Which of the following uses an 80-bit key on 64-bit blocks?

Skipjack is indeed the encryption algorithm that uses an 80-bit key on 64-bit blocks. Developed by the National Security Agency (NSA), it was specifically designed for secure voice and data communication, fitting well into applications that required efficient encryption with a moderate level of security.

The 80-bit key length offers a balance between speed and security, making it suitable for the hardware at the time it was conceptualized. The algorithm processes 64-bit blocks of data, meaning that data is encrypted in segments of that size, which aligns with the specifics of the question.

In contrast to Skipjack, AES utilizes a larger key size (typically 128, 192, or 256 bits) and operates on 128-bit blocks, thereby not fitting the criteria of the question. Similarly, 3DES (Triple DES) operates with a key length of 112 or 168 bits and processes 64-bit blocks, while Blowfish uses key sizes from 32 bits up to 448 bits but also processes 64-bit blocks. These differences highlight why Skipjack is the correct answer, as it uniquely matches the specifications provided in the question.