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

Question

Options

• AOFB mode
• BCFB mode
• CCBC mode
• DPCBC mode

• A
  72% (21)
• B
  7% (2)
• C
  17% (5)
• D
  3% (1)

Explanation

OFB (Output Feedback) mode generates its keystream entirely from the cipher's own output, independent of both plaintext and ciphertext. This means a bit error in the transmitted ciphertext produces exactly one corresponding bit error in the decrypted plaintext - errors don't spread or multiply, so error correcting codes applied before encryption survive decryption intact and can still do their job.

CFB (Cipher Feedback) feeds ciphertext back into the keystream generator, so a single ciphertext bit error corrupts that bit and garbles a full shift-register's worth of subsequent plaintext - ECC can't reliably handle that burst pattern. CBC (Cipher Block Chaining) XORs each ciphertext block into the next decryption, so one corrupted block destroys an entire plaintext block and flips a bit in the following one - two affected regions instead of one. PCBC (Propagating CBC) is even worse: errors cascade through all subsequent blocks, making error correction essentially impossible.

Memory tip: In OFB, the "feedback" never touches the data stream - think of it as an error firewall. If errors can't spread, a pre-applied ECC works normally. Any mode where ciphertext feeds back into decryption (CFB, CBC, PCBC) lets errors propagate, disqualifying it.

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