# Checksum

In the information technology one is **checksum** ( English *checksum* ) is a value with which the integrity of data can be checked. Checksums are typically used for data transmission or data backup .

Basically, a checksum is a value that was calculated from the output data and is able to detect at least one bit error in the data. Depending on how complex the calculation rule for the checksum is, several errors can be recognized or corrected .

The calculated checksum is saved or transmitted by the transmitter with the data. The receiver now calculates the checksum from the received data using the same procedure and compares it with the checksum transmitted by the sender:

- if the two checksums differ, there is a transmission error;
- if the two checksums are identical, there is a high probability that the message was transmitted correctly.

## Simple procedures

A simple example of a checksum is the checksum or parity . However, these methods do not recognize, for example, “ rotated numbers ”. Fletcher's checksum is a little more difficult to calculate, but also offers greater security.

Check sum methods that work with a weighted mean value of the data are quite secure against accidental changes, such as character swaps, doublings or omissions. They are used, for example, with the ISBN ( International Standard Book Number ) and the EAN codes.

## More complex procedures

The term “checksum” is also used for more complex test procedures that carry out more complex calculations instead of simply adding up the data values, for example for the cyclical redundancy check (CRC). A cyclic redundancy check uses a polynomial division instead of simple addition and is generally more effective than a primitive checksum at detecting random errors.

Although a conventional checksum is useful to protect against unintentional changes, it does not offer any security against intentional data changes (manipulation), since it is trivial to circumvent. It is therefore often necessary to use cryptographically stronger algorithms, such as one-way hash functions (e.g. the secure hash algorithm ), instead of a simple checksum method. These continue to be the basis for electronic signatures .

## See also

- Checksums are part of most error correction procedures .
- The mathematical model of many checksums is the dihedral group .
- Check digit
- Hash function