POSIX shell implementation of TOTP using OpenSSL

totp.sh

#!/bin/sh

TOTP() {
    TOTP_SECRET="${1:-GEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQ}"
    TOTP_DIGITS="${2:-6}"
    TOTP_STEP="${3:-30}"
    TOTP_TIME="${4:-"$(date '+%s')"}"
    TOTP_COUNTER="$((TOTP_TIME / TOTP_STEP))"
    printf '%016x' "${5:-$TOTP_COUNTER}" | xxd -r -p \
    | openssl dgst -sha1 -hmac "$(printf '%s' "$TOTP_SECRET" | base32 -d)" \
    | sed 's/^.*\([0-9a-f]\{39\}\)\([0-9a-f]\)$/\1\2 0x\2/' \
    | if read -r HMAC POS; then
        OFFSET="$((2 * POS))"
        MODULO="$(printf "1%0${TOTP_DIGITS}d" 0)"
        VALUE="0x$(printf '%s' "$HMAC" | cut -b "$((OFFSET+1))-$((OFFSET+8))")"
        printf "%0${TOTP_DIGITS}d\n" "$(((VALUE & 0x7FFFFFFF) % MODULO))"
    fi
}

TOTP "$(printf '12345678901234567890' | base32)" 8 30 123456789 # 54781240