zebra/zebra-chain/src/orchard/shielded_data.rs

//! Orchard shielded data for `V5` `Transaction`s.

use std::{
    cmp::{Eq, PartialEq},
    fmt::Debug,
    io,
};

use byteorder::{ReadBytesExt, WriteBytesExt};
use halo2::pasta::pallas;

use crate::{
    amount::{Amount, NegativeAllowed},
    block::MAX_BLOCK_BYTES,
    orchard::{tree, Action, Nullifier, ValueCommitment},
    primitives::{
        redpallas::{self, Binding, Signature, SpendAuth},
        Halo2Proof,
    },
    serialization::{
        AtLeastOne, SerializationError, TrustedPreallocate, ZcashDeserialize, ZcashSerialize,
    },
};

/// A bundle of [`Action`] descriptions and signature data.
#[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
pub struct ShieldedData {
    /// The orchard flags for this transaction.
    pub flags: Flags,
    /// The net value of Orchard spends minus outputs.
    pub value_balance: Amount,
    /// The shared anchor for all `Spend`s in this transaction.
    pub shared_anchor: tree::Root,
    /// The aggregated zk-SNARK proof for all the actions in this transaction.
    pub proof: Halo2Proof,
    /// The Orchard Actions, in the order they appear in the transaction.
    pub actions: AtLeastOne<AuthorizedAction>,
    /// A signature on the transaction `sighash`.
    pub binding_sig: Signature<Binding>,
}

impl ShieldedData {
    /// Iterate over the [`Action`]s for the [`AuthorizedAction`]s in this
    /// transaction, in the order they appear in it.
    pub fn actions(&self) -> impl Iterator<Item = &Action> {
        self.actions.actions()
    }

    /// Collect the [`Nullifier`]s for this transaction.
    pub fn nullifiers(&self) -> impl Iterator<Item = &Nullifier> {
        self.actions().map(|action| &action.nullifier)
    }

    /// Calculate the Action binding verification key.
    ///
    /// Getting the binding signature validating key from the Action description
    /// value commitments and the balancing value implicitly checks that the
    /// balancing value is consistent with the value transferred in the
    /// Action descriptions, but also proves that the signer knew the
    /// randomness used for the Action value commitments, which
    /// prevents replays of Action descriptions that perform an output.
    /// In Orchard, all Action descriptions have a spend authorization signature,
    /// therefore the proof of knowledge of the value commitment randomness
    /// is less important, but stills provides defense in depth, and reduces the
    /// differences between Orchard and Sapling.
    ///
    /// The net value of Orchard spends minus outputs in a transaction
    /// is called the balancing value, measured in zatoshi as a signed integer
    /// cv_balance.
    ///
    /// Consistency of cv_balance with the value commitments in Action
    /// descriptions is enforced by the binding signature.
    ///
    /// Instead of generating a key pair at random, we generate it as a function
    /// of the value commitments in the Action descriptions of the transaction, and
    /// the balancing value.
    ///
    /// https://zips.z.cash/protocol/protocol.pdf#orchardbalance
    pub fn binding_verification_key(&self) -> redpallas::VerificationKeyBytes<Binding> {
        let cv: ValueCommitment = self.actions().map(|action| action.cv).sum();
        let cv_balance: ValueCommitment =
            ValueCommitment::new(pallas::Scalar::zero(), self.value_balance);

        let key_bytes: [u8; 32] = (cv - cv_balance).into();
        key_bytes.into()
    }

    /// Provide access to the `value_balance` field of the shielded data.
    ///
    /// Needed to calculate the sapling value balance.
    pub fn value_balance(&self) -> Amount<NegativeAllowed> {
        self.value_balance
    }

    /// Collect the cm_x's for this transaction, if it contains [`Action`]s with
    /// outputs, in the order they appear in the transaction.
    pub fn note_commitments(&self) -> impl Iterator<Item = &pallas::Base> {
        self.actions().map(|action| &action.cm_x)
    }
}

impl AtLeastOne<AuthorizedAction> {
    /// Iterate over the [`Action`]s of each [`AuthorizedAction`].
    pub fn actions(&self) -> impl Iterator<Item = &Action> {
        self.iter()
            .map(|authorized_action| &authorized_action.action)
    }
}

