Do not error out on version mismatch in the standard code path

auth/callback.go

@@ -531,7 +531,7 @@ func handleBaseFormat(cmixMsg format.Message, grp *cyclic.Group) (baseFormat,
 
 	baseFmt, err := unmarshalBaseFormat(cmixMsg.GetContents(),
 		grp.GetP().ByteLen())
-	if err != nil {
+	if err != nil && baseFmt == nil {
 		return baseFormat{}, nil, errors.WithMessage(err, "Failed to"+
 			" unmarshal auth")
 	}
@@ -542,5 +542,5 @@ func handleBaseFormat(cmixMsg format.Message, grp *cyclic.Group) (baseFormat,
 	}
 	partnerPubKey := grp.NewIntFromBytes(baseFmt.pubkey)
 
-	return baseFmt, partnerPubKey, nil
+	return *baseFmt, partnerPubKey, nil
 }

auth/fmt.go

@@ -65,18 +65,18 @@ func buildBaseFormat(data []byte, pubkeySize int) baseFormat {
 	return f
 }
 
-func unmarshalBaseFormat(b []byte, pubkeySize int) (baseFormat, error) {
+func unmarshalBaseFormat(b []byte, pubkeySize int) (*baseFormat, error) {
 	if len(b) < pubkeySize {
-		return baseFormat{}, errors.New("Received baseFormat too small")
+		return nil, errors.New("Received baseFormat too small")
 	}
 	bfmt := buildBaseFormat(b, pubkeySize)
 	version := bfmt.GetVersion()
 	if version != requestFmtVersion {
-		return baseFormat{}, errors.Errorf(
+		return &bfmt, errors.Errorf(
 			"Unknown baseFormat version: %d", version)
 	}
 
-	return bfmt, nil
+	return &bfmt, nil
 }
 
 func (f baseFormat) Marshal() []byte {

auth/fmt_test.go

@@ -151,10 +151,10 @@ func TestBaseFormat_MarshalUnmarshal(t *testing.T) {
 			"Could not unmarshal into baseFormat: %v", err)
 	}
 
-	if !reflect.DeepEqual(newMsg, baseMsg) {
+	if !reflect.DeepEqual(*newMsg, baseMsg) {
 		t.Errorf("unmarshalBaseFormat() error: "+
 			"Unmarshalled message does not match originally marshalled message."+
-			"\n\tExpected: %v\n\tRecieved: %v", baseMsg, newMsg)
+			"\n\tExpected: %v\n\tRecieved: %v", baseMsg, *newMsg)
 	}
 
 	// Unmarshal error test: Invalid size parameter