What is defined as an exhaustive attempt to decrypt ciphertext by trying every possible key?

• A. Brute-force attack
• B. Dictionary attack
• C. Man-in-the-middle attack
• D. Replay attack

Answer: (A)

The term that refers to an exhaustive attempt to decrypt ciphertext by trying every possible key is known as a brute-force attack. This method works by systematically checking all possible combinations of keys until the correct one is found, making it one of the most straightforward yet resource-intensive approaches to breaking encryption.

In a brute-force attack, the attacker does not utilize any specific intelligence or understanding of the potential key, relying solely on the sheer computational power to discover the correct key. This can be particularly effective against weak encryption algorithms or those with short key lengths, where the number of combinations is manageable.

Other types of attacks mentioned, such as dictionary attacks, work differently. A dictionary attack utilizes a predefined list of likely passwords or keys rather than trying all possibilities. Similarly, man-in-the-middle attacks and replay attacks involve intercepting or reusing data transmissions without focusing on decrypting ciphertext through key attempts. Thus, the defining characteristics and methodology of brute-force attacks distinctly categorize it as the correct answer to this question.