Which of the following algorithms has the smallest key size?

The algorithm with the smallest key size among those listed is Data Encryption Standard (DES). DES uses a fixed key size of 56 bits for encryption, making it relatively small compared to the key sizes used by the other algorithms provided.

Twofish supports key sizes of up to 256 bits, making it significantly larger than DES. RSA, which is an asymmetric encryption algorithm used primarily for secure key exchange, typically employs key sizes of at least 1024 bits, and often 2048 bits or more, for adequate security. AES (Advanced Encryption Standard) can utilize key sizes of 128, 192, or 256 bits, which are also larger than DES's 56-bit key.

Understanding the practical implications of key sizes is important in the realm of cryptography. A smaller key size like that of DES may be easier to crack due to advances in computing power and brute-force attacks, which is part of why DES has largely fallen out of favor in modern applications.