Feature Request: Plugin Model for authorizing public keys

[Mark Cavage]

I would like to implement a feature whereby users can write their own
plugins for authorizing use of a public key.  I've got a private branch of
this working, but would like feedback before submitting a patch (starting
with whether the community would want this).  Essentially, I've added a line
in sshd_config like:

PubKeyPlugin ~/local/dev/plugin/libsshplugin.so/dylib/.<http://libsshplugin.so/dylib/>
..

Which dlopen's said plugin and looks for a function that checks access for
RSA public keys (function defined below). It would not be a stretch to add
DSA et al., but I didn't want to bother unless this gets blessed. The
function gets invoked in auth2-pubkey only if the authorized_keys file
checks failed.

I did this as the only other functionality like this I saw were things like
the LPK/Fed-SSH patches that assume one is running LDAP; this mechanism
allows a user to run whatever arbitrary things they want to determine what
keys map to what users (or by group, etc.).

Thank you!

/**
 * NAME:
 *     int sshd_user_rsa_key_allowed(RSA *rsa, struct passwd *pwd, const
char *fingerprint)
 *
 * DESCRIPTION:
 *     Determines whether or not the specified key is allowed to
authenticate as the user in pwd
 *
 * INPUTS:
 *      RSA * rsa                      The RSA public key used by the remote
party (signature check has already passed)
 *      struct passwd * pwd       The user record the remote party is
attempting to login as
 *      const char * fingerprint    MD5 fingerprint of the RSA public key
(for convenience)
 *
 * OUTPUTS:
 *     [0]    Not Allowed
 *     [1]    Allowed
 *
 * NOTES:
 *     Developers are expected to link against OpenSSL, and
include<openssl/rsa.h>.  There is no dependency on OpenSSH.
 */
int sshd_user_rsa_key_allowed(RSA *rsa, struct passwd *pwd, const char
*fingerprint);