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6.2.3 What is Capstone?

Capstone has been the U.S. government's long-term project to develop a set of standards for publicly available cryptography, as authorized by the Computer Security Act of 1987. The primary agencies responsible for Capstone were NIST and the NSA (see Question 6.2.2). The plan called for the elements of Capstone to become official U.S. government standards, in which case both the government itself and all private companies doing business with the government would have been required to use Capstone. However, Capstone is no longer an active development initiative.

There are four major components of Capstone: a bulk data encryption algorithm, a digital signature algorithm, a key exchange protocol, and a hash function. The data encryption algorithm is called Skipjack, often referred to as Clipper (see Question 6.2.4), which was the encryption chip that included the Skipjack algorithm. The digital signature algorithm is DSA (see Section 3.4) and the hash function used is SHA-1 (see Question 3.6.5). The key exchange protocol is not published, but is generally considered to be related to Diffie-Hellman (see Question 3.6.1).

The Skipjack algorithm and the concept of a Law Enforcement Access Field (LEAFs, see Question 7.13) have been accepted as FIPS 185; DSS has been published as FIPS 186, and finally SHS has been published as FIPS 180.

All parts of Capstone were aimed at the 80-bit security level. The symmetric-keys involved were 80 bits long and other aspects of the algorithm suite were designed to withstand an ``80-bit'' attack, that is, an effort equivalent to 280 operations.

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