Use dedicated exception class when ECPointFormat is wrong

This makes it easier to catch these specific types of errors if the
user wants to, and makes it clearer what is happening.

pgpy/errors.py

@@ -4,6 +4,7 @@
 __all__ = ('PGPError',
            'PGPEncryptionError',
            'PGPDecryptionError',
+           'PGPIncompatibleECPointFormat',
            'PGPOpenSSLCipherNotSupported',
            'PGPInsecureCipher',
            'WontImplementError',)
@@ -24,6 +25,11 @@ class PGPDecryptionError(Exception):
     pass
 
 
+class PGPIncompatibleECPointFormat(Exception):
+    """Raised when the point format is incompatible with the elliptic curve"""
+    pass
+
+
 class PGPOpenSSLCipherNotSupported(Exception):
     """Raised when OpenSSL does not support the requested cipher"""
     pass

pgpy/packet/fields.py

@@ -55,6 +55,7 @@
 
 from ..errors import PGPDecryptionError
 from ..errors import PGPError
+from ..errors import PGPIncompatibleECPointFormat
 
 from ..symenc import _decrypt
 from ..symenc import _encrypt
@@ -565,8 +566,7 @@ def parse(self, packet):
 
         self.p = ECPoint(packet)
         if self.p.format != ECPointFormat.Standard:
-            raise NotImplementedError("Expected: standard Elliptic curve point format (0x40). Got: 0x{:02X}".format(self.p.format))
-
+            raise PGPIncompatibleECPointFormat("Only Standard format is valid for ECDSA")
 
 
 class ECDHPub(PubKey):
@@ -637,7 +637,7 @@ def parse(self, packet):
 
         self.p = ECPoint(packet)
         if self.p.format != ECPointFormat.Standard:
-            raise NotImplementedError("Expected: standard Elliptic curve point format (0x40). Got: 0x{:02X}".format(self.p.format))
+            raise PGPIncompatibleECPointFormat("Only curves using Standard format are currently supported for ECDH")
         self.kdf.parse(packet)