Which encryption method uses an 80 bit key on 64 bit blocks?

The encryption method that uses an 80-bit key on 64-bit blocks is Skipjack. Skipjack was designed by the National Security Agency (NSA) and was used in the Clipper chip. The algorithm encrypts data in 64-bit blocks and operates with an 80-bit key, which includes a 64-bit key for the main encryption process and additional bits for control purposes.

While Twofish and AES are modern encryption algorithms, they utilize different block sizes and key lengths. Twofish operates on 128-bit blocks with key sizes ranging from 128 to 256 bits, while AES also works with 128-bit blocks but supports key lengths of 128, 192, or 256 bits. DES, on the other hand, uses a 56-bit key on 64-bit blocks; it is not associated with the 80-bit key characteristic specified in the question.

Thus, Skipjack uniquely meets the criteria of using an 80-bit key on 64-bit blocks, making it the correct answer.