[krbdev.mit.edu #1415] subkey fubar description

[Tom Yu via RT]

First, clarification on my message of 20030416 -- (1) and (2) are
swapped in 1.0.x only for rd_safe/rd_priv, not mk_safe/mk_priv.

Conclusions from discussion yesterday with Sam and Ken:

There will be three subkeys stored in the auth_context: "subkey",
"send_subkey", and "recv_subkey" (or similarly named things).

"send_subkey" will be what "local_subkey" is now, and "recv_subkey"
will be what "remote_subkey" is now, except that they won't be set
anymore.  New APIs may be written to allow the application to set
"send_subkey" and "recv_subkey".

On the client side, mk_req and rd_rep will set "subkey" only.
mk_safe/mk_priv will prefer "send_subkey", followed by "subkey".
rd_safe/rd_priv will prefer "recv_subkey", followed by "subkey".

In the future, new APIs will allow a client application to extract the
different subkeys by querying the subkey either after mk_req or after
rd_rep.  mk_req will set "subkey" if requested, and send it in the
AP-REQ message.  rd_rep will read the subkey from the AP-REP message,
and store it in "subkey", possibly overwriting one stored there by
mk_req.

On the server side, rd_req will set "subkey", and mk_rep may
eventually allow an application to set a new "subkey", which will also
be transmitted by mk_rep.