Signer: Add a helper function for loading a private key file

This keeps the key type determination logic in one place.

lib/Mail/DKIM/Signer.pm

@@ -196,13 +196,9 @@ sub init {
         $self->{'Algorithm'} = 'rsa-sha1';
     }
 
-    my $type = 'rsa'; # default
-    $type = 'ed25519' if ( $self->{'Algorithm'} =~ /^ed25519/ );
-
     if ( defined $self->{KeyFile} ) {
         $self->{Key} ||=
-          Mail::DKIM::PrivateKey->load( File => $self->{KeyFile},
-            Type => $type );
+          load_private_key( $self->{KeyFile}, $self->{Algorithm} );
     }
 
     unless ( $self->{'Method'} ) {
@@ -322,10 +318,7 @@ sub finish_body {
           || $self->{Key}
           || $self->{KeyFile};
         if ( defined($key) && !ref($key) ) {
-            my $type = 'rsa'; # default
-            $type = 'ed25519' if ( $signature->algorithm =~ /^ed25519/ );
-            $key = Mail::DKIM::PrivateKey->load( File => $key,
-                Type => $type );
+            $key = load_private_key( $key, $signature->algorithm );
         }
         $key
           or die "no key available to sign with\n";
@@ -342,6 +335,17 @@ sub finish_body {
     }
 }
 
+# Load a private key file for the given algorithm.
+sub load_private_key {
+    my $key_file = shift;
+    my $algorithm = shift;
+
+    my $type = 'rsa'; # default
+    $type = 'ed25519' if ( $algorithm =~ /^ed25519/ );
+
+    return Mail::DKIM::PrivateKey->load( File => $key_file, Type => $type );
+}
+
 =head1 METHODS
 
 =head2 PRINT()