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) -- And supports negative rotation amounts, now, too.
 proc rot {text {amount 13}} {
   if {abs($amount) > 25} {
     set amount [expr {$amount % 26}]
   }
 
   set alphabet "ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz"
   set res ""
   set length [string length $text]
   set find_command [expr {$amount > 0 ? "first" : "last"}]

   for {set index 0} {$index < $length} {incr index} {
     set char [string index $text $index]
     set pos [string $find_command $char $alphabet]
     append res [expr {$pos == -1 ? $char : [string index $alphabet [expr {$pos + $amount}]]}]
   }
   return $res
 }

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2002-12-10 JJM, The one-liner approach to this problem:

 regsub -all -- {([a-z])} [string tolower $text] {[format "%c" [expr {[scan "a" "%c"] + ((([scan \1 "%c"] - [scan "a" "%c"]) + 13) % 26)}]]} text; set text [subst $text]

Or for those of us with Tcl 8.4.x:

 set text [subst [regsub -all -- {([a-z])} [string tolower $text] {[format "%c" [expr {[scan "a" "%c"] + ((([scan \1 "%c"] - [scan "a" "%c"]) + 13) % 26)}]]}]]

This doesn't do upper/lower case, though.  Just making note.  -[FW]

2002-12-10 JJM, Yes, but this one does:

 set text [subst [regsub -all -- {([a-zA-Z])} $text {[if {[string is lower "\1"] != "0"} then {set A "a"; set dummy ""} else {set A "A"; set dummy ""}][format "%c" [expr {[scan $A "%c"] + ((([scan "\1" "%c"] - [scan $A "%c"]) + 13) % 26)}]]}]]

Slightly more generic/entertaining:

 proc rotate { text {amount 13} } { 
  return [subst [regsub -all -- {([a-zA-Z])} $text {[if {[string is lower "\1"] != "0"} then {set A "a"; set dummy ""} else {set A "A"; set dummy ""}][format "%c" [expr {[scan $A "%c"] + ((([scan "\1" "%c"] - [scan $A "%c"]) + $amount) % 26)}]]}]] 
 }

Suggested by dkf and modified slightly by JJM:

 set text [subst [regsub -all {([a-zA-Z])} $text {[format "%c" [expr {[string is lower "\1"] ? (97 + (([scan "\1" "%c"] - 84) % 26)) : (65 + (([scan "\1" "%c"] - 52) % 26))}]]}]]

As short as possible so far:

 set text [subst [regsub -all {[a-zA-Z]} $text {[format %c [expr [set c [scan & %c]] \& 96 | (($c \& 31) + 12) % 26 + 1]]}]]

[DKF] - Though these [subst]-based solutions are vulnerable to attacks from malicious and unfortunate input strings, forcing a slightly longer solution:

 set text [subst [regsub -all {[a-zA-Z]} [regsub -all "\[\[$\\\\\]" $text {\\&}] {[format %c [expr [set c [scan & %c]]\&96|(($c\&31)+12)%26+1]]}]]

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See also [vignere], [caesar], [uuencode], [base64] 

[Category Package] [Category Cryptography]