Cryptography

diagram showing shift three alphabetic cypher D becomes A and E becomes B
Alphabet shift ciphers are believed to have been used by Julius Caesar over 2,000 years ago.[5] This is an example with k=3. In other words, the letters in the alphabet are shifted three in one direction to encrypt and three in the other direction to decrypt.
The first use of the term cryptograph (as opposed to cryptogram) dates back to the 19th century - it originated in The Gold-Bug, a novel by Edgar Allan Poe.[10]

Until modern times, cryptography referred almost exclusively to encryption, which is the process of converting ordinary information (called plaintext) into unintelligible text (called ciphertext).[11] Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext. A cipher (or cypher) is a pair of algorithms that create the encryption and the reversing decryption. The detailed operation of a cipher is controlled both by the algorithm and in each instance by a "key". The key is a secret (ideally known only to the communicants), usually a short string of characters, which is needed to decrypt the ciphertext. Formally, a "cryptosystem" is the ordered list of elements of finite possible plaintexts, finite possible cyphertexts, finite possible keys, and the encryption and decryption algorithms which correspond to each key. Keys are important both formally and in actual practice, as ciphers without variable keys can be trivially broken with only the knowledge of the cipher used and are therefore useless (or even counter-productive) for most purposes. Historically, ciphers were often used directly for encryption or decryption without additional procedures such as authentication or integrity checks. There are two kinds of cryptosystems: symmetric and asymmetric. In symmetric systems the same key (the secret key) is used to encrypt and decrypt a message. Data manipulation in symmetric systems is faster than asymmetric systems as they generally use shorter key lengths. Asymmetric systems use a public key to encrypt a message and a private key to decrypt it. Use of asymmetric systems enhances the security of communication.[12] Examples of asymmetric systems include RSA (Rivest-Shamir-Adleman), and ECC (Elliptic Curve Cryptography).

Source: http://www.navodayaengg.in/wp-content/uploads/2015/12/4.2-Terminology-in-cryptography.pdf

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