Consider the following encryption scheme to provide confidentiality. A and B sha
ID: 3813697 • Letter: C
Question
Consider the following encryption scheme to provide confidentiality. A and B share a secret key K for an encryption scheme E which works with 4-bit blocks. A wants to send an English text message M to B. A is going to encrypt M one character at a time. To encrypt a single character x, we look at the 8 bit ASCII representation of x: b_7, b_6, b_3, b_2, b_1, b_0. The ciphertext C will be C = E_K (b_7, b_3, b_3, b_1) and this what A will transmit to B Is this a good scheme? i. Give a YES/NO answer. ii. If you said NO explain what you think is the single biggest weakness of the scheme.Explanation / Answer
To provide confidentiality the role of secret key is very improtant. In this example A and B shares a secret key K for an encryption seceme E which works with 4 bit block. Now according to this encryption algorithm A is going to encrypt one character at a time. So this algorithm is very slow and very time consuming. Again to encrypt a single character lets say X it will see its corrosponding ASCII value then it will convert this value into 8 bit binary form. Now as the encryption algorithm works with 4 bit block so from 8 bit representation of X we have to choose 4 bit as a encrypted ciphertext. Now this 4 bit is basically permutation of 8 bits of X. Now at this point a broute force attack can be done. Though it takes a lot of time to decode the message but it will do. So there is not much of confidentiality. As the 4 bit of encrypted ciphertext can be obtained using different combinations. That's why it is not a good encryption scheme to provide confidentiality.