In cryptography, a cipher is a series of defined instructions that one can follow in order to encrypt or decrypt a text message. The process of encryption consists of converting clear information into an unreadable or inaccessible version. The original text that can be clearly understood is known as plaintext, while its encrypted form is called ciphertext. Both texts contain the same information, the only difference is that the ciphertext is written in a format that can only be read or accessed by the ones that have the correct decryption mechanism.
Most cipher algorithms involve the use of a particular piece of secret information, which is commonly referred to as a cryptographic key. The encryption scheme varies according to the key model, and ciphers algorithms can be defined as symmetric or asymmetric. Symmetric ciphers use only one key for both encryption and decryption, while asymmetric ciphers make use of different keys for each operation.
Although most modern encryption techniques are done by computers, ciphers were already being used to encode messages since before the ancient Greeks, around 400 BC. The well-known Roman politician Julius Caesar made great use of substitution ciphers, replacing each letter in a message with the letter that was located three places further down the alphabet.
For instance, if we use the same technique to encrypt the word BINANCE, the resulting ciphertext would be ELQDQFH. This message looks meaningless to a third party but can be easily decrypted by the intended recipient since he already knows the shift number employed by the sender to encrypt it. Those are basically what cipher and cryptographic key are about.
