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October 2024

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2 discussions

Origin line issues
by Niels Möller 11 Oct '24

11 Oct '24

Hi, I said during the witnessing breakout at tdev that I'm afraid that the origin line as id is coming back to bite us. Let me expand a little on the problem I see. For a start, when adding keys for logs or witnesses to the trust policy for log users, it's clear that care and due diligence is required. That's natural, and I don't think origin line or other non-cryptographic ids in the picture is much of a problem. However, I think it's desirable that logs and witnesses are decoupled. When an operator of a log or witness gets an email "please add my new shiny witness/log to your config", I think it's rather important that no subtle or complicated due diligence is required, and that there are no severe or surprising consequences for temporarily adding a malicious entry. Main problem is for witness operators, and in particular if trying to run a witness on a device like the tkey with minimal configuration. For a sigsum log, with origin line based on keyhash, the only irreversible consequence when a witness operator adds a new log to the witness config is a commitment to storing a record (on the order of 100 bytes) for that log for the entire lifetime of the witness. If the log causes other operational problems, it could be rate limited or completely removed from the config, and that's it. The tkey app could happily accept any add-checkpoint request + log pubkey from the host, verify everything, and store the (pubkey, treehead) on success. A compromised host could fill up the tkey storage, preventing the witness from adding new logs later on, but that's about the worst it could do. On the other hand, if the host provides a pubkey and a checkpoint with an arbitrary origin line, that mapping needs to be authenticated. If not, an attacker could make it's own keypair for anyone else origin line, push a tree head with new leaves to the witness, and the witness would then refuse cosiging genuine tree heads for that origin. If we envision that people will start lots of application specific logs, and want them cosigned by public witnesses (e.g., consider a thousand github projects doing "serverless logs" in their github actions (if that makes sense, I'm not that familiar with github)), it's clear that adding logs to a witness config must be easy. The witness needs an authority mapping origin lines to keys. In the tkey case, the simple solution would be to have that authority sign some kind of certs defining this binding, and embed the ca pubkey in the tkey app binary. (For key rotation, one could consider accepting certs signed by previous key rather than the ca, but the ca is still needed for new logs). And then origin line owners should demand transparency for those certs, and we're down the rabbit whole. Finally, on the log side, for a non-sigsum log that publishes cosignatures as checkpoints, we have a related issue with the witness key names. I think that's less severe: if a bad witness is added, the log might publish cosignatures where the pubkey doens't belong to the "proper" owner of a key name, which will look invalid to users that have the right keys for that name, but that is a recoverable problem; once the problem is pointed out, the log can just drop that witness, and it will not appear on later checkpoints. So what would be my take aways, accepting that the "ship has sailed" on radical changes, and this isn't the right time for cosignature/v2? 1. We should define a origin line scheme similar to sigsum.org/v1/tree for use by non-sigsum logs that don't need key rotation. And strongly recommend that logs that don't have an urgent need for key rotation use that scheme. The advantage for logs that follow this scheme is that it's very cheap for witnesses to add their logs, since no due diligence on who's the proper "owner" of that origin line is needed. We will in effect get two classes of logs: Those with self-authenticating origin lines, and those that need require additional data or context to establish an authentic mapping between name and keys. (Again, this is an issue in the context of making witness operation easy; defining a proper trust policy will always require more information about a log than just what its key is). 2. Witness operators need to document what procedures they use to validate origin lines of logs they are asked to witness, and how they validate new public keys to be added for that origin line. 3. It would be nice if those operating logs identified by arbitrary origin lines, like "go.sum database tree", outline what procedures they'd like a witness to take before accepting a new public key for that log. For example, say Debian wants to run a witness for the go checksum database and patch go tooling to require that as an additional witness in the trust policy, how will they get authentic information about key rotation events? (Making a replica of the log and witness that, instead of witnessing the upstream log, as was suggested at the breakout, doesn't seem like an attractive alternative to me. With a new origin line, witness cosignatures on the upstream log won't verify, so they are effectively lost. And one would still need that authentic key for the upstream log, when building the replica. But maybe that alternative could be fleshed out). 4. How should we act when we find that a witness used a "wrong" or unexpected public key to verify a checkpoint for some origin? The witness clearly can't cosign any later tree heads for the proper view of that log, and it should be removed from policies involving that log. But perhaps we should make it very clear that this is possible result of an honest mistake, and not hold that against the witness operator's reputation? 5. We could think of alternative ways to do key rotation, e.g., starting a new log under a fresh key, and adding a special leaf at index 0 including the signed tree head of the old log + needed metadata. And possibly with a corresponding forward link in the tombstone message of the old log. /nisse

