Provide specialized error message when signing or verifying ed25519

signatures and pynacl is not installed (LP: #1854475)

ChangeLog

@@ -1,6 +1,8 @@
 Version 1.0.1
     - Follow CNAMES when looking up key records when using DNS (pydns)
       (LP: #1856421)
+    - Provide specialized error message when signing or verifying ed25519
+      signatures and pynacl is not installed (LP: #1854475)
 
 2019-12-09 Version 1.0.0
     - Add support for RFC 8460 tlsrpt DKIM signature processing (LP: #1847020)

README.md

@@ -139,6 +139,11 @@ need not be considered experimental.  The dkimpy implementation has
 successfully interoperated with three other implementations and the technical
 parameters for ed25519-sha256 are defined and stable.
 
+To install from pypi with the required optional depenencies, use the ed25519
+option:
+
+```pip install -e '.[ed25519]'```
+
 ## DKIM SCRIPTS
 
 Three helper programs are also supplied: dknewkey, dkimsign and

dkim/__init__.py

@@ -184,7 +184,6 @@ class NaClNotFoundError(DKIMException):
     """ Nacl package not installed, needed for ed25119 signatures """
     pass
 
-
 class UnknownKeyTypeError(DKIMException):
     """ Key type (k tag) is not known (rsa/ed25519) """
 
@@ -443,7 +442,10 @@ def evaluate_pk(name, s):
       pass
   try:
       if pub[b'k'] == b'ed25519':
+          try:
               pk = nacl.signing.VerifyKey(pub[b'p'], encoder=nacl.encoding.Base64Encoder)
+          except NameError:
+              raise NaClNotFoundError('pynacl module required for ed25519 signing, see README.md')
           keysize = 256
           ktag = b'ed25519'
   except KeyError:
@@ -826,7 +828,10 @@ class DKIM(DomainSigner):
         except UnparsableKeyError as e:
             raise KeyFormatError(str(e))
     elif self.signature_algorithm == b'ed25519-sha256':
+        try:
             pk = nacl.signing.SigningKey(privkey, encoder=nacl.encoding.Base64Encoder)
+        except NameError:
+            raise NaClNotFoundError('pynacl module required for ed25519 signing, see README.md')
 
     if identity is not None and not identity.endswith(domain):
         raise ParameterError("identity must end with domain")