In cryptography, an attack occurs when the attacker has ciphertext and knows the plaintext. This is best described as which type of attack?

• A. Known Plaintext Attack
• B. Chosen Plaintext Attack
• C. Ciphertext-Only Attack
• D. Brute Force Attack

Answer: (A)

A known-plaintext attack is being described. Here, having both the ciphertext and its corresponding plaintext for some messages lets the attacker analyze how the cipher maps plaintext to ciphertext and use that information to infer the key or decrypt other messages encrypted with the same key. This differs from ciphertext-only scenarios, where only ciphertext is available, and from chosen-plaintext scenarios, where the attacker can obtain ciphertexts for plaintexts they select. It also isn’t brute force, which relies on trying many possible keys without using any known plaintext–ciphertext pairs. Knowing a plaintext–ciphertext pair can reveal patterns or mappings in the cipher, which can then be exploited to uncover the key or accelerate decrypting additional data.