storage: Add common amounts commitment lookup table (#323)

Add common ammounts commitment lookup table

- Implements `compute_zero_commitment` function in `cuprate-helper::crypto` module.
- Added test that compare the function output with the correct calculation.
- Use of a constant-time algorithm for the lookup table.
- Added according documentation

helper/Cargo.toml

@@ -16,6 +16,7 @@ atomic    = ["dep:crossbeam"]
 asynch    = ["dep:futures", "dep:rayon"]
 cast      = []
 constants = []
+crypto    = ["dep:curve25519-dalek", "dep:monero-serai", "std"]
 fs        = ["dep:dirs"]
 num       = []
 map       = ["cast", "dep:monero-serai", "dep:cuprate-constants"]
@@ -26,8 +27,9 @@ tx        = ["dep:monero-serai"]
 [dependencies]
 cuprate-constants = { path = "../constants", optional = true, features = ["block"] }
 
-crossbeam    = { workspace = true, optional = true }
 chrono           = { workspace = true, optional = true, features = ["std", "clock"] }
+crossbeam        = { workspace = true, optional = true }
+curve25519-dalek = { workspace = true, optional = true }
 dirs             = { workspace = true, optional = true }
 futures          = { workspace = true, optional = true, features = ["std"] }
 monero-serai     = { workspace = true, optional = true }

