A cipher can refer generally to a coded message. It can also refer more specifically to a cryptographic system in which symbols or symbol groups represent a section of text — either single letters or longer — and/or units of text are rearranged according to a set of rules that have been predetermined. The cipher key is the information needed to use the cipher.
Ciphers start with a plain text message, the words that someone wants to conceal. The plaintext is manipulated twice. First, it is enciphered to hide the meaning. The plaintext message is then said to be enciphered or encrypted and may be called ciphertext. Then it is deciphered to reveal the plaintext again. In each case, the cipher key is necessary.
There are several levels of cipher complexity. The most basic cipher has a plaintext alphabet, which is the usual English language alphabet, and a ciphertext alphabet, which is the same alphabet starting on a letter other than a and with the letters that were removed from the front tagged on the back. An early and famous use of a cipher was made by Julius Caesar, who used this encryption method to send messages to his generals.
Plaintext Alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Ciphertext Alphabet: DEFGHIJKLMNOPQRSTUVWXYZABC
A total of 25 different ciphers can be made by shifting the cipher alphabet to start with every letter besides a. But even with that many possibilities, an unauthorized person who get hold of the enciphered message and knows that it is a cipher has a straight, if tedious, path to figuring it out, even without access to the cipher key.
To send a message, one uses the cipher key to determine the ciphertext alphabet. Then for each plaintext letter that one wishes to convey, one writes the ciphertext letter in its place. Thus, if one decided to encipher wiseGEEK, one would end up with:
zlvhJHHN
To decipher the ciphertext, one would use the cipher key to construct the ciphertext alphabet and substitute the plaintext letter for each ciphertext letter.
There are several obvious clues in the cipher itself that help to reveal the message in this type of simple cipher. One is that a ciphertext message will be nonsense, which will be a strong clue that a cipher was used. Second, the letter frequencies that are widely known to exist in English — making E, T, A, O, and N the most common letters — are likely to help reveal some strong clues about the cipher used, if the message is of any length.
Although the simple type of cipher described can be broken and is not in serious use anymore, there are more complex ones. With these ciphers as well as simple ones, the security of the cipher key is essential to the proper transmission of an enciphered message. Failure of the recipient to obtain the key means that the message will go unread. The cipher key falling into the wrong hands destroys the security of the message. For this reason, cipher key management is an essential element of cryptography.