Model of Conventional Cryptosystem
We assume that it is impractical to decrypt a message on the basis of the ciphertext plus knowledge of the encryption/decryption algorithm. In other words, we do not need to keep the algorithm secret; we need to keep only the key secret. This feature of symmetric encryption is what makes it feasible for widespread use. The fact that the algorithm need not be kept secret means that manufacturers can and have developed low-cost chip implementations of data encryption algorithms. These chips are widely available and incorporated into a number of products. With the use of symmetric encryption, the principal security problem is maintaining the secrecy of the key.
Let us take a closer look at the essential elements of a symmetric encryption scheme. A source produces a message in plaintext, X = [ X 1 , X 2 , …, X M ]. The M elements of X are letters in some finite alphabet. Traditionally, the alphabet usually consisted of the 26 capital letters . Nowadays, the binary alphabet {0, 1} is typically used. For encryption, a key of the form K = [ K 1 , K 2 , …, K J ] is generated. If the key is generated at the message source, then it must also be provided to the destination by means of some secure channel. Alternatively, a third party could generate the key and securely deliver it to both source and destination.
With the message X and the encryption key K as input, the encryption algorithm forms the ciphertext Y = [ Y 1 , Y 2 , …, Y N ]. We can write this as
Y = E( K , X )
This notation indicates that Y is produced by using encryption algorithm E as a function of the plaintext X , with the specific function determined by the value of the key K .
The intended receiver, in possession of the key, is able to invert the transformation:
X = D( K , Y )
An opponent, observing Y but not having access to K or X , may attempt to recover X or K or both X and K . It is assumed that the opponent knows the encryption (E) and decryption (D) algorithms. If the opponent is interested in only this particular message, then the focus of the effort is to recover X by generating a plaintext estimate . Often, however, the opponent is interested in being able to read future messages as well, in which case an attempt is made to recover K by generating an estimate.
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