|First published||1998 (declassifed)|
|Key sizes||80 bits|
|Block sizes||64 bits|
|Structure||unbalanced Feistel network|
|Best public cryptanalysis|
|31 rounds are susceptible to impossible differential cryptanalysis.|
In cryptography, Skipjack is a block cipher — an algorithm for encryption — developed by the U.S. National Security Agency (NSA). Initially classified, it was originally intended for use in the controversial Clipper chip. Subsequently, the algorithm was declassified and now provides a unique insight into the cipher designs of a government intelligence agency.
Skipjack was proposed as the encryption algorithm in a US government-sponsored scheme of key escrow, and the cipher was provided for use in the Clipper chip, implemented in tamperproof hardware. Skipjack is used only for encryption; the key escrow is achieved through the use of a separate mechanism known as the Law Enforcement Access Field (LEAF).
The design was initially secret, and was regarded with considerable suspicion by many in the public cryptography community for that reason. It was declassified on 24 June 1998.
To ensure public confidence in the algorithm, several academic researchers from outside the government were called in to evaluate the algorithm (Brickell et al., 1993). The researchers found no problems with either the algorithm itself or the evaluation process. Moreover, their report gave some insight into the (classified) history and development of Skipjack:
Eli Biham and Adi Shamir discovered an attack against 16 of the 32 rounds within one day of declassification, and (with Alex Biryukov) extended this to 31 of the 32 rounds within months using impossible differential cryptanalysis.
Truncated differentials and later a complementation slide attack was published against all 32 rounds of Skipjack cipher. Biham, Shamir and Biryukov's attack continues to be the best cryptanalysis of Skipjack known to the public.
An algorithm named Skipjack forms part of the back-story to Dan Brown's 1998 novel Digital Fortress. In Brown's novel, Skipjack is proposed as the new "public-key encryption standard", along with a back door secretly inserted by the NSA ("a few lines of cunning programming") which would have allowed them to decrypt Skipjack using a secret password and thereby "read the world's email". However, when Skipjack is released for public peer review, a programmer discovers and announces the existence of the back door, effectively ending the chances of the standard being adopted.