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Primary and secondary
by Niels Möller 07 Oct '24

07 Oct '24

I've been thinking a bit about roles and responsibilities for the primary and secondary nodes of a log. Here I'm sketching a model that is mostly compatible with the current replication protocol, and which makes the nodes a bit more independent (e.g., could be run by different organizations). A log instance consists of a primary node and (ideally) several secondary nodes. A log is identified by its key, i.e., the key that is used to sign the log's advertised tree heads (the same tree heads that are cosigned by witnesses, and for which the log operator's states intended reliability etc). Each node is identified by a separate node key. * Local trees Each node (including the primary) keeps its own local tree. That tree is possibly larger (but not smaller, except when a new node is starting up) than the log's advertised tree. Each node is identified by its node key. The node key is used to sign the tree heads of its local tree. These signatures must not be confused with the log's signed tree heads; if it's not enough that separate keys are used, they could use a separate signature namespace. The semantics of the signatures on local trees is that the node promises that it's local tree is append-only, and that all data covered by the signed tree head is committed to local storage. I.e., the tree should survive events like a local power outage. However, reliability is best effort. If the node suffers a disk failure, or is decommissioned for any other reason, the contents of the tree may be lost (except for parts of it replicated elsewhere, as described below). * Primary node The primary node's responsibility is to accept new leaves from users, commit into its local tree, and sign resulting local tree using its node key. Periodically, it queries the signed tree heads of the secondary nodes' trees, checks consistency, and publishes new versions of the *log*'s signed tree head once data is replicated to all secondaries. (If we have a larger number of secondaries, we could consider allowing the primary to proceed even in the case that a single secondary is behind or unreachable). * Secondary nodes Secondaries only accept new leaves from the primary. A secondary that is new or for some reason is behind, will first get the log's signed tree head, and retrieve all leaves it is missing. It must check inclusion and consistency before committing the leaves to its local tree and underlying storage. Next, it will periodically get the primary node's local tree head (verifying the signature using the node key of the node that is the current primary), and similarly incorporate after inclusion and consistency checks pass. Periodically, or when asked by the primary, it will sign the head of its local tree using its own node key. So at all time we have this relation between tree sizes: log's tree <= each secondary node tree <= primary node tree Extensions: It may be useful to enable secondaries to also act as mirrors, republishing the latest tree head it has received from the primary node, together with available cosignatures. It may be possible to distribute new leaves in more of a peer-to-peer fashion, instead of each secondary retrieving them directly from the primary. * Migration on primary failure What needs to happen when a primary fails or is to be replaced? We need the following steps: 0. If possible, the primary node's access to the log signing key should be removed. 1. Each secondary must be configured that the primary is down. This must likely be a manual procedure, with a human determining that the primary should no longer be used. On each secondary, this means that the node key of the old primary is removed from the configuration. 2. Once all the secondaries agree that there is no longer any primary node, one of the secondaries can become new primary. If the local trees of the secondaries are of different sizes, the one with the largest tree should be selected as the new (interrim) primary, but not yet with access to the log's signing key. (If, for some reason, a different node is chosen, the nodes that are ahead of the chosen node must be reset: Discard the extra leaves, destroy previous node key and create a new one). 3. The secondaries that were not chosen as primary are now reconfigured to use the chosen node (identified by node key, as usual) as primary, and retrieve all leaves and commit them to their local trees. 4. After some time, nodes should all be in sync. If desired, the chosen node can now be demoted back to secondary (after which all the other secondaries will again be reconfigured that there is no primary), and a new primary node can be selected. 5. Finally, the new primary should be given access to the log's signing key and start normal operation (accept leaves from users, advertise new tree heads, request cosignatures, etc). If we are willing to have secondaries coordinate with eachother, part of this process could potentially be automated. If all nodes are connected to each other (with the exception of the failing primary, which is explicitly and manually removed from the set of nodes in step (1) above), we could maybe have a protocol that lets nodes first agree that there is no primary, and then elect a new primary based on tree size, and which nodes are configured as candidates for getting access to the log's signing key. Regards, /Niels

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