Question

Computer science encryption please refer to the chart to help

For the remaining questions, consider a 4-bit block cipher, described in hexadecimal by the following table: Plaintext Ciphertext Plaintext Ciphertext 4 You can think of this as a simple substitution cipher for hexadecimal digits. The table itself serves as the "key"

Answer 1

Answer is as follows :

A)

As we have 4 block message i.e. 0xfeed or we can say that only "feed".

So according to plain text "feed" the corresponding cipher text

" feed " = " 199b " (cipher text for feed in given table)

So as per CBC 199b is converted to binary as follows :

0001 1001 1001 1011

and we have IV of 6 i.e. 0110.

So we need to perform XOR operation

as

0001 1001 1001 1011 $\oplus$ 0110 0110 0110 0110

= 0111 1111 1111 1101 i.e. equal to 7FFD in hexadecimal.

So feed is encrypted to "7ffd"

B)

As we have cipher text 0x0a25 or we can simply "0a25"

For decrypt it first convert it to binary as follows :

0000 1010 0010 0101

and after performing XOR operation with IV 6 (0110)

we get

0000 1010 0010 0101 $\oplus$ 0110 0110 0110 0110

= 0110 1100 0100 0011 i.e. equal to "6C53"

So data after decryption we get 6c53 (Plain text of 0a25)

if there is any query please ask in comments...