What is the process called wherein the ciphertext block is encrypted and then XOR'd back with the plaintext to produce the current ciphertext block?

The process described is known as Cipher Feedback (CFB). In CFB mode, the encryption of a previous ciphertext block feeds into the encryption algorithm to produce the next ciphertext block. This mode allows the encryption of smaller segments of data than the standard block size used by the encryption algorithm.

Specifically, in CFB, the most recent ciphertext block is used as input to the encryption function, and the output of that function is then XOR'd with the next plaintext block to produce the current ciphertext block. This chaining effect means that each ciphertext block is dependent on the preceding ones, contributing to the security of the encryption.

Other modes mentioned in the options, like Output Feedback (OFB), Electronic Codebook (ECB), and Cipher-block chaining (CBC), employ different mechanisms for processing plaintext and ciphertext and do not utilize the XOR process on the plaintext in the same immediate manner as CFB. For instance, in ECB, each block of plaintext is independently encrypted into its corresponding ciphertext block without any dependency on previous blocks, which can lead to patterns and vulnerabilities. In CBC, while previous ciphertext blocks influence the encryption of the current block, the process does not involve XOR'ing the encrypted output back with the plaintext directly as described.