dkim: reject tag-lists with duplicate tag names

Closes emersion#69

Author: mimi89999

dkim/header.go

@@ -83,7 +83,12 @@ func parseHeaderParams(s string) (map[string]string, error) {
 			return params, errors.New("dkim: malformed header params")
 		}
 
-		params[strings.TrimSpace(key)] = strings.TrimSpace(value)
+		trimmedKey := strings.TrimSpace(key)
+		_, present := params[trimmedKey]
+		if present {
+			return params, errors.New("dkim: duplicate tag name")
+		}
+		params[trimmedKey] = strings.TrimSpace(value)
 	}
 	return params, nil
 }

dkim/query.go

@@ -100,7 +100,7 @@ func queryDNSTXT(domain, selector string, txtLookup txtLookupFunc) (*queryResult
 func parsePublicKey(s string) (*queryResult, error) {
 	params, err := parseHeaderParams(s)
 	if err != nil {
-		return nil, permFailError("key syntax error: " + err.Error())
+		return nil, permFailError("key record error: " + err.Error())
 	}
 
 	res := new(queryResult)