Applying policy results in Bad encryption type

[Ken Hornstein]

>We did a server replacement of our master KDC that had been on RHEL7
>for years to finally upgrade to RHEL8.  We did a dump of the database
>prior to the swap, we still have the old server sitting around as
>well.  Principal database is on disk in old db2 style.  Kerberos
>version is 1.18 for RHEL8, RHEL7 version is 1.15.
>
>Everything went smooth, except any attempt to change a password results in:
>
>"change_password: Bad encryption type while changing password for < principal >"
>
>Doesn't matter if it is done over the network or with kadmin.local.

What is the key type of the password history principal?  That is in your
database as kadmin/history at REALM.

If it's something like single-DES, then that's your problem because
the old keys are encrypted in the database with the history key and
"Bad encryption key" is coming from the attempt to check the password
history.  If that's the case then you can change the history key to a
modern algorithm using the command detailed here:

  https://web.mit.edu/kerberos/krb5-latest/doc/admin/database.html#updating-the-history-key

But as detailed there that will invalidate your password history (much
like modprinc -clearpolicy).

In THEORY you could do some mangling on the database dump and try to
re-encrypt the old keys with a new key; when I ran into this situation I
decided that I didn't care THAT much about the old password history and
I didn't bother doing that.

--Ken