Add a GPGSigner

The GPGSigner is an implementation of the new "Signer" interface
that was merged in secure-systems-lab#319
and could be found in securesystemslib.signer.py

This is the next logical step given that securesystemslib supports GPG
and we want this interface to have implementations for all signature
types which are already supported by securesystemslib.

While implementing the GPGSigner, I wanted to make sure that one can
easily sign a portion of data, receive a Signature object and use the
information stored in that object to verify the signature.

To verifty a GPG signature, one have to use
securesystemslib/gpg/functions.verifty_signature().
There, the signature_object argument should be in the
securesystemslib.formats.GPG_SIGNATURE_SCHEMA format.
I searched for a way to easily retrieve the additional fields in the
GPG_SIGNATURE_SCHEMA -"other_headers" and "info" from the keyid stored
in the "Signature" object returned by the "sign" operation.
Unfortunately, right now there is no function that I can use for that
purpose.
The only option I was left with, was to create a new class:
"GPGSignature" where we can store those additional fields returned
from securesystemslib.gpg.functions.create_signature() which we call
during the "sign" process.

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

securesystemslib/signer.py

@@ -7,6 +7,7 @@
 
 import abc
 import securesystemslib.keys as sslib_keys
+import securesystemslib.gpg.functions as gpg
 from typing import Any, Dict, Optional, Mapping
 
 
@@ -88,6 +89,57 @@ def to_dict(self) -> Dict:
 
 
 
+class GPGSignature(Signature):
+    """A container class containing information about a gpg signature.
+
+    Besides the signature, it also contains other meta information
+    needed to uniquely identify the key used to generate the signature.
+
+    Attributes:
+        keyid: HEX string used as a unique identifier of the key.
+        signature: HEX string representing the signature.
+        other_headers: HEX representation of additional GPG headers.
+    """
+    def __init__(
+        self,
+        keyid: str,
+        signature: str,
+        other_headers: str,
+    ):
+        super().__init__(keyid, signature)
+        self.other_headers = other_headers
+
+
+    @classmethod
+    def from_dict(cls, signature_dict: Dict) -> "Signature":
+        """Creates a GPGSignature object from its JSON/dict representation.
+
+        Args:
+            signature_dict: Dict containing valid "keyid", "signature" and
+                "other_fields" fields.
+
+        Raises:
+            KeyError: If any of the "keyid", "sig" or "other_headers" fields
+                are missing from the signature_dict.
+
+        Returns:
+            GPGSignature instance.
+        """
+
+        return cls(signature_dict["keyid"], signature_dict["sig"],
+                signature_dict["other_headers"], signature_dict.get("info"))
+
+
+    def to_dict(self) -> Dict:
+        """Returns the JSON-serializable dictionary representation of self."""
+        return {
+            "keyid": self.keyid,
+            "signature": self.signature,
+            "other_headers": self.other_headers
+        }
+
+
+
 class Signer:
     """Signer interface created to support multiple signing implementations."""
 
@@ -160,3 +212,62 @@ def sign(self, payload: bytes) -> "Signature":
 
         sig_dict = sslib_keys.create_signature(self.key_dict, payload)
         return Signature(**sig_dict)
+
+
+
+class GPGSigner(Signer):
+    """A securesystemslib gpg implementation of the "Signer" interface.
+
+    Provides a sign method to generate a cryptographic signature with gpg, using
+    an RSA, DSA or EdDSA private key identified by the keyid on the instance.
+
+    Args:
+        keyid: The keyid of the gpg signing keyid. If not passed the default
+              key in the keyring is used.
+
+        homedir: Path to the gpg keyring. If not passed the default keyring
+            is used.
+
+    """
+    def __init__(
+            self, keyid: Optional[str] = None, homedir: Optional[str] = None
+    ):
+        self.keyid = keyid
+        self.homedir = homedir
+
+
+    def sign(self, payload: bytes) -> "GPGSignature":
+        """Signs a given payload by the key assigned to the GPGSigner instance.
+
+        Calls the gpg command line utility to sign the passed content with the
+        key identified by the passed keyid from the gpg keyring at the passed
+        homedir.
+
+        The executed base command is defined in
+        securesystemslib.gpg.constants.GPG_SIGN_COMMAND.
+
+        Arguments:
+            payload: The bytes to be signed.
+
+        Raises:
+            securesystemslib.exceptions.FormatError:
+                If the keyid was passed and does not match
+                securesystemslib.formats.KEYID_SCHEMA.
+
+            ValueError: the gpg command failed to create a valid signature.
+            OSError: the gpg command is not present or non-executable.
+            securesystemslib.exceptions.UnsupportedLibraryError: thehe gpg
+                command is not available, or the cryptography library is
+                not installed.
+            securesystemslib.gpg.exceptions.CommandError: the gpg command
+                returned a non-zero exit code.
+            securesystemslib.gpg.exceptions.KeyNotFoundError: the used gpg
+                version is not fully supported and no public key can be found
+                for short keyid.
+
+        Returns:
+            Returns a "GPGSignature" class instance.
+        """
+
+        sig_dict = gpg.create_signature(payload, self.keyid, self.homedir)
+        return GPGSignature(**sig_dict)