Update zcash_script to support V5 transactions

[jvff]

Motivation

With the NU5 activation, a new transaction format was introduced (V5). Zebra is currently in the process of adding support for verifying those transactions (#1981). To validate transparent inputs and outputs, Zebra uses the zcash_script crate. However, it seems that it doesn't yet support V5 transactions, because it segfaults with a V5 test case.

Specifications

The new transaction encoding is specified here.

Designs

Solution

Once the zcash_script issue is closed, update Zebra to use the new version.

The segmentation fault itself could be caused by calling zcash_script_verify_precomputed with a nullptr returned when zcash_script_new_precomputed_tx failed to deserialize the V5 transaction, so it would be good to check for null after creating the precomputed transaction and returning an error to avoid reaching the segfault.

https://github.com/ZcashFoundation/zebra/blob/main/zebra-script/src/lib.rs#L80-L82

Alternatives

We'll need V5 transaction validation for the NU5 activation, and updating zcash_script is likely the simplest solution. An alternative could be to write our own verifier, but that will likely require too much effort and time.

Related Work

Issue #21 in thezcash_script repository tracks the support for V5.