onchaind: Adjust the witness weight estimation to be a bit more conservative #5669
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As discussed here lightningdevkit/rust-lightning#1783 (comment) the spec is likely a bit too conservative with its 73 byte signatures, given that the low-S rule ensures we don't have 73 bytes, but 72 bytes. |
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rustyrussell
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Nov 10, 2022
We were missing the OP_PUSH for the pubkeys, and the spec mentions we should be using 73 bytes to estimate the witness weight. Effectively this adds 4 bytes which really just matters in case fees hit the floor, and computing the weight becomes important. Changelog-Fixed: onchaind: Witness weight estimations could be slightly lower than the VLS signer
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This came up while debugging the VLS signer for Greenlight. In case of the feerate hitting the floor it is important we agree on the size of the transaction, and in particular the witness weight which is estimated, not measured. If the estimation of VLS is larger than the one in CLN we end up with too small a fee according to VLS, and it'll refuse to sign.
The differences we found were due to a) a missing
OP_PUSH33for the pubkeys, and b) the lower expected signature weight of 72 instead of the spec suggestion of 73.