V5 transaction rfc (#1886)
* propose a v5 transaction rfc * define orchard flags * Update test sections * Add security section * move some code into sapling and orchard crates, do renames * Move sapling value balance into shielded data * Add AuthorizedAction type * Order fields based on last serialized data * Add the proofsOrchard field * Delete visibility modifiers for readability All those `pub`s make the design harder to read. * Model anchor variants as generic trait types * Be specific about the network upgrade * Specify a file for each new type * Document how the Orchard flags are used Co-authored-by: teor <teor@riseup.net>
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Alfredo Garcia
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- Feature Name: `v5_transaction`
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- Start Date: 2021-03-11
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- Design PR: [ZcashFoundation/zebra#1886](https://github.com/ZcashFoundation/zebra/pull/1886)
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- Zebra Issue: [ZcashFoundation/zebra#1863](https://github.com/ZcashFoundation/zebra/issues/1863)
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# Summary
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[summary]: #summary
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Network Upgrade number 5 (`NU5`) introduces a new transaction type (transaction version 5). This document is a proposed design for implementing such a transaction version.
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# Motivation
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[motivation]: #motivation
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The Zebra software wants to be a protocol compatible Zcash implementation. One of the tasks to do this includes the support of the new version 5 transactions that will be implemented in Network Upgrade 5 (NU5).
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# Definitions
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[definitions]: #definitions
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- `NU5` - the 5th Zcash network upgrade, counting from the `Overwinter` upgrade as upgrade zero.
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- `Orchard` - a new shielded pool introduced in `NU5`.
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- `Sapling` - a new shielded pool introduced in the 1st network upgrade. (`Sapling` is also the name of that network upgrade, but this RFC is focused on the `Sapling` shielded pool.)
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- `orchard data` - Data types needed to support orchard transactions.
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- `sapling data` - Data types needed to support sapling transactions.
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- `orchard transaction version` - Transactions that support orchard data. Currently only V5.
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- `sapling transaction version` - Transactions that support sapling data. Currently V4 and V5 but the data is implemented differently in them.
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# Guide-level explanation
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[guide-level-explanation]: #guide-level-explanation
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V5 transactions are described by the protocol in the second table of [Transaction Encoding and Consensus](https://zips.z.cash/protocol/nu5.pdf#txnencodingandconsensus).
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All of the changes proposed in this document are only to the `zebra-chain` crate.
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To highlight changes most of the document comments from the code snippets in the [reference section](#reference-level-explanation) were removed.
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## Sapling Changes Overview
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[sapling-changes-overview]: #sapling-changes-overview
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V4 and V5 transactions both support sapling, but the underlying data structures are different. So need to make the sapling data types generic over the V4 and V5 structures.
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In V4, anchors are per-spend, but in V5, they are per-transaction.
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For consistency, also we move some fields into the `ShieldedData` type, and rename some fields and types.
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## Orchard Additions Overview
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[orchard-additions-overview]: #orchard-additions-overview
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V5 transactions are the only ones that will support orchard transactions with `Orchard` data types.
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Orchard uses `Halo2Proof`s with corresponding signature type changes. Each Orchard `Action` contains a spend and an output. Placeholder values are substituted for unused spends and outputs.
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## Other Transaction V5 Changes
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[other-transaction-v5-changes]: #other-transaction-v5-changes
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The order of some of the fields changed from V4 to V5. For example the `lock_time` and `expiry_height` were moved above the transparent inputs and outputs.
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Zebra enums and structs put fields in serialized order. Composite fields are ordered based on **last** data deserialized for each field.
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# Reference-level explanation
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[reference-level-explanation]: #reference-level-explanation
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## Sapling Changes
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[sapling-changes]: #sapling-changes
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We know by protocol (2nd table of [Transaction Encoding and Consensus](https://zips.z.cash/protocol/nu5.pdf#txnencodingandconsensus)) that V5 transactions will support sapling data however we also know by protocol that spends ([Spend Description Encoding and Consensus](https://zips.z.cash/protocol/nu5.pdf#spendencodingandconsensus), See †) and outputs ([Output Description Encoding and Consensus](https://zips.z.cash/protocol/nu5.pdf#outputencodingandconsensus), See †) fields change from V4 to V5.
