Provide more specific error message when ed25519 private key is invalid

ChangeLog

@@ -7,6 +7,8 @@ Version 1.1
       with no DKIM signatures
     - Update dnsplug for DNS Python (dns) 2.0 compatibility (LP: #1888583)
     - Fix @param srv_id typos (LP: #1890532)
+    - Provide more specific error message when ed25519 private key is invalid
+      (See LP 1901569 for background)
 
 2019-12-31 Version 1.0.2
     - dknewkey: On posix operating systems set file permissions to 600 for

dkim/__init__.py

@@ -832,6 +832,8 @@ class DKIM(DomainSigner):
             pk = nacl.signing.SigningKey(privkey, encoder=nacl.encoding.Base64Encoder)
         except NameError:
             raise NaClNotFoundError('pynacl module required for ed25519 signing, see README.md')
+        except nacl.exceptions.ValueError:
+            raise KeyFormatError('invalid ed25519 private key or format')
 
     if identity is not None and not identity.endswith(domain):
         raise ParameterError("identity must end with domain")