Compute the tx SIGHASH only once per tx verification

[upbqdn]

The tx SIGHASH is currently computed twice per tx verification:

• once for shielded data verification: https://github.com/zcashfoundation/zebra/blob/05a9b6171ccbc06bcc888470e4be8a60b1d574ef/zebra-consensus/src/transaction.rs#L974-L979, and
• once for transparent data verification: https://github.com/zcashfoundation/zebra/blob/05a9b6171ccbc06bcc888470e4be8a60b1d574ef/zebra-script/src/lib.rs#L200-L218.

It is currently not possible to compute the SIGHASH only once because the transparent data verification is done over FFI, and the FFI API doesn't allow passing a precomputed SIGHASH. Once we have the new transparent script engine implemented in Rust, it would be beneficial to be able to pass a precomputed SIGHASH to it, and refactor Zebra's code so that the SIGHASH is computed only once.

[conradoplg]

It is currently not possible to compute the SIGHASH only once because the transparent data verification is done over FFI, and the FFI API doesn't allow passing a precomputed SIGHASH.

It does! After the last refactoring, zcash_script takes the same sighash context that we use in Rust, because the sighash is computed by the caller via a callback. I noticed that too but forgot to create an issue about it.

[upbqdn]

Great! I didn't look thoroughly enough. I think this is worth doing some time.