#!/usr/bin/env python3
# Copyright (C) 2020 Oscar Benedito <oscar@oscarbenedito.com>
# License: Affero General Public License version 3 or later

# This script simply translates hexadecimal strings into Base64 strings (by
# converting them into bits and then reading the bits as Base64) and the inverse
# process.
#
# The goal of this program is make a password containing only characters in the
# Base64 alphabet and then being able to split the secret between different
# parties using ssss (<http://point-at-infinity.org/ssss/>) with the hexadecimal
# option. With this, an attacker can't get any advantage by discarding unvalid
# answers, since they are all valid (when running the program normally, you can
# get "binary" secrets or "ASCII" secrets).
#
# All this trouble is due to the fact that I am not sure if there is a way for
# an attacker with some shares of the secret to avoid making a brute-force
# attack by knowing the implementation of ssss and anticipate binary (and
# therefore invalid) results.

import sys
from getpass import getpass


def char_to_bits(c):
    n = ord(c)
    if n >= 65 and n <= 90:
        return bin(n - 65)[2:].zfill(6)
    elif n >= 97 and n <= 122:
        return bin(n - 71)[2:].zfill(6)
    elif n >= 48 and n <= 57:
        return bin(n + 4)[2:].zfill(6)
    elif (c == '+'):
        return bin(62)[2:].zfill(6)
    elif (c == '/'):
        return bin(63)[2:].zfill(6)
    else:
        sys.exit('Error, ' + c + ' is not a Base64 character.', file=sys.stderr)


def bits_to_char(s):
    n = int(s, 2)
    if n < 26:
        return chr(n + 65)
    elif n < 52:
        return chr(n + 71)
    elif n < 62:
        return chr(n - 4)
    elif n == 62:
        return '+'
    elif n == 63:
        return '/'
    else:
        sys.exit('Error, ' + s + ' (' + str(n) + ') is not a binary number lower than 64.', file=sys.stderr)


def base64_to_hex(s):
    if len(s) % 2:
        print('WARNING: Number of Base64 characters is not multiple of 2. Adding zeros to string.', file=sys.stderr)
        s = 'A' + s

    ret = ''
    carry = ''
    while len(s) > 0:
        cs = s[:2]
        bs = char_to_bits(s[0]) + char_to_bits(s[1])
        ret += hex(int(bs, 2))[2:].zfill(3)
        s = s[2:]

    return ret


def hex_to_base64(s):
    if len(s) % 3:
        print('WARNING: Number of hexadecimal values is not a multiple of 3. Adding zeros to string.', file=sys.stderr)
        s = '0'*(3 - (len(s) % 3)) + s

    ret = ''
    while len(s) > 0:
        bs = bin(int(s[:3], 16))[2:].zfill(12)
        ret += bits_to_char(bs[:6])
        ret += bits_to_char(bs[6:])
        s = s[3:]

    return ret


if __name__ == '__main__':
    if len(sys.argv) != 2 or (sys.argv[1] != 'base64-to-hex' and sys.argv[1] != 'hex-to-base64'):
        sys.exit('Usage: ' + sys.argv[0] + ' base64-to-hex | hex-to-base64.')

    if sys.argv[1] == 'base64-to-hex':
        inp = getpass(prompt = 'Base64 secret: ')
        print('')
        out = base64_to_hex(inp)
        print('-'*80)
        print('Secret in hexadecimal:', out)
        print('-'*80)

    elif sys.argv[1] == 'hex-to-base64':
        inp = getpass(prompt = 'Hexadecimal secret: ')
        print('')
        out = hex_to_base64(inp)
        print('-'*80)
        print('Secret in Base64:', out)
        print('-'*80)
        if inp[0] == '0' and len(inp) % 2 == 0:
            out = hex_to_base64(inp[1:])
            print('-'*80)
            print('Due to SSSS having an output with an even number of characters, your secret could be:', out)
            print('-'*80)