Debian OpenSSL Bugs and Foolscap

As you've probably heard, last week (13 May 2008) the debian project published a security advisory that described how the openssl package has been using a 17-bit random number generator for the last two years. As a result, keys and certificates generated by the affected versions (or used for DSA signatures by affected versions) should be considered compromised and immediately replaced. This affects the last 3 or 4 Ubuntu releases too (Dapper was correct, Edgy was broken). All affected releases now have fixed versions available for download, and obviously everyone using an affected release needs to upgrade immediately.

​http://lists.debian.org/debian-security-announce/2008/msg00152.html

Since the bug affected key generation, keys which were created by the broken versions will remain broken even after openssl itself is upgraded. The Debian and Ubuntu projects have released tools to detect weak ssh keys that were generated by these versions and disable them. Other projects (including Foolscap) use generated keys for various purposes, so the effects of this bug will linger even after openssl has been fixed.

How does this affect Foolscap? The short answer: Weak keys generated by the broken versions of openssl will enable a man-in-the-middle attack, which (if successful) would reveal the private swissnums that protect access to Referenceables. Foolscap users who have created Tubs while they had the broken versions installed should stop using the associated FURLs and generate new certificates/FURLs.

The longer answer: There are three things in Foolscap that use random numbers:

1. TubIDs [COMPROMISED]. These are a hash of the public key that corresponds to a Tub's private key, and are included in FURLs (as the base32 part immediately following the pb:// prefix). This provides protection against a man-in-the-middle attack: when connecting to a FURL, the certificate of the remote end is compared against the TubID in the FURL, and the connection is dropped with extreme prejudice if they do not match. This validation is performed before any secrets are passed over the wire.

2. Swissnums [SAFE]. These are a randomly-generated name for a reference, created when tub.registerReference(obj) is called without a name= argument, and when a Referenceable is sent over the wire without being explicitly registered. They appear at the right-hand end of the FURL, after the last slash. Most Referenceables are given these unguessable names, because that limits access to parties that have been explicitly granted access. (when registerReference() is called with a guessable name, anyone who can guess that name will get reference to the given object). The capability-based security of foolscap Referenceables depends upon these swissnums remaining secret.

3. SSL transport sessions [VULNERABLE]. Foolscap runs over SSL, which uses random numbers to generate the Diffie-Hellman key agreement parameters. An eavesdropper who can guess one of these parameters will be able to determine the session key, and can decrypt the remainder of the conversation.

TubIDs are derived from an SSL certificate that OpenSSL generates on demand, and if the broken version of OpenSSL was used for this purpose, than that certificate will be guessable. (at some point it would be great to write a tool that would generate the 128000-ish TubIDs that can be generated by the broken SSL, to create a blacklist like the ones created by debian/ubuntu for ssh keys).

So if you've generated a TubID with the broken version of openssl, it is pretty easy to guess the corresponding private key. This allows an attacker to mount an undetectable man-in-the-middle attack: if they can convince you to make your TCP connection to their server instead of the right one, then they can impersonate the real server, and they'll get to see the unencrypted conversation (revealing the private swissnums).

Swissnums are generated by calling os.urandom(), which was not affected by the openssl bug. As long as one of the other problems (man-in-the-middle attack) doesn't reveal your swissnums to an attacker, access to Referenceables will remain limited to the parties to whom you've explicitly granted it (by telling them the swissnum, in a FURL).

Foolscap uses SSL for its encrypted transport, and SSL frequently uses Diffie-Hellman key negotiation to establish a session key. This key negotiation process depends upon random numbers, generated by the same buggy RNG that is used to create keys. So if one of the two ends of an encrypted session was using the buggy version of openssl, then an attacker may be able to guess the session key, and thus decrypt the conversation. (note, I'm not a cryptographer, and I'm not as confident about this attack as I am about the others). So like the MitM attack, it seems likely that swissnums used by sessions established during the buggy openssl era may be compromised: an eavesdropper who recorded that encrypted conversation may be able to decrypt it after the fact.

Of course, the whole idea of capabilities is to implement the Principle Of Least Authority: each FURL gives access to a specific Referenceable that will only do some specific limited task. If an MitM/eavesdropper manages to guess your supposedly-unguessable swissnums, they'll still only be able to do whatever the Referenceable will let them do.

So to summarize: if you've been using the buggy openssl libraries, here's what you need to do:

1. upgrade openssl
2. throw out your old FURLs (tell everyone that you've given them to to stop using them): this protects clients from the MitM attack.
3. throw out your old Tub certificates: the certFile= argument to Tub() provides a persistent location for this.. just delete that file. This prevents clients from ever successfully using the old FURLs to connect to your server.
4. restart the Tub, allowing it to create a new certificate
5. regenerate and distribute new FURLs

You can also just upgrade openssl (and keep using your old certs and FURLs) if you're willing to remain vulnerable to man-in-the-middle attacks or eavesdroppers who have recorded your earlier conversations.