Re-using Raven's password database

Raven currently provides a usable authentication system for interactive, web-based applications but doesn't support non-interactive web-based applications, nor non-web ones, and this covers many potential uses: Windows/MacOS/Unix logon, IMAP, POP, SMTP, LDAP, WebDAV (and so CalDAV), etc., etc. Since Raven of necessity has a database containing username/password pairs for most people in the University, and most of them know their Raven password, it is tempting to assume that extending it to support these other uses would be simple.

This paper explores various ways in which password-based authentication could be provided and attempts to point out the advantages and drawbacks of each proposal. In reading this, it is important to keep in mind what security properties you might actually want from such a system (hint: the important thing isn't normally that legitimate users have to provide their password before gaining access, it is that people can not realistically gain access using an identity that is not their own - these are not the same thing). It is also important to consider who might be attempting to attack what, and how much we care about it: are we talking about a) a bored University student, b) a tabloid journalist, c) an organised criminal, or d) the American NSA, and are they trying to gain access to a) their mate's photo archive, b) the next heir to the throne's email account, c) the University financial system, or d) everything.

"At first he thought that the whole world had blown up. Then he thought that only the Forest part of it had; and then he thought that only he had..." Piglet, Winnie-The-Pooh, A.A. Milne, Methuen 1926.

Option the first: forget passwords

Stepping back a bit, we could just forget passwords and simply ask people to tell us what their CRSid was. This would even simplify Raven itself:

This would save people from having a password that they had to remember, and would save the UCS from having to issue them. Both of these would be significant advantages. Anyone coud easily set up almost any system 'protected' in this way - the hard part might actually be stopping it from asking for a password.

The obvious downside is that we'd have to trust everyone who can get to a login prompt not to lie (and for many networked services this would mean everyone in the entire world), and so this is rather unrealistic.

Option the second: use a single fixed password

Rather than forgetting passwords completely, we could could use a single, fixed, 'well known' password to protect all accounts.

This would be easy to remember (and easy to recover if you forgot it). It would also be fairly easy to set up a system protected in this way.

The problem is that we would still have to trust everyone who knew the password not to lie. It is fair to assume that a 'secret' known by 55,000 (and rising) people would not stay a secret for long so we'd have to trust quite a lot of people not to lie. It would also be next to impossible ever to change this password. Again, this is probably not a realistic option (though this doesn't actually stop people using it, though usually on a small scale - think of the codes used to arm and disarm intruder alarm systems).

Option the third: distribute a list of user name/plaintext password pairs

We could (in theory, though not currently in practice) extract a list of user names and corresponding plain text passwords from Raven, one for each user, and then distribute this list to every system that needs to authenticate users.

Users could then quote their user name and password, the system could look up the the user name and compare the offered password with the one on the list - if it matches they would get in, if not they wouldn't. Implementing authentication in this way would be fairly easy, and system administrators could post-process the list into whatever form their system actually needed.

This would avoid having to trust most users not to lie - the vast majority of users would only be able to authenticate as themselves. It would still possible for users to give away their password, but that's a feature of password-based authentication that's effectively impossible to avoid.

The list itself, however, would now be a major problem. Anyone with access to it could authenticate as any user on any system using this authentication service. Worse, since many people use the same password in multiple places, anyone with access to the list could probably also forge authentication on entirely unrelated systems.

All of the managers of all of the systems using the authentication service would inherently have access to the list, so we would have to trust them. Even if we assume that none of these administrators would be actively malicious, there would still the danger that they might accidentally or recklessly allow the list to leak (from a hacked server, on a memory stick, on a laptop left on a train, etc.). So the security of each system under this model would still depend on a whole group of people that each system's administrator has no particular reason to trust. In practice the only option would be to severely restrict the systems that could participate, to the point where it could never provide a 'universal' authentication system for the University.

Option the fourth: distribute a list of user name/'crypted' password pairs

Rather than distributing plaintext passwords, we could distribute them 'non-reversibly encrypted' (or 'hashed') - for example using the 'crypt' or 'md5' password format used in Unix password files. In principle this would make it possible to check that a proffered password is correct (by encrypting it and checking the encrypted versions match), without exposing all the passwords on the list.

