src/pqc_kv_cache/encryptor.py
| 1 | """CacheEncryptor / CacheDecryptor - the AES-256-GCM wrappers.""" |
| 2 | |
| 3 | from __future__ import annotations |
| 4 | |
| 5 | import json |
| 6 | import os |
| 7 | |
| 8 | from cryptography.hazmat.primitives.ciphers.aead import AESGCM |
| 9 | |
| 10 | from pqc_kv_cache.entry import EncryptedEntry, EntryMetadata, KVCacheEntry |
| 11 | from pqc_kv_cache.errors import ( |
| 12 | DecryptionError, |
| 13 | NonceReplayError, |
| 14 | TenantIsolationError, |
| 15 | ) |
| 16 | from pqc_kv_cache.session import TenantSession |
| 17 | |
| 18 | NONCE_SIZE = 12 |
| 19 | |
| 20 | |
| 21 | def _aad(metadata: EntryMetadata, sequence_number: int, key_len: int) -> bytes: |
| 22 | """Associated data binding metadata + sequence + key_len to the ciphertext.""" |
| 23 | payload = { |
| 24 | "metadata": metadata.to_dict(), |
| 25 | "sequence_number": sequence_number, |
| 26 | "key_len": key_len, |
| 27 | } |
| 28 | return json.dumps( |
| 29 | payload, sort_keys=True, separators=(",", ":"), ensure_ascii=False |
| 30 | ).encode("utf-8") |
| 31 | |
| 32 | |
| 33 | class CacheEncryptor: |
| 34 | """Wraps a TenantSession to produce EncryptedEntry objects.""" |
| 35 | |
| 36 | def __init__(self, session: TenantSession): |
| 37 | self.session = session |
| 38 | |
| 39 | def encrypt_entry(self, entry: KVCacheEntry) -> EncryptedEntry: |
| 40 | self.session.check_valid() |
| 41 | if entry.metadata.tenant_id != self.session.tenant.tenant_id: |
| 42 | raise TenantIsolationError( |
| 43 | f"entry tenant {entry.metadata.tenant_id} != session tenant " |
| 44 | f"{self.session.tenant.tenant_id}" |
| 45 | ) |
| 46 | seq = self.session.consume_sequence() |
| 47 | nonce = os.urandom(NONCE_SIZE) |
| 48 | plaintext = entry.key_tensor_bytes + entry.value_tensor_bytes |
| 49 | key_len = len(entry.key_tensor_bytes) |
| 50 | aad = _aad(entry.metadata, seq, key_len) |
| 51 | aes = AESGCM(self.session.symmetric_key) |
| 52 | ct = aes.encrypt(nonce, plaintext, aad) |
| 53 | return EncryptedEntry( |
| 54 | metadata=entry.metadata, |
| 55 | nonce=nonce.hex(), |
| 56 | ciphertext=ct.hex(), |
| 57 | key_len=key_len, |
| 58 | sequence_number=seq, |
| 59 | ) |
| 60 | |
| 61 | |
| 62 | class CacheDecryptor: |
| 63 | """Wraps a TenantSession to decrypt EncryptedEntry objects. |
| 64 | |
| 65 | Enforces strict tenant isolation: refuses to decrypt entries whose |
| 66 | metadata.tenant_id does not match the session's tenant_id, even if the |
| 67 | symmetric key would somehow work. |
| 68 | """ |
| 69 | |
| 70 | def __init__(self, session: TenantSession): |
| 71 | self.session = session |
| 72 | self._seen_nonces: set[str] = set() |
| 73 | |
| 74 | def decrypt_entry(self, enc: EncryptedEntry) -> KVCacheEntry: |
| 75 | self.session.check_valid() |
| 76 | if enc.metadata.tenant_id != self.session.tenant.tenant_id: |
| 77 | raise TenantIsolationError( |
| 78 | f"entry tenant {enc.metadata.tenant_id} != session tenant " |
| 79 | f"{self.session.tenant.tenant_id}" |
| 80 | ) |
| 81 | if enc.nonce in self._seen_nonces: |
| 82 | raise NonceReplayError(f"nonce {enc.nonce} already consumed") |
| 83 | aad = _aad(enc.metadata, enc.sequence_number, enc.key_len) |
| 84 | aes = AESGCM(self.session.symmetric_key) |
| 85 | try: |
| 86 | pt = aes.decrypt( |
| 87 | bytes.fromhex(enc.nonce), bytes.fromhex(enc.ciphertext), aad |
| 88 | ) |
| 89 | except Exception as exc: |
| 90 | raise DecryptionError(f"AES-GCM decrypt failed: {exc}") from exc |
| 91 | self._seen_nonces.add(enc.nonce) |
| 92 | key_bytes = pt[: enc.key_len] |
| 93 | val_bytes = pt[enc.key_len:] |
| 94 | return KVCacheEntry( |
| 95 | metadata=enc.metadata, |
| 96 | key_tensor_bytes=key_bytes, |
| 97 | value_tensor_bytes=val_bytes, |
| 98 | ) |
| 99 | |