Which of the following encryption modes has the property to allow many error correcting codes to function normally even when applied before encryption?

Output Feedback (OFB) mode transforms a block cipher into a synchronous stream cipher. The keystream is generated by encrypting an initialization vector (IV) repeatedly, independent of the plaintext and ciphertext. Encryption is performed by XORing the plaintext with this keystream. Consequently, a bit error introduced in a ciphertext block during transmission will only affect the corresponding bit in the decrypted plaintext block. This property of not propagating errors allows error-correcting codes, which are designed to handle a limited number of bit errors, to function effectively after decryption.

B. CFB mode: A single-bit error in a ciphertext block corrupts the corresponding plaintext bit and also garbles the entirety of the next decrypted plaintext block.

C. CBC mode: A single-bit error in a ciphertext block completely randomizes the corresponding plaintext block and inverts the identical bit in the subsequent plaintext block.

D. PCBC mode: This mode is explicitly designed for error propagation; an error in one ciphertext block renders all subsequent plaintext blocks incorrect after decryption.

1. Dworkin

M. (2001). Recommendation for Block Cipher Modes of Operation: Methods and Techniques (NIST Special Publication 800-38A). National Institute of Standards and Technology.

Page 15

Section 6.3 (OFB Mode): "A distinguishing property of the OFB mode is that the corruption of any bit of the ciphertext block only affects the corresponding bit of the decrypted plaintext block." This directly supports the choice of OFB.

Page 13

Section 6.2 (CBC Mode): "The corruption of a ciphertext block Cj affects the decryption of two plaintext blocks

Pj and Pj+1." This explains why CBC is incorrect.

Page 17

Section 6.4 (CFB Mode): "...a 1-bit error in a ciphertext segment Cj affects the decryption of the corresponding plaintext segment Pj and the s succeeding plaintext segments..." This explains why CFB is incorrect.

2. Katz

J.

& Lindell

Y. (2014). Introduction to Modern Cryptography (2nd ed.). Chapman and Hall/CRC.

Page 93

Section 3.6.4: Discusses modes of operation

noting that for OFB mode

"flipping the i-th bit of c results in flipping the i-th bit of the recovered plaintext m

and has no other effect." This confirms the non-propagating nature of errors in OFB.

3. Stallings

W. (2017). Cryptography and Network Security: Principles and Practice (7th ed.). Pearson.

Chapter 6

Section 6.5 (Output Feedback Mode): "An advantage of the OFB method is that bit errors in transmission do not propagate. For example

if a bit error occurs in C1

only the recovered P1 is affected..." This contrasts with CBC and CFB

where errors propagate to subsequent blocks.