Unfortunately hashed passwords can be recovered by a 'dictionary attack', in which an attacker generates a dictionary of words and their hashed equivalents and then searches the hashed passwords for matches. Since users have a bad habit of choosing common words (or trivial variations of them) as passwords, such an attack will normally recover a reasonable proportion of passwords. So even with hashed passwords the list would be vulnerable to compromise and would still have to be treated as confidential. Worse, some authentication schemes (particularly those using challenge-response) need access to plaintext passwords, or at least particular hashes of the passwords, and so can't be supported under this sort of scheme.

Option the fifth: central password verification

Even if a list could be made to work distributing it would be difficult, especially if it needs to be done in a timely manner. One solution would be to have a 'central password verification service'. In this model, a system using the authentication service solicits a user name and password for a user and forwards them to the central service which returns a match/no match response. Almost any network protocol could be used for this, but it is common to use LDAP and overload LDAP's 'user login' process to provide authentication. Alternatives include POP, IMAP, RADIUS, and home-grown protocols. Configuring these so that they don't expose plaintext passwords on the network, and so the authentication server can't be spoofed, makes them somewhat more complicated and is a step which is often omitted.

This approach would successfully avoid exposing the list of everyone's plaintext or hashed passwords to system administrators (and potentially others) which significantly reduces the exposure. However everyone logging-in would still have to offer their plaintext password for verification, at which point it would still be vulnerable to a malicious or negligent system administrator. Consider two systems 'A' and 'B', both using such a system and with some users in common. What grounds can system 'A's administrators have for believing that system 'B's administrators will not (accidentally or deliberately) capture, and disclose or use, user name/password pairs that would allow forged logins on system 'A'. What if system 'A' were on the Student Run Computer System web site and system 'B' was the University Financial System, or vice-versa?

There is unfortunately a further problem. If asked for their 'central authentication system password', how could a user know that it is safe to quote it? Passwords are only safe if you don't disclose them, but to use them that is exactly what you have to do. If you disclose them to a malicious system then you've lost. This is the basis of 'phishing' attacks aimed (with significant success) at getting people to disclose the electronic banking or mail system passwords. Given all of the following, which are the bogus sites that is just trying to capture your password?

There is no obvious solution to this.

Option the sixth: Kerberos (or similar)

Kerberos was designed precisely to overcome this problem. It allows a central verification service to assert that a user has correctly quoted a password, and so has authenticated themselves, without the user having to disclose their password to anything other than their local workstation. It uses assorted cryptographic sleights of hand to do this and has numerous other important properties, such as preventing the recipient of an assertion from using that to impersonate the user on any other service.

In principle Raven could be extended to become a Kerberos central verification service (a 'KDC'). But using Kerberos comes at a cost. Firstly the user's local workstation needs Kerberos software installed and configured. Secondly the Kerberos protocol is significantly more complicated than a user name/password exchange, so unless the systems to be protected already support then there are likely to be difficulties. Further Keberos, like all other password systems, assumes that you can trust your local workstation. If it has been compromised, for example with a virus that installs a key logger or by a malicious or inept system administrator, then even Kerberos can not keep password authentication secure.

The only upside is that Microsoft have adopted Kerberos (or at least, a version of Kerberos) for authentication under Windows and so many computer users, especially those participating in a well run Active Domain, may already have valid Kerberos credentials and these may work with some Microsoft software.

Note that some software (for example this 'Kerberos' module for Apache) actually just misuse the initial Kerberos user authentication process by soliciting the user's user name and plaintext password and then seeing if it can authenticate as the user. In this they are just using the Kerberos system as a central password verification service with all the problems that this entails described earlier. Again, the problem of how to enable users to safely detect this seems insurmountable.

Where does Raven fit into this?

The current Ucam WebAuth system used by Raven, and a whole range of similar web-redirect-based systems, depends on the client software in use being a web browsers and on browsers including as standard a fortuitous combination of features:

• Support for HTTP redirects, allowing the client to be instructed to contact the authentication server direct, allowing communication that bypasses the server initiating authentication;
• Support for the https: protocol, providing both security for the user's password on the wire and a way for users to positively confirm that they are communicating with the real authentication server and not an imposter (and so that it is safe to disclose their password); and
• Provision of a user interface which can solicit a user and and password.

Ucam WebAuth has some features in common with Kerberos, but without the need to distribute or configure client software, though its use does require much more investment at the server end than a simple user name/password system. It does, crucially, only require the user to disclose their password to the central authentication server and provides a way for users to easily identify it. As a result it, just, manages to provide a reasonably reliable authentication system using passwords, but of course only in a web environment.