MD5 hash values are a 16 byte digital signature. We use them to help prove that a file has not changed in any way since first read. The usual time this is important is if the data is part of a police investigation.
The MD5 algorithm is an industry standard, so that anyone at any time can recreate the hash value from any file. If the value is the same as the original, then it means that not a single bit of the file has been changed. It can therefore verify that any process that has happened to the file since the original hash value was created, and this could include sending it via e-mail or FTP, being compressed, written to a hard disk or CD, has not changed the file.
It is argued that no two documents can ever exist with the same hash value. This may sound a bit grand, but what is certain is that it is virtually impossible to modify a spread sheet, a word document, and image to distort the contents, without the hash value changing.
One big joy of a hash value is that all that is required to prove a file has not been changed is a 16 byte number - typically displayed as a 32 character string - and this is valid for a single byte file, or a 1TB file. There are also many tools for anybody to run on a file, and produce an MD5 value for that file, and therefore produce independent verification.
The hash values are stored in logs that we produce whenever we do any data recovery
CnW Recovery Lewes East Sussex UK