Just because I had most of this from my Caesar shift cipher node.

Plain    a b c d e f g h i j k l m n o p q r s t u v w x y z
ROT13    N O P Q R S T U V W X Y Z A B C D E F G H I J K L M
Or this might be simpler:
a b c d e f g h i j k l m
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n o p q r s t u v w x y z