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`ShieldedData` is currently defined and implemented in `zebra-chain/src/transaction/shielded_data.rs`. As this is Sapling specific we propose to move this file to `zebra-chain/src/sapling/shielded_data.rs`.
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### Changes to V4 Transactions
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[changes-to-v4-transactions]: #changes-to-v4-transactions
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Here we have the proposed changes for V4 transactions:
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* make `sapling_shielded_data` use the `PerSpendAnchor` anchor variant
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* rename `shielded_data` to `sapling_shielded_data`
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* move `value_balance` into the `sapling::ShieldedData` type
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* order fields based on the **last** data deserialized for each field
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```rust
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enum Transaction::V4 {
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inputs: Vec<transparent::Input>,
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outputs: Vec<transparent::Output>,
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lock_time: LockTime,
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expiry_height: block::Height,
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joinsplit_data: Option<JoinSplitData<Groth16Proof>>,
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sapling_shielded_data: Option<sapling::ShieldedData<PerSpendAnchor>>,
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}
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```
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### Anchor Variants
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[anchor-variants]: #anchor-variants
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We add an `AnchorVariant` generic type trait, because V4 transactions have a per-`Spend` anchor, but V5 transactions have a shared anchor. This trait can be added to `sapling/shielded_data.rs`:
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```rust
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struct PerSpendAnchor {}
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struct SharedAnchor {}
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impl AnchorVariant for PerSpendAnchor {
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type Shared = ();
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type PerSpend = tree::Root;
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}
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impl AnchorVariant for SharedAnchor {
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type Shared = tree::Root;
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type PerSpend = ();
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}
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trait AnchorVariant {
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type Shared;
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type PerSpend;
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}
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```
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### Changes to Sapling ShieldedData
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[changes-to-sapling-shieldeddata]: #changes-to-sapling-shieldeddata
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We use `AnchorVariant` in `ShieldedData` to model the anchor differences between V4 and V5:
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```rust
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struct sapling::ShieldedData<AnchorV: AnchorVariant> {
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value_balance: Amount,
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anchor: AnchorV::Shared,
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first: Either<Spend<AnchorV>, Output>,
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rest_spends: Vec<Spend<AnchorV>>,
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rest_outputs: Vec<Output>,
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binding_sig: Signature<Binding>,
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}
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```
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### Adding V5 Sapling Spend
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[adding-v5-sapling-spend]: #adding-v5-sapling-spend
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Sapling spend code is located at `zebra-chain/src/sapling/spend.rs`. We use `AnchorVariant` to model the anchor differences between V4 and V5:
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```rust
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struct Spend<AnchorV: AnchorVariant> {
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cv: commitment::ValueCommitment,
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anchor: AnchorV::PerSpend,
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nullifier: note::Nullifier,
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rk: redjubjub::VerificationKeyBytes<SpendAuth>,
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zkproof: Groth16Proof,
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spend_auth_sig: redjubjub::Signature<SpendAuth>,
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}
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```
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### No Changes to Sapling Output
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[no-changes-to-sapling-output]: #no-changes-to-sapling-output
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In Zcash the Sapling output representations are the same for V4 and V5 transactions, so no variants are needed. The output code is located at `zebra-chain/src/sapling/output.rs`:
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```rust
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struct Output {
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cv: commitment::ValueCommitment,
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cm_u: jubjub::Fq,
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ephemeral_key: keys::EphemeralPublicKey,
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enc_ciphertext: note::EncryptedNote,
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out_ciphertext: note::WrappedNoteKey,
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zkproof: Groth16Proof,
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}
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```
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## Orchard Additions
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[orchard-additions]: #orchard-additions
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### Adding V5 Transactions
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[adding-v5-transactions]: #adding-v5-transactions
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Now lets see how the V5 transaction is specified in the protocol, this is the second table of [Transaction Encoding and Consensus](https://zips.z.cash/protocol/nu5.pdf#txnencodingandconsensus) and how are we going to represent it based in the above changes for Sapling fields and the new Orchard fields.
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We propose the following representation for transaction V5 in Zebra:
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```rust
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enum Transaction::V5 {
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lock_time: LockTime,
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expiry_height: block::Height,
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inputs: Vec<transparent::Input>,
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outputs: Vec<transparent::Output>,
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sapling_shielded_data: Option<sapling::ShieldedData<SharedAnchor>>,
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orchard_shielded_data: Option<orchard::ShieldedData>,
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}
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```
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To model the V5 anchor type, `sapling_shielded_data` uses the `SharedAnchor` variant located at `zebra-chain/src/transaction/sapling/shielded_data.rs`.
