A vigenere encoder and decoder.

vigenere

#!/usr/bin/env bash

numeric() {
  printf %d "$(( $(printf %d \'"${1,}") - $(printf %d \'a) ))"
}

vigenere_enc() {
  local key=$1 plain=$2 idx {plain,key}_pos incr offset

  for (( idx = 0; idx < ${#plain}; idx += 1 )); do
    if [[ ${plain:idx:1} = [[:alpha:]] ]]; then
      plain_pos=$(numeric "${plain:idx:1}")
      key_pos=$(numeric "${key:0:1}")

      if [[ ${plain:idx:1} = [[:lower:]] ]]; then
        offset=97
      else
        offset=65
      fi

      (( incr = ((plain_pos + key_pos) % 26) + offset ))
      printf \\"$(printf %03o "$incr")"
      key=${key:1}${key:0:1}
    else
      printf %c "${plain:idx:1}"
    fi
  done

  printf "\n"
}

vigenere_dec() {
  local key=$1 enc=$2 idx {enc,key}_pos decr offset

  for (( idx = 0; idx < ${#enc}; idx += 1 )); do
    if [[ ${enc:idx:1} = [[:alpha:]] ]]; then
      enc_pos=$(numeric "${enc:idx:1}")
      key_pos=$(numeric "${key:0:1}")

      if [[ ${enc:idx:1} = [[:lower:]] ]]; then
        offset=97
      else
        offset=65
      fi

      (( decr = ((enc_pos - key_pos + 26) % 26) + offset ))
      printf \\"$(printf %03o "$decr")"
      key=${key:1}${key:0:1}
    else
      printf %c "${enc:idx:1}"
    fi
  done

  printf "\n"
}

if [[ -n $1 && encrypt = $1* ]]; then
  vigenere_enc "$2" "$3"
elif [[ -n $1 && decrypt = $1* ]]; then
  vigenere_dec "$2" "$3"
fi