CompTIA
CAS-002 · Question #810
CAS-002 Question #810: Real Exam Question with Answer & Explanation
The correct answer is E: Use RC4 with a nonce generated IV. This question tests selection of encryption algorithms and operational modes that minimize performance overhead for streaming video transport across heterogeneous devices.
Question
A security administrator has been asked to select a cryptographic algorithm to meet the criteria of a new application. The application utilizes streaming video that can be viewed both on computers and mobile devices. The application designers have asked that the algorithm support the transport encryption with the lowest possible performance overhead. Which of the following recommendations would BEST meet the needs of the application designers? (Select TWO).
Options
- AUse AES in Electronic Codebook mode
- BUse RC4 in Cipher Block Chaining mode
- CUse RC4 with Fixed IV generation
- DUse AES with cipher text padding
- EUse RC4 with a nonce generated IV
- FUse AES in Counter mode
Explanation
This question tests selection of encryption algorithms and operational modes that minimize performance overhead for streaming video transport across heterogeneous devices.
Common mistakes.
- A. AES in Electronic Codebook mode encrypts each block independently, causing identical plaintext blocks to produce identical ciphertext and leaking patterns in the stream, making it both insecure and unsuitable for streaming media.
- B. RC4 is a stream cipher and is architecturally incompatible with Cipher Block Chaining mode, which is an operation mode defined only for block ciphers - the combination is not a valid cryptographic construction.
- C. A fixed IV with RC4 means the same keystream is generated every session, allowing an attacker who captures two ciphertexts to XOR them and recover plaintext, rendering the encryption broken.
- D. AES with ciphertext padding is characteristic of block cipher modes such as CBC and introduces computational and size overhead that is unnecessary and counterproductive for low-latency streaming transport.
Concept tested. Stream cipher and block cipher mode selection for video transport encryption
Reference. https://csrc.nist.gov/publications/detail/sp/800-38a/final
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