38. cryptography
The art or science encompassing the principles and methods of transforming an intelligible message into one that is unintelligible, and then retransforming that message back to its original form the original intelligible message the transformed message .An algorithm for transforming an intelligible message into one that is unintelligible by transposition and/or substitution methods .

some critical information used by the cipher, known only to the sender & eceiver .the process of converting plaintext to ciphertext using a cipher and a key the process of converting ciphertext back into plaintext using a cipher and a key .the study of principles and methods of transforming an unintelligible message back into an intelligible message without knowledge of the key.
The goal of cryptanalysis is to find some weakness or insecurity in a cryptographic scheme. Cryptanalysis might be undertaken by a hostile attacker, attempting to subvert a system, or by the system's designer (or others) attempting to evaluate whether a system has vulnerabilities. In modern practice, however, quality cryptographic algorithms and protocols usually come with proofs that establish practical security of the system (at least, under clear -- and hopefully reasonable -- assumptions).
In cryptography the term plaintext refers to the actual text, encrypted into ciphertext. A cipher is a method for changing the plaintext into ciphertext. Encryption is the implementation of the cipher. Decryption is the more difficult process of reversing the cipher method, i.e., transforming the ciphertext back into the original text, the plaintext. The entire study of cryptography is known as Cryptology. These terms are useful in describing the many methods of encryption and decryption.