google-authenticator-2fa-totp.sh

#!/bin/bash

# (C) 2017, Mateusz Adamowski
# public domain
#
# (compare with reference JS implementation: http://jsfiddle.net/russau/ch8PK/ )

SECRET32="jbswY3DPEHPK3PXP"

base32_decode(){
    arg="$1"
    result=0
    
    for value in $( echo -n "$arg" | tr a-z A-Z | xxd -ps | tr a-z A-Z | sed -e 's/../\0 /g' )
    do
        value2=$( echo "$value" | sed -e '
            s/41/00/g; s/42/01/g; s/43/02/g; s/44/03/g; 
            s/45/04/g; s/46/05/g; s/47/06/g; s/48/07/g; 
            s/49/08/g; s/4A/09/g; s/4B/0A/g; s/4C/0B/g; 
            s/4D/0C/g; s/4E/0D/g; s/4F/0E/g; s/50/0F/g; 
            s/51/10/g; s/52/11/g; s/53/12/g; s/54/13/g;
            s/55/14/g; s/56/15/g; s/57/16/g; s/58/17/g;
            s/59/18/g; s/5A/19/g; s/32/1A/g; s/33/1B/g;
            s/34/1C/g; s/35/1D/g; s/36/1E/g; s/37/1F/g;' )
        result=$( echo "32 $result * 16 i $value2 + p" | dc )
    done
    resulthex=$( echo "$result 16 o p" | dc )
    echo $resulthex
}

secrethex=$( echo -n 00000000000000000000$( base32_decode "$SECRET32" ) | tail -c 20 )
T=$( echo -n 0000000000000000$( echo "$( date +%s ) 30 / 16 o p" | dc ) | tail -c 16 )
HMAC=$( echo -n "$T" | xxd -ps -r | openssl dgst -sha1 -mac HMAC -macopt "hexkey:$secrethex" -binary | xxd -ps )
offsethex=$( echo -n $HMAC | tail -c 1 )
offsetdec=$( echo "10 o 16 i $( echo -n $offsethex | tr a-f A-F ) p" | dc )
offsetdouble=$(( $offsetdec * 2 ))
codebasehex=$( echo $HMAC | sed -e "s/^.\{$offsetdouble\}\(........\).*$/\1/" | tr a-f A-F )
codebasedec=$( echo "10 o 16 i $codebasehex p" | dc )
code=$(( ( 0x$codebasehex & 0x7FFFFFFF ) % 1000000 ))
#code2=$( echo "$codebasedec 2147483648 % 1000000 % p" | dc )

echo "Secret: $SECRET32"
echo "Secrethex: $secrethex"
echo "Time: $T"
echo "HMAC: $HMAC"
echo "offsethex: $offsethex"
echo "offsetdec: $offsetdec"
echo "codebasehex: $codebasehex"
echo "codebasedec: $codebasedec"
echo "code: $code"