Mod11tarkisteet

Mod11tarkisteet is a simple checksum algorithm used primarily in Finland for generating and validating identification numbers. It is a type of check digit calculation, meaning a single digit is appended to a number to detect errors that may occur during manual data entry or transmission. The algorithm is based on a weighted sum of the digits of the number.

The calculation involves multiplying each digit of the base number by a specific weight. These weights are sequential, typically starting from 2 and increasing for each subsequent digit. For example, if the base number is ABCD, the weighted sum would be (2*A) + (3*B) + (4*C) + (5*D). The result of this sum is then divided by 11. The remainder of this division is used to calculate the check digit.

If the remainder is 0, the check digit is 0. If the remainder is greater than 0, the check digit is 11 minus the remainder. In cases where 11 minus the remainder results in 10 or 11, these are often represented by a specific character, such as 'X' for 10, to maintain a single character for the check digit. This process ensures that a modified number will have a different check digit, thus flagging the error.

Mod11tarkisteet is commonly found in various Finnish identification systems, including some older versions of social security numbers and identification numbers for vehicles and businesses. Its simplicity makes it easy to implement and verify manually, contributing to its historical use.

---