Question

N blocks of plaintext (P1 to PN) are being encrypted in CFB mode into ciphertext (C1 to CN). Assume that two bits of C3 are erroneous (their binary values have flipped). What bits of C3, C4, and C20 are erroneous_____

Answer 1

In CFB mode, two erroneous bits in C3 of the ciphertext will affect the corresponding bits in C4. However, the bits in C20 will remain unchanged.

Step-by-step explanation:

In CFB (Cipher Feedback) mode, the ciphertext is generated by encrypting the previous ciphertext block and then XORing it with the plaintext block.

In this case, two bits of C3 are erroneous. This means that during encryption, two bits of the previous ciphertext block (C2) were incorrect. Let's assume that the correct bits in C3 are B1 and B2, and the incorrect bits are B3 and B4.

Since C3 is XOR'ed with P3 to generate C4, the erroneous bits in C3 (B3 and B4) will affect the corresponding bits in C4. Therefore, the erroneous bits in C4 will be the inverted values of B3 and B4, i.e., ~B3 and ~B4.

As for C20, since the error occurred in C3, the bits in C20 will remain unchanged.