CertiGuide to Security+  9  Chapter 4:  Basics of Cryptography (Domain 4.0; 15%)       9  4.1  Algorithms

4.1  Algorithms 1 Message Digest as a Message Fingerprint

Hashing is the process of creating a long alphanumeric string, called a hash value or “message digest”, which functions as a relatively unique identifier of that file.

The idea behind hashing is to run a large set of data, like a document, through an algorithm, and get a result -- a small piece of data that somewhat-uniquely identifies the contents of the original document. This hash value, or sum, can then be used as a smaller, easier to handle unique identifier of the document. Anyone with a copy of a document whose author computed a hash value for it can use the same algorithm to compute a hash value on their copy, to ensure that they have the same version of the document as the author. Changing even one character of the document will change the resulting hash value in an unpredictable way, so if someone computes a hash value for a document and gets a different result than the originator did, they’ll know the document has been tampered with.

The most popular type of hashing is the MD5 “message digest” (another term for “hashing”) function developed by Ronald Rivest, so named because it creates a 128-bit “digest” (smaller or summarized) version of a message. The footnote takes you to a freeware version know as MD5Sums³⁸⁸. Another popular hashing algorithm is SHA-1, the Secure Hash Algorithm (revision 1), which was developed by NIST and later standardized by ANSI. SHA-1 produces a 160-bit “digest” of a message.

4.1  Algorithms 1 Message Digest as a Message Fingerprint