Final answer:
A ciphertext-only attack is what most people mean when they refer to breaking an encryption system, where only the ciphertext is known by the attacker. It is distinguished from other attacks like known-plaintext, chosen-plaintext, and chosen-ciphertext attacks. Historical encryption methods such as the transposition cipher used in the Zimmerman Telegram are an example of how plaintext can be encrypted into ciphertext based on a systematic rule.
Step-by-step explanation:
The correct answer to the student's question is A) Ciphertext-only attack. In a ciphertext-only attack, the attacker attempts to decrypt a message having only observed or intercepted the ciphertext, which is indeed the most challenging form of cryptanalysis because no additional information such as plaintext or encryption keys is available. This contrasts with other types of attacks such as a known-plaintext attack, where the attacker has both the plaintext and its corresponding ciphertext, a chosen-plaintext attack, where the attacker can choose a plaintext and receive its corresponding ciphertext, and a chosen-ciphertext attack, where the attacker can choose ciphertexts to be decrypted to plaintexts.
As part of historical cryptographic methods, the Zimmerman Telegram utilized a form of encryption known as a transposition cipher. In this methodology, the letters in a plaintext are rearranged according to a rule. A common technique is to divide the plaintext into blocks and change the order of the letters in each block. Sometimes a keyword is used to determine both the size of the blocks and the specific change made to the order of the letters within each block.