Legal ways to check the validity of a bank card

[chushpan]

Checking the validity of a bank card can only be done for legitimate and legal purposes, such as developing payment processing software, testing payment systems, or training. Below are legal ways to check the validity of a card that do not violate the law and do not involve using the card data for illegal activities.

1. Checking by the Luhn algorithm​

The Luhn Algorithm is a mathematical method used to check the correctness of a card number. It does not require access to sensitive data and works only with the card number.

How it works:​

• The algorithm checks whether the card number complies with generally accepted standards.
• This is a basic method used by many payment systems for preliminary verification.

Example of implementation:​

If you are developing software or want to check your own card number, you can use the Luhn algorithm. Example in Python:

def luhn_checksum(card_number):
def digits_of(n):
return [int(d) for d in str(n)]
digits = digits_of(card_number)
odd_digits = digits[-1::-2]
even_digits = digits[-2::-2]
checksum = sum(odd_digits)
for d in even_digits:
checksum += sum(digits_of(d * 2))
return checksum % 10

def is_luhn_valid(card_number):
return luhn_checksum(card_number) == 0

# Usage example
card_number = "4539148803436467" # Replace with your card number
if is_luhn_valid(card_number):
print("Card number is valid.")
else:
print("Card number is invalid.")

This code only checks the mathematical correctness of the card number, but does not guarantee that the card exists or is active.

The Luhn algorithm is a mathematical method that is used to check the correctness of a bank card number. It applies to most payment systems, such as Visa, MasterCard, American Express and others.

Luhn algorithm verification steps:​

1. Write down the card number.
   For example: 4539 1488 0343 6467.
2. Turn over the card number
   Write the numbers in reverse order: 7646 3430 8841 9354.
3. Double every other digit
   Starting with the first digit (from the right), double every other digit:
   • 7 → 7 (does not double)
   • 6 → 12 (doubled)
   • 4 → 4 (does not double)
   • 6 → 12 (doubled)
   • Continue this process for all numbers.
4. If the result of doubling is greater than 9, subtract 9.
   If after doubling the number becomes a two-digit number (for example, 12), subtract 9:
   • 12 → 12 - 9 = 3.
   • Example: 7, 3, 4, 3, 3, 9, 7, 9, 8, 7, 3, 9, 9, 1, 5, 4.
5. Add up all the numbers
   Add up all the numbers you got:
   • 7 + 3 + 4 + 3 + 3 + 9 + 7 + 9 + 8 + 7 + 3 + 9 + 9 + 1 + 5 + 4 = 80.
6. Check divisibility by 10
   If the sum is divisible by 10 without a remainder, the card number is considered valid:
   • 80 % 10 = 0 → Card number is valid.

Using online tools​

There are legal online services that allow you to check the validity of a card number using the Luhn algorithm. These tools are useful for developers or testers of payment systems. An example of such a service: Luhn Algorithm Validator

2. Using test cards from payment systems​

Payment systems such as Visa, MasterCard, PayPal, Stripe and others provide test card numbers for developers and testers. These numbers are specially created to check the functionality of payment gateways and are not real.

Examples of test cards:​

• Visa: 4242 4242 4242 4242
• MasterCard: 5555 5555 5555 4444
• American Express: 3782 822463 10005
• Discover: 6011 1111 1111 1117

These numbers can only be used for testing in the payment system sandbox. They will never work for real transactions.

3. Checking BIN/IIN through legal services​

The first 6 digits of the card number are called Bank Identification Number (BIN) or Issuer Identification Number (IIN). They indicate the issuing bank, country of issue, and card type. You can use legitimate online services to check BIN/IIN.

Examples of legal services:​

• Binlist - Free service for checking BIN/IIN information.
• BIN Checker Tools - Provides detailed information about the bank, country and card type.

How it works:​

1. Enter the first 6 digits of the card number.
2. The service will show information about the bank, country of issue and card type.

This is useful for developers or companies who want to ensure that a card is issued in a specific country or bank.

4. Checking through the official bank application​

If you want to check your own card, the safest way is to use your bank's official mobile app. Most banks provide features to check your balance, card status, and transaction history.

How to do it:​

1. Download your bank's official app.
2. Log in using your login and password.
3. Check the card status in the "Cards" or "Accounts" section.

5. Contacting the bank's support service​

If you have any doubts about the validity of your card or believe it may have been compromised, please contact your bank directly. Customer support operators can help you:

• Check the card status.
• Block the card if it is stolen or lost.
• Confirm the authenticity of the card.

How to contact:​

• Call the bank's hotline.
• Write to the support chat via the mobile application or the bank's website.

6. Verification via payment gateways​

If you are a business owner and accept payments through payment gateways (e.g. Stripe, PayPal, Square), you can check the validity of the card through their API. These systems automatically check the card number, expiration date, and CVV code.

Example of verification via Stripe:​

Stripe provides an API to verify payment data. If the card is invalid, the API will return an error.

import stripe

stripe.api_key = "your_stripe_api_key"

try:
token = stripe.Token.create(
card={
"number": "4242424242424242",  # Тестовый номер карты
"exp_month": 12,
"exp_year": 2025,
"cvc": "123",
 },
)
print("Card is valid.")
except stripe.error.CardError as e:
print("Card is invalid:", e)

7. Checking the length of the card number​

Different payment systems have strict rules regarding the length of the card number:

Payment system	Number length	First digits (IIN/BIN)
Visa	16–19	4
MasterCard	16	51–55
American Express	15	34, 37
Discover	16–19	6011, 65
JCB	16–19	35

If the card number does not comply with these rules, it is obviously invalid.

However, be careful when using third-party sites and never enter real card details.

Legality of use​

All of the above methods are legal if used for the following purposes:

• Software development and testing.
• Checking your own cards through official channels.
• Using test cards to test payment systems.

Using these methods to check someone else's cards without the owner's permission or for the purpose of fraud is a criminal offense.

Important note​

If you want to check your card, always use official channels (bank, mobile app, customer support). Do not enter card details on third-party sites or in unverified apps.

The main advice: protect your card details and never share them with third parties.