WI-5 #832: Receipt-challenge protocol (compaction-refresh, milestone 188) (#845)

packages/mosaic/framework/tools/lease-broker/receipt_challenge.py

@@ -0,0 +1,43 @@
+#!/usr/bin/env python3
+"""Broker-side construction and verification for one-time receipt challenges."""
+
+from __future__ import annotations
+
+import hashlib
+import hmac
+import struct
+from typing import Final
+
+
+LATEST_ASSISTANT_DOMAIN_SEPARATOR: Final = b"MOSAIC/H_LATEST_ASSISTANT/v1\x00"
+
+
+def _length_frame(value: bytes) -> bytes:
+    return struct.pack(">Q", len(value)) + value
+
+
+def receipt_for(challenge: str, binding: dict[str, object]) -> str:
+    """Return the sole receipt text a model may copy for this broker cycle."""
+
+    return (
+        "MOSAIC-RECEIPT{"
+        f"challenge={challenge}; "
+        f"H_payload={binding['h_payload']}; "
+        f"gen={binding['runtime_generation']}; "
+        f"cep={binding['compaction_epoch']}"
+        "}"
+    )
+
+
+def is_verbatim_receipt(message: str, challenge: str, binding: dict[str, object]) -> bool:
+    """Require the exact one current-cycle receipt, not a transcript substring."""
+
+    expected = receipt_for(challenge, binding)
+    return hmac.compare_digest(message, expected)
+
+
+def latest_assistant_digest(message: str) -> str:
+    """Record the broker-computed digest of the exact observed assistant entry."""
+
+    encoded = message.encode("utf-8")
+    return hashlib.sha256(LATEST_ASSISTANT_DOMAIN_SEPARATOR + _length_frame(encoded)).hexdigest()