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"))

Thanks, Ashu for your quick response. I really appreciate it. It's really strange. I use the same Secret Key, Merchant ID for the APIs "Initiate Transaction, GET OTP, Validate OTP", and it works perfectly fine however fails here in this way with "Invalid Checksum". I tried using the new checksum file which Paytm has updated in github ( https://github.com/paytm/Paytm_Python_Checksum) as well. NO Luck though. Also, I verified the generated checksum using verify_checksum, it matched. Well, I used the same keys to generate the checksum and verify, maybe that's the reason it matches at my end. I am just curious If I am missing something to replace from your article. I have also raised a support ticket, let's see what happens. I will keep debugging. Cheers! Deepti Patro

…

On Thu, Jun 18, 2020 at 8:47 PM Ashutosh Chauhan ***@***.***> wrote: ***@***.**** commented on this gist. ------------------------------ Invalid checksum could only mean wrong Secret Key or merchant Id. You can try editing the *main* in this gist. with sample details and verify the checksum is verified. Also don't use the same order id twice, that could be the issue. — You are receiving this because you commented. Reply to this email directly, view it on GitHub <https://gist.github.com/f90b8ba6d4324d0a7d65ab0f2d8306c0#gistcomment-3347169>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AEALHELIQ42VL4DK25W5ZVTRXLNU3ANCNFSM4M7AHYIQ> .

Thanks Ashu for your quick response. I really appreciate it. I figured out that the redirect view has extra quotes in the value i.e. value="{{MID}}". Removing these quotes work fine.

ModuleNotFoundError: No module named 'PayTm'
I got this error ,please help me out.

Use import <filename_without_extension> to import this.
For example if you are using paytm_checksum.py, use import paytm__checksum

@masterashu
Bro i was asking because i wrote
from PayTm import Checksum in views.py
And then i got that error please help me in this way

Make sure you have this file in the same folder. Replace PayTm with the file name(without extension) in the from PayTm import Checksum. Note: python import are case sensitive.

Hey, I am a new user to PayTM Businesses, and it seems they have deprecated this API, and used a new one. Still I am trying to integrate it, Can you let me know if it will work. I am stuck on invalid checksum error when i send the data to staging

@mayankwadhwani Yes, Thanks for pointing that out. I will create a post/sample for the new one and mention it here.

I am getting error at Incorrect AES key length (23 bytes)....i am using windows7 32 bit 4 gb ram...
please help me i am absulute begginer.

ModuleNotFoundError: No module named 'PayTm'
I got the same error ,please help me out.