diff --git a/compiler/modulepaths.nim b/compiler/modulepaths.nim
index fa8fab08a..63b0cb44d 100644
--- a/compiler/modulepaths.nim
+++ b/compiler/modulepaths.nim
@@ -73,6 +73,17 @@ proc checkModuleName*(conf: ConfigRef; n: PNode; doLocalError=true): FileIndex =
   else:
     result = fileInfoIdx(conf, fullPath)
 
+proc rot13(result: var string) =
+  # don't mangle .nim
+  let finalIdx =
+    if result.endsWith(".nim"): result.len - 4
+    else: result.len
+  for i, c in result[0..<finalIdx]:
+    case c
+    of 'a'..'m', 'A'..'M': result[i] = char(c.uint8 + 13)
+    of 'n'..'z', 'N'..'Z': result[i] = char(c.uint8 - 13)
+    else: discard
+
 proc mangleModuleName*(conf: ConfigRef; path: AbsoluteFile): string =
   ## Mangle a relative module path to avoid path and symbol collisions.
   ##
@@ -81,9 +92,11 @@ proc mangleModuleName*(conf: ConfigRef; path: AbsoluteFile): string =
   ##
   ## Example:
   ## `foo-#head/../bar` becomes `@foo-@hhead@s..@sbar`
-  "@m" & relativeTo(path, conf.projectPath).string.multiReplace(
+  result = "@m" & relativeTo(path, conf.projectPath).string.multiReplace(
     {$os.DirSep: "@s", $os.AltSep: "@s", "#": "@h", "@": "@@", ":": "@c"})
+  rot13(result)
 
 proc demangleModuleName*(path: string): string =
   ## Demangle a relative module path.
   result = path.multiReplace({"@@": "@", "@h": "#", "@s": "/", "@m": "", "@c": ":"})
+  rot13(result)
diff --git a/compiler/msgs.nim b/compiler/msgs.nim
index ae4bcfcb8..1ad7e5c08 100644
--- a/compiler/msgs.nim
+++ b/compiler/msgs.nim
@@ -661,8 +661,10 @@ proc uniqueModuleName*(conf: ConfigRef; fid: FileIndex): string =
   for i in 0..<trunc:
     let c = rel[i]
     case c
-    of 'a'..'z':
-      result.add c
+    of 'a'..'m':
+      result.add char(c.uint8 + 13)
+    of 'n'..'z':
+      result.add char(c.uint8 - 13)
     of {os.DirSep, os.AltSep}:
       result.add 'Z' # because it looks a bit like '/'
     of '.':