Managing Code Signing Digital IDs for Open Source?

Saqib Ali asks: "What are the best practices for managing Code Signing Digital IDs for Open Source projects, where the developers are dispersed throughout the globe? For our project there is NO central office, where we can secure the private key for the Code Signing Digital ID. Who should have the possession of the private key? Multiple people, or just the project manager? What Key Escrow (recovery) techniques can be used, in case the private key holder is not available? Who should be allowed to digitally sign the build? Currently one person handles the signing responsibility, but I think that is surely a single point of failure. Any thoughts/ideas?"

Just checksum

[Anonymous Coward]

and be done.

Sharing the public key

[xmath]

There are so-called secret-sharing [wikipedia.org] systems, which allow you to distribute a secret (such as a private key) over some number of people such that a specified number of people (the threshold) must work together to recover the secret.

This way, you can avoid a single person being able to sign, while at the same time making sure that no single person is critical for the signing.

Share split with a minimum number

[m50d]

Share split the key among a number of trusted project leaders, and require over half but not all of them to restore the key. Maybe give the project manager more than one share, but ensure that s/he doesn't have enough shares to make them essential - that way you don't need any escrow, if someone goes AWOL you can recombine the rest of your shares and then split it again without them. That's probably the best way to do things. However, I think having a single person sign the build is probably "good enough" unless it's an extremely sensitive application.

Split secrets

[lewis2]

In high assurance scenarios like commercial CA operations private-keys are never controlled by an individual. Typically an N of M scheme is used to activate crypto-hardware.

There as been some interesting work done demonstrating the generation of partial signatures using partial keys - this probably meeds one of your needs. Each of the trusted core of developers gets part of the private-key and uses that to sign part of the release, all the signature parts are assembled and voila you're done. Key recovery works well here as each key part can be encrypted and backed up elsewhere (USB token somewhere else). This may be way overkill for your needs.

Why not just use an OTS code signing certificate and use the Mozilla or Java or OpenSSL tools to manage signing? If you lose the key you can just get a (free) replacement. This way your key chains up to a well known root that ships with FF, Java, Opera etc. Also if you find your key has been compromised for whatever reason, CRLs or OCSP will be available to prevent use of the compromised key by whomever it is you want to defend against.

Why is this presumed to be an Open Source issue?

[ScentCone]

Certainly the risks are higher, in open source development, of people bailing out of the effort. But pretty much any organization of any size engaged in such projects ("closed" or otherwise) has issues like this.

I've run into problems with departing web admins and SSL cert renewals, domain management absent the original admin/tech contacts, or just simple stuff like having to crack ZIP files because the project manager has gone on to greener pastures. So far I have yet to beat the paper backup in the company management's private safe, with the In Case Of Death, Open Me label on it. For multi-developer projects, there's usually a central figure - sort of an Alpha Dog - no matter how peer-ish the project is supposed to be.

Thoroughly off-topic, but...

[BaudKarma]

...isn't it about time we shelved that idiotic phrase "best practices"? Can't you just ask for the best way, or the best method? People are using "best practices" for everything from EMail management to installing a new kitchen sink. It's hokey and pretentious, and I'll be really glad if I never see it again.

Okay, I feel better now. Thank you.

P.S. My karma is right on the edge, so if you mod me down I'll probably lose all my bonuses and everything.

Don't make work for yourself

[TrumpetPower!]

What are the best practices for managing Code Signing Digital IDs for Open Source projects

I'd have to say that you're over-thinking this. I doubt you need digital signatures at all.

First, should there be any questions at all, well--Use The Source, Luke! You've got it, so examine it and compile it yourself. That's one of the big selling points of open source, no?

For binary releases, just do it the OpenBSD way. Official releases are created and hosted on trusted servers, along with the hashes. A bunch of mirror sites copy the releases and the hashes. (And, there're these nifty CDs that come pre-packaged with the release and the hashes.)

Anybody who has any questions can verify the hashes on their own copies in any number of ways. You could get the hash off the trusted site, several of the mirrors, etc. You could email somebody you trust, asking them to confirm them. You could even use a telephone or meet in person.

Belive me, if there's any hanky-panky going on, it'll show up real quick. All sorts of people will raise a ruckus.

So, the end result is that you get secure code that everybody trusts and you don't have to muck around with digital signatures, secret sharing, and all that.

Don't get me worng--all those things have their places. Distributing free software just ain't one of them.

Cheers,

b&

Re:Split secrets

[Conare]

Good post! One nit pick - it's usually called M of N (just in case someone wants to google it). Also hardware scheme won't work for them here, due to the distribted nature of the organization.

I may be misunderstanding you, but generally in digital signing, you don't recover the signing private key, because there is no point to it. Just issue a new one. Key Recovery is only useful for encryption purposes when there is data that is encrypted and will be lost unless you can recover the key. With digital signing, losing the private key (or compromising it) just means you can't use it to sign anymore. You can still validate signatures created with the compromised or lost private key using the public portion of it, which is usually included in the signed object itself. If you have an associated trusted time stamp that is, and the signature predates the compromise event.

I think the OTS code signing certificate is a good idea, and you could entrust the use of it (private key password) to a small group. if you do this, I would highly recommend that if someone that is trusted with the signing duties, or holds part of your multi-part key as detailed above, leaves your organization, that you revoke that certificate (or whatever PGP uses) and issue a new one. I would also recommend trusted timestamping if you are concerned about continuity of validations following a compromise.

It's a social trust problem, not a technical one

[arete]

Code signing is a mechanism for proving "who" is endorsing that code as something you can trust. Your problem is defining "who" and that's not really a technical problem.

If somebody forks you, you shouldn't sign their code. Not because it's bad, but because you can't vouch for it. THEY should sign their code.

Letting somebody else have the key to sign code is endorsing that your good name should go on ANYTHING that they decide to put out. Certainly, a project above a certain size with a community of maintainers should distribute this responsibility.

If I had a small project, I would make sure the key was left to somebody in my will (and I'd probably leave it with some close friends) - hopefully it'd get to somebody who'd be nominated to takeover the probject - because if I die presumeably I'm less worried about someone pretending to be me. This is a form of key escrow, but it's not a very arbitrary one.

For a larger project, it must be almost the highest level of trust, and it doubtless has to be learned. Those levels of trust would go something like:
bank accounts & corp documents
CVS log modification (auditability erasing)
code signing
CVS commit (but at least you can track it after the fact)
fast-track patch submission
anyone (normal patch submission)

These levels are a pyramid; fewer people should be trusted at each level - and fewer people are needed.

Re:Don't make work for yourself

[cs]

I'd have to say that you're over-thinking this. I doubt you need digital signatures at all.
First, should there be any questions at all, well--Use The Source, Luke! You've got it, so examine it and compile it yourself.

I'd have to say you're underthinking this. Nobody has the time to source review every app and update of that app. Many lack the skills, and almosy everyone lacks sufficient attention span to ready and perfectly comprehend all the source code.

A great deal of stuff is done on trust, and digital signatures at least tell the user that they are in fact placing their trust in who they think they're placing their trust. If the keys for signature are improperly managed then that confidence (that the people you trust are the authors) may not be maintained.

So if you're signing stuff, this is an important issue. Signing stuff lets users adopt a higher level trust method than "read the source". Overkill example: how recently have you read the entire Linux source code and verified it for correctness? (You must read it all, including the drivers, because everything runs with full privileges, and so a hack may be inserted in any code sequence that can run.)

Even small libraries are infeasible to personally verify if you're not intimate with the code. Therefore users must trust the authors for most things, and signatures are essential to being able to do so.