Certified Ethical Hacker (CEH v12) — Question 302

Harper, a software engineer, is developing an email application. To ensure the confidentiality of email messages, Harper uses a symmetric-key block cipher having a classical 12- or 16-round Feistel network with a block size of 64 bits for encryption, which includes large 8 × 32-bit S-boxes (S1, S2, S3, S4) based on bent functions, modular addition and subtraction, key-dependent rotation, and XOR operations. This cipher also uses a masking key (Km1) and a rotation key (Kr1) for performing its functions.

What is the algorithm employed by Harper to secure the email messages?

Answer options

• A. CAST-128
• B. AES
• C. GOST block cipher
• D. DES

Correct answer: A

Explanation

The correct answer is A, CAST-128, as it is a symmetric-key block cipher that fits the description given, including the use of a Feistel network and large S-boxes. The other options, such as AES and DES, do not match the specific characteristics mentioned, particularly the 12- or 16-round Feistel structure and the specific use of masking and rotation keys.