sign_verify_ok(identities, body=b"original")
+    with pytest.raises(ACTSignatureError):
+        verify_request(
+            method="POST",
+            target_uri="http://127.0.0.1:8765/mcp",
+            body=b"tampered",  # different body → different digest → no match
+            wimse_ect_header="ect.placeholder.compact",
+            content_digest_header=signed.content_digest,
+            signature_input_header=signed.signature_input,
+            signature_header=signed.signature,
+            expected_audience="mcp-server",
+            public_key=agent.public_key,
+        )
+
+
+def test_signature_fails_on_wrong_audience(identities):
+    agent = identities["agent"]
+    signed = sign_request(
+        method="POST",
+        target_uri="http://example/mcp",
+        body=b"{}",
+        wimse_ect="ect.placeholder",
+        wimse_aud="the-wrong-workload",  # signed for the wrong audience
+        keyid=agent.kid,
+        private_key=agent.private_key,
+    )
+    with pytest.raises(ACTSignatureError):
+        verify_request(
+            method="POST",
+            target_uri="http://example/mcp",
+            body=b"{}",
+            wimse_ect_header="ect.placeholder",
+            content_digest_header=signed.content_digest,
+            signature_input_header=signed.signature_input,
+            signature_header=signed.signature,
+            expected_audience="mcp-server",
+            public_key=agent.public_key,
+        )