key: refactor SSlibKey.verify_signature

Signature verification for "securesystemslib keys" was previously
implemented in 'rsa_keys', 'ecdsa_keys' and 'ed25519_keys' modules,
which were called from `SSlibKey.verify_signature` via the legacy
interface function `keys.verify_signature()`.

This commit moves the entire implementation to SSlibKey, which will
allow us (in a subsequent commit) to drastically decrease LOC count and
drop 'nacl' optional dependency for ed25519 keys, in favour of
'pyca/cryptography', which we already use for all other sslib keys.

An alternative design for this refactor used separate RSAKey, ECDSAKey
and ED25510Key classes to replace SSlibKey, but that mostly added
redundant boilerplate code. To the user it shouldn't matter, so let's do
what makes sense from maintainer perspective.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

securesystemslib/signer/_key.py

@@ -1,12 +1,47 @@
 """Key interface and the default implementations"""
 import logging
 from abc import ABCMeta, abstractmethod
-from typing import Any, Dict, Optional, Tuple, Type
+from typing import Any, Dict, Optional, Tuple, Type, cast
 
-import securesystemslib.keys as sslib_keys
 from securesystemslib import exceptions
+from securesystemslib._vendor.ed25519.ed25519 import (
+    SignatureMismatch,
+    checkvalid,
+)
 from securesystemslib.signer._signature import Signature
 
+CRYPTO_IMPORT_ERROR = None
+try:
+    from cryptography.exceptions import InvalidSignature
+    from cryptography.hazmat.primitives.asymmetric.ec import (
+        ECDSA,
+        EllipticCurvePublicKey,
+    )
+    from cryptography.hazmat.primitives.asymmetric.ed25519 import (
+        Ed25519PublicKey,
+    )
+    from cryptography.hazmat.primitives.asymmetric.padding import (
+        MGF1,
+        PSS,
+        PKCS1v15,
+    )
+    from cryptography.hazmat.primitives.asymmetric.rsa import (
+        AsymmetricPadding,
+        RSAPublicKey,
+    )
+    from cryptography.hazmat.primitives.asymmetric.types import PublicKeyTypes
+    from cryptography.hazmat.primitives.hashes import (
+        SHA224,
+        SHA256,
+        SHA384,
+        SHA512,
+        HashAlgorithm,
+    )
+    from cryptography.hazmat.primitives.serialization import load_pem_public_key
+except ImportError:
+    CRYPTO_IMPORT_ERROR = "'pyca/cryptography' library required"
+
+
 logger = logging.getLogger(__name__)
 
 # NOTE Key dispatch table is defined here so it's usable by Key,
@@ -180,22 +215,80 @@ def from_dict(cls, keyid: str, key_dict: Dict[str, Any]) -> "SSlibKey":
     def to_dict(self) -> Dict[str, Any]:
         return self._to_dict()
 
+    def _from_pem(self) -> "PublicKeyTypes":
+        """Helper to load public key instance from PEM-formatted keyval."""
+        public_bytes = self.keyval["public"].encode("utf-8")
+        return load_pem_public_key(public_bytes)
+
+    @staticmethod
+    def _hash_algo(name) -> Type["HashAlgorithm"]:
+        """Helper to return hash algorithm class for name."""
+        algos = {
+            "sha224": SHA224,
+            "sha256": SHA256,
+            "sha384": SHA384,
+            "sha512": SHA512,
+        }
+        return algos[name]
+
     def verify_signature(self, signature: Signature, data: bytes) -> None:
         try:
-            if not sslib_keys.verify_signature(
-                self.to_securesystemslib_key(),
-                signature.to_dict(),
-                data,
+            sig = bytes.fromhex(signature.signature)
+
+            if CRYPTO_IMPORT_ERROR:
+                if self.scheme == "ed25519":
+                    # Verify using vendored ed25519 implementation
+                    pub = bytes.fromhex(self.keyval["public"])
+                    checkvalid(sig, data, pub)
+                    return
+
+                raise exceptions.UnsupportedLibraryError(CRYPTO_IMPORT_ERROR)
+
+            key: PublicKeyTypes
+            if self.scheme in [
+                "rsassa-pss-sha224",
+                "rsassa-pss-sha256",
+                "rsassa-pss-sha384",
+                "rsassa-pss-sha512",
+                "rsa-pkcs1v15-sha224",
+                "rsa-pkcs1v15-sha256",
+                "rsa-pkcs1v15-sha384",
+                "rsa-pkcs1v15-sha512",
+            ]:
+                key = cast(RSAPublicKey, self._from_pem())
+                padding_name, algo_name = self.scheme.split("-")[1:]
+                algo = self._hash_algo(algo_name)()
+                padding: AsymmetricPadding
+                if padding_name == "pss":
+                    padding = PSS(mgf=MGF1(algo), salt_length=PSS.AUTO)
+                else:
+                    padding = PKCS1v15()
+                key.verify(sig, data, padding, algo)
+
+            elif self.scheme in ["ecdsa-sha2-nistp256", "ecdsa-sha2-nistp384"]:
+                key = cast(EllipticCurvePublicKey, self._from_pem())
+                algo_name = f"sha{self.scheme[-3:]}"
+                algo = self._hash_algo(algo_name)()
+                key.verify(sig, data, ECDSA(algo))
+
+            elif self.scheme in ["ed25519"]:
+                public_bytes = bytes.fromhex(self.keyval["public"])
+                key = Ed25519PublicKey.from_public_bytes(public_bytes)
+                key.verify(sig, data)
+
+            else:
+                raise ValueError(f"unknown scheme '{self.scheme}'")
+
+        # Workaround for 'except (SignatureMismatch, InvalidSignature)' to
+        # conditionally evaluate the optional 'InvalidSignature':
+        except Exception as e:
+            if isinstance(e, SignatureMismatch) or (
+                not CRYPTO_IMPORT_ERROR and isinstance(e, InvalidSignature)
             ):
                 raise exceptions.UnverifiedSignatureError(
                     f"Failed to verify signature by {self.keyid}"
-                )
-        except (
-            exceptions.CryptoError,
-            exceptions.FormatError,
-            exceptions.UnsupportedAlgorithmError,
-            exceptions.UnsupportedLibraryError,
-        ) as e:
+                ) from e
+
             logger.info("Key %s failed to verify sig: %s", self.keyid, str(e))
             raise exceptions.VerificationError(
                 f"Unknown failure to verify signature by {self.keyid}"