SSH, The Secure Shell: The Definitive Guide, 2nd Edition

Algorithms Used by SSH

We now summarize each of the algorithms we have mentioned. Don’t treat these summaries as complete analyses, however. You can’t necessarily extrapolate from characteristics of individual algorithms (positive or negative) to whole systems without considering the other parts. Security is complicated that way.

3.8.1 Public-Key Algorithms

3.8.1.1 Rivest-Shamir-Adleman (RSA)

The Rivest-Shamir-Adleman (RSA) public-key algorithm is the most widely used asymmetric cipher. It derives its security from the difficulty of factoring large integers

Figure 3-4. scp operation

that are the product of two large primes of roughly equal size. Factoring is widely believed to be intractable (i.e., infeasible, admitting no efficient, polynomial-time solution), although this isn’t proven. RSA can be used for both encryption and signatures.

Until September 2000, RSA was claimed to be patented in the U.S. states by Public Key Partners, Inc., a company in which RSA Security, Inc. is a partner. (The algorithm is now in the public domain.) While the patent was in force, PKP claimed that it controlled the use of the RSA algorithm in the U.S., and that the use of unauthorized implementations was illegal. Until the mid-1990s, RSA Security provided a freely available reference implementation, RSAref, with a license allowing educational and broad commercial use (as long as the software itself was ...