helper/src/crypto.rs

@@ -0,0 +1,122 @@
+//! Crypto related functions and runtime initialized constants
+
+//---------------------------------------------------------------------------------------------------- Use
+use std::sync::LazyLock;
+
+use curve25519_dalek::{
+    constants::ED25519_BASEPOINT_POINT, edwards::VartimeEdwardsPrecomputation,
+    traits::VartimePrecomputedMultiscalarMul, EdwardsPoint, Scalar,
+};
+use monero_serai::generators::H;
+
+//---------------------------------------------------------------------------------------------------- Pre-computation
+
+/// This is the decomposed amount table containing the mandatory Pre-RCT amounts. It is used to pre-compute 
+/// zero commitments at runtime.
+/// 
+/// Defined at:
+/// - <https://github.com/monero-project/monero/blob/893916ad091a92e765ce3241b94e706ad012b62a/src/ringct/rctOps.cpp#L44>
+#[rustfmt::skip]
+pub const ZERO_COMMITMENT_DECOMPOSED_AMOUNT: [u64; 172] = [
+    1,                   2,                   3,                   4,                   5,                   6,                   7,                   8,                   9,
+    10,                  20,                  30,                  40,                  50,                  60,                  70,                  80,                  90,
+    100,                 200,                 300,                 400,                 500,                 600,                 700,                 800,                 900,
+    1000,                2000,                3000,                4000,                5000,                6000,                7000,                8000,                9000,
+    10000,               20000,               30000,               40000,               50000,               60000,               70000,               80000,               90000,
+    100000,              200000,              300000,              400000,              500000,              600000,              700000,              800000,              900000,
+    1000000,             2000000,             3000000,             4000000,             5000000,             6000000,             7000000,             8000000,             9000000,
+    10000000,            20000000,            30000000,            40000000,            50000000,            60000000,            70000000,            80000000,            90000000,
+    100000000,           200000000,           300000000,           400000000,           500000000,           600000000,           700000000,           800000000,           900000000,
+    1000000000,          2000000000,          3000000000,          4000000000,          5000000000,          6000000000,          7000000000,          8000000000,          9000000000,
+    10000000000,         20000000000,         30000000000,         40000000000,         50000000000,         60000000000,         70000000000,         80000000000,         90000000000,
+    100000000000,        200000000000,        300000000000,        400000000000,        500000000000,        600000000000,        700000000000,        800000000000,        900000000000,
+    1000000000000,       2000000000000,       3000000000000,       4000000000000,       5000000000000,       6000000000000,       7000000000000,       8000000000000,       9000000000000,
+    10000000000000,      20000000000000,      30000000000000,      40000000000000,      50000000000000,      60000000000000,      70000000000000,      80000000000000,      90000000000000,
+    100000000000000,     200000000000000,     300000000000000,     400000000000000,     500000000000000,     600000000000000,     700000000000000,     800000000000000,     900000000000000,
+    1000000000000000,    2000000000000000,    3000000000000000,    4000000000000000,    5000000000000000,    6000000000000000,    7000000000000000,    8000000000000000,    9000000000000000,
+    10000000000000000,   20000000000000000,   30000000000000000,   40000000000000000,   50000000000000000,   60000000000000000,   70000000000000000,   80000000000000000,   90000000000000000,
+    100000000000000000,  200000000000000000,  300000000000000000,  400000000000000000,  500000000000000000,  600000000000000000,  700000000000000000,  800000000000000000,  900000000000000000,
+    1000000000000000000, 2000000000000000000, 3000000000000000000, 4000000000000000000, 5000000000000000000, 6000000000000000000, 7000000000000000000, 8000000000000000000, 9000000000000000000,
+    10000000000000000000
+];
+
+/// Runtime initialized [`H`] generator.
+static H_PRECOMP: LazyLock<VartimeEdwardsPrecomputation> =
+    LazyLock::new(|| VartimeEdwardsPrecomputation::new([*H, ED25519_BASEPOINT_POINT]));
+
+/// Runtime initialized zero commitment lookup table
+///
+/// # Invariant
+/// This function assumes that the [`ZERO_COMMITMENT_DECOMPOSED_AMOUNT`]
+/// table is sorted.
+pub static ZERO_COMMITMENT_LOOKUP_TABLE: LazyLock<[EdwardsPoint; 172]> = LazyLock::new(|| {
+    let mut lookup_table: [EdwardsPoint; 172] = [ED25519_BASEPOINT_POINT; 172];
+
+    for (i, amount) in ZERO_COMMITMENT_DECOMPOSED_AMOUNT.into_iter().enumerate() {
+        lookup_table[i] = ED25519_BASEPOINT_POINT + *H * Scalar::from(amount);
+    }
+
+    lookup_table
+});
+
+//---------------------------------------------------------------------------------------------------- Free functions
+
+/// This function computes the zero commitment given a specific amount.
+///
+/// It will first attempt to lookup into the table of known Pre-RCT value.
+/// Compute it otherwise.
+#[expect(clippy::cast_possible_truncation)]
+pub fn compute_zero_commitment(amount: u64) -> EdwardsPoint {
+    // OPTIMIZATION: Unlike monerod which execute a linear search across its lookup
+    // table (O(n)). Cuprate is making use of an arithmetic based constant time
+    // version (O(1)). It has been benchmarked in both hit and miss scenarios against
+    // a binary search lookup (O(log2(n))). To understand the following algorithm it
+    // is important to observe the pattern that follows the values of
+    // [`ZERO_COMMITMENT_DECOMPOSED_AMOUNT`].
+
+    // First obtain the logarithm base 10 of the amount. and extend it back to obtain
+    // the amount without its most significant digit.
+    let Some(log) = amount.checked_ilog10() else {
+        // amount = 0 so H component is 0.
+        return ED25519_BASEPOINT_POINT;
+    };
+    let div = 10_u64.pow(log);
+
+    // Extract the most significant digit.
+    let most_significant_digit = amount / div;
+
+    // If the *rounded* version is different than the exact amount. Then
+    // there aren't only trailing zeroes behind the most significant digit.
+    // The amount is not part of the table and can calculated apart.
+    if most_significant_digit * div != amount {
+        return H_PRECOMP.vartime_multiscalar_mul([Scalar::from(amount), Scalar::ONE]);
+    }
+
+    // Calculating the index back by progressing within the powers of 10.
+    // The index of the first value in the cached amount's row.
+    let row_start = u64::from(log) * 9;
+    // The index of the cached amount
+    let index = (most_significant_digit - 1 + row_start) as usize;
+
+    ZERO_COMMITMENT_LOOKUP_TABLE[index]
+}
+
+//---------------------------------------------------------------------------------------------------- Tests
+#[cfg(test)]
+mod test {
+    use curve25519_dalek::{traits::VartimePrecomputedMultiscalarMul, Scalar};
+
+    use crate::crypto::{compute_zero_commitment, H_PRECOMP, ZERO_COMMITMENT_DECOMPOSED_AMOUNT};
+
+    #[test]
+    /// Compare the output of `compute_zero_commitment` for all
+    /// preRCT decomposed amounts against their actual computation.
+    ///
+    /// Assert that the lookup table returns the correct commitments
+    fn compare_lookup_with_computation() {
+        for amount in ZERO_COMMITMENT_DECOMPOSED_AMOUNT {
+            let commitment = H_PRECOMP.vartime_multiscalar_mul([Scalar::from(amount), Scalar::ONE]);
+            assert!(commitment == compute_zero_commitment(amount));
+        }
+    }
+}

helper/src/lib.rs

@@ -30,6 +30,9 @@ pub mod time;
 
