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backend/shared/crypto.py

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+"""
+AES-256-GCM Cookie encryption / decryption utilities.
+"""
+
+import base64
+import hashlib
+
+from Crypto.Cipher import AES
+
+
+def derive_key(raw_key: str) -> bytes:
+    """Derive a 32-byte key from an arbitrary string using SHA-256."""
+    return hashlib.sha256(raw_key.encode("utf-8")).digest()
+
+
+def encrypt_cookie(plaintext: str, key: bytes) -> tuple[str, str]:
+    """
+    Encrypt a cookie string with AES-256-GCM.
+
+    Returns:
+        (ciphertext_b64, iv_b64) — both base64-encoded strings.
+    """
+    cipher = AES.new(key, AES.MODE_GCM)
+    ciphertext, tag = cipher.encrypt_and_digest(plaintext.encode("utf-8"))
+    # Append the 16-byte tag to the ciphertext so decryption can verify it
+    ciphertext_with_tag = ciphertext + tag
+    ciphertext_b64 = base64.b64encode(ciphertext_with_tag).decode("utf-8")
+    iv_b64 = base64.b64encode(cipher.nonce).decode("utf-8")
+    return ciphertext_b64, iv_b64
+
+
+def decrypt_cookie(ciphertext_b64: str, iv_b64: str, key: bytes) -> str:
+    """
+    Decrypt a cookie string previously encrypted with encrypt_cookie.
+
+    Raises ValueError on decryption failure (wrong key, corrupted data, etc.).
+    """
+    raw = base64.b64decode(ciphertext_b64)
+    nonce = base64.b64decode(iv_b64)
+    # Last 16 bytes are the GCM tag
+    ciphertext, tag = raw[:-16], raw[-16:]
+    cipher = AES.new(key, AES.MODE_GCM, nonce=nonce)
+    plaintext = cipher.decrypt_and_verify(ciphertext, tag)
+    return plaintext.decode("utf-8")