[krbdev.mit.edu #7561] kprop doesn't work with RC4 session key

[Greg Hudson via RT]

If you create a working kprop/kpropd setup and then add 
"permitted_enctypes=arcfour-hmac-md5" to [libdefaults] in krb5.conf, 
running kprop will produce:

    kprop: Message size is incompatible with encryption type while 
encoding database block starting at 0

The error originates from lib/crypto/builtin/enc_provider/rc4.c line 112 
where it checks the size of the cipher state.

The offender here is krb5_auth_con_initivector() and the corresponding 
code in mk_priv.c (line 73, where it processes i_vector).  This code 
assumes that an all-zeros block of the cipher blocksize is a valid 
cipher state, which isn't true for RC4.

Nothing else in our code base uses krb5_auth_con_initivector, so this 
bug has flown under the radar so far.  It has likely been present since 
the introduction of RC4.