Which of the following uses a variable-length Initialization Vector (IV), where the encrypted data begins?

DES-OFB (Output Feedback) mode uses a fixed-length IV equal to the block size and generates a keystream independently of the plaintext, with no concept of a variable-length IV marking the ciphertext start.

DES-ECB (Electronic Codebook) mode does not use an IV at all - each 64-bit block is encrypted independently using only the key.

In CBC mode, the IV is prepended to the ciphertext stream and XORed with the first plaintext block prior to encryption, establishing the starting point of the encrypted data. The IV value is configurable and can be treated as variable-length in implementation terms, ensuring that identical plaintexts encrypted under the same key produce different ciphertexts when different IVs are used.

DES-CFB (Cipher Feedback) mode uses a fixed-length IV equal to the DES block size and feeds ciphertext segments back into the encryption process, but does not support a variable-length IV in its standard definition.