calculate SteamGuard OTP in shell script (standard TOTP, last step use base26 charset, truncate to 5 chars)

steamguard_otp.sh

#!/bin/bash

tohex() { od -An -t x1 | tr -d '[:space:]'; }
fromhex() { printf %b "$(echo "$1" | sed 's/../\\x&/g')"; }
steamguard_otp() {
	SECRET_HEX=$(base32 -d <<<"$1" | tohex)
	DIGEST_HEX=$(fromhex "$(printf "%016x" "$(($(date +%s)/30))")" | openssl dgst -sha1 -mac HMAC -macopt hexkey:"${SECRET_HEX}" -binary | tohex)
	OFFSET=$((0x${DIGEST_HEX:39:1}))
	X=$((0x${DIGEST_HEX:$((OFFSET*2)):8}&0x7FFFFFFF))

	CHARSET=23456789BCDFGHJKMNPQRTVWXY
	RESULT=''
	while [ ${#RESULT} -lt 5 ]; do
		RESULT+=${CHARSET:$((X%26)):1}
		X=$((X/26))
	done
	echo -n "${RESULT}"
}

steamguard_otp "$1"