Skipjack (cipher)

Skipjack is a block cipher developed by the U.S. National Security Agency (NSA) and released in 1998. It was initially classified but was later declassified, making its specification publicly available. Skipjack operates on 64-bit blocks of data using an 80-bit key.

The cipher utilizes a complex, Feistel-network structure consisting of 32 rounds. A unique aspect of Skipjack is the use of a keyed table lookup using the "F-table," which is derived from the key. This F-table is a permutation table that's key-dependent and significantly impacts the cipher's security.

Skipjack was designed to be implemented in hardware efficiently and was used in the Clipper chip, a device intended for secure voice and data communication. The Clipper chip's key escrow system, which allowed government access to encrypted communications under specific circumstances, sparked considerable controversy regarding privacy and security.

Although Skipjack was once considered a strong cipher, its security has been the subject of research since its declassification. While no practical attacks have been found that break the full 32-round Skipjack with an 80-bit key, theoretical vulnerabilities and cryptanalytic techniques have been identified that raise concerns about its long-term security, especially when considering the advances in computational power and cryptanalysis. Modern cryptographic systems often favor newer ciphers with larger key sizes and more robust designs.