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Eccouncil 212-81 Exam - Topic 1 Question 71 Discussion

The mode makes a block cipher into a synchronous stream cipher. It generates keystream blocks, which are then XORed with the plaintext blocks to get the ciphertext.
C) Output feedback (OFB)
A) Cipher-block chaining (CBC)
B) Electronic codebook (ECB)
D) Cipher feedback (CFB)

Eccouncil 212-81 Exam - Topic 1 Question 71 Discussion

Actual exam question for Eccouncil's 212-81 exam
Question #: 71
Topic #: 1
[All 212-81 Questions]

The mode makes a block cipher into a synchronous stream cipher. It generates keystream blocks, which are then XORed with the plaintext blocks to get the ciphertext.

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Suggested Answer: C

Output feedback (OFB)

https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Output_feedback_(OFB)

The output feedback (OFB) mode makes a block cipher into a synchronous stream cipher. It generates keystream blocks, which are then XORed with the plaintext blocks to get the ciphertext. Just as with other stream ciphers, flipping a bit in the ciphertext produces a flipped bit in the plaintext at the same location. This property allows many error-correcting codes to function normally even when applied before encryption.

Incorrect answers:

Cipher feedback (CFB)- mode, a close relative of CBC, makes a block cipher into a self-synchronizing stream cipher.

Electronic codebook (ECB)- the simplest of the encryption modes (named after conventional physical codebooks). The message is divided into blocks, and each block is encrypted separately.

Cipher-block chaining (CBC)- Ehrsam, Meyer, Smith and Tuchman invented the cipher block chaining (CBC) mode of operation in 1976. In CBC mode, each block of plaintext is XORed with the previous ciphertext block before being encrypted. This way, each ciphertext block depends on all plaintext blocks processed up to that point. To make each message unique, an initialization vector must be used in the first block.


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Kerry
5 hours ago
That's definitely true for OFB!
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Kasandra
5 days ago
I keep mixing up ECB and CBC, but I think they don't generate keystreams like OFB does. So, maybe it's C?
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Ilona
11 days ago
I practiced a question like this, and I feel like C) and D) are both related to stream ciphers, but I can't recall the exact differences.
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Shelia
16 days ago
I'm not entirely sure, but I remember something about CBC being more about chaining blocks rather than generating a keystream.
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Sherita
2 months ago
I think the answer might be C) Output feedback, since it turns the block cipher into a stream cipher by generating keystream.
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