The Caesar cipher is a monoalphabetic cipher where the plaintext is ecrypted by character substitution against a rotated alphabet. Supposedly first used by Julius Caesar and more recently popularised by usenet as rot13.
References
http://www.trincoll.edu/depts/cpsc/cryptography/caesar.html
Usage
% caesar::caesar -rot 3 {The Tcl'ers Wiki}
Wkh Wfo'huv Zlnl
% caesar::caesar -rot 3 -mode decode {Wkh Wfo'huv Zlnl}
The Tcl'ers Wiki
% caesar::rot13 {The Tcl'ers Wiki}
Gur Gpy'ref Jvxv
% caesar::rot13 -mode decode {Gur Gpy'ref Jvxv}
The Tcl'ers Wiki# caesar.tcl - Copyright (C) 2002 Pat Thoyts <[email protected]> # # Provide a Tcl only implementation of the caesar or ROT-13 cipher. # # The Caesar cipher is a monoalphabetic cipher where the plaintext is ecrypted # by character substitution against a rotated alphabet. Supposedly first used # by Julius Caesar and more recently popularised by usenet as rot-13. # # Re: # http://www.trincoll.edu/depts/cpsc/cryptography/caesar.html # # ------------------------------------------------------------------------- # See the file "license.terms" for information on usage and redistribution # of this file, and for a DISCLAIMER OF ALL WARRANTIES. # ------------------------------------------------------------------------- # @(#)$Id: 2885,v 1.1 2002-06-21 04:30:02 jcw Exp $
namespace eval caesar {
namespace export caesar rot13
# The rot13(1) cipher is simply the 13 rotation version of the caesar
# cipher.
interp alias {} [namespace current]::rot13 \
{} [namespace current]::caesar -rotation 13}
# ------------------------------------------------------------------------- # Description: # Tcl implementation of the Caesar algorithm. Takes the input data and # enciphers by mapping against a rotated alphabet. Ignores non-alphabetic # characters. # proc caesar::encode {s {n 13}} {
set r {}
binary scan $s c* d
foreach {c} $d {
append r [format %c [expr {
(($c ^ 0x40) & 0x5F) < 27 ?
(((($c ^ 0x40) & 0x5F) + $n - 1) % 26 + 1) | ($c & 0xe0)
: $c
}]]
}
return $r}
proc caesar::decode {s {n 13}} {
set n [expr {abs($n - 26)}]
return [encode $s $n]
}
# -------------------------------------------------------------------------
# Description:
# Encrypt data using the caesar cipher (also known as rot or rot13). For this
# cipher decryption is achieved by repeating the algorithm over the
# encrypted data.
# Notes:
# insecure
#
proc caesar::caesar {args} {
array set opts {filename {} rotation 13 mode encode}
while {[string match -* [lindex $args 0]]} {
switch -glob -- [lindex $args 0] {
-m* {
set opts(mode) [lindex $args 1]
set args [lreplace $args 0 0]
}
-r* {
set opts(rotation) [lindex $args 1]
set args [lreplace $args 0 0]
}
-f* {
set opts(filename) [lindex $args 1]
set args [lreplace $args 0 0]
}
-- {
set args [lreplace $args 0 0]
break
}
default {
return -code error "bad option [lindex $args 0]:\
must be -mode, -rotation, -filename or --"
}
}
set args [lreplace $args 0 0]
}
if {$opts(filename) != {}} {
set f [open $opts(filename) r]
#fconfigure $f -translation binary
set data [read $f]
close $f
} else {
if {[llength $args] != 1} {
return -code error "wrong \# args: should be\
\"caesar ?-mode mode? ?-rotation n? -file name | data\""
}
set data [lindex $args 0]
}
if {[string match "encode" $opts(mode)]} {
return [encode $data $opts(rotation)]
} else {
return [decode $data $opts(rotation)]
}
}
# -------------------------------------------------------------------------
package provide caesar 1.0
# -------------------------------------------------------------------------
#
# Local variables:
# mode: tcl
# indent-tabs-mode: nil
# End: