Which of the following is not an asymmetric system?

DES (Data Encryption Standard) is the correct answer as it is not an asymmetric encryption system; rather, it is a symmetric encryption algorithm. In symmetric encryption, both the sender and receiver use the same key for encryption and decryption of the data, which means that key management can be more challenging, particularly for secure communication between multiple parties.

In contrast, SSL (Secure Sockets Layer), PGP (Pretty Good Privacy), and RSA (Rivest-Shamir-Adleman) all involve asymmetric encryption methods. Asymmetric encryption, also known as public-key cryptography, utilizes a pair of keys—one public and one private. The public key can be shared with anyone to encrypt data, while the private key is kept secret by the recipient to decode the information. This system enhances security and simplifies key management, especially in scenarios where secure communication between multiple users is required, as it eliminates the need to distribute the private key.

Thus, DES stands out as a symmetric system, making it the appropriate choice for this question.