JWE: Add explicit decrypt step

It may be useful to perform the deserialization and decryption steps
separately.

The decrypt() step assumes the caller already called the deserialize()
function, and accepts only one key.

The decrypt step will allow a caller to try multiple keys, by simply
calling decrypt() multiple times.
Signed-off-by: Simo Sorce <simo@redhat.com>

jwcrypto/jwe.py

@@ -606,6 +606,29 @@ class JWE(object):
         else:
             raise ValueError('Unknown compression')
 
+    def decrypt(self, key):
+        if not isinstance(key, JWK):
+            raise ValueError('key is not a JWK object')
+        if 'ciphertext' not in self.objects:
+            raise InvalidJWEOperation("No available ciphertext")
+        self.decryptlog = list()
+
+        if 'recipients' in self.objects:
+            for rec in self.objects['recipients']:
+                try:
+                    self._decrypt(key, rec)
+                except Exception as e:  # pylint: disable=broad-except
+                    self.decryptlog.append('Failed: [%s]' % repr(e))
+        else:
+            try:
+                self._decrypt(key, self.objects)
+            except Exception as e:  # pylint: disable=broad-except
+                self.decryptlog.append('Failed: [%s]' % repr(e))
+
+        if not self.plaintext:
+            raise InvalidJWEData('No recipient matched the provided '
+                                 'key' + repr(self.decryptlog))
+
     def deserialize(self, raw_jwe, key=None):
         """ Destroys any current status and tries to import the raw
             JWS provided.
@@ -664,24 +687,4 @@ class JWE(object):
             raise InvalidJWEData('Invalid format', repr(e))
 
         if key:
-            if not isinstance(key, JWK):
-                raise ValueError('key is not a JWK object')
-            if 'ciphertext' not in self.objects:
-                raise InvalidJWEOperation("No available ciphertext")
-            self.decryptlog = list()
-
-            if 'recipients' in self.objects:
-                for rec in self.objects['recipients']:
-                    try:
-                        self._decrypt(key, rec)
-                    except Exception as e:  # pylint: disable=broad-except
-                        self.decryptlog.append('Failed: [%s]' % repr(e))
-            else:
-                try:
-                    self._decrypt(key, self.objects)
-                except Exception as e:  # pylint: disable=broad-except
-                    self.decryptlog.append('Failed: [%s]' % repr(e))
-
-            if not self.plaintext:
-                raise InvalidJWEData('No recipient matched the provided '
-                                     'key' + repr(self.decryptlog))
+            self.decrypt(key)