🚧 Add CA and server cert management

backend/app/crypto.py

@@ -0,0 +1,59 @@
+"""Key encryption at rest (spec sections 43-44).
+
+Private keys that the platform must keep (CA and server keys) are stored
+in PostgreSQL ONLY in encrypted form, using AES-256-GCM authenticated
+encryption. The master key comes from the SECRET_ENCRYPTION_KEY
+environment variable (Docker secret in production) and never touches
+the database.
+
+Wire format: "v1:<nonce-hex>:<ciphertext-hex>" — the version prefix
+keeps the format evolvable without breaking existing rows.
+"""
+
+from __future__ import annotations
+
+import os
+
+from cryptography.hazmat.primitives.ciphers.aead import AESGCM
+
+from app.config import get_settings
+
+_FORMAT = "v1"
+
+
+def _master_key() -> bytes:
+    """Derive the 32-byte AES key from the configured secret.
+
+    The secret is hashed to exactly 32 bytes (AES-256) regardless of the
+    length the operator chose for SECRET_ENCRYPTION_KEY.
+    """
+    import hashlib
+
+    secret = get_settings().secret_encryption_key.encode("utf-8")
+    return hashlib.sha256(secret).digest()
+
+
+def encrypt_secret(plaintext: str) -> str:
+    """Encrypt a PEM string into the storage format."""
+    nonce = os.urandom(12)  # 96-bit nonce, GCM standard
+    aesgcm = AESGCM(_master_key())
+    ciphertext = aesgcm.encrypt(nonce, plaintext.encode("utf-8"), None)
+    return f"{_FORMAT}:{nonce.hex()}:{ciphertext.hex()}"
+
+
+def decrypt_secret(blob: str) -> str:
+    """Decrypt a stored secret back to plaintext PEM."""
+    try:
+        version, nonce_hex, ciphertext_hex = blob.split(":", 2)
+    except ValueError as exc:
+        raise ValueError("corrupt secret blob: bad format") from exc
+    if version != _FORMAT:
+        raise ValueError(f"unsupported secret blob version: {version}")
+    aesgcm = AESGCM(_master_key())
+    try:
+        plaintext = aesgcm.decrypt(
+            bytes.fromhex(nonce_hex), bytes.fromhex(ciphertext_hex), None
+        )
+    except Exception as exc:
+        raise ValueError("secret decryption failed (wrong master key?)") from exc
+    return plaintext.decode("utf-8")