Paytm Payment Gateway: Checksum Generator for Python3

paytm_checksum.py

import base64
import string
import random
import hashlib
from Crypto.Cipher import AES


IV = "@@@@&&&&####$$$$"
BLOCK_SIZE = 16


def generate_checksum(param_dict, merchant_key, salt=None):
    params_string = __get_param_string__(param_dict)
    salt = salt if salt else __id_generator__(4)
    final_string = '%s|%s' % (params_string, salt)

    hasher = hashlib.sha256(final_string.encode())
    hash_string = hasher.hexdigest()

    hash_string += salt

    return __encode__(hash_string, IV, merchant_key)


def generate_refund_checksum(param_dict, merchant_key, salt=None):
    for i in param_dict:    
        if("|" in param_dict[i]):
            param_dict = {}
            exit()
    params_string = __get_param_string__(param_dict)
    salt = salt if salt else __id_generator__(4)
    final_string = '%s|%s' % (params_string, salt)

    hasher = hashlib.sha256(final_string.encode())
    hash_string = hasher.hexdigest()

    hash_string += salt

    return __encode__(hash_string, IV, merchant_key)


def generate_checksum_by_str(param_str, merchant_key, salt=None):
    params_string = param_str
    salt = salt if salt else __id_generator__(4)
    final_string = '%s|%s' % (params_string, salt)

    hasher = hashlib.sha256(final_string.encode())
    hash_string = hasher.hexdigest()

    hash_string += salt

    return __encode__(hash_string, IV, merchant_key)


def verify_checksum(param_dict, merchant_key, checksum):
    # Remove checksum
    if 'CHECKSUMHASH' in param_dict:
        param_dict.pop('CHECKSUMHASH')

    # Get salt
    paytm_hash = __decode__(checksum, IV, merchant_key)
    salt = paytm_hash[-4:]
    calculated_checksum = generate_checksum(param_dict, merchant_key, salt=salt)
    return calculated_checksum == checksum


def verify_checksum_by_str(param_str, merchant_key, checksum):
    # Remove checksum
    #if 'CHECKSUMHASH' in param_dict:
        #param_dict.pop('CHECKSUMHASH')

    # Get salt
    paytm_hash = __decode__(checksum, IV, merchant_key)
    salt = paytm_hash[-4:]
    calculated_checksum = generate_checksum_by_str(param_str, merchant_key, salt=salt)
    return calculated_checksum == checksum


def __id_generator__(size=6, chars=string.ascii_uppercase + string.digits + string.ascii_lowercase):
    return ''.join(random.choice(chars) for _ in range(size))


def __get_param_string__(params):
    params_string = []
    for key in sorted(params.keys()):
        if "REFUND" in params[key] or "|" in params[key]:
            respons_dict = {}
            exit()
        value = params[key]
        params_string.append('' if value == 'null' else str(value))
    return '|'.join(params_string)


__pad__ = lambda s: s + (BLOCK_SIZE - len(s) % BLOCK_SIZE) * chr(BLOCK_SIZE - len(s) % BLOCK_SIZE)
__unpad__ = lambda s: s[0:-ord(s[-1])]


def __encode__(to_encode, iv, key):
    # Pad
    to_encode = __pad__(to_encode)
    # Encrypt
    c = AES.new(key.encode('utf-8'), AES.MODE_CBC, iv.encode('utf-8'))
    to_encode = c.encrypt(to_encode.encode('utf-8'))
    # Encode
    to_encode = base64.b64encode(to_encode)
    return to_encode.decode("UTF-8")


def __decode__(to_decode, iv, key):
    # Decode
    to_decode = base64.b64decode(to_decode)
    # Decrypt
    c = AES.new(key.encode('utf-8'), AES.MODE_CBC, iv.encode('utf-8'))
    to_decode = c.decrypt(to_decode)
    if type(to_decode) == bytes:
        # convert bytes array to str.
        to_decode = to_decode.decode()
    # remove pad
    return __unpad__(to_decode)


if __name__ == "__main__":
    params = {
        "MID": "mid",
        "ORDER_ID": "order_id",
        "CUST_ID": "cust_id",
        "TXN_AMOUNT": "1",
        "CHANNEL_ID": "WEB",
        "INDUSTRY_TYPE_ID": "Retail",
        "WEBSITE": "xxxxxxxxxxx"
    }

    print(verify_checksum(
        params, 'xxxxxxxxxxxxxxxx',
        "CD5ndX8VVjlzjWbbYoAtKQIlvtXPypQYOg0Fi2AUYKXZA5XSHiRF0FDj7vQu66S8MHx9NaDZ/uYm3WBOWHf+sDQAmTyxqUipA7i1nILlxrk="))

    # print(generate_checksum(params, "xxxxxxxxxxxxxxxx"))