Question

Consider a simple system with 8-bit block size. Assume the encryption (and decryption) to be a simple XOR of the key with the input. In other words, to encrypt x with k we simply perform x⊕k giving y. Similarly, to decrypt y, we perform y⊕k giving x.

You are given the following 16-bit input 1A2Bin hexadecimal. You are provided IV as 9D in hexadecimal. The key to be used (where appropriate) is 7C in hexadecimal. Compute the encrypted output with the following methods. Express your final answer, for each of them, as 4 hexadecimal characters so it is easy to read.

A. ECB

B. CBC

C. OFB

D. CFB

Answer 1

Note: Done accordingly. Please comment for any problem. Please Uprate. Thanks

Solution:

Plaintext:1111000011110000 (F0F0)

IV: 00001111

KEY: 11000111

Encryption: XOR

XOR TABLE:

A	B	R
0	0	0
0	1	1
1	0	1
1	1	0

i) ECB

i.e. Plaintext(each block) XOR Key:

Plaintext:1111000011110000 (F0F0)

KEY: 11000111

Therefore:

Block1	11110000
KEY	11000111
XOR
Ciphertext1	00110111

Block2	11110000
KEY	11000111
XOR
Ciphertext2	00110111

So cipher text= Ciphertext1Ciphertext2

Cipher text=0011011100110111 or in hex (3737)

ii) CBC

Block1	11110000
IV	00001111
XOR
R1	11111111
KEY	11000111
CipherText1	00111000

Block2	11110000
CipherText1	00111000
XOR
R2	11001000
KEY	11000111
CipherText2	00001111

So cipher text=CipherText1CipherText2

Cipher text=0011100000001111 or in hex (380F)

iii)

KEY	11000111
IV	00001111
XOR
R1	11001000
Block1	11110000
CipherText1	00111000

KEY	11000111
R1	11001000
XOR
R2	00001111
Block2	11110000
CipherText2	11111111

So cipher text=CipherText1CipherText2

Cipher text=0011100011111111 or in hex (38FF)

iv)

KEY	11000111
IV	00001111
XOR
R1	11001000
Block1	11110000
CipherText1	00111000

KEY	11000111
CipherText1	00111000
XOR
R2	11111111
Block2	11110000
CipherText2	00000000

So cipher text=CipherText1CipherText2

Cipher text=0011100000000000 or in hex (3800)

v) CTR

Therefore:

KEY	11000111
Counter	11001100
XOR
R1	00001011
Block1	11110000
CipherText1	11111011

KEY	11000111
Counter	11001100
XOR
R2	00001011
Block2	11110000
CipherText2	11111011

So cipher text=CipherText1CipherText2

Cipher text=1111101111111011 or in hex (FBFB)

i hope it helps..

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