Key Management - Codes: The Guide to Secrecy from Ancient to Modern Times - page 232

Chapter 6

Key Management

Every time I make an appointment, I create a hundred malcontents and one
ingrate.

King Louis XIV ( 1638 - 1715 )
— from Voltaire Siecle de Louis XIV , 1768 edition

6.1 Authentication, Exchange, and Distribution

Since any (properly implemented) cryptosystem is only as strong as its keys,
we need to be concerned about key management , the secure generation, distri-
bution, and storage of keys. Generation of cryptographic keys is vital in any
cryptosystem. A real-world example, illustrating what can go wrong, is given
by SSL discussed in Section 5.7. In the early days of SSL, implementations
released by Netscape failed due to weak keys . We saw how this was a problem
with DES, and related ciphers, in Section 3.2. We encountered numerous secure
key-generation schemes such as Blowfish in Section 3.4; AES in Section 3.5; RC4
in Section 3.7; RSA in Section 4.2; and ElGamal in Section 4.4. Thus, we have
su7cient illustrations of the mechanisms for doing so.
As for key agreement we saw, on page 180, how an impersonation attack can
be launched, and how Di7e-Hellman key exchange is particularly vulnerable to
this type of fraud. Also, on page 199, we saw how three-pass protocols can be
effective means of authentication. Now we look at a scheme that is considered to
be a three-pass variant of the Di7e-Hellman scheme, which solves the problem
with the original scheme. The following appeared in [71] in 1992. This is an
example of an authenticated key-agreement protocol , which means that the key-
agreement protocol itself, authenticates the parties, in this case, Alice and Bob,
with Trent's help.

Station-to-Station Protocol (STS)
Background Assumptions : In the following, it is assumed that
(sig A , ver A ) and (sig B , ver B ) are Alice and Bob's respective signature and ver-
ification algorithms (see Section 4.3); and that Trent has compiled and made