Tools to check Luhn generated numbers. The Luhn algorithm (also called modulo 10 or mod 10) is a checksum formula for numbers/digits used with credit card or administrative numbers.

Luhn Number Checksum - dCode

Tag(s) : Checksum, Arithmetics

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Luhn's algorithm calculates, from a number (or a sequence of digits), a check key (called checksum), this key is a digit which is dependent on the others.

Luhn makes it possible to check numbers (credit card, SIRET, etc.) thanks to its control key (a digit which makes it possible to check the others digits). If a character is misread or badly written, then Luhn's algorithm will detect this error.

Luhn is known because MasterCard, American Express (AMEX), Visa and all credit cards use it.

__Example:__ `12345674` is a valid card number, `1234567` is the initial number and `4` is the checksum.

__Example:__ If a user enter `13245674` (2 and 3 are switched), then the program calculates the luhn checksum for `1324567` and finds `5` instead of `4` expected, the number is invalid and so the code has been badly typed.

The Luhn algorithm starts by the end of the number, from the last right digit to the first left digit. Multiplying by 2 all digits of even rank. If the double of a digit is equal or superior to 10, replace it by the sum of its digits. Realize the sum $ s $ of all digits found. The control digit $ c $ is equal to $ c = (10 - ( s \mod 10 ) \mod 10) $.

__Example:__ The number `853X`, with `X=0`, the digit to calculate.

Take the digit 3, doubled, 3*2 = 6.

Takes the digit 5, not multiplied by 2

Take the 8, multiplies it by 2: 8*2=16 and 1+6=7 to get 7.

The sum is 6+5+7 = 18. As 18 modulo 10 = 8, one calculated (10 - 8) %10 = 2, 2 is the digit checksum control. So `8532` is valid according to Luhn.

8 | 5 | 3 | 0 |

8*2=16 | stays 5 | 3*2=6 | stays 0 |

1+6=7 | 5 | 6 | 0 |

s=7 | +5 | +6 | +0=18 |

c=10-(18%10)=2 |

The verification algorithm does not allow the detection of certain permutations of digits. This is the case for pairs `09` and `90`: any number containing a `0` replaced by a `9` and a `9` replaced by a `0` has an identical checksum.

__Example:__ The numbers `0123456789` and `9123456780` both have a checksum of value `7`

Another weakness is the failure to detect a double error like `22` from/to `55` or `33` from/to `66` or even `44` from/to `77`.

__Example:__ `001122` and `001155` have the same checksum.

The presence or absence of leading zeros `0` at the beginning of the number does not modify the checksum. It can be an advantage as well as a disadvantage.

__Example:__ `000123` and `123` have the same checksum.

CVC (Card Validation Code) or CVV (Card Validation Value) or *verification codes* are 3-digits located on the back of bank cards. Generated by the banks, Visa and MasterCard have their own algorithm (based on private key) using the personal account number, the expiry date of the card and the service code, this information is then compared by those calculated by the bank. There are even banks that offer to change these numbers at will. These numbers are therefore impossible to compute without knowing the algorithm and the key, and there is no relation to Luhn's algorithm.

The expiration date (expr/expiry date) is not a value dependent on Luhn's algorithm, it is not computable. Its value is written on the front of the card in mm/yy format.

The best practice for generating a gift card code is to generate a random number and associate a checksum with it, such as Luhn's algorithm. The code of the gift card is then stored in a database with its data (money, name, loyalty points, etc.). The use of Luhn makes it possible to ensure that the gift code is well written if it must be typed by a human or read by a machine. A person wanting to find the generation algorithm can not then achieve this, the random number ensuring too low probability of success.

NB: all gift card codes are unfortunately not based on a random generation and if they are deterministic, they are then subject to a security vulnerability allowing reverse engineering and potential generation at will.

No, in the magnetic strip is the information of the credit card completed by a different checksum control: the Longitudinal redundancy check.

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Cite as source (bibliography):

*Luhn Number Checksum* on dCode.fr [online website], retrieved on 2022-08-08,

- Numbers generated by Luhn Algorithm
- Valid Number Generator
- Number with Missing Digits
- What is the Luhn Algorithm? (Definition)
- What is the Luhn Algorithm for?
- How to verify a number with Luhn? (Validity check)
- What are Luhn's algorithm weaknesses?
- How to calculate a CVC/CVC1/CVV1/CVC2/CVV2 code?
- How to calculate the expiry date for a card?
- Can Luhn be used for gift cards codes?
- Is Luhn used for credit cards magnetic stripe?

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