5.1 Upgrading to Triple-DES Encryption Keys

Beginning with the 1.2 release from MIT, Kerberos includes a stronger encryption algorithm called “triple DES” – essentially, three applications of the basic DES encryption algorithm, greatly increasing the resistance to a brute-force search for the key by an attacker. This algorithm is more secure, but encryption is much slower.

Release 1.1 had some support for triple-DES service keys, but with release 1.2 we have added support for user keys and session keys as well. Release 1.0 had very little support for multiple cryptosystems, and some of that software may not function properly in an environment using triple-DES as well as plain DES.

In the 1.3 release from MIT, Kerberos also includes the RC4 encryption alogorithm, a stream cipher symmetric key algorithm developed in 1987 by Ronald Rivest at RSA Data Security. Please note that RC4 is not part of the IETF standard.

Because of the way the MIT Kerberos database is structured, the KDC will assume that a service supports only those encryption types for which keys are found in the database. Thus, if a service has only a single-DES key in the database, the KDC will not issue tickets for that service that use triple-DES or RC4 session keys; it will instead issue only single-DES session keys, even if other services are already capable of using triple-DES or RC4. So if you make sure your application server software is updated before adding a triple-DES or RC4 key for the service, clients should be able to talk to services at all times during the updating process.

Normally, the listed supported_enctypes in kdc.conf are all used when a new key is generated. You can control this with command-line flags to kadmin and kadmin.local. You may want to exclude triple-DES and RC4 by default until you have updated a lot of your application servers, and then change the default to include triple-DES and RC4. We recommend that you always include des-cbc-crc in the default list.