Many situations arise when some smaller form of a file must be generated. In situations such as this, hashing algorithms are used. This is then used to validate a particular file. If the hash of a file matches a different hash, the other hash must have come from the same file. The standard hashing algorithm is called MD5.
Security here means that it is reasonably difficult to find two different files with the same hash.
Certificates A certificate is a file which contains certain data about an entity. That data is stored in a particular format that allows many different programs to identify the contents of that information. It could also contain information such as home address, work address, telephone numbers, etc. The Public Key section stored one half of an asymmetric key. Digital Signatures Digital signatures are the combination of most of the above technologies.