3. Alice is sending a DES encrypted message to Bob. She does the encryption corr

Question

3. Alice is sending a DES encrypted message to Bob. She does the encryption correctly obtaining a ciphertext of length 50 x 64 bits = 3200 bits. During transmission the 65th bit is corrupted and all the other bits are received correctly by Bob (recall from the discussion in class, that there is a difference if the error happens during encryption, or during transmission; in this problem the error is at transmission). What bits (give their rank in the plaintext, such as, for example, bits 73 to 136 – these are made up numbers, meant to only indicate the format of your answer) of the plaintext will be garbled when Bob is doing the decryption if:

a. Alice is using the ECB mode.

b. Alice is using CBC.

c. Alice is using CFB, the 8-bit version.

Explanation / Answer

a) in ECB mode the input text is divided into equal blocks and each block is encrypted in the exact same manner. So the error in one block will result in garbling up of only one bit. That is 65th bit will be garbled up during decryption.

b) for CBC, each block of plaintext is XORed with previous block, so each block depends on all previous blocks, since 65th bit is corrupted, and since block size is 64, 1st bit of second block is corrupted, so in final text, 1st bit of every block except the first block will be corrupted, ie 65, 64*2 + 1, 64*3 + 1, 64*4 + 1 ....for all blocks.

c) for 8-bit version of CFB, its the same as above, except that the block size is 8 bits, since 65th bit is corrupted, 1st bit of 9th block is corrupted first (since each block is 8 bits, 65 is in 9th block), and again, subsequent blocks have 1st bit corrupted, ie 65, 8*9 + 1, 8*10 + 1, 8*11 + 1 ...

Related Questions

Navigate

• Browse (All)
• Browse 3
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.