Which stream cipher uses a variable length key from 1 to 256 bytes?

The correct choice is based on the characteristics of the RC4 stream cipher, which is notable for its use of a variable-length key that can range from 1 to 256 bytes. This flexibility in key length allows users to choose a level of security that corresponds to their particular needs, making it versatile for various applications.

RC4 uses a simple algorithm that operates on byte by byte rather than in fixed block sizes, which is a hallmark of stream ciphers. The variable key length allows for a greater complexity and adaptability in encryption, making it suitable for environments where the security needs might change or require customization.

In contrast, other options have fixed or specific key lengths that do not provide the same range of variability as RC4. For instance, DES uses a specific 56-bit (7-byte) key length, while AES operates with fixed key lengths of 128, 192, or 256 bits. DESx is also based on DES but includes additional transformations for enhanced security, without featuring the same level of key length flexibility as RC4. This makes RC4 distinct in its ability to accommodate varying key sizes for encryption purposes.