Make new API compatible with the Signing interface

In the securesystemslib pr secure-systems-lab/securesystemslib#319
I added a new Signer interface with the purpose of supporting multiple
signing implementations.
Additionally, I added the SSlibSigner implementation of that interface
which implements the signing operation for rsa, ed25519 and ecdsa
schemes.
With this commit, I integrate the SSlibSigner into the new API in tuf.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

setup.py

@@ -113,7 +113,7 @@
   python_requires="~=3.6",
   install_requires = [
     'requests>=2.19.1',
-    'securesystemslib>=0.18.0',
+    'securesystemslib>=0.20.0',
     'six>=1.11.0'
   ],
   packages = find_packages(exclude=['tests']),

tests/test_api.py

@@ -153,8 +153,9 @@ def test_sign_verify(self):
         self.assertTrue(metadata_obj.verify(
                 self.keystore['targets']['public']))
 
+        sslib_signer = SSlibSigner(self.keystore['snapshot']['private'])
         # Append a new signature with the unrelated key and assert that ...
-        metadata_obj.sign(self.keystore['snapshot']['private'], append=True)
+        metadata_obj.sign(sslib_signer, append=True)
         # ... there are now two signatures, and
         self.assertTrue(len(metadata_obj.signatures) == 2)
         # ... both are valid for the corresponding keys.
@@ -163,16 +164,17 @@ def test_sign_verify(self):
         self.assertTrue(metadata_obj.verify(
                 self.keystore['snapshot']['public']))
 
+        sslib_signer.key_dict = self.keystore['timestamp']['private']
         # Create and assign (don't append) a new signature and assert that ...
-        metadata_obj.sign(self.keystore['timestamp']['private'], append=False)
+        metadata_obj.sign(sslib_signer, append=False)
         # ... there now is only one signature,
         self.assertTrue(len(metadata_obj.signatures) == 1)
         # ... valid for that key.
         self.assertTrue(metadata_obj.verify(
                 self.keystore['timestamp']['public']))
 
         # Assert exception if there are more than one signatures for a key
-        metadata_obj.sign(self.keystore['timestamp']['private'], append=True)
+        metadata_obj.sign(sslib_signer, append=True)
         with self.assertRaises(tuf.exceptions.Error) as ctx:
             metadata_obj.verify(self.keystore['timestamp']['public'])
         self.assertTrue(

tuf/api/metadata.py

@@ -19,7 +19,8 @@
     persist_temp_file
 )
 from securesystemslib.storage import StorageBackendInterface
-from securesystemslib.keys import create_signature, verify_signature
+from securesystemslib.keys import verify_signature
+from securesystemslib.signer import Signer, Signature
 
 import tuf.formats
 import tuf.exceptions
@@ -92,12 +93,17 @@ class also that has a 'from_dict' factory method. (Currently this is
         else:
             raise ValueError(f'unrecognized metadata type "{_type}"')
 
+        signatures = []
+        for signature in metadata['signatures']:
+            new_signature = Signature(signature['keyid'], signature['sig'])
+            signatures.append(new_signature)
+
         # NOTE: If Signature becomes a class, we should iterate over
         # metadata['signatures'], call Signature.from_dict for each item, and
         # pass a list of Signature objects to the Metadata constructor intead.
         return cls(
                 signed=inner_cls.from_dict(metadata['signed']),
-                signatures=metadata['signatures'])
+                signatures=signatures)
 
 
     @classmethod
@@ -146,8 +152,13 @@ def from_json_file(
     # Serialization.
     def to_dict(self) -> JsonDict:
         """Returns the JSON-serializable dictionary representation of self. """
+
+        signatures = []
+        for sig in self.signatures:
+            signatures.append(sig.to_dict())
+
         return {
-            'signatures': self.signatures,
+            'signatures': signatures,
             'signed': self.signed.to_dict()
         }
 
@@ -184,11 +195,12 @@ def to_json_file(
 
 
     # Signatures.
-    def sign(self, key: JsonDict, append: bool = False) -> JsonDict:
+    def sign(self, signer: Signer, append: bool = False) -> JsonDict:
         """Creates signature over 'signed' and assigns it to 'signatures'.
 
         Arguments:
-            key: A securesystemslib-style private key object used for signing.
+            singer: An object implementing the securesystemslib.signer.Signer
+                interface.
             append: A boolean indicating if the signature should be appended to
                 the list of signatures or replace any existing signatures. The
                 default behavior is to replace signatures.
@@ -203,7 +215,7 @@ def sign(self, key: JsonDict, append: bool = False) -> JsonDict:
             A securesystemslib-style signature object.
 
         """
-        signature = create_signature(key, self.signed.to_canonical_bytes())
+        signature = signer.sign(self.signed.to_canonical_bytes())
 
         if append:
             self.signatures.append(signature)
@@ -232,7 +244,7 @@ def verify(self, key: JsonDict) -> bool:
 
         """
         signatures_for_keyid = list(filter(
-                lambda sig: sig['keyid'] == key['keyid'], self.signatures))
+                lambda sig: sig.keyid == key['keyid'], self.signatures))
 
         if not signatures_for_keyid:
             raise tuf.exceptions.Error(
@@ -244,7 +256,7 @@ def verify(self, key: JsonDict) -> bool:
                     f'{key["keyid"]}, not sure which one to verify.')
 
         return verify_signature(
-            key, signatures_for_keyid[0],
+            key, signatures_for_keyid[0].to_dict(),
             self.signed.to_canonical_bytes())