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

The choice indicating Output Feedback (OFB) and Counter Mode (CTR) is correct because both modes effectively turn a block cipher into a stream cipher by enabling the encryption of data in a manner similar to that of stream ciphers.

In OFB mode, the block cipher encrypts an initial value called an initialization vector (IV), generating a keystream that can then be XORed with plaintext to produce ciphertext. This allows for the processing of data in a continuous stream, as the output of the block cipher is independent of the plaintext, making it suitable for scenarios where data needs to be streamed, such as in real-time communications.

Similarly, in CTR mode, a counter value is encrypted by the block cipher, which results in another keystream. The counter is incremented for each block of plaintext that needs encryption. This method allows for simultaneous encryption and decryption, making CTR mode efficient and conducive to parallel processing.

The other options involve modes that either do not convert a block cipher into a stream cipher or serve different purposes in encryption. For example, ECB mode is not suitable for stream cipher conversion due to its lack of diffusion properties and the potential for revealing patterns in plaintext. Likewise, while PCBC is a block cipher mode, it does