Which method turns a block cipher into a stream cipher by generating a keystream?

The method that accurately turns a block cipher into a stream cipher by generating a keystream is output feedback (OFB). In OFB mode, the block cipher is used to generate a pseudorandom keystream by repeatedly encrypting an initial value (often called an initialization vector or IV). This keystream is then combined with the plaintext by using a bitwise XOR operation, which transforms the block cipher into a stream cipher.

The key to understanding why OFB is the right choice lies in its ability to produce an uninterrupted stream of bits (the keystream) that can be used for encryption, rather than processing fixed-size blocks as traditional block ciphers inherently do. Thanks to this flexibility, OFB can be effectively used in scenarios where data needs to be encrypted in a continuous flow.

Furthermore, both counter mode (CTR) and OFB accomplish similar goals and can also generate keystreams; however, CTR does so by combining the block cipher output with incrementing counter values, which is a distinct operational method. Electronic Codebook (ECB) and Cipher-block chaining (CBC) do not generate keystreams in the same way; rather, they operate directly on data blocks for encryption, leading to different use cases and