S27: HPKE-Base content wrap + public-key directory; reject xor on send

Routing/error fields stay clear. Content is X25519-HKDF-SHA256-ChaCha20.

python/secure_messaging/pubkey_dir.py

@@ -0,0 +1,93 @@
+"""Public-key directory for NPE/HPKE endpoint encryption.
+
+Only *public* material is listed. Private keys stay in private/ (0600).
+Every service that needs E2E looks up handles here (HTTP or files).
+"""
+
+from __future__ import annotations  # annotations
+
+import json  # records
+import os  # chmod
+import time  # updated_ts
+from pathlib import Path  # dir root
+from typing import Any, Dict, List, Optional  # types
+
+from .hpke import generate_keypair  # X25519
+
+
+class PubKeyDir:
+    """File-backed directory: public/<handle>.json + private/<handle>.sk."""
+
+    def __init__(self, root: Path):
+        self.root = Path(root)  # /opt/pfc/etc/npe-dir
+        self.public = self.root / "public"
+        self.private = self.root / "private"
+        self.public.mkdir(parents=True, exist_ok=True)
+        self.private.mkdir(parents=True, exist_ok=True)
+
+    def create_endpoint(self, handle: str) -> Dict[str, Any]:
+        """Generate X25519 keypair; write public record + private sk."""
+        handle = _safe(handle)
+        sk, pk = generate_keypair()
+        rec = {
+            "handle": handle,
+            "kem": "x25519",
+            "kdf": "hkdf-sha256",
+            "aead": "chacha20poly1305",
+            "suite": "HPKE-Base X25519-HKDF-SHA256-ChaCha20-Poly1305",
+            "enc_pk": pk.hex(),
+            "inbox": "npe.inbox." + pk[:16].hex(),
+            "updated_ts": time.time(),
+        }
+        (self.public / (handle + ".json")).write_text(json.dumps(rec, indent=2) + "\n")
+        skp = self.private / (handle + ".sk")
+        skp.write_bytes(sk)
+        os.chmod(skp, 0o600)
+        return rec
+
+    def get(self, handle: str) -> Optional[Dict[str, Any]]:
+        p = self.public / (_safe(handle) + ".json")
+        if not p.exists():
+            return None
+        return json.loads(p.read_text())
+
+    def list(self) -> List[Dict[str, Any]]:
+        rows = []
+        for p in sorted(self.public.glob("*.json")):
+            rows.append(json.loads(p.read_text()))
+        return rows
+
+    def put_public(self, rec: Dict[str, Any]) -> Dict[str, Any]:
+        """Publish someone else's public key (no private)."""
+        handle = _safe(str(rec.get("handle") or ""))
+        if not handle or not rec.get("enc_pk"):
+            raise ValueError("handle and enc_pk required")
+        rec = dict(rec)
+        rec["handle"] = handle
+        rec["updated_ts"] = time.time()
+        (self.public / (handle + ".json")).write_text(json.dumps(rec, indent=2) + "\n")
+        return rec
+
+    def load_sk(self, handle: str) -> bytes:
+        p = self.private / (_safe(handle) + ".sk")
+        if not p.exists():
+            raise FileNotFoundError("no private key for " + handle)
+        return p.read_bytes()
+
+    def available(self) -> Dict[str, Any]:
+        """What every E2E service can see: handles + public keys only."""
+        recs = self.list()
+        return {
+            "ok": True,
+            "directory": str(self.root),
+            "suite": "HPKE-Base X25519-HKDF-SHA256-ChaCha20-Poly1305",
+            "handles": [r["handle"] for r in recs],
+            "keys": recs,
+        }
+
+
+def _safe(handle: str) -> str:
+    h = "".join(c for c in handle if c.isalnum() or c in "-_")
+    if not h:
+        raise ValueError("empty handle")
+    return h