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X509 Directories
X.500 is a ITU-T recommendation which defines a
directory (ie, a distributed set of servers) which
maintain a database of information about users.
X.509 defines a framework for authentication services -- usually
as a repository of public key certificates. Such a
certificate contains the public key of a user signed with the
private key of a trusted certificate authority.
The standard does not specify a particular cryptographic
algorithm, although an informative annex of the standard describes
the RSA algorithm.
A certificate must, in general, be purchased from a trusted
Certificate Authority. For some years, the only
authority operating commercially was VeriSign Inc -- in fact, early
versions of Netscape would only accept certificates issued by
VeriSign. There are now several (but still not very many) companies
operating as certificate authorities. Commercial certificates cost
in the range of several hundred $US per annum. It's also possible
to purchase a private certificate, but takeup rates have been low.
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