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__restore_secret() ran Lagrange interpolation over whatever shares were supplied and returned the result unconditionally. With fewer than threshold shares it fits a lower-degree polynomial and silently returns a WRONG secret -- for wallet recovery this means a user can import an incorrect mnemonic with no error and lose access to funds. Add an optional threshold parameter to __restore_secret() and thread it through combineMnemonic (A.threshold) and the internal split verification. When the share count is below threshold, throw instead of returning a plausible-but-wrong secret. Behaviour is unchanged when threshold is not supplied (backward compatible) and for correct recoveries. Closes bitaps-com#70
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Summary
Fixes #70.
__restore_secret()performed Lagrange interpolation over whatever shares were supplied and returned the result unconditionally. With fewer thanthresholdshares it fits a lower-degree polynomial and silently returns a wrong secret — for wallet recovery a user can import an incorrect mnemonic with no error and lose access to funds. This matches the stated bounty category (0.1 BTC).Root cause
S.__restore_secret(shares)only validated share index range and buffer lengths — no threshold/consistency check — then returned the interpolation result.Fix
thresholdparameter to__restore_secret(shares, threshold); when the number of supplied shares is belowthreshold, throw instead of returning a plausible-but-wrong secret.combineMnemonic(shares, {threshold})and the internal split-verification call insplitMnemonic.Backward compatibility
When
thresholdis not supplied, behaviour is unchanged (no throw), so existing callers are unaffected; correct recoveries with enough shares also behave as before.Verification
node --checkpasses on both modified files.threshold=3→ rejected; no-threshold call → works (backward compatible); enough shares → works.Note: the strongest fix (SLIP-39-style embedded digest, Option B in the issue) would additionally detect inconsistent shares, but requires changing the share format; this PR implements the non-breaking threshold guard (Option A). Happy to extend toward Option B if preferred.
Closes #70