Cryptography is an activity for hiding information. Cryptography combines computer science, mathematics and engineering in its implementation. Cryptography basically means encryption, involving the coding of information using cyphers and decoding encrypted text back to plain text. Modern cryptography involves the use of sophisticated algorithms implemented in cyphers to encrypt and decrypt text.

Cryptography started long before the introduction of computers. People were able to communicate messages that could only be interpreted by the recipient, who knew the key to the message. Many nations used cryptography to communicate with their troops in time of war. Spies and diplomats also used cryptography to send messages across to only the intended recipients. Cryptography today however does more than just encoding a message. It also applies methods for checking the integrity of the message, authenticating the identity of the recipient, and security of the message.

Older forms of cryptography use simple pen and paper, with the cypher determined by human minds. One common method used was simple transposition of letters of the message. A message "hello" could possibly say "olleh". This however became very easy to decipher introducing the substitution method, where a letter is substituted with another, "hello" could read "ifmmp", by replacing each letter with the one following it in the alphabets. The Caesar cipher, the first cipher that makes use of some form of calculation came to be used. It works like the substitution, but instead of substituting with the next letter, substitution is by a letter which is a number of positions from the one being substituted.

Though these ciphers served their purpose for some time, it takes very little intelligence to be able to decode them. Once a person is able to determine the cypher, the message is decoded, so when computers came, better and complex ways of cryptography were introduced. Ciphers became much more difficult and required more effort to determine. There are many cyphers used today and most of them use keys for encryption which could either be public or private.

The symmetric-key algorithm is one of the modern methods of cryptography. In this, both the sender and recipient share a key, which can be the same or different but related in a way. Symmetric cyphers are widely applied in the block ciphers and stream ciphers. Another modern cryptography implementation is the public-key cryptography. In this, the key used for encryption is the same used for the decryption. This method has a fundamental flaw in managing the keys. Since each pair communicating must have its own set of keys, increasing the number of communicants increases the pair of key, needing complex key management methods.

With the increasing rate at which computer systems are hacked and broken into, cryptography may play a role in combating the menace. One motivation for people who intrude into other people's system is that, they obtain information they need in readily readable form. If every message on computers is encrypted, hacking will no longer be lucrative, as the information obtained will be of no use to whoever stole it.