Prompted by our colleagues in the Networks team, we will be posting a series of blog entries about the work that we’re currently undertaking to improve the University’s Kerberos infrastructure.
What is Kerberos?
I’m not going to try and explain Kerberos here – it’s been explained elsewhere on the internet far better than I can. I recommend the “Explain like I’m 5” guide. Wikipedia’s description is a bit technical. And MIT have a dialogue about designing a system like Kerberos which is very readable.
The first (public) version of Kerberos was Kerberos 4. The current version is Kerberos 5, which expands on the previous version and improves security, and this is the version we’re using.
Where do we use Kerberos?
Kerberos underpins Oxford’s Single Sign-On stack. Your Oxford username is actually a Kerberos principal. Kerberos underpins WebAuth (which is where most people will be familiar with it) – WebAuth basically implements the ticket part of Kerberos using cookies, and authentication is done against the Kerberos infrastructure.
Kerberos is also used by Nexus (although indirectly; it uses Active Directory which uses Kerberos, but it doesn’t hook directly into Oxford SSO. Instead passwords are synchronized), as well as for cross-realm trusts.
Other things Kerberos is used for in the University include:
- LDAP
- NFS
- AFS
- GSSAPI-protected web services
- SSH
- SMTP
What are some of the advantages of using Kerberos?
Single password
For each account, you only have a single password. Note that you might have more than one account – for example, I might have a ‘ouit0144’ account for normal work, and an ‘ouit0144/itss’ account for carrying out more privileged tasks.
Single sign-on
Kerberos uses a ticket cache, so once you’ve authenticated to the KDC you can keep getting tickets to connect to new services without reauthenticating, as long as you’re within a given time period. (For anyone thinking this sounds like WebAuth, that works in the same way – as I said above, it basically implements that aspect of Kerberos but using cookies.)
Your password never goes over the network
If you’re using kerberized software, you run ‘kinit’ or similar on your desktop. which uses your password generate a key to (symmetrically) encrypt a timestamp, and sends it to the KDC. The KDC validates that, and uses the same key (which it has stored in a database) to encrypt a response. This means that your password never has to go over the network.
What will this work involve?
We’re dividing the work up into a series of individual tasks, both to make it more manageable and to make it easier for us to be confident that our changes aren’t breaking things (and if something does break, we only have a single change to look at, rather than a collection of changes).
In (approximate) order the tasks are:
- Upgrading kdc-admin to new hardware, new software and a different data centre
- Rekeying some “high-value” Kerberos principals (such as krbtgt/OX.AC.UK) to remove single DES
- Dropping single DES from the default list of encryption types offered
- Upgrading slave KDCs to newer software (and possibly new hardware)
- Enable incremental database propagation
- Wait for all user principals to be updated to not have single DES portions
- Disable single DES support entirely
We will be writing further blog posts to explain more these steps more fully.
Why are we doing it?
Single DES has been deprecated for a long time (NIST withdrew it as a standard in 2005), but in 2012 RFC6649 was published explicitly stating that DES should be considered weak. MIT have also updated their Kerberos advice to say that DES is deprecated and should not be used. Given all that, we’re looking to retire single DES across the OX.AC.UK realm.
Rekeying the krbtgt/OX.AC.UK principal will allow easier cross-realm trusts with Windows domains. Currently, while krbtgt/OX.AC.UK supports stronger encryption types than single DES, they’re not supported by Windows. Recent versions of Windows have become increasingly less happy about talking single DES, and rekeying will allow us to add strong encryption types supported by all systems (such as AES256).
kdc-admin is running software that is less supported than we would like, and so we want to take this opportunity to upgrade it to the latest version of Debian. It’s also running a backported and patched version of Kerberos, which was required to get some functionality we needed – functionality that’s built-in in the latest version of Kerberos in Debian, so we won’t have the overhead of maintaining a separate package.
Currently when changes are made to principals they are pushed in bulk from kdc-admin to the slave KDCs. This bulk dump and propagation runs every 5 minutes, and locks the database for a brief period while it takes place. This causes problems for users and systems attempting to carry out actions on those principals (eg setting passwords, changing expiration dates, etc), and requires retrying the action. Moving to incremental propagation should reduce the amount that the database needs to be locked, reducing the number of failures end-users see, and also reducing the number of incidents where we have to get involved to reconcile data. It should also lead to faster password changes.
When are we doing it?
Starting as soon as we can after the end of Trinity term. While we are doing thorough testing, there will need to be downtime of services during maintenance periods while we carry out some of the work, and we would rather avoid that during busy times of the year (such as exam season). We also need to give ITSS enough notice about major changes to allow them to check their systems and make changes if required.
I think my services may require single-DES, what do I do?
Drop us a line at iam@it.ox.ac.uk. We can have a look at the logs and the principals in question and tell you if you are currently using it, and work with you to mitigate the impact of the work.
Another reason for annual password resets
Annual password resets of user principals aren’t just for the reasons listed here – they also allow us to add new encryption types and remove deprecated encryption types from users’ Kerberos principals. If we didn’t have the annual expiry policy in place, we would have to have more disruptive procedures when we needed to modify encryption types, such as forcing mass password expiries – not a move likely to make us popular!
Glossary
This is not by any means a comprehensive list of terms related to Kerberos; however, it should help explain most of the terms mentioned here. See the links in the first section of this blog post for deeper explanation of the terms.
- DES: Data Encryption Standard. A type of encryption, now generally considered weak.
- 3DES: encryption that applies DES three times to each block. Stronger than DES.
- KDC: Key Distribution Centre. Generates Kerberos tickets and authenticates users.
- kadmin: the Kerberos administrative daemon – the software that clients talk to in order to create, delete or modify principals (including password changes)
- MIT: Original developers of Kerberos. There are now more variants available (Heimdal is another open-source implementation, and Windows AD uses its own implementation it under the hood), but in Oxford we use MIT Kerberos.
- Principal: an entry in the authentication database. The most obvious case is usernames – eg ouit0144@OX.AC.UK – but there are other principals too, such as host/webauth1.ox.ac.uk@OX.AC.UK. They are qualified with a realm, and if you don’t have a realm the default realm is assumed (so here, OX.AC.UK).
- Realm: an authentication administrative domain. By convention, it is an upper-cased version of an organization’s DNS name. In Oxford’s case, we use OX.AC.UK. (Note that the realm is case-sensitive.)