See also: Signature | Data structure | Distributed hash table

Includes: Digital Signature, Message Digest, or Secure Cryptographic Hash

A unique identifier generated from the bits of a file, used to verify that a file is authentic. Akin to using a fingerprint or retinal scan to verify a person, hashes verify files. Hash systems grants you more security.

Hash files have a variety of uses. Among them:

  • Allow users to digitally “sign” documents, or verify the sender.
  • Prevent the distribution of virus-infected files, making sure that an executable file is authentic without downloading the original file all over again.
  • Check a CD to see if a scratch harmed data saved to disc.
  • Checking the fingerprint in Public-Key Cryptosystem.

A good hash makes it very difficult to know what file a hash was generated from yet easy to validate that a file matches a particular hash. This means a file’s hash can be passed around the Internet instead of the file itself. See hash-link.

Depending on the strength of the hash, it may be used for other purposes, such as dividing objects into buckets for fast lookup (a Hash Table or distributed hash table) or to cause a user to spend compute time before using a resoure (Hash Cash).

Hash types include

Not all hashes are “secure crytographic identifiers.” These are very computationally expensive and time-consuming to generate and overkill for most purposes. However, this is probably the most commonly usage of the term.

Creating a hash

Although there are a host of programs that create and use Hash files, here are a list of items that do it file-by-file. Usually hashes are made of a single, compressed archive to allow the hashing of only one file.

  • MD5sum – One of the easiest and most common, a simple textmode program that generates a short string of code unique to an individual file. This is common for many Internet-distributed files.
  • PGP or GPG – two very similar programs that can create a hash specific to one person or public key. This hash not only verifies data but verifies the sender as well.
  • BitTorrent – as well as being type of file sharing, BitTorrent files can also serve as a hash file.


“For example, the industry disclosed its use of a library of digital fingerprints, called “hashes,” that it said can uniquely identify MP3 music files that had been traded on the Napster service as far back as May 2000

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