High marks for encryption algorithm.

The security of an encryption scheme depends in part on the quality of the mathematical procedure, or algorithm, used to scramble digitized speech or text into unintelligible strings of digits. Only recipients with the appropriate "key" should be able to decipher the coded message. Earlier this year, the White House proposed a novel "key-escrow" cryptographic system based on an encryption algorithm developed in secret by the National Security Agency (NSA). This represented the first time that classified encryption technology had been offered for public use (SN: 6/19/93, p.394).

To help allay fears that the secret algorithm, known as SKIPJACK, may contain a loophole or exhibit some other kind of weakness that could undermine the system, NSA gave five cryptography experts a chance to assess the algorithm's quality. "The government's new encryption algorithm is first-rate," concludes computer scientist Dorothy E. Denning of Georgetown University in Washington, D.C., who participated in this independent review of the algorithm.

Incorporated in an integrated-circuit chip placed in a security device attached to a telephone, the algorithm handles digitized speech in 64-bit chunks. In essence, it converts each incoming string 64 ones and zeros into a scrambled sequence of the same length. It also requires the use of an 80-bit key as part of the encryption process.

Starting in late June, each of the five experts independently tested the SKIPJACK algorithm in a variety of ways, looking for potential flaws in the scheme. These tests failed to turn up any weaknesses. Indeed, the algorithm behaves "like a high-quality random-number generator," says Denning.

In a joint report, the five experts concluded that, even with tremendous increases in computer power, there was no significant risk that SKIPJACK could be broken in the next 30 or 40 years by an exhaustive search based on trying every possible key. They also dismissed the possibility that a shortcut method of attack would succeed.

Denning presented the group's findings at a meeting of the Computer System Security and Privacy Advisory Group, held late last month at the National Institute of Standards and Technology in Gaithersburg, Md. The other members of the SKIPJACK review panel are Ernest F. Brickell of Sandia National Laboratories in Albuquerque, N.M., Stephen T. Kent of BBN Communications Corp. in Cambridge, Mass., David P. Maher of AT&T in Andover, Mass., and Walter Tuchman of Amperif Corp. in Chatsworth, Calif.

Because SKIPJACK is just one component of a large, complex encryption system, these experts plan to assess the strength of the entire key-escrow scheme as soon as the federal government settles various technical details. "When it's ready, we'll evaluate it," Denning says.