Simplify and strengthen AWSSigner.import_ (WIP)

**Simplify**
- remove redundant keytype/scheme switches
- remove try/except, which catches an exception only to re-raise it

**Strengthen**
- choose default scheme based on schemes supported by the key

Roughly implements the algorithm suggested in:
#724 (review)

**Unrelated changes**
- fix #765 fix typo AND disable scheme until SSlibKey adds support
- fail in __init__ if public_key.scheme is not in aws_algos

TODO:
* add tests

Many thanks to @ianhundere for the solid ground work!

Signed-off-by: Lukas Puehringer <[email protected]>

securesystemslib/signer/_aws_signer.py

@@ -4,8 +4,10 @@
 from typing import Optional, Tuple
 from urllib import parse
 
-from securesystemslib import exceptions
-from securesystemslib.exceptions import UnsupportedLibraryError
+from securesystemslib.exceptions import (
+    UnsupportedAlgorithmError,
+    UnsupportedLibraryError,
+)
 from securesystemslib.signer._key import Key, SSlibKey
 from securesystemslib.signer._signer import SecretsHandler, Signature, Signer
 from securesystemslib.signer._utils import compute_default_keyid
@@ -57,10 +59,12 @@ class AWSSigner(Signer):
 
     SCHEME = "awskms"
 
-    aws_signing_algorithms = {
+    # Ordered dict of securesystemslib schemes to aws signing algorithms
+    # NOTE: the order matters when choosing a default (see _get_default_scheme)
+    aws_algos = {
         "ecdsa-sha2-nistp256": "ECDSA_SHA_256",
         "ecdsa-sha2-nistp384": "ECDSA_SHA_384",
-        "ecdsa-sha2-nistp512": "ECDSA_SHA_512",
+        # "ecdsa-sha2-nistp521": "ECDSA_SHA_512", # FIXME: needs SSlibKey support
         "rsassa-pss-sha256": "RSASSA_PSS_SHA_256",
         "rsassa-pss-sha384": "RSASSA_PSS_SHA_384",
         "rsassa-pss-sha512": "RSASSA_PSS_SHA_512",
@@ -76,9 +80,7 @@ def __init__(self, aws_key_id: str, public_key: Key):
         self.aws_key_id = aws_key_id
         self._public_key = public_key
         self.client = boto3.client("kms")
-        self.aws_algo = AWSSigner.aws_signing_algorithms.get(
-            self.public_key.scheme
-        )
+        self.aws_algo = self.aws_algos[self.public_key.scheme]
 
     @property
     def public_key(self) -> Key:
@@ -98,6 +100,23 @@ def from_priv_key_uri(
 
         return cls(uri.path, public_key)
 
+    @classmethod
+    def _get_default_scheme(cls, supported_by_key: list[str]) -> Optional[str]:
+        # Iterate over supported AWS algorithms, pick the first one also
+        # supported by the key, and return the related securesystemslib scheme.
+        for scheme, algo in cls.aws_algos.items():
+            if algo in supported_by_key:
+                return scheme
+        return None
+
+    @staticmethod
+    def _get_keytype_for_scheme(scheme: str) -> str:
+        if scheme.startswith("ecdsa"):
+            return "ecdsa"
+        if scheme.startswith("rsa"):
+            return "rsa"
+        raise RuntimeError
+
     @classmethod
     def import_(
         cls, aws_key_id: str, local_scheme: Optional[str] = None
@@ -126,30 +145,28 @@ def import_(
         if AWS_IMPORT_ERROR:
             raise UnsupportedLibraryError(AWS_IMPORT_ERROR)
 
-        if local_scheme and local_scheme not in cls.aws_signing_algorithms:
-            raise ValueError(f"Unsupported scheme: {local_scheme}")
+        if local_scheme:
+            if local_scheme not in cls.aws_algos:
+                raise ValueError(f"Unsupported scheme '{local_scheme}'")
 
-        try:
-            client = boto3.client("kms")
-            request = client.get_public_key(KeyId=aws_key_id)
-        except (BotoCoreError, ClientError) as e:
-            logger.error(
-                "Failed to import key using AWS KMS key ID %s: %s",
-                aws_key_id,
-                str(e),
-            )
-            raise e
+        client = boto3.client("kms")
+        request = client.get_public_key(KeyId=aws_key_id)
+        key_algos = request["SigningAlgorithms"]
 
-        keytype = cls._get_keytype_from_aws_response(request)
-        aws_scheme = request["SigningAlgorithms"][0]
+        if local_scheme:
+            if cls.aws_algos[local_scheme] not in key_algos:
+                raise UnsupportedAlgorithmError(
+                    f"Unsupported scheme '{local_scheme}' for AWS key"
+                )
 
-        if not local_scheme:
-            if keytype == "ecdsa":
-                local_scheme = cls._get_ecdsa_scheme(aws_scheme)
-            elif keytype == "rsa":
-                local_scheme = "rsassa-pss-sha256"
-            else:
-                raise ValueError(f"Unsupported key type: {keytype}")
+        else:
+            local_scheme = cls._get_default_scheme(key_algos)
+            if not local_scheme:
+                raise UnsupportedAlgorithmError(
+                    f"Unsupported AWS key algorithms: {key_algos}"
+                )
+
+        keytype = cls._get_keytype_for_scheme(local_scheme)
 
         kms_pubkey = serialization.load_der_public_key(request["PublicKey"])
 
@@ -163,42 +180,6 @@ def import_(
         public_key = SSlibKey(keyid, keytype, local_scheme, keyval)
         return f"{cls.SCHEME}:{aws_key_id}", public_key
 
-    @staticmethod
-    def _get_keytype_from_aws_response(reponse: dict) -> str:
-        """Determines the key type from the AWS KMS get_public_key response.
-
-        Arguments:
-            response (dict): The response from AWS KMS get_public_key request.
-
-        Returns:
-            str: The key type, either 'ecdsa' or 'rsa'.
-        """
-        algo = reponse["SigningAlgorithms"][0]
-        if "ECDSA" in algo:
-            return "ecdsa"
-        if "RSASSA" in algo:
-            return "rsa"
-        raise exceptions.UnsupportedAlgorithmError(
-            f"Unsupported algorithm in AWS response: {algo}"
-        )
-
-    @staticmethod
-    def _get_ecdsa_scheme(aws_algo: str) -> str:
-        """Returns ECDSA signing scheme based on AWS signing algorithm.
-
-        Arguments:
-            aws_algo (str): The AWS ECDSA signing algorithm.
-
-        Returns:
-            str: The Secure Systems Library ECDSA scheme.
-        """
-        ecdsa_signing_algorithms = {
-            "ECDSA_SHA_256": "ecdsa-sha2-nistp256",
-            "ECDSA_SHA_384": "ecdsa-sha2-nistp384",
-            "ECDSA_SHA_512": "ecdsa-sha2-nistp512",
-        }
-        return ecdsa_signing_algorithms[aws_algo]
-
     def sign(self, payload: bytes) -> Signature:
         """Sign the payload with the AWS KMS key