Which mode of encryption generates a keystream using an initialization vector and does not require padding?

The mode of encryption that generates a keystream using an initialization vector and does not require padding is Counter mode (CTR). In CTR mode, a counter value is combined with an initialization vector, and this combination is then encrypted to create a stream of pseudo-random bits known as the keystream. This keystream can be XORed with the plaintext to produce ciphertext.

One of the significant features of CTR mode is that it functions similarly to stream ciphers in that it encrypts data in smaller segments rather than fixed-size blocks. As a result, it doesn't require padding to complete blocks, which is a common requirement for block cipher modes that operate on fixed-length data units.

In contrast, other modes like Cipher Block Chaining (CBC) and Triple DES (3DES) typically require padding because they encrypt entire blocks of fixed size and need to handle plaintext that may not fill these blocks completely. Output Feedback (OFB) also generates a stream of keystream values but functions differently, leading to different characteristics regarding initialization vectors and padding requirements. Thus, CTR's distinct mechanism and flexibility regarding data sizes make it the appropriate answer.