Now let’s look at a more modern symmetric encryption. Assume that the algorithm

Question

Now let’s look at a more modern symmetric encryption. Assume that the algorithm for this system is to rotate the bits in the message right 4 positions, XOR the bits with the key, and rotate the bits 2 positions to the left. The key is C3

1.Using the key and the above algorithm, decrypt the following message which contains a name (given in hex): 1D 1F 5C 5C 1E 5A. Remember that you’ll have to reverse the order and direction of operations in order to decrypt the name (i.e. Start by rotating 2 positions right). What is name (as it appears exactly in the message)?

2.The answer to Part 1 is the name of a fictional character. Find (and give in upper case) the name of the fictional character who was in love with the character in Part (b). Using the key and the above algorithm, encrypt the answer (in upper case) for Part (2) and give your rsult in hex format (not ASCII).  

Explanation / Answer

1. Cipher text - 1D 1F 5C 5C 1E 5A

Find the binary value of the chipher text - 00011101 00011111 01011100 01011100 00011110 01011010

Decryption

i) Start by rotating cipher text two position right. The resultant binary value is

10000111 01000111 11010111 00010111 00000111 10010110

ii) XOR with the key (C3)

10000111 01000111 11010111 00010111 00000111 10010110

11000011 11000011 11000011 11000011 11000011 11000011

-------------------------------------------------------------------------------------

01000100 10000100 00010100 11010100 11000100 01010101

iii) Rotate the XOR'ed result left 4 positions. The resultant binary value is

01001000 01000001 01001101 01001100 01000101 01010100 - Plain text

Find the ascii value of the plain text - H A M L E T

2. OPHELIA  was in love with HAMLET.

Plain text - OPHELIA

Find the binary value of the plain text - 01001111 01010000 01001000 01000101 01001100 01001001 01000001

Encryption

i) Start by rotating plain text right 4 positions. The resultant binary value is

00010100 11110101 00000100 10000100 01010100 11000100 10010100

ii) XOR with the key (C3)

00010100 11110101 00000100 10000100 01010100 11000100 10010100

11000011 11000011 11000011 11000011 11000011 11000011 11000011

----------------------------------------------------------------------------------------------------

11010111 00110110 11000111 01000111 10010111 00000111 01010111

iii) Rotate the XOR'ed result 2 positions right. The resultant binary value is

11110101 11001101 10110001 11010001 11100101 11000001 11010101 - Cipher text

Find the Hex vale of the cipher text - F5 CD B1 D1 E5 C1 D5

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