Parse Sapling data in Transaction Version 5

[teor2345]

Is your feature request related to a problem?

In #1824, we parse the transaction header and transparent data for transaction version 5.

Transaction version 5 also contains Sapling data, which we need to parse:

• Design: https://github.com/ZcashFoundation/zebra/blob/main/book/src/dev/rfcs/0010-v5-transaction.md#sapling-changes
• Spec: https://zips.z.cash/protocol/nu5.pdf#txnencodingandconsensus

Describe the solution you'd like

Implementation

Spend

Implement serialization and deserialization for:

• Spend<PerSpendAnchor> (moved from the pre-RFC Spend)
• SpendPrefixInTransactionV5 (new)
• redjubjub::Signature<redjubjub::SpendAuth> (new - for v5 spend auth sig arrays)

Test Arbitrary Spend (both variants):

• round-trip serialization and deserialization
• trusted vector deserialization

Output

Implement serialization and trusted vector deserialization for:

• OutputInTransactionV4 (moved from Output)
• OutputPrefixInTransactionV5 (new)

Test Arbitrary Output (both transaction versions):

• round-trip serialization and deserialization
• trusted vector deserialization

ShieldedData

Documentation

• read the relevant consensus rules in the spec
• read the relevant RFC section
• summarise them in each struct and function's docs

Implement serialization and deserialization for:

• Option<sapling::ShieldedData<SharedAnchor>>
  • unlike v4 transactions, v5 sapling shielded data can be serialized into continuous bytes
  • https://github.com/ZcashFoundation/zebra/blob/main/book/src/dev/rfcs/0010-v5-transaction.md#changes-to-sapling-shieldeddata
  • for an example, see Option<sprout::JoinSplitData> at https://github.com/ZcashFoundation/zebra/blob/main/zebra-chain/src/transaction/serialize.rs#L34

TestArbitrary Option<sapling::ShieldedData<SharedAnchor>>

• round-trip serialization and deserialization

Test manual implementations:

• Proptest the PartialEq impl for ShieldedData<SharedAnchor>
  • see the ShieldedData<PerSpendAnchor> tests in https://github.com/ZcashFoundation/zebra/blob/main/zebra-chain/src/sapling/tests/prop.rs
  • these tests should be simpler, because ShieldedData<SharedAnchor> has serialization impls, so it doesn't need to be wrapped in a transaction for serialization

Transaction

• Parse the Sapling section of V5 transactions using ShieldedData
• return an empty iterator for transaction version 5 in sprout_nullifiers
• implement transaction version 5 in sapling_nullifiers

TestArbitrary Transaction::V5

• round-trip serialization and deserialization

Cleanups and Refactoring

• Redesign data model (again)
  • shared anchor only present when there are v5 spends Redesign Sapling data model for V5 shared anchor and spends #2021
  • simplify "at least one" constraint Use the AtLeastOne generic type to enforce field counts #2028

Testing

Test round-trip serialization and deserialization for transaction v4 and v5 (without orchard)

Unit tests

• an empty transaction v5, with no Sapling or Transparent data
  • empty transaction are invalid, but Zebra only checks this rule in zebra_consensus::transaction::Verifier
• v5 with empty sapling spends, some outputs
  • covers the shared anchor serialization condition
• an empty transaction v4, with no Sapling, Sprout, or Transparent data
  • empty transaction are invalid, but Zebra only checks this rule in zebra_consensus::transaction::Verifier

Test vectors

• "Fake" Sapling-only and Sapling/Transparent v5 transactions based on zebra_test::vectors::block::BLOCKS

  • provides Sapling only, Transparent only, and Sapling/Transparent test vectors
  • we can write a test utility function to "convert" transactions from V4 to V5 format
  • some transactions won't convert because they have multiple sapling anchors

• work out why these tests fail on blocks, but not transactions (fake_v5_round_trip test fails on blocks but not in transactions #2023)

• transaction v5 sapling-only test vectors from zcashd (or created by us and shared with zcashd)

Describe alternatives you've considered

This is a required change for NU5.

Follow Up Tasks

"Fake" Sapling-only and Sapling/Transparent v5 transactions based on zebra_test::vectors::block::BLOCKS

• work out why these tests fail on blocks, but not transactions (fake_v5_round_trip test fails on blocks but not in transactions #2023)

Test vectors

• transaction v5 sapling-only test vectors from zcashd (or created by us and shared with zcashd) - Create Sapling-only transaction v5 test vectors #2047

[oxarbitrage]

Blocked on #1863

[mpguerra]

I just wanted to highlight this item from the spec which may be relevant here also:

For a v5 transaction, vbalanceSapling is implicitly zero if the transaction has no Spend descriptions
or Output descriptions.

[teor2345]

I just wanted to highlight this item from the spec which may be relevant here also:

For a v5 transaction, vbalanceSapling is implicitly zero if the transaction has no Spend descriptions
or Output descriptions.

Thanks Pili, that was a bug in the draft PR, I've added a comment at https://github.com/ZcashFoundation/zebra/pull/1860/files#r595643213

[teor2345]

I just wanted to highlight this item from the spec which may be relevant here also:

For a v5 transaction, vbalanceSapling is implicitly zero if the transaction has no Spend descriptions
or Output descriptions.

Thanks Pili, that was a bug in the draft PR, I've added a comment at https://github.com/ZcashFoundation/zebra/pull/1860/files#r595643213

These was actually also a missing related Orchard spec rule as well:

I've opened #1917 to implement those rules, and I'll also update the transaction RFC.

[teor2345]

I opened #2023 and #2047 for follow-up, so we can close this ticket now.