Checksum and verification

Checksum programs are used to generate a relatively small unique key from files. We can recalculate the key to confirm that a file has not changed. Files may be modified deliberately (adding a new user changes the password file), accidentally (a data read error from a CD-ROM drive), or maliciously (a virus is inserted). Checksums let us verify that a file contains the data we expect it to.

Checksums are used by backup applications to check whether a file has been modified and needs to be backed up.

Most software distributions also have a checksum file available. Even robust protocols such as TCP can allow a file to be modified in transit. Hence, we need to know whether the received file is the original one or not by applying some kind of test.

By comparing the checksum of the file we downloaded with the checksum calculated by the distributer, we can verify that the received file is correct. If the checksum calculated from the original file at the source location matches the one calculated at the destination, the file has been received successfully.

Some system validation suites maintain a checksum of the critical files. If malware modifies a file, we can detect this from the changed checksum.

In this recipe, we will see how to compute checksums to verify the integrity of data.