Final answer:
Diffie-Hellman is not based on the Feistel cipher structure; it is a key exchange algorithm, unlike Skipjack, CAST-128, and Blowfish, which are block ciphers utilizing the Feistel network.
Step-by-step explanation:
The question asks which algorithm is NOT based on the Feistel cipher. The Feistel cipher is a symmetric structure used in the construction of block ciphers, named after the cryptographer Horst Feistel. Options A, C, and D, namely Skipjack, CAST-128, and Blowfish are all based on the Feistel network or structure. However, option B, Diffie-Hellman, is not a block cipher at all; it is actually a key exchange algorithm used to securely exchange cryptographic keys over a public channel and does not rely on the Feistel structure. Therefore, Diffie-Hellman is NOT based on the Feistel cipher.