A Cryptanalytic View of the NSA’s Skipjack Block Cipher Design

Abstract

Skipjack is a block cipher designed by the NSA for use in US government phones, and commercial mobile and wireless products by AT&T. Among its initial implementations in hardware were the Clipper chip and Fortezza PC cards, which have since influenced the private communications market to be compatible with this technology. For instance, the Fortezza card comes in PCMCIA interface and is a very easy plug-n-play device to add on to mobile and wireless systems to provide encryption for wireless transmissions. Initially classified when it was first proposed, Skipjack was declassified in 1998 and sparked numerous security analyses from security researchers worldwide because it provides insight into the state-of-the-art security design techniques used by a highly secretive government intelligence agency such as the NSA. In this paper, commemorating over a decade since Skipjack's public revelation, we revisit the security of Skipjack against cryptanalytic results and discuss why certain attack approaches fare better with reference to Skipjack's design structure.