Correct Answer: C
Acipher is a cryptographic transformation that operates on characters or bits. In different words, a cipher is defined as a cryptographic algorithm or mathematical function that operates on characters or bits and implements encryption or decryption. In contrast, a code operates with words, phrases and sentences. In a code, a word may be the encipherment of a sentence or phrase. For example, the word SCARF may be the code for the term BEWARE OF DUTCH TRAITOR IN YOUR MIDST.
*a cryptosystem is a set of transformations from a message space to a ciphertext space. This system includes all cryptovariables (keys), plaintexts and ciphertexts associated with the transformation algorithm. The difference between answers a and c is that answer c, the correct answer, refers to the algorithm alone and answer a refers to the algorithm and all plaintexts, ciphertexts and cryptovariables associated with this algorithm.
*cryptanalysis, refers to being able to break the cipher so that the encrypted message can be read. Cryptanalysis may be accomplished by exploiting weaknesses in the cipher or, in some fashion, determining the key. This act of obtaining the plaintext or key from the ciphertext can be used to recover sensitive or classified information and, perhaps, to pass on altered or fake messages in order to deceive the original intended recipient.
*the key or cryptovariable, is used with a particular algorithm to encipher or decipher the plaintext message. By using the key, the algorithm can be publicly known and evaluated for its strength against attack. The key associated with a particular transformation or algorithm can take on many values and the range of all of these possible values is called the keyspace. Ideally, an enciphered plaintext message using a specific algorithm will produce a unique ciphertext message for each different key that is used with that algorithm. The situation in which a plaintext message generates identical
ciphertext messages using the same transformation algorithm, but with different cryptovariables, is called key clustering. Obviously, this is not a desirable situation, since it effectively reduces the number of keys that have to be tried by an attacker in order to recover the plaintext.
