chilldkg: tighten API and internal representation#248
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Each Contributor now saves the parameters it was set up with (encryption keys, contributor nonce, threshold, role, slot) and verifies the aggregated input against that saved view rather than trusting whatever the coordinator returns. cert_bytes() incorporates the saved keysets, so a malicious coordinator that shows different parties different views of the receiver- or aux-keyset produces different cert_bytes per victim and mutual certification fails. The biggest consequences: - Receiver encryption keys leave the encpedpop wire form (they were embedded next to each encrypted share) and aux contributor keys stop being a `finalize` parameter; both move onto the contributor at gen_keygen_input time and feed into cert_bytes() via the verified state. - encpedpop::AggKeygenInput length consistency is enforced on deserialize via a private wire type, so downstream code can assume the embedded simplepedpop contributor count matches the encryption-nonce count. - Contributor is type-parameterized by role (ShareReceiver | AuxContributor). The role decides slot-index translation at compile time and selects the verify shape; ShareReceiver's verify atomically pairs the secret share, and the share is gated behind a final certificate check. - Errors are split into per-function variants so each call site enumerates exactly the conditions it can produce; an EncryptionCheckError sub-enum is shared between aux and share-receiver verify paths so simplepedpop errors flow through one Inner path with no ambiguity.
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Summary
Stacked on #247. A single squashed commit that tightens the chilldkg API.
The core idea: each
Contributornow saves the parameters it was set up with (encryption keys, contributor nonce, threshold, role, slot) and verifies the aggregated input against that saved view rather than trusting whatever the coordinator returns.cert_bytes()incorporates the saved keysets, so a malicious coordinator that shows different parties different views of the receiver- or aux-keyset produces differentcert_bytesper victim and mutual certification fails.The biggest consequences:
Receiver encryption keys leave the encpedpop wire form (they were embedded next to each encrypted share) and aux contributor keys stop being a
finalizeparameter; both move onto the contributor atgen_keygen_inputtime and feed intocert_bytes()via the verified state.encpedpop::AggKeygenInputlength consistency is enforced on deserialize via a private wire type, so downstream code can assume the embedded simplepedpop contributor count matches the encryption-nonce count.Contributoris type-parameterized by role (ShareReceiver|AuxContributor). The role decides slot-index translation at compile time and selects the verify shape;ShareReceiver's verify atomically pairs the secret share, and the share is gated behind a final certificate check.Errors are split into per-function variants so each call site enumerates exactly the conditions it can produce; an
EncryptionCheckErrorsub-enum is shared between aux and share-receiver verify paths so simplepedpop errors flow through oneInnerpath with no ambiguity.The only thing missing from this PR is a way to "keep around" the final agg keygen so you can actually recover from it. We don't use that in frostsnap but the feature has been soft-deleted since it's hard to create a
VerifiedAggKeygenInput(which is the thing you can actually recover from) and you can't serialize it.Frostsnap is updated against this branch in frostsnap/frostsnap#chilldkg-api-update.