Sign Sparkle appcasts in Python

Scripts/sparkle/sign-appcast-kms.py

@@ -3,7 +3,7 @@
 
 Google Cloud KMS Ed25519 signing can only sign small payloads directly. Sparkle
 requires a standard EdDSA signature over the full update archive, so normal
-release artifacts use Sparkle's signing tool with the decrypted release key.
+release artifacts sign directly from the decrypted release seed.
 """
 
 from __future__ import annotations
@@ -58,32 +58,39 @@ def kms_sign(args: argparse.Namespace, payload: Path) -> str:
         return base64.b64encode(signature_path.read_bytes()).decode("ascii")
 
 
-def sign_update(args: argparse.Namespace, payload: Path) -> str:
+def read_sparkle_seed(key_path: Path) -> bytes:
+    key_data = key_path.read_bytes().strip()
+    if len(key_data) == 32:
+        return key_data
+    try:
+        decoded = base64.b64decode(key_data, validate=True)
+    except ValueError as exc:
+        raise ValueError(f"Sparkle key is not raw or base64 Ed25519 seed data: {key_path}") from exc
+    if len(decoded) != 32:
+        raise ValueError(f"Sparkle key must decode to a 32-byte Ed25519 seed: {key_path}")
+    return decoded
+
+
+def sign_with_release_seed(args: argparse.Namespace, payload: Path) -> str:
     key_path = Path(args.ed_key_path) if args.ed_key_path else None
     if key_path is None or not key_path.is_file():
         raise FileNotFoundError(
             "Sparkle artifact is too large for direct Google KMS Ed25519 signing "
             "and SPARKLE_EDDSA_KEY_PATH does not point to a key file"
         )
-    result = run([
-        args.sign_update,
-        "--ed-key-file",
-        str(key_path),
-        "-p",
-        str(payload),
-    ])
-    signature = result.stdout.strip().splitlines()[-1].strip()
-    if not signature:
-        raise ValueError(f"Sparkle sign_update produced no signature for {payload}")
-    return signature
+    from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
+
+    seed = read_sparkle_seed(key_path)
+    signature = Ed25519PrivateKey.from_private_bytes(seed).sign(payload.read_bytes())
+    return base64.b64encode(signature).decode("ascii")
 
 
 def sparkle_signature(args: argparse.Namespace, payload: Path) -> str:
     if args.ed_key_path:
-        return sign_update(args, payload)
+        return sign_with_release_seed(args, payload)
     if payload.stat().st_size <= args.kms_max_input_bytes:
         return kms_sign(args, payload)
-    return sign_update(args, payload)
+    return sign_with_release_seed(args, payload)
 
 
 def enclosure_path(enclosure: ET.Element, artifact_dir: Path) -> Path:
@@ -130,7 +137,6 @@ def parse_args(argv: list[str]) -> argparse.Namespace:
     parser.add_argument("--kms-max-input-bytes", type=int, default=DEFAULT_KMS_MAX_INPUT_BYTES)
     parser.add_argument("--gcloud", default=os.environ.get("GCLOUD", "gcloud"))
     parser.add_argument("--ed-key-path", default=os.environ.get("SPARKLE_EDDSA_KEY_PATH"))
-    parser.add_argument("--sign-update", default=os.environ.get("SPARKLE_SIGN_UPDATE", "sign_update"))
     return parser.parse_args(argv)