CREST Practitioner Security Analyst (CPSA) Practice

Which algorithm is known for its 128-bit block size and key sizes of up to 256 bits?

1. 3DES

2. RC4

3. AES

4. Blowfish

The correct answer is: AES

The algorithm recognized for its 128-bit block size and key sizes ranging up to 256 bits is the Advanced Encryption Standard (AES). AES was established as a symmetric encryption standard by the National Institute of Standards and Technology (NIST) and has become widely adopted due to its strength and efficiency. It operates using block ciphers, which process fixed-size blocks of data—in this case, 128 bits. One of the key features of AES is its flexibility in key sizes, which can be 128, 192, or 256 bits. The option of using a 256-bit key size offers enhanced security and is particularly useful in environments where higher security levels are necessary. While the other algorithms listed also serve as encryption methods, they either do not match the specified block size or key size or are used in different circumstances. For example, 3DES uses a block size of 64 bits, RC4 is a stream cipher and utilizes variable-size keys but does not work with fixed block sizes, and Blowfish has a block size of 64 bits as well. Therefore, AES stands out as the correct choice based on the specific attributes noted in the question.