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Encryption Types Order in Kerberos

Things I don’t wish to forget the next time I need them.  In a Kerberos Realm with multiple encryption types available the KDC will use the first type in the list that’s compatible with the client.  So let’s say you are adding encryption types to your KDC, of course you need to add new keys to the krbtgt/<REALM> principle.  Once you do so check out the list of keys.  What will the KDC chose to use if things are in this order?

Key: vno 2, DES cbc mode with CRC-32, no salt
Key: vno 2, AES-256 CTS mode with 96-bit SHA-1 HMAC, no salt
Key: vno 2, ArcFour with HMAC/md5, no salt
Key: vno 1, DES cbc mode with CRC-32, no salt

You guested it, des-cbc-crc!  The weakest type.  Even when the AES encryption is compatible with the client.

This is where I began to bang my head on my desk.  The order is, of course, set when you use the change_password command in kadmin or via the kpasswd tool.  The order is read from the supported_enctypes variable in your kdc.conf.  So make sure the encryption types listed for that variable are listed in the order of their strength.

Unfortunately, there’s no way to fix the order.  You need to correct the order in your kdc.conf file, restart the kadmin server, then use the change_password command again (with -keepold) to rekey the krbtgt/<REALM> principle.  Or use the kpasswd tool to update your principle.  Yes, the restart in there is required.

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