diff --git a/Lib/encodings/rot_13.py b/Lib/encodings/rot_13.py
index 1f2f47bb34f..f0b4186dc87 100755
--- a/Lib/encodings/rot_13.py
+++ b/Lib/encodings/rot_13.py
@@ -106,7 +106,7 @@ def getregentry():
 ### Filter API
 
 def rot13(infile, outfile):
-    outfile.write(infile.read().encode('rot-13'))
+    outfile.write(codecs.encode(infile.read(), 'rot-13'))
 
 if __name__ == '__main__':
     import sys
diff --git a/Misc/NEWS b/Misc/NEWS
index 0b558727ba1..949d8e69772 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -34,6 +34,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #21171: Fixed undocumented filter API of the rot13 codec.
+  Patch by Berker Peksag.
+
 - Issue #20539: Improved math.factorial error message for large positive inputs
   and changed exception type (OverflowError -> ValueError) for large negative
   inputs.