Which cipher from the list below is an "unbalanced" Feistel cipher where blocks of plaintext are split into unequal parts?

The correct answer is associated with the Skipjack cipher, as it is indeed an "unbalanced" Feistel cipher, where the plaintext is divided into unequal parts. In a conventional Feistel cipher, the blocks of plaintext are typically split into two equal halves, but Skipjack deviates from this standard by using a different structure that allows for unequal partitioning of the data. This unique approach can provide distinct advantages in certain encryption scenarios, such as optimizing performance or enhancing security against specific types of attacks.

In contrast, the other options represent different types of ciphers. DES, for instance, is a standard Feistel cipher that splits the data into equal halves for its operation. AES operates on fixed-size blocks using a substitution-permutation network (SPN) structure rather than a Feistel network, employing alternatives to the half-splitting method. Lastly, RC4 is a stream cipher that does not utilize block partitioning in any form, as it processes data in a continuous stream rather than in discrete blocks like the Feistel structure. Thus, Skipjack’s unique method of handling plaintext blocks makes it distinct as an unbalanced Feistel cipher.