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### Adding Orchard ShieldedData
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[adding-orchard-shieldeddata]: #adding-orchard-shieldeddata
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The new V5 structure will create a new `orchard::ShieldedData` type. This new type will be defined in a new `zebra-chain/src/orchard/shielded_data.rs` file:
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```rust
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struct orchard::ShieldedData {
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flags: Flags,
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value_balance: Amount,
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anchor: tree::Root,
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proof: Halo2Proof,
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/// An authorized action description.
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///
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/// Storing this separately ensures that it is impossible to construct
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/// an invalid `ShieldedData` with no actions.
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first: AuthorizedAction,
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rest: Vec<AuthorizedAction>,
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binding_sig: redpallas::Signature<redpallas::Binding>,
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}
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```
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The fields are ordered based on the **last** data deserialized for each field.
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### Adding Orchard AuthorizedAction
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[adding-orchard-authorizedaction]: #adding-orchard-authorizedaction
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In `V5` transactions, there is one `SpendAuth` signature for every `Action`. To ensure that this structural rule is followed, we create an `AuthorizedAction` type in `orchard/shielded_data.rs`:
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```rust
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/// An authorized action description.
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///
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/// Every authorized Orchard `Action` must have a corresponding `SpendAuth` signature.
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struct orchard::AuthorizedAction {
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action: Action,
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spend_auth_sig: redpallas::Signature<redpallas::SpendAuth>,
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}
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```
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Where `Action` is defined as [Action definition](https://github.com/ZcashFoundation/zebra/blob/68c12d045b63ed49dd1963dd2dc22eb991f3998c/zebra-chain/src/orchard/action.rs#L18-L41).
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### Adding Orchard Flags
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[adding-orchard-flags]: #adding-orchard-flags
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Finally, in the V5 transaction we have a new `orchard::Flags` type. This is a bitfield type defined as:
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```rust
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bitflags! {
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/// Per-Transaction flags for Orchard.
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///
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/// The spend and output flags are passed to the `Halo2Proof` verifier, which verifies
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/// the relevant note spending and creation consensus rules.
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struct orchard::Flags: u8 {
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/// Enable spending non-zero valued Orchard notes.
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///
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/// "the `enableSpendsOrchard` flag, if present, MUST be 0 for coinbase transactions"
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const ENABLE_SPENDS = 0b00000001;
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/// Enable creating new non-zero valued Orchard notes.
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const ENABLE_OUTPUTS = 0b00000010;
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// Reserved, zeros (bits 2 .. 7)
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}
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}
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```
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This type is also defined in `orchard/shielded_data.rs`.
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## Test Plan
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[test-plan]: #test-plan
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- All renamed, modified and new types should serialize and deserialize.
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- The full V4 and V5 transactions should serialize and deserialize.
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- Prop test strategies for V4 and V5 will be updated and created.
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- Before NU5 activation on testnet, test on the following test vectors:
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- Hand-crafted Orchard-only, Orchard/Sapling, Orchard/Transparent, and Orchard/Sapling/Transparent transactions based on the spec
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- "Fake" Sapling-only and Sapling/Transparent transactions based on the existing test vectors, converted from V4 to V5 format
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- We can write a test utility function to automatically do these conversions
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- An empty transaction, with no Orchard, Sapling, or Transparent data
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- Any available `zcashd` test vectors
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- After NU5 activation on testnet:
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- Add test vectors using the testnet activation block and 2 more post-activation blocks
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- After NU5 activation on mainnet:
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- Add test vectors using the mainnet activation block and 2 more post-activation blocks
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# Security
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To avoid parsing memory exhaustion attacks, we will make the following changes across all `Transaction`, `ShieldedData`, `Spend` and `Output` variants, V1 through to V5:
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- Check cardinality consensus rules at parse time, before deserializing any `Vec`s
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- In general, Zcash requires that each transaction has at least one Transparent/Sprout/Sapling/Orchard transfer, this rule is not currently encoded in our data structures (it is only checked during semantic verification)
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- Stop parsing as soon as the first error is detected
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These changes should be made in a later pull request, see [#1917](https://github.com/ZcashFoundation/zebra/issues/1917) for details.
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