/// An authorized action description.
///
/// Every authorized Orchard `Action` must have a corresponding `SpendAuth` signature.
#[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
pub struct AuthorizedAction {
    /// The action description of this Action.
    pub action: Action,
    /// The spend signature.
    pub spend_auth_sig: Signature<SpendAuth>,
}

impl AuthorizedAction {
    /// Split out the action and the signature for V5 transaction
    /// serialization.
    pub fn into_parts(self) -> (Action, Signature<SpendAuth>) {
        (self.action, self.spend_auth_sig)
    }

    // Combine the action and the spend auth sig from V5 transaction
    /// deserialization.
    pub fn from_parts(action: Action, spend_auth_sig: Signature<SpendAuth>) -> AuthorizedAction {
        AuthorizedAction {
            action,
            spend_auth_sig,
        }
    }
}

/// The size of a single Action
///
/// Actions are 5 * 32 + 580 + 80 bytes so the total size of each Action is 820 bytes.
/// [7.5 Action Description Encoding and Consensus][ps]
///
/// [ps] https://zips.z.cash/protocol/nu5.pdf#actionencodingandconsensus
pub const ACTION_SIZE: u64 = 5 * 32 + 580 + 80;

/// The size of a single Signature<SpendAuth>
///
/// Each Signature is 64 bytes.
/// [7.1 Transaction Encoding and Consensus][ps]
///
/// [ps] https://zips.z.cash/protocol/nu5.pdf#actionencodingandconsensus
pub const SPEND_AUTH_SIG_SIZE: u64 = 64;

/// The size of a single AuthorizedAction
///
/// Each serialized `Action` has a corresponding `Signature<SpendAuth>`.
pub const AUTHORIZED_ACTION_SIZE: u64 = ACTION_SIZE + SPEND_AUTH_SIG_SIZE;

/// The maximum number of orchard actions in a valid Zcash on-chain transaction V5.
///
/// If a transaction contains more actions than can fit in maximally large block, it might be
/// valid on the network and in the mempool, but it can never be mined into a block. So
/// rejecting these large edge-case transactions can never break consensus.
impl TrustedPreallocate for Action {
    fn max_allocation() -> u64 {
        // Since a serialized Vec<AuthorizedAction> uses at least one byte for its length,
        // and the signature is required,
        // a valid max allocation can never exceed this size
        (MAX_BLOCK_BYTES - 1) / AUTHORIZED_ACTION_SIZE
    }
}

impl TrustedPreallocate for Signature<SpendAuth> {
    fn max_allocation() -> u64 {
        // Each signature must have a corresponding action.
        Action::max_allocation()
    }
}

bitflags! {
    /// Per-Transaction flags for Orchard.
    ///
    /// The spend and output flags are passed to the `Halo2Proof` verifier, which verifies
    /// the relevant note spending and creation consensus rules.
    ///
    /// Consensus rules:
    ///
    /// - "In a version 5 transaction, the reserved bits 2..7 of the flagsOrchard field MUST be zero."
    /// ([`bitflags`](https://docs.rs/bitflags/1.2.1/bitflags/index.html) restricts its values to the
    /// set of valid flags)
    /// - "In a version 5 coinbase transaction, the enableSpendsOrchard flag MUST be 0."
    /// (Checked in zebra-consensus)
    #[derive(Deserialize, Serialize)]
    pub struct Flags: u8 {
        /// Enable spending non-zero valued Orchard notes.
        ///
        /// "the `enableSpendsOrchard` flag, if present, MUST be 0 for coinbase transactions"
        const ENABLE_SPENDS = 0b00000001;
        /// Enable creating new non-zero valued Orchard notes.
        const ENABLE_OUTPUTS = 0b00000010;
    }
}

impl ZcashSerialize for Flags {
    fn zcash_serialize<W: io::Write>(&self, mut writer: W) -> Result<(), io::Error> {
        writer.write_u8(self.bits())?;

        Ok(())
    }
}

impl ZcashDeserialize for Flags {
    fn zcash_deserialize<R: io::Read>(mut reader: R) -> Result<Self, SerializationError> {
        // Consensus rule: "In a version 5 transaction,
        // the reserved bits 2..7 of the flagsOrchard field MUST be zero."
        // https://zips.z.cash/protocol/protocol.pdf#txnencodingandconsensus
        Flags::from_bits(reader.read_u8()?)
            .ok_or(SerializationError::Parse("invalid reserved orchard flags"))
    }
}