Add the Signer interface and the SSlibSigner

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

securesystemslib/signer.py

@@ -0,0 +1,130 @@
+"""The goal of this module is to provide a signing interface supporting multiple
+    signing implementations and a couple of example implementations.
+"""
+
+import abc
+import securesystemslib.keys as sslib_keys
+from typing import Dict
+
+
+class Signature:
+    """A container class containing information about a signature.
+
+    Contains a signature and the keyid
+        uniquely identifying the key used to generate the signature.
+    Provides utility methods to easily create an object from a dictionary
+        and return the dictionary representation of the object.
+
+    Attributes:
+        keyid: HEX string used as a unique identifier of the key.
+        signature: HEX string representing the signature.
+
+    """
+    def __init__(self, keyid: str, sig: str):
+        self.keyid = keyid
+        self.signature = sig
+
+
+    @classmethod
+    def from_dict(cls, signature_dict: Dict) -> 'Signature':
+        """Creates a Signature object from its JSON/dict representation.
+
+        Arguments:
+            signature_dict:
+                A dict containing a valid keyid and a signature.
+                Note that the fields in it should be named "keyid" and "sig"
+                respectively.
+
+        Raises:
+            KeyError: If any of the "keyid" and "sig" fields are missing from
+                the signature_dict.
+
+        Returns:
+            A "Signature" instance.
+        """
+
+        return cls(signature_dict["keyid"], signature_dict["sig"])
+
+
+    def to_dict(self) -> Dict:
+        """Returns the JSON-serializable dictionary representation of self."""
+
+        return {
+            "keyid": self.keyid,
+            "sig": self.signature
+        }
+
+
+
+class Signer:
+    """Signer interface created to support multiple signing implementations."""
+
+    __metaclass__ = abc.ABCMeta
+
+    @abc.abstractmethod
+    def sign(payload: bytes) -> 'Signature':
+        """Signs a given payload by the key assigned to the Signer instance.
+
+        Arguments:
+            payload: The bytes to be signed.
+
+        Returns:
+            Returns a "Signature" class instance.
+        """
+        raise NotImplementedError # pragma: no cover
+
+
+
+class SSlibSigner(Signer):
+    """A securesystemslib signer implementation.
+
+    Provides a sign method to generate a cryptographic signature with a
+    securesystemslib-style rsa, ed25519 or ecdsa private key on the instance.
+    The signature scheme is determined by the key and must be one of:
+
+    - rsa(ssa-pss|pkcs1v15)-(md5|sha1|sha224|sha256|sha384|sha512) (12 schemes)
+    - ed25519
+    - ecdsa-sha2-nistp256
+
+    See 'securesystemslib.interface' for functions to generate and load keys.
+
+    Attributes:
+        key_dict:
+            A securesystemslib-style key dictionary, which includes a keyid,
+            key type, signature scheme, and the public and private key values,
+            e.g.::
+
+                {
+                    "keytype": "rsa",
+                    "scheme": "rsassa-pss-sha256",
+                    "keyid": "f30a0870d026980100c0573bd557394f8c1bbd6...",
+                    "keyval": {
+                        "public": "-----BEGIN RSA PUBLIC KEY----- ...",
+                        "private": "-----BEGIN RSA PRIVATE KEY----- ..."
+                    }
+                }
+
+            The public and private keys are strings in PEM format.
+    """
+    def __init__(self, key_dict: Dict):
+        self.key_dict = key_dict
+
+
+    def sign(self, payload: bytes) -> 'Signature':
+        """Signs a given payload by the key assigned to the SSlibSigner instance.
+
+        Arguments:
+            payload: The bytes to be signed.
+
+        Raises:
+            securesystemslib.exceptions.FormatError: Key argument is malformed.
+            securesystemslib.exceptions.CryptoError, \
+                securesystemslib.exceptions.UnsupportedAlgorithmError:
+                Signing errors.
+
+        Returns:
+            Returns a "Signature" class instance.
+        """
+
+        sig_dict = sslib_keys.create_signature(self.key_dict, payload)
+        return Signature(**sig_dict)