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An essential component of IT security involves the use of cryptography. In this chapter we will look at the basics of cryptography and provide an overview of a number of different types of cryptography.

== Hashing Algorithms ==

A ''hash'' is a mathematical algorithm designed to perform ''one-way encryption''. When we say ''one-way'' we mean that once the information has been encrypted there is no way to retrieve the original information from the hashed form. Hashing is commonly used in password files and for ensuring the integrity of data. As an example, a hash may be created for an email message in the form of a Message Authentication Code (MAC). When the message is received the receiver would also generate a hash from the message. If the recipient's hash matches the code which accompanied the message the receiver knows the message is authentic and has not been tampered with during transmission.

Once a document has been signed with a digital signature it is essential that a message digest be created using a hash function. This ensures that if the document is modified the change will be detected because the hash will fail verification by the recipient. Asymmetric encryption is also used to verify that the signature was indeed signed by the apparent document sender. To achieve this the hash is typically encrypted using the senders private key. If the recipient is able to decrypt the hash using the senders public key then the message is deemed to be authentic. It will not go unnoticed to those who have read the previous sections that this involves ''authentication'', ''integrity'' and ''nonrepudiation''.

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