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signify - sign and verify

One of the things OpenBSD has never done is sign releases, for whatever reasons. But 2014 is a new year, time to make a change. The first thing you need to start signing OS releases (besides the release itself) is a signing tool. Other projects use a variety of tools for this, but unfortunately none of them were invented here. signify is a small tool I wrote to fill that gap. Here’s a few notes about it, working from the top down.

There are only two useful portmanteaus to be made from the words sign and verify. Verisign charges a lot of money, so we went with the free option.

The UI/UX is pretty spartan. It does lots of error checking, but makes no effort at error recovery. Early versions of signify refused to overwrite existing files, based on the theory that we’re going to stick this in a careful workflow and not make mistakes; i.e., overwriting a signature file by signing the same file twice indicates the workflow has failed. This was later changed, based on feedback from the poor souls who had to use the thing.

The signify file format is also really simple. At its core, it consists of a two byte algorithm identifier (“Ed“) and the signature or key data, plus a bit more data in secret keys to support encrypting them. The core data is base64 encoded (so you don’t poke your eye out) and appended after a one line comment. The comment is ignored and just there so you can tell the files apart if you accidentally rename them.

Initially, I resisted adding identifiers to the keys. One way signature schemes break is user error, as in the evil people trick the user into believing their evil key is legit. By omitting identifying information, you can’t be tricked into believing a key is OpenBSD official just because it says “OpenBSD official”. Does this matter? Probably not, but since signify itself would never use such identifiers for verification, there wasn’t any reason to add them. Soon after we started using signify however, we had too many keys running around and so a compromise was reached: keys have 64-bit random fingerprints to enable differentiation between the data being compromised and the wrong key being used. (I think we would have been ok without the identifiers eventually; it was only that first week with people experimenting and also the version changing from 5.4 to 5.5 when we were drowning in keys. Once more of the process was worked out, things settled down and key mismatches went away.)

Moving on to the fun crypto bits, we wanted a tool that would fit on installation media, which meant minimizing code size and external dependencies.

-rw-r--r--  1 anonftp  wheel    1474560 Jul 30 12:27 floppy54.fs
-rw-r--r--  1 anonftp  wheel    2728452 Jul 22 18:40 gnupg-2.0.19p3.tgz

The one and only supported algorithm is Ed25519. It has a lot of very nice properties, though I really like the deterministic signatures. Anything that makes it harder to screw up is great. The implementation, currently built by reaching over into the ssh directory, is small and independent with a simple interface that’s hard to screw up.

Ed25519 keys are really small. This makes them easy to work with. Public keys, in particular, are only 32 bytes. With some additional data signify adds and base64 encoded, the relevant figure is still 56 bytes, small enough to fit on a tshirt or even the back of your hand. Compare that to djm’s new PGP key which is more than twice the size of all the text on this page.

I did run into one downside in the Ed25519 implementation interface. It signs messages by creating a complete copy of the message. Verification creates another complete copy of the message. The rationale for this decision is pretty obvious: You sign a message and get back a (theoretically opaque) string which you transmit; then the receiver verifies it to get back the original message. You can’t accidentally use a message that fails verification because they are transmitted as part of the same bundle. Unfortunately, this design works better for small network packets than large files. We would prefer to keep the signatures detached from the message (this means it’s now our responsibility to not accidentally use an unverified message.) signify deals with this internally by creating lots of copies. In the future we may open up the implementation for surgery, but that voids the manufacturer’s warranty, so for now we’re using the code exactly as it is with the workarounds in our code.

Thanks to espie@, the pkg_add tools now use signify instead of the openssl command. The OpenBSD installer scripts are also done thanks to halex@. Some refining work is still in progress. Verifying sets requires enough disk space to store the sets, which means tiny 1GB CF installs may not immediately benefit. There is a workaround, which is to download the sets ahead of time (which can even be done in the installer itself by escaping to shell via !) and choose the option to install from disk. And after your upgrade is done, sysmerge will verify things as well, thanks to rpe@ and ajacoutot@.

Where do you get public keys from? The current and next releases’ keys are in /etc/signify. Once you have OpenBSD installed, you should have all the pieces you need to securely upgrade to the next version. (The keys themselves will not be signed, but there is an implicit trust chain from one release to the next via the etcXX.tgz sets.) If you don’t have OpenBSD installed, you need to get the public key yourself somehow. That’s the hard part, though I’m thinking if we build a large enough bonfire, we should be able to generate smoke signals visible from space. Good thing the public keys are small.

The signify source lives in CVS. Some last minute changes to the command line interface meant I bobbled the initial commit. How embarrassing. Fortunately, the internals were all factored out so that main can be rearranged as desired without messing things up too badly, which has happened a few more times as we started using it.

I think I’ve covered the important bits here. For some additional commentary, the BSD Now folks chatted with me about signify.

More information is also contained in the presentation about signify at BSDCan 2015.

Posted 31 Dec 2013 15:37 by tedu Updated: 31 Dec 2016 21:23
Tagged: openbsd project security software