This is an encryption algorithm commonly used for hiding information in [usenet] posts that is open to all but not immediately readable. An example is posting about film endings, or example answers.

The algorithm is actually a [caesar] cipher with a 13 place rotation. This number is special because in an alphabet with 26 letters, caesar 13 and caesar -13 use the same map, so the same call can be used for encoding and decoding. ([RS])

Using '''tr''' from [Example scripts everybody should have], a rot13 cipher can be written as easily as
 puts [tr A-Za-z N-ZA-Mn-za-m {Guvf vf n grfg}]
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So, using rot13, "fnynq" is caesar salad. --[CLN]
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[KBK] - Any monoalphabetic substitution cipher is, of course, trivial
to break.  [Solving cryptograms] written in such ciphers is well within
the compass of Tcl's capabilities.

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[FW] gives a version that doesn't require any additional procedures, and rotates by any amount (specified by an optional second argument, defaults to 13 of course):
 proc rot {text {amount 13}} {
   if {$amount > 25} {
     set amount [expr {$amount % 26}]
   }
 
   set alphabet "ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz"
   set res ""
   set length [string length $text]

   for {set index 0} {$index < $length} {incr index} {
     set char [string index $text $index]
     set pos [string first $char $alphabet]
     append res [expr {$pos == -1 ? $char : [string index $alphabet [expr {$pos + $amount}]]}]
   }
   return $res
 }
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See also [vignere], [caesar], [uuencode], [base64] 

[Category Package] [Category Cryptography]