String attributes feature (project review)

[Nico Williams]

I believe that building an extension data keyed by text feature over
TL data is a great idea.

I'm less sure about the idea that the data must be text as well, but
it's certainly most expedient.  The alternative would be to make
kadmin pluggable, but that'd also mean distributing plugins to the
kadmin clients...

A few comments:

a) the key should be all US-ASCII;
b) the key namespace needs more guidance/definition (e.g.,
feature at domainname, a la SSHv2);
c) should any type information be included for the data part?
d) how should binary data be encoded for storage as text data?  There
are many options, but it'd be nice if there was a single common
recommendation (e.g., base64) and utility functions for it.

Nico
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