Make a Base64 decoder

A certain Crackme came out with a string encoded with something called custom_base64 ... To get the Flag, you have to decode it to find the original string. What is custom_base64 ... To find the answer, we went to the Amazon hinterland ...

Overview

There is a dictionary in which 000000 to 111111 are replaced with characters in the Base64 mechanism. In normal Base64, a dictionary to which ʻABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789 + /` is applied is used in order. The contents of the custom_base64 function were replaced with the specified dictionary. [Article implementing Base64] In this article, the decoder is implemented in Python with reference to 1.

How Base64 works

Encode

Roughly check the Base64 encoding process. See other sites for details.

Convert the character string (ASCII) you want to change to binary (binary)
Divide the binary into 6 bits
When splitting, the last is less than 6 bits, so add 0 so that it becomes 6 bits.
Convert 6bit to characters using conversion table
Add "=" so that the number of characters is a multiple of 4 in order to output in Base64 by 4 bits each.
Completion of base64 string !!

Decode

Decoding is easy if you understand the encoding mechanism! Basically, just follow the reverse procedure!

Delete the added "="

SG9nZUhvZ2U= → SG9nZUhvZ2U

Convert the characters to binary using the conversion table and connect them.

SG9nZUhvZ2U → 010010 000110 111101 100111 011001 010100 100001 101111 011001 110110 010100

Divide the binary into 8 bits, and delete the 0s added in Encoding 3.

01001000 01101111 01100111 01100101 01001000 01101111 01100111 01100101 00

Convert binary bits to ASCII

01001000 → H
01101111 → o
01100111 → g
01100101 → e
01001000 → H
01101111 → o
01100111 → g
01100101 → e

Decoding is complete!

Try to program

`custom_base64_decoder.py`


import sys
import argparse
BYTE_SIZE = 8

# 000000 ->A function that creates a dictionary-type list character by character up to 111111
def makeDict(base64Dict_seed):
    dictionary = {}

    for i in range(0, 64):
        dictionary[format(i, '06b')] = base64Dict_seed[i]

    return dictionary


#A function that lists the string s separated by n characters
def split(string, n):
    split_list = []

    for i in range(0, len(string), n):
        split_list.append(string[i:i+n])

    return split_list


#If the string does not have n characters, it will be n characters`c`Add
def fillBlank(s, n, c):
    mod = len(s) % n

    if mod == 0:
        return s
    else:
        margin = n - mod
        return s + c * margin


#Passing a dictionary value returns the dictionary key
def getValue(key, items):
    for v in items.items():
        # print(v[1])
        if v[1] == key:
            # print(v)
            return v[0]
    return ''


def main():
    # -You can enter a custom dictionary by adding k
    parser = argparse.ArgumentParser(
    description='custom Base64 Decoder')
    parser.add_argument('-k', '--key', help="Use custom Seed to encrypt in base64 ", \
        default="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/")
    parser.add_argument('text', help='base text')
    args = parser.parse_args()


    # 0.Make a dictionary
    base64Dict = makeDict(args.key)


    # 1. '='Remove
    text = args.text.replace("=", '')

    binStr = ""


    # 2.Use the conversion table to convert characters to binary and connect them.
    for i in text:
        binStr += getValue(i, base64Dict)

    # 3.Divide the binary into 8 bits, encode 3.Since the 0s added by are left over, delete them.
    splitCount = 8
    s = split(binStr, splitCount)

    if (len(s[-1]) != 8):
        s.pop(-1)


    # 4.Convert binary bits to ASCII
    result =""

    for c in s:
        print(c + " → " + chr(int(c, 2)))
        result += chr(int(c, 2))

    print(result)

if __name__ == "__main__":
    main()

How to use

$ python3 customBase64Decoder.py <Base64 text>

$ python3 customBase64Decoder.py -k <Custom dictionary> <Base64 text>

$ python3 customBase64Decoder.py SG9nZUhvZ2U=

$ python3 customBase64Decoder.py -k xEPOKnvADqeG0m1VkZ47CM653jrtbzLsTc2ypoYUSWJ9ludQig+awf8XF/RNHBhI 4vBUjCcQj8C=

HogeHoge

Summary

Base64 I fully understood. With this, you can make an original Base64 and make secret communication, you did it

Sample code can be found on GitHub

--Encoder - https://github.com/itiB/sandpit/blob/master/tools/customBase64Encoder.py --Decoder - https://github.com/itiB/sandpit/blob/master/tools/customBase64Decoder.py

References

What is base64? ?? I implemented it for understanding --qiita https://qiita.com/PlanetMeron/items/2905e2d0aa7fe46a36d4

[PYTHON] Make a Base64 decoder