feat(config): add AES-256-GCM crypto primitives

Add encrypt/decrypt functions using AES-256-GCM for secret storage.
Master key loaded from file path specified in bootstrap config.

Features:
- 32-byte key from base64-encoded file
- 12-byte random nonce per encryption
- AEAD authentication (detects tampering)
- Key caching with clear_key_cache() for rotation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_crypto.py

@@ -0,0 +1,175 @@
+"""Tests for cryptographic primitives."""
+
+import base64
+import os
+from pathlib import Path
+
+import pytest
+
+from central.crypto import (
+    KEY_SIZE,
+    DecryptionError,
+    KeyLoadError,
+    clear_key_cache,
+    decrypt,
+    encrypt,
+)
+
+
+@pytest.fixture
+def master_key(tmp_path: Path) -> Path:
+    """Create a valid master key file."""
+    key = os.urandom(KEY_SIZE)
+    key_path = tmp_path / "master.key"
+    key_path.write_text(base64.b64encode(key).decode())
+    clear_key_cache()
+    return key_path
+
+
+@pytest.fixture
+def wrong_key(tmp_path: Path) -> Path:
+    """Create a different master key file."""
+    key = os.urandom(KEY_SIZE)
+    key_path = tmp_path / "wrong.key"
+    key_path.write_text(base64.b64encode(key).decode())
+    return key_path
+
+
+class TestEncryptDecrypt:
+    """Test encrypt/decrypt round-trip."""
+
+    def test_round_trip(self, master_key: Path) -> None:
+        """Encrypting then decrypting returns original plaintext."""
+        plaintext = b"Hello, Central!"
+
+        ciphertext = encrypt(plaintext, key_path=master_key)
+        decrypted = decrypt(ciphertext, key_path=master_key)
+
+        assert decrypted == plaintext
+
+    def test_round_trip_empty(self, master_key: Path) -> None:
+        """Empty plaintext encrypts and decrypts correctly."""
+        plaintext = b""
+
+        ciphertext = encrypt(plaintext, key_path=master_key)
+        decrypted = decrypt(ciphertext, key_path=master_key)
+
+        assert decrypted == plaintext
+
+    def test_round_trip_large(self, master_key: Path) -> None:
+        """Large plaintext encrypts and decrypts correctly."""
+        plaintext = os.urandom(1024 * 1024)  # 1MB
+
+        ciphertext = encrypt(plaintext, key_path=master_key)
+        decrypted = decrypt(ciphertext, key_path=master_key)
+
+        assert decrypted == plaintext
+
+    def test_ciphertext_different_each_time(self, master_key: Path) -> None:
+        """Same plaintext produces different ciphertext (random nonce)."""
+        plaintext = b"test"
+
+        ct1 = encrypt(plaintext, key_path=master_key)
+        ct2 = encrypt(plaintext, key_path=master_key)
+
+        assert ct1 != ct2
+        # But both decrypt to same plaintext
+        assert decrypt(ct1, key_path=master_key) == plaintext
+        assert decrypt(ct2, key_path=master_key) == plaintext
+
+
+class TestDecryptionFailures:
+    """Test AEAD authentication catches tampering."""
+
+    def test_wrong_key_fails(self, master_key: Path, wrong_key: Path) -> None:
+        """Decryption with wrong key raises DecryptionError."""
+        plaintext = b"secret"
+        ciphertext = encrypt(plaintext, key_path=master_key)
+
+        clear_key_cache()  # Clear cache so wrong_key is loaded
+        with pytest.raises(DecryptionError):
+            decrypt(ciphertext, key_path=wrong_key)
+
+    def test_tampered_ciphertext_fails(self, master_key: Path) -> None:
+        """Modified ciphertext is detected and rejected."""
+        plaintext = b"secret"
+        ciphertext = encrypt(plaintext, key_path=master_key)
+
+        # Flip a bit in the ciphertext (after nonce, before tag)
+        tampered = bytearray(ciphertext)
+        tampered[15] ^= 0x01  # Flip one bit
+        tampered = bytes(tampered)
+
+        with pytest.raises(DecryptionError):
+            decrypt(tampered, key_path=master_key)
+
+    def test_tampered_tag_fails(self, master_key: Path) -> None:
+        """Modified authentication tag is detected and rejected."""
+        plaintext = b"secret"
+        ciphertext = encrypt(plaintext, key_path=master_key)
+
+        # Flip a bit in the last byte (part of the tag)
+        tampered = bytearray(ciphertext)
+        tampered[-1] ^= 0x01
+        tampered = bytes(tampered)
+
+        with pytest.raises(DecryptionError):
+            decrypt(tampered, key_path=master_key)
+
+    def test_truncated_ciphertext_fails(self, master_key: Path) -> None:
+        """Truncated ciphertext is rejected."""
+        ciphertext = b"tooshort"
+
+        with pytest.raises(DecryptionError, match="too short"):
+            decrypt(ciphertext, key_path=master_key)
+
+
+class TestKeyLoading:
+    """Test master key loading."""
+
+    def test_missing_key_file(self, tmp_path: Path) -> None:
+        """Missing key file raises KeyLoadError."""
+        clear_key_cache()
+        missing = tmp_path / "nonexistent.key"
+
+        with pytest.raises(KeyLoadError, match="not found"):
+            encrypt(b"test", key_path=missing)
+
+    def test_invalid_key_size(self, tmp_path: Path) -> None:
+        """Key file with wrong size raises KeyLoadError."""
+        clear_key_cache()
+        bad_key = tmp_path / "bad.key"
+        bad_key.write_text(base64.b64encode(b"tooshort").decode())
+
+        with pytest.raises(KeyLoadError, match="Invalid master key size"):
+            encrypt(b"test", key_path=bad_key)
+
+    def test_invalid_base64(self, tmp_path: Path) -> None:
+        """Invalid base64 in key file raises KeyLoadError."""
+        clear_key_cache()
+        bad_key = tmp_path / "bad.key"
+        bad_key.write_text("not valid base64!!!")
+
+        with pytest.raises(KeyLoadError):
+            encrypt(b"test", key_path=bad_key)
+
+    def test_key_cached(self, master_key: Path) -> None:
+        """Key is cached after first load."""
+        # First encryption loads the key
+        encrypt(b"test1", key_path=master_key)
+
+        # Delete the file
+        master_key.unlink()
+
+        # Second encryption should still work (cached)
+        ciphertext = encrypt(b"test2", key_path=master_key)
+        assert len(ciphertext) > 0
+
+    def test_cache_clear(self, master_key: Path) -> None:
+        """clear_key_cache forces reload."""
+        encrypt(b"test", key_path=master_key)
+        master_key.unlink()
+        clear_key_cache()
+
+        with pytest.raises(KeyLoadError, match="not found"):
+            encrypt(b"test", key_path=master_key)