 #[cfg(feature = "tx")]
 pub mod tx;
+
+#[cfg(feature = "crypto")]
+pub mod crypto;
 //---------------------------------------------------------------------------------------------------- Private Usage
 
 //----------------------------------------------------------------------------------------------------

storage/blockchain/Cargo.toml

@@ -20,7 +20,7 @@ service     = ["dep:thread_local", "dep:rayon", "cuprate-helper/thread"]
 [dependencies]
 cuprate-database         = { path = "../database" }
 cuprate-database-service = { path = "../service" }
-cuprate-helper           = { path = "../../helper", features = ["fs", "map"] }
+cuprate-helper           = { path = "../../helper", features = ["fs", "map", "crypto"] }
 cuprate-types            = { path = "../../types", features = ["blockchain"] }
 cuprate-pruning          = { path = "../../pruning" }
 

storage/blockchain/src/ops/output.rs

@@ -1,12 +1,13 @@
 //! Output functions.
 
 //---------------------------------------------------------------------------------------------------- Import
-use curve25519_dalek::{constants::ED25519_BASEPOINT_POINT, edwards::CompressedEdwardsY, Scalar};
+use curve25519_dalek::edwards::CompressedEdwardsY;
-use monero_serai::{generators::H, transaction::Timelock};
+use monero_serai::transaction::Timelock;
 
 use cuprate_database::{
     RuntimeError, {DatabaseRo, DatabaseRw},
 };
+use cuprate_helper::crypto::compute_zero_commitment;
 use cuprate_helper::map::u64_to_timelock;
 use cuprate_types::OutputOnChain;
 
@@ -155,9 +156,7 @@ pub fn output_to_output_on_chain(
     amount: Amount,
     table_tx_unlock_time: &impl DatabaseRo<TxUnlockTime>,
 ) -> Result<OutputOnChain, RuntimeError> {
-    // FIXME: implement lookup table for common values:
+    let commitment = compute_zero_commitment(amount);
-    // <https://github.com/monero-project/monero/blob/c8214782fb2a769c57382a999eaf099691c836e7/src/ringct/rctOps.cpp#L322>
-    let commitment = ED25519_BASEPOINT_POINT + *H * Scalar::from(amount);
 
     let time_lock = if output
         .output_flags

storage/blockchain/src/ops/tx.rs

@@ -2,10 +2,10 @@
 
 //---------------------------------------------------------------------------------------------------- Import
 use bytemuck::TransparentWrapper;
-use curve25519_dalek::{constants::ED25519_BASEPOINT_POINT, Scalar};
 use monero_serai::transaction::{Input, Timelock, Transaction};
 
 use cuprate_database::{DatabaseRo, DatabaseRw, RuntimeError, StorableVec};
+use cuprate_helper::crypto::compute_zero_commitment;
 
 use crate::{
     ops::{
@@ -136,12 +136,9 @@ pub fn add_tx(
             .enumerate()
             .map(|(i, output)| {
                 // Create commitment.
-                // <https://github.com/Cuprate/cuprate/pull/102#discussion_r1559489302>
+
-                // FIXME: implement lookup table for common values:
-                // <https://github.com/monero-project/monero/blob/c8214782fb2a769c57382a999eaf099691c836e7/src/ringct/rctOps.cpp#L322>
                 let commitment = if miner_tx {
-                    ED25519_BASEPOINT_POINT
+                    compute_zero_commitment(output.amount.unwrap_or(0))
-                        + *monero_serai::generators::H * Scalar::from(output.amount.unwrap_or(0))
                 } else {
                     proofs
                         .as_ref()