#!/usr/bin/env bash # the quick brown fox jumps over the lazy dog # 5 rails # # t k f s h d : tkfshd # h c _ _ o p _ t e _ o : hc__co_te_o # e i b n x m o _ _ y g : eibnxmo__yg # _ u r w _ u v r l z : _urw_uvrlz # q o j e a : qojea # # # the quick brown fox jumps over the lazy dog # tkfshdhc co te oeibnxmo yg urw uvrlzqojea main() { local OPTIND OPTARG local func rails while getopts :e:d: opt; do case $opt in e) func=encode; rails=$OPTARG;; d) func=decode; rails=$OPTARG;; :) die "missing argument for -$OPTARG";; ?) die "unknown option -$OPTARG";; esac done [[ -z $func ]] && die "specify -e or -d" (( rails <= 0 )) && die "must have positive number of rails" shift $((OPTIND - 1)) $func $rails "$1" } encode() { local -i n=$1 local plaintext=$2 local ciphertext="" # length of a "down & up cycle" local -i cycle=$(( 2 * (n - 1) )) local -i i j idx for ((i=0; i < n; i++)); do for ((j=0; j <= ${#plaintext}/n; j++)); do idx=$(( i + j * cycle )) (( idx >= ${#plaintext} )) && break ciphertext+=${plaintext:idx:1} if (( 0 < i && i < n-1 )); then idx=$(( (cycle - i) + j * cycle )) (( idx >= ${#plaintext} )) && break ciphertext+=${plaintext:idx:1} fi done done echo "$ciphertext" } decode() { local -i n=$1 local ciphertext=$2 local plaintext # a string spaces of the appropriate length printf -v plaintext "%*s" ${#ciphertext} "" local -i cycle=$(( 2 * (n - 1) )) local -i i j idx k=0 for ((i=0; i < n; i++)); do for ((j=0; j <= ${#plaintext}/n; j++)); do idx=$(( i + j * cycle )) (( idx >= ${#ciphertext} )) && break putAt $idx "${ciphertext:k++:1}" plaintext if (( 0 < i && i < n-1 )); then idx=$(( (cycle - i) + j * cycle )) (( idx >= ${#ciphertext} )) && break putAt $idx "${ciphertext:k++:1}" plaintext fi done done echo "$plaintext" } # In a string variable, set the character at a specific index # to be the given new character. # First char is at index 0 putAt() { local -i idx=$1 local char=${2:- } local -n str=$3 if (( 0 <= idx && idx < ${#str} )); then str=${str:0:idx}${char}${str:idx+1} fi } die() { echo "$*" >&2; exit 1; } main "$@"