The word cipher has a specific and a general meaning. Cipher is used loosely to refer to any way of encoding a message. More specifically, a cipher is a cryptographic system in which an algorithm, or set of predetermined instructions or procedures, is used to substitute symbols or symbol groups for sections of text in order to make them indecipherable, that is, unreadable. The original material that a cipher is used on is called plaintext. The result, after it is enciphered or encrypted, is ciphertext.
Ciphertext has been used for military operations since the at least the time of Julius Caesar, and today is commonly used for messages sent over the Internet for security purposes. In order to send a message using ciphertext, the message first must be encrypted using a cipher key. When the message arrives at the address to which it was sent, it needs to be deciphered or decrypted to reveal the plaintext. Again, a cipher key is needed.
Caesar’s cipher was comparatively simple. It uses a plaintext alphabet which is just the usual alphabet — in Caesar’s time, it would have been the Latin alphabet; in our time, it can easily be visualized using any modern alphabet. The ciphertext alphabet is created by shifting D to the position usually held by A as the first letter and placing the first three letters at the end of the string like this:
Caesar’s generals simply had to substitute letter for letter in order to decipher what their emperor’s instructions.
Twenty-five different ciphers can be created by shifting the alphabet in this manner, but they are all simple ciphers. With only 25 possibilities, it only takes time to crack the cipher and have access to the ciphertext. In addition, frequently occurring words, like definite articles, could easily be picked out, knowledge of the letter frequencies of the language could be readily employed, and with this type of analysis used, there wouldn’t be a need to try many possibilities to find the right one.
The greater the size of the key, the stronger the cipher. For example, the Data Encryption Standard (DES), which was formerly used for Internet messages, had a key of only 56 bits. The replacement, Advanced Encryption Standard (AES), which was introduced in 2000 and employs the Rijndael algorithm, can use 128-bit, 192-bit, and 256-bit keys. AES is used in Symmetric-Key Encryption, one of the types of encryption used for Internet messages.