String attributes feature (project review)

[ghudson@MIT.EDU]

I've written up a proposal for adding string attributes to principals.
It's at:

http://k5wiki.kerberos.org/wiki/Projects/Principal_entry_string_mapping

The immediate use case for this feature is per-principal configuration
for the OTP KDC preauth plugin.

I'll likely commit an implementation in a day or so if there's no
initial negative feedback on the design.