The design of a private key cryptosystem

In order to design a private key cryptosystem using a layered approach, we must first examine (he basic characteristics of a conventional cryptosystem. A very simple cryplosyslcm has plaintext, a key, ciphertext, a deciphering transformation and an enciphering transformation. Plaintext is defined to be the original text in readable form; for example, (he material you arc now reading. The key is defined as a set of symbols that is used repeatedly when performing the transformations in a deterministic way. An enciphering transformation takes the key and the plaintext, performs a specific function, and outputs the ciphertext. Ciphertext is defined to be the result of this transformation. It cannot be read in the same way as this text because it won’t make any sense. In order to do this the ciphertext must be transformed using a deciphering transformation. When this is done properly, the plaintext is recovered in its original form. Let us now construct a simple cryptosystem using these definitions. The following algorithm will be used: define key = K of 64 bits in length define plaintext = message = M of L in length define ciphertext = C define input buffer = I define output buffer = O while ( <= L) /* message not completely processed */ (get next 64 bits of M and put them in a buffer I XOR (he bits in I with the key K to get C put C in 0 put 0 back in the same position (overwrite) in M | With this simple algorithm, we can do both the encryption and the decryption. This is due to the properties of the XOR operator. The property we are taking advantage of is this: a XOR b -> x, x XOR b -> a. But how do we know that the message is ncrypted? Well, just try to read it, it’s that simple! If gibberish comes up on the screen, you know that the file is encrypted. Just run the algorithm