bxor.bash: bitwise exclusive OR (XOR) of two numbers represented in hexadecimal numerals

bxor.bash

#!/bin/bash

# bxor.bash: bitwise exclusive OR (XOR) of two numbers represented in hexadecimal numerals. Result is padded with leading zeros if the number is smaller than 2**256.
# The script could be used to encrypt or decrypt (Vernam cipher).
# Examples: 
# $ bash bxor.bash "000ffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364139" "fedc09"
# $ for i in {1..8}; do printf "%08X" "$SRANDOM" >> b256random.key; done && bash ./bxor.bash 012345b $( < b256random.key )
# $ head -c 32 /dev/random >> b256random.key && bash ./bxor.bash $( head -c 32 plaindata.bin | basenc --base16 -w 0 ) $( basenc --base16 -w 0 < b256random.key )
# $ zsh bxor.bash "ffff" "011"
# $ sh bxor.bash "ffff" "011"

export LC_ALL=C
CHAR_COUNT1=${#1}
CHAR_COUNT2=${#2}

if [ "${CHAR_COUNT1}" -lt "${CHAR_COUNT2}" ]; then
  SHORTER_INPUT_STRING="$1"
  LONGER_INPUT_STRING="$2"
else
  SHORTER_INPUT_STRING="$2"
  LONGER_INPUT_STRING="$1"
  char_count=${CHAR_COUNT2}
  CHAR_COUNT2=${CHAR_COUNT1}
  CHAR_COUNT1=${char_count}
fi
result=""
min_size_in_bits=256
count_of_bits_in_nibble=4
position=1
nibble=""
char_s=""
char_l=""

prefix_substring_s=${SHORTER_INPUT_STRING%?}
prefix_substring_l=${LONGER_INPUT_STRING%?}

while [ "${position}" -le "${CHAR_COUNT1}" ] ; do
  char_s=$( printf "%c" "${SHORTER_INPUT_STRING##${prefix_substring_s}}" )
  char_l=$( printf "%c" "${LONGER_INPUT_STRING##${prefix_substring_l}}" )
  nibble=$( printf "%01X" $(( 0x${char_s} ^ 0x${char_l} )) )
  result="${nibble}${result}"
  prefix_substring_s=${prefix_substring_s%?}
  prefix_substring_l=${prefix_substring_l%?}
  position=$(( ${position} + 1 ))
done
while [ "${position}" -le "${CHAR_COUNT2}" ] ; do
  char_l=$( printf "%c" "${LONGER_INPUT_STRING##${prefix_substring_l}}" )
  nibble=$( printf "%01X" $(( 0x${char_l} )) )
  result="${nibble}${result}"
  prefix_substring_l=${prefix_substring_l%?}
  position=$(( ${position} + 1 ))
done
while [ "${position}" -le $(( $min_size_in_bits / $count_of_bits_in_nibble )) ] ; do
  result="0${result}"
  position=$(( ${position} + 1 ))
done
echo "${result}"