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Managing Code Signing Digital IDs for Open Source?

Posted by Cliff on from the project-security dept.

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?"

3 of 103 comments (clear)

  1. Sharing the public key by xmath · · Score: 5, Insightful

    There are so-called secret-sharing 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.

  2. Split secrets by lewis2 · · Score: 5, Insightful

    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.

  3. Secret Sharing and Verifiable Secret Sharing by HidingMyName · · Score: 5, Informative
    Key escrow/recovery schemes where there is a sort of "backdoor" built in to allow for key recovery via trusted third parties fell out of favor in the late 1990's, as can be seen at: in this paper.

    My research is currently looking into approaches to related areas (as a user, not necessarily as a cryptographer), you may wish to look into "secret sharing", where given a secret (e.g. a private key), a set of participants, and what the literature calls an access structure which is a collection of subsets of participants that you wish to be able to easily recover the secret (called a qualified subset), establishes a two stage protocol:

    1. Share - a trusted entity called the dealer takes the secret and encodes it into a set of shares, securely awarding each participant a unique share.
    2. Reconstruct - some subset of the participants presents their shares, if the shares are valid and the subset is a qualified subset, the secret is recovered and securely distributed to that subset of participants, otherwise the secret should not be revealed.
    Now, there are (t,n) theshold schemes where any subset of t or more participants where t is between 2 and n are qualified to recover the key otherwise they are not.

    There are proactive variants that periodically recut the shares to prevent accumulated leaking of shares over time from forming a qualified subset.

    Also there are verified secret sharing schemes which support a verify operation, where a share can be checked for correctness without trying to reconstruct the secret (so that bad dealers can be caught and that at reconstruct time invalid shares can be found prior to reconstruction).

    Finally there are "cheating immune" schemes. A cheater is a participant who gives a bogus share at reconstruct time. If they know something about the reconstruction step and can assume the other participants are giving valid shares, some schemes may allow the cheaters to learn something about the secret. In cheating immune schemes, this is prevented.

    Finally there are schemes that use verifiable threshold schemes and verifiable secret sharing for digital signatures.

    If you are interested in some references, Doug Stinson's bibliography on Secret Sharing (he has some recent work too). Tal Rabin has done some good work, as has Markus Stadler. Recent work by Stanislaw Jarecki has caught my eye.