decode_breakme.py

import binascii
import base64


def __decode_flag(encodedFlag, startChar):
    decodedBytes = bytearray(binascii.unhexlify(encodedFlag))
    print(len(decodedBytes))

    output_bytes = []
    i = 0
    baseChar = base64.b64encode(startChar.encode('utf-8'))[0]
    while i < len(decodedBytes):
        nextItem = decodedBytes[((i + 1) % len(decodedBytes))]
        newChar = baseChar ^ nextItem
        output_bytes.append(newChar)
        i = i + 1
        baseChar = newChar
    print(len(output_bytes))

    b64dencoded = base64.b64decode(bytearray(output_bytes))#, altchars="-_")
    output = b64dencoded.decode('utf-8')

    return output

    print('You are suppose to work a bit')


def main():
    input = b"646a310e2d261f1121013717516236040e01161b183b2a0a0f54095126112f300e1d231b500705690e20103e01163423010e251727775331112f000c27013b200e1d2308390c3d0e2d2616330e1567570f2b3215390a3e202b5a006b"
    alphanums = r'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
    for num in alphanums:
        print("char: "+str(num))
        decoded = __decode_flag(input, num)
        print(decoded)

if __name__ == '__main__':
    main()