<P> In a symmetric key algorithm (e.g., DES and AES), the sender and receiver must have a shared key set up in advance and kept secret from all other parties; the sender uses this key for encryption, and the receiver uses the same key for decryption . In an asymmetric key algorithm (e.g., RSA), there are two separate keys: a public key is published and enables any sender to perform encryption, while a private key is kept secret by the receiver and enables only him to perform correct decryption . </P> <P> Symmetric key ciphers can be divided into block ciphers and stream ciphers . Block ciphers operate on fixed - length groups of bits, called blocks, with an unvarying transformation . Stream ciphers encrypt plaintext digits one at a time on a continuous stream of data and the transformation of successive digits varies during the encryption process . </P> <P> Cryptanalysis is the study of methods for obtaining the meaning of encrypted information, without access to the secret information that is normally required to do so . Typically, this involves knowing how the system works and finding a secret key . Cryptanalysis is also referred to as codebreaking or cracking the code . Ciphertext is generally the easiest part of a cryptosystem to obtain and therefore is an important part of cryptanalysis . Depending on what information is available and what type of cipher is being analyzed, crypanalysts can follow one or more attack models to crack a cipher . </P> <Ul> <Li> Ciphertext - only: the cryptanalyst has access only to a collection of ciphertexts or codetexts </Li> <Li> Known - plaintext: the attacker has a set of ciphertexts to which he knows the corresponding plaintext </Li> <Li> Chosen - plaintext attack: the attacker can obtain the ciphertexts corresponding to an arbitrary set of plaintexts of his own choosing <Ul> <Li> Batch chosen - plaintext attack: where the cryptanalyst chooses all plaintexts before any of them are encrypted . This is often the meaning of an unqualified use of "chosen - plaintext attack". </Li> <Li> Adaptive chosen - plaintext attack: where the cryptanalyst makes a series of interactive queries, choosing subsequent plaintexts based on the information from the previous encryptions . </Li> </Ul> </Li> <Li> Chosen - ciphertext attack: the attacker can obtain the plaintexts corresponding to an arbitrary set of ciphertexts of his own choosing <Ul> <Li> Adaptive chosen - ciphertext attack </Li> <Li> Indifferent chosen - ciphertext attack </Li> </Ul> </Li> <Li> Related - key attack: like a chosen - plaintext attack, except the attacker can obtain ciphertexts encrypted under two different keys . The keys are unknown, but the relationship between them is known; for example, two keys that differ in the one bit . </Li> </Ul>

The process to discover plain text or key is known as