Final answer:
Symmetric encryption requires the secrecy of the key to be maintained by both the sender and receiver, as exemplified by the Zimmerman Telegram's use of a transposition cipher.
Step-by-step explanation:
For the encryption method where it is critical that the secrecy of the key be maintained by both the sending and receiving parties in order for the encryption to be effective, a symmetric encryption method is used. A famous historical example of symmetric encryption is the Zimmerman Telegram, which utilized a transposition cipher. In this type of cipher, the letters in the plaintext are rearranged according to a rule. Often, the plaintext is divided into blocks, and a keyword can be applied to determine the size of the blocks and the order change in each block. Maintaining the secrecy of the encryption key is essential since having the key would allow others to decrypt the message and compromise confidentiality.