Address Lukas comments for "Add the Signer..."

Signed-off-by: Martin Vrachev <[email protected]>

securesystemslib/signer.py

@@ -36,6 +36,21 @@ def __init__(self, keyid, sig):
     self.signature = sig
 
 
+  @classmethod
+  def from_dict(cls, signature_dict):
+    """
+    <Purpose>
+      Creates a Signature object from its JSON/dict representation.
+
+    <Arguments>
+      signature_dict: A dict containing that should contain valid keyid
+        and signature.
+        Note that the fields should be named "keyid" and "sig" respectively.
+    """
+
+    return cls(signature_dict["keyid"], signature_dict["sig"])
+
+
   def to_dict(self):
     """
     <Purpose>
@@ -60,7 +75,7 @@ class Signer:
   def sign(payload):
     """
     <Purpose>
-      Abstract function used for signig a given payload by the key assigned
+      Abstract function used for signing a given payload by the key assigned
       to the Signer instance.
 
     <Arguments>
@@ -70,15 +85,15 @@ def sign(payload):
       Returns a "Signature" class instance containing the signature and the
       the keyid which uniquely identifies the key used for signature generation.
     """
-    pass
+    raise NotImplementedError # pragma: no cover
 
 
 
 class SSlibSigner(Signer):
   """
     <Purpose>
       Securesystemslib default implementation of the "Signer" interface.
-      With this implementation the following signature schemas are supported:
+      With this implementation the following signature schemes are supported:
 
       'RSASSA-PSS'
       RFC3447 - RSASSA-PSS
@@ -88,12 +103,14 @@ class SSlibSigner(Signer):
       ed25519 - high-speed high security signatures
       http://ed25519.cr.yp.to/
 
+      'ecdsa-sha2-nistp256'
+      https://tools.ietf.org/html/rfc5656
+
     <Attributes>
       key_dict:
         A dictionary containing the keys. Both private and public keys are
         needed.
-        Which signature to generate is determined by the key type of 'key_dict'
-        and the available cryptography library specified in 'settings'.
+        Which signature to generate is determined by the key type of 'key_dict'.
 
         An example RSA key dict has the form:
 
@@ -120,7 +137,7 @@ def sign(self, payload):
 
     <Returns>
       Returns a "Signature" class instance containing the signature and the
-      the keyid which uniquely identifies the key used for signature generation.
+      keyid which uniquely identifies the key used for signature generation.
     """
 
     sig_dict = sslib_keys.create_signature(self.key_dict, payload)