Vigenère cipher in pure bash

#!/usr/local/bin/bash
# vigenere.sh
# http://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher

a="ABCDEFGHIJKLMNOPQRSTUVWXYZ"

[[ "${*/-d/}" != "" ]] &&
echo "Usage: $0 [-d]" && exit 1
m=${1:+-}

printf "string: ";read t
printf "keyphrase: ";read -s k
printf "\n"
for ((i=0;i<${#t};i++)); do
p1=${a%%${t:$i:1}*}
p2=${a%%${k:$((i%${#k})):1}*}
d="${d}${a:$(((${#p1}${m:-+}${#p2})%${#a})):1}"
done
echo "$d"