Hans-Christoph Steiner <hans@at.or.at> writes: > Thanks for digging in here, its very important work! I'd be happy to > contribute where I can. I'm a DD and a core contributor to F-Droid, > where we wrestle with basically the same issues. So we've thought a > lot about these kinds of things, but definitely do not have all the > answers. Since F-Droid started much later than Debian, we were able > to build in some nice protections from the beginning, like requiring > HTTPS. We've also made the decision to stick with a single > jurisdiction for the singing keys, even though it is not the best one > for our needs. We'd of course love to move them to the best > jurisdiction but that is actually quite a bit of work, so we have to > stay grounded in reality. > > For me the hard part of all this is how to be transparent as possible > without putting a giant target on the heads of the people who control > the keys. I think it is clear that publishing private key usage > information is safe, like this key signed these things at these times. > Publishing all the details about how the key was generated could help > those who want to attack it more than those who rely on it. But that > kind of information would be good to share internally to the project > at the very least. Good aspect -- and indeed this is a trade-off, and thanks for caring about these issues for f-droid! It seems that if you would verify signatures via a transparency log, you would reduce the risk on the people controlling the keys? Then you (and they) could feel more comfortable publishing information how your process work. That would be valuable to publish (even de-personalized or generalized) as a best practices approach. The reason is that anyone stealing the keys from these persons would ALSO have to upload signatures of whatever they wanted to use these keys for into the transparency log, which would provide evidence to what they did, so they may be less likely to follow through on their plans. What would be involved is to 1) during signing of artifacts, also sign and upload into Sigstore/Sigsum, and 2) during verification in the f-droid app, also verify that the signature has been committed to the Sigstore/Sigsum logs. Both projects have clients written in Go which should work on Android, but the rest of the details are sketchy to me. I'm happy to continue discuss and help with design if you are interested, to understand what the limitations of your environments are and how to resolve them. /Simon > And publishing the jurisdictions could be enough info to deanonymize > the key holder, e.g. if it is Germany, then there are many DDs there. > If it is Iceland, then govs can easily find who to target. > > .hc > >> Hi >> I'm exploring how to defend against an attacker who can create valid >> signatures for cryptographic private keys (e.g., PGP) that users need to >> trust when using an operating system such as Debian. A signature like >> that can be used in a targetted attacks against one victim. >> For example, apt does not have any protection against this threat >> scenario, and often unprotected ftp:// or http:// transports are used, >> which are easy to man-in-the-middle. Even with https:// there is a >> large number of mirror sites out there that can replace content you get. >> There is a risk that use of a compromised trusted apt PGP key will not >> be noticed. Attackers are also in a good position to deny themselves >> out of their actions if they are careful. >> Part of my effort is to inventory all trusted private keys that a >> distribution needs to manage on their own, or depend on that are managed >> by others, and gain some insight how these keys are handled. >> Does the Debian project, or any members, publish information on >> these >> topics? Has this been discussed before? >> Some questions that I think would be useful to answer include: >> 1) List of relevant private keys, held by the organization, its >> members, >> or someone else. As far as I can tell from Debian, the list includes >> the following PGP trust anchors: >> B8B8 0B5B 623E AB6A D877 5C45 B7C5 D7D6 3509 47F8 >> 05AB 9034 0C0C 5E79 7F44 A8C8 254C F3B5 AEC0 A8F0 >> 4D64 FEC1 19C2 0290 67D6 E791 F8D2 585B 8783 D481 >> 1F89 983E 0081 FDE0 18F3 CC96 73A4 F27B 8DD4 7936 >> AC53 0D52 0F2F 3269 F5E9 8313 A484 4904 4AAD 5C5D >> A428 5295 FC7B 1A81 6000 62A9 605C 66F0 0D6C 9793 >> 80D1 5823 B7FD 1561 F9F7 BCDD DC30 D7C2 3CBB ABEE >> 5E61 B217 265D A980 7A23 C5FF 4DFA B270 CAA9 6DFA >> 6D33 866E DD8F FA41 C014 3AED DCC9 EFBF 77E1 1517 >> Compromising any single key on this list leads to trouble. >> However I >> don't think this list is complete. What other keys are there? >> I believe there are keys used to sign some component of the boot >> phase, compare the 'shim-signed' and 'grub-efi-amd64-signed' >> packages. What private keys held by Debian are involved here? What >> keys held by others are involved? What other similar packages >> exists? >> 2) For each private key, information about its management and >> lifecycle. >> Relevant questions include: >> a) How was the key generated? By whom? On what hardware? What >> software? In what environment? What legal jurisdiction apply to >> people involved? >> b) How is the key stored and protected during its lifetime? What >> media >> is used? Who control the physical storage of the key? How are they >> stored and transported? What jurisdiction? >> c) Under what policy is the key used? What should it sign? Who >> authorize the signing? What hardware and software is used? What >> jurisdiction? >> d) For externally held keys, what are the legal terms we use the >> keys >> under? What insight into key transparency questions do we have? >> What of those can we make public? How do they restrict what we are >> allowed to do? >> 3) Which less visible private keys are indirectly trusted by users >> of >> the distribution? For example, all DD PGP keys are indirectly >> trusted since they are permitted to make uploads into the archive. >> Host keys used to authorized access to sensitive systems may also be >> relevant. I'm primarily thinking SSH private keys to a system that >> may have online access to a private key signing oracle. Indirectly, >> questions about authentication protocols and authorization methods >> are relevant. >> To the extent that symmetric shared secrets (rather than asymmetric >> public/private keys) are involved, the same question applies. >> Other aspects worth exploring? >> I understand commercial distributions have different incentives >> related >> to making this information public. They have a commercial interest to >> defend, and has usually entered various legal arrangements that create >> obstacles related to releasing information. As we've seen with the >> WebPKI CA business, that model does not inspire trust and leads to >> systematic failures. More transparent approaches like Certificate >> Transparency and LetsEncrypt evolved as a consequence. >> I believe that Debian is in a good position to improve things and, >> if >> there is interest, could lead the way by documenting a better approach, >> and describe how to deal with these concerns in a more transparent way >> than what we do today. >> Thoughts? >> /Simon > >
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