What is the method for turning a block cipher into a stream cipher by generating a keystream block?

The method for turning a block cipher into a stream cipher by generating a keystream block is known as Output Feedback (OFB). In this mode, a block cipher is utilized to create a sequence of output bits that can be used as a keystream. The keystream is generated by encrypting an initial value (often called an initialization vector or IV) and feeding the output back into the encryption process to produce the next block of output. This allows the keystream to be derived in a way that can be combined with plaintext to produce ciphertext or vice versa.

OFB operates independently of the plaintext, which means that the same keystream can be applied to different plaintexts without any dependency on them. This characteristic makes it suitable for applications requiring continuous data encryption, as it effectively transforms the block-based approach of a cipher into a more flexible stream format.

In contrast, the other methods listed do not function in the same way. Cipher-block chaining (CBC) and electronic codebook (ECB) are modes of operation that do not yield a continuous keystream but rather process blocks of data with specific chaining or independent attributes. Cipher feedback (CFB) also generates keystream but does so by feeding back portions of the ciphertext