Final answer:
The cipher mode described in the question is Propagating Cipher Block Chaining (PCBC), used for block encryption and differing from CBC by including an XOR with the previous plaintext block.
Step-by-step explanation:
The cipher mode being described in this question is Propagating Cipher Block Chaining (PCBC). This is a mode of operation for block ciphers, which is similar to the more common Cipher Block Chaining (CBC) mode. However, PCBC differs in that each block of plaintext is XORed with both the previous plaintext block and the previous ciphertext block before encryption. This chaining mechanism ensures that small changes in the plaintext or ciphertext will propagate indefinitely. An initialization vector is used for the first block, much like it is in CBC mode.
In the context of historical ciphers, the Zimmerman Telegram famously used a transposition cipher. Such a cipher rearranges the order of letters in the plaintext. Plaintext is typically divided into blocks, and the same reordering is applied to each block. It can be governed by a keyword which defines the block size and the specific ordering used within each block, as mentioned.