Calculate the Yubikey serial number from the OTP (bash only)

yubikey_otp_to_serial.sh

#!/usr/bin/env bash
set -Eeuo pipefail
IFS=$'\n\t'

# adapted from Python: https://gist.github.com/href/dad244512c320bd9b5d6ff50e7c7e0b2

declare -ra ALPHABET=(c b d e f g h i j k l n r t u v)

index_of() {
  local -r element="$1"
  local -i index=-1

  for i in "${!ALPHABET[@]}" ; do
    if [[ "${ALPHABET[$i]}" = "$element" ]] ; then
      index=$i
      printf '%i' "$index"
      break
    fi
  done
}

yubikey_otp_to_serial() {
  local -r otp_str="cccc${1:0:12}"

  # splits into array bash trick
  [[ "$otp_str" =~ ${otp_str//?/(.)} ]]
  declare -ar otp_arr=("${BASH_REMATCH[@]:1}")

  local -i index=-1
  local -i keep=0
  local -i toggle=0

  local -a bytesarray=()

  for char in ${otp_arr[*]} ; do
    index=$(index_of "$char")

    if [ "$toggle" -eq 0 ] ; then
      toggle=1
      keep="$index"
    else
      toggle=0
      bytesarray+=($(( (keep << 4) | index )))
    fi
  done

  local -i value=0

  for index in {0..7} ; do
    value+=$(( (${bytesarray[$index]} & 255) << ( ( (4 - 1 - index) * 8) & 0x1f ) ))
  done

  printf '%i\n' "$value"
}

printf 'touch your yubikey: '
read -r otp

yubikey_otp_to_serial "$otp"