f41ebc2149 Add test for input weight predictions (conduition)
4514a80a23 Fix the InputWeightPrediction constants for DER signatures (conduition)
b5ce219c62 add weight method to InputWeightPrediction (conduition)
Pull request description:
The `P2WPKH_MAX` constant assumed DER signatures in the witness have a max length of 73. In practice, their maximum length is 72, because [BIP62](https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki) forbids nodes from relaying transactions which contain non-canonical ECDSA signatures (i.e. TX sigs must have an $s$ value of less than $\frac{n}{2}$).
This means $s$ is never encoded with a leading zero byte, and the signature as a whole never exceeds 72 bytes in total encoded length. The `ground_p2wpkh` function was already correct; only the constant needed to be corrected.
Technically 73 bytes *is* the upper limit for signatures, as nothing forbids miners from including such non-standard transaction signatures in blocks, but for the purposes of fee estimation and input weight prediction, 72 is the number which 99.9% of implementations should use as their ceiling. We already use it as the ceiling for the `ground_p2wpkh` function - `ground_p2wpkh(0)` returns a prediction which uses a witness signature of length 72.
Reference:
- https://bitcoin.stackexchange.com/questions/77191/what-is-the-maximum-size-of-a-der-encoded-ecdsa-signature
- https://bitcoin.stackexchange.com/questions/106435/are-high-s-ecdsa-signatures-forbidden-in-segwit-witnesses
- https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki
To enable testing, I added a `weight()` method to `InputWeightPrediction` and made it public but i'm not sure whether it has a use-case. Let me know if I should make it private instead.
ACKs for top commit:
tcharding:
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apoelstra:
ACK f41ebc2149
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