Certified Ethical Hacker (CEH v11) — Question 347

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 CAST-128, which is a symmetric-key block cipher that uses a Feistel network and fits the description provided. The other options, while also being block ciphers, do not match the specific characteristics mentioned, such as the use of a 12- or 16-round Feistel network and the specific S-box structure.