Which of the following modes can be used to turn a block cipher into a stream cipher?

The correct answer highlights that both Output Feedback (OFB) and Counter Mode (CTR) are modes of operation that allow a block cipher to function as a stream cipher.

In Output Feedback mode, the block cipher is used to generate a keystream, which is then XORed with the plaintext to create ciphertext. This means that it continuously encrypts outputs based on previous ciphertext blocks, effectively creating a stream of encrypted bits. This method of generating a keystream does not require padding and allows for the processing of plaintext data in a bit-by-bit fashion, akin to a stream cipher.

Counter mode also effectively converts a block cipher into a stream cipher by generating a sequence of counter values that are encrypted with the block cipher. Each block of the counter can be thought of as a unique input to the block cipher, creating a keystream that is then used to XOR with the plaintext. This not only enables efficient data encryption and decryption but also supports random access to encrypted data, which is a beneficial feature in many applications.

The other options are not correct because they either do not provide a way to convert block ciphers to stream ciphers or involve different concepts that do not relate to the operation of stream ciphers. For instance, Electronic