Both __________ produce output that is independent of both the plaintext and the ciphertext. This makes them natural candidates for stream ciphers that encrypt plaintext by XOR one full block at a time.

Question

Both __________ produce output that is independent of both the plaintext and the ciphertext.  This makes them natural candidates for stream ciphers that encrypt plaintext by XOR one full block at a time.

BlackTom AI · Accepted Answer

OFB and CTR both produce keystream blocks that are functions only of the key and an initial value (IV or nonce/counter), not of plaintext or ciphertext.

1) Relevant concepts:
- Block cipher modes: ways to use a block cipher to encrypt variable-length data. Some modes produce a keystream (suitable for XORing with plaintext), others mix plaintext/ciphertext into subsequent blocks.
- Stream-cipher-like modes: Output Feedback (OFB) and Counter (CTR) produce keystream blocks independent of plaintext/ciphertext, making them natural stream-cipher modes.
- Cipher Feedback (CFB), Cipher Block Chaining (CBC), and Electronic Codebook (ECB) behave differently: some depend on ciphertext chaining or are not keystream generations.

2) Step-by-step reasoning and formulas:
- OFB: Let S0 = IV. For i ≥ 1, Si = E_k(S_{i-1}). Keystream block = Si. Encryption: Ci = Pi XOR Si. Si depends only on key k and IV (S0), never on Pi or Ci.
- CTR: For block i, Si = E_k(Nonce || (Counter + i)). Keystream block = Si. Encryption: Ci = Pi XOR Si. Si depends only on key k, nonce and counter value, not on Pi or Ci.

3) Intermediate steps shown:
- OFB example: S0 = IV → S1 = E_k(IV) → C1 = P1 XOR S1; S2 = E_k(S1) → C2 = P2 XOR S2. At no point does P1 or C1 influence S2.
- CTR example: S1 = E_k(Nonce||1) → C1 = P1 XOR S1; S2 = E_k(Nonce||2) → C2 = P2 XOR S2. Counters advance independently of plaintext/ciphertext.

4) Why the correct answer is correct:
Because both OFB and CTR generate keystream blocks solely from repeated encryption of internal state or counters (key + IV/nonce), their outputs are independent of plaintext and ciphertext, satisfying the requirement.

5) Why other options are incorrect:
- "OFB and CFB": CFB uses previous ciphertext as feedback (Si depends on Ci-1), so it depends on ciphertext.
- "CTR and CBC": CBC chains ciphertext or plaintext into subsequent blocks (Ci = E_k(Pi XOR Ci-1)), so output depends on plaintext/ciphertext.
- "CBC and ECB": CBC depends on chaining; ECB encrypts blocks independently but does not produce a keystream for XORing whole blocks and is not a stream mode.

Conclusion: The correct choice is OFB and CTR, matching the provided answer.

Both __________ produce output that is independent of both the plaintext and the ciphertext. This makes them natural candidates for stream ciphers that encrypt plaintext by XOR one full block at a time.单项选择题

类似问题

__________ mode is suitable for parallel operation. Because there is no chaining, multiple blocks can be encrypted or decrypted simultaneously. Unlike CTR mode, this mode includes a nonce as well as a counter.

It is possible to convert a block cipher into a stream cipher using cipher feedback, output feedback and counter modes.

Why is AES encryption in ECB mode considered insecure, despite AES itself being secure?

Question at position 12 Which mode allows full parallelization for both encryption and decryption?CFB - Cipher Feedback ModeCTR - Counter ModeCBC - Cipher Block ChainingOFB - Output Feedback ModeClear my selection

[7.15] In what mode is the AES256 algorithm used for encrypted backups?

What was the world's first widely adopted biodiversity policy?

The aim of the Paris Agreement is to ensure that the earth’s preindustrial temperature is not exceeded by more than º Celsius.

The IPCC's mission is to Blank ______.

更多留学生实用工具

智能学习助手

风格化写作助手

论文查重助手

文献引用助手

课堂转译助手

课堂笔记助手

Quiz搜索助手

学校历年真题

智能刷题助手

智能匹配练习

希望你的学习变得更简单