Ciphertext can also be referred as encrypted text. Before encryption we had plaintext. The term "cipher" is sometimes used as an alternative term for ciphertext. In cryptography, cipher is an algorithm that is applied on the plain text to get the ciphertext. Another name for ciphertext is encrypted or encoded information because it is unreadable by a human or computer without the proper algorithm. The reverse of encryption is decryption. It is the process of turning ciphertext into readable plaintext. Codetext and ciphertext are completely different. Codetext is a result of a code, not a cipher.
The art of cryptography emerged thousands of years ago. Earlier ciphers or algorithms were performed manually and were entirely different from modern algorithms which are generally executed by a machine. There are different types of ciphers such as substitution cipher, transposition cipher, polyalphabetic substitution cipher, permutation cipher, public-key cryptography and public-key cryptography.
Ciphertext substitutes the plaintext. This type of cipher is also known as Caesar cipher and One-time Pad Cipher.
This ciphertext is a permutation of the plaintext. It is also known as Rail Fence Cipher.
A substitution cipher using multiple substitution alphabets is also known as Vigenère Cipher and Enigma Machine.
A transposition cipher is a type in which the key to decryption is a permutation.
Modern ciphers are much more secure than classical ciphers and can withstand a wide range of attacks. The cipher is designed in such a way that an attacker cannot crack the key, even if he is aware of the plaintext and corresponding ciphertext. Private-key cryptography is also known as "symmetric key algorithm". Here the same key is used for encryption as well as decryption. The receiver and the sender must have a pre-shared key. The key is kept secret from all other parties; this key is used by for encryption by the sender, and the same key is used by the receiver for decryption. An example for this type of cipher is DES and AES algorithms.
It is also known as asymmetric key algorithm. Here two different keys are used for encryption and decryption. There are 2 distinct keys: public key and private key. The public key is published and hence is possible for any sender to perform encryption, whereas, the private key is kept secret or hidden from the receiver. Example for this type of cipher is the RSA algorithm.
Cryptanalysis is the study of obtaining encrypted information, without accessing to the secret information. This method involves knowing how the system works and finding the secret key. Cryptanalysis is also referred to as code breaking or cracking the code. Converting plaintext to ciphertext is the easiest and the most significant part of cryptanalysis. Depending on the available information and the type of cipher is being analyzed, crypanalysts can follow various attack models to crack a cipher. The various attack models are –