feat(precompile): use Aurora modexp lib. (#769)

* Aurora modexp precompile wip

* Clean input parsing and comments

* cargo toml, rm num add modexp rev

* bump aurora modexp to crate version

* Update crates/precompile/src/utilities.rs

* Update crates/precompile/src/utilities.rs

crates/precompile/Cargo.toml

@@ -11,10 +11,11 @@ version = "2.2.0"
 [dependencies]
 revm-primitives = { path = "../primitives", version = "1.3.0", default-features = false }
 bn = { package = "substrate-bn", version = "0.6", default-features = false }
-num = { version = "0.4.0", default-features = false, features = ["alloc"] }
 once_cell = { version = "1.17", default-features = false, features = ["alloc"] }
 ripemd = { version = "0.1", default-features = false }
 sha2 = { version = "0.10", default-features = false }
+# modexp precompile
+aurora-engine-modexp = { version = "1.0", default-features = false }
 
 # Optional KZG point evaluation precompile
 c-kzg = { version = "0.1.1", default-features = false, optional = true }
@@ -26,12 +27,12 @@ secp256k1 = { version = "0.27.0", default-features = false, features = [
     "recovery",
 ], optional = true }
 
+
 [features]
 default = ["std", "c-kzg", "secp256k1"]
 std = [
     "revm-primitives/std",
     "k256/std",
-    "num/std",
     "once_cell/std",
     "ripemd/std",
     "sha2/std",
@@ -41,7 +42,7 @@ std = [
 
 optimism = ["revm-primitives/optimism"]
 
-# This library may not work on all no_std platforms as they depend on C libraries.
+# These libraries may not work on all no_std platforms as they depend on C.
 
 # Enables the KZG point evaluation precompile.
 c-kzg = ["dep:c-kzg", "revm-primitives/c-kzg"]

crates/precompile/src/lib.rs

@@ -17,6 +17,7 @@ mod identity;
 pub mod kzg_point_evaluation;
 mod modexp;
 mod secp256k1;
+pub mod utilities;
 
 use alloc::{boxed::Box, vec::Vec};
 use core::fmt;

crates/precompile/src/modexp.rs

@@ -1,12 +1,11 @@
 use crate::{
-    primitives::U256, Error, Precompile, PrecompileAddress, PrecompileResult, StandardPrecompileFn,
+    primitives::U256,
+    utilities::{get_right_padded, get_right_padded_vec, left_padding, left_padding_vec},
+    Error, Precompile, PrecompileAddress, PrecompileResult, StandardPrecompileFn,
 };
 use alloc::vec::Vec;
-use core::{
-    cmp::{max, min, Ordering},
-    mem::size_of,
-};
-use num::{BigUint, One, Zero};
+use aurora_engine_modexp::modexp;
+use core::cmp::{max, min};
 
 pub const BYZANTIUM: PrecompileAddress = PrecompileAddress(
     crate::u64_to_address(5),
@@ -32,121 +31,96 @@ pub fn berlin_run(input: &[u8], gas_limit: u64) -> PrecompileResult {
     })
 }
 
-fn calculate_iteration_count(exp_length: u64, exp_highp: &BigUint) -> u64 {
+fn calculate_iteration_count(exp_length: u64, exp_highp: &U256) -> u64 {
     let mut iteration_count: u64 = 0;
 
-    if exp_length <= 32 && exp_highp.is_zero() {
+    if exp_length <= 32 && *exp_highp == U256::ZERO {
         iteration_count = 0;
     } else if exp_length <= 32 {
-        iteration_count = exp_highp.bits() - 1;
+        iteration_count = exp_highp.bit_len() as u64 - 1;
     } else if exp_length > 32 {
-        iteration_count = (8 * (exp_length - 32)) + max(1, exp_highp.bits()) - 1;
+        iteration_count = (8 * (exp_length - 32)) + max(1, exp_highp.bit_len() as u64) - 1;
     }
 
     max(iteration_count, 1)
 }
 
-macro_rules! read_u64_with_overflow {
-    ($input:expr, $from:expr, $to:expr, $overflow_limit:expr) => {{
-        const SPLIT: usize = 32 - size_of::<u64>();
-        let len = $input.len();
-        let from_zero = min($from, len);
-        let from = min(from_zero + SPLIT, len);
-        let to = min($to, len);
-        let overflow_bytes = &$input[from_zero..from];
-
-        let mut len_bytes = [0u8; size_of::<u64>()];
-        len_bytes[..to - from].copy_from_slice(&$input[from..to]);
-        let out = u64::from_be_bytes(len_bytes) as usize;
-        let overflow = !(out < $overflow_limit && overflow_bytes.iter().all(|&x| x == 0));
-        (out, overflow)
-    }};
-}
-
 fn run_inner<F>(input: &[u8], gas_limit: u64, min_gas: u64, calc_gas: F) -> PrecompileResult
 where
-    F: FnOnce(u64, u64, u64, &BigUint) -> u64,
+    F: FnOnce(u64, u64, u64, &U256) -> u64,
 {
-    let len = input.len();
-    let (base_len, base_overflow) = read_u64_with_overflow!(input, 0, 32, u32::MAX as usize);
-    let (exp_len, exp_overflow) = read_u64_with_overflow!(input, 32, 64, u32::MAX as usize);
-    let (mod_len, mod_overflow) = read_u64_with_overflow!(input, 64, 96, u32::MAX as usize);
-
-    if base_overflow || mod_overflow {
-        return Err(Error::ModexpBaseOverflow);
+    // If there is no minimum gas, return error.
+    if min_gas > gas_limit {
+        return Err(Error::OutOfGas);
     }
-
-    if mod_overflow {
+    // The format of input is:
+    // <length_of_BASE> <length_of_EXPONENT> <length_of_MODULUS> <BASE> <EXPONENT> <MODULUS>
+    // Where every length is a 32-byte left-padded integer representing the number of bytes
+    // to be taken up by the next value
+    const HEADER_LENGTH: usize = 96;
+
+    // Extract the header.
+    let base_len = U256::from_be_bytes(get_right_padded::<32>(input, 0));
+    let exp_len = U256::from_be_bytes(get_right_padded::<32>(input, 32));
+    let mod_len = U256::from_be_bytes(get_right_padded::<32>(input, 64));
+
+    // cast base and modulus to usize, it does not make sense to handle larger values
+    let Ok(base_len) = usize::try_from(base_len) else {
+        return Err(Error::ModexpBaseOverflow);
+    };
+    let Ok(mod_len) = usize::try_from(mod_len) else {
         return Err(Error::ModexpModOverflow);
+    };
+
+    // Handle a special case when both the base and mod length is zero
+    if base_len == 0 && mod_len == 0 {
+        return Ok((min_gas, Vec::new()));
     }
 
-    let (r, gas_cost) = if base_len == 0 && mod_len == 0 {
-        if min_gas > gas_limit {
-            return Err(Error::OutOfGas);
-        }
-        (BigUint::zero(), min_gas)
-    } else {
-        // set limit for exp overflow
-        if exp_overflow {
-            return Err(Error::ModexpExpOverflow);
-        }
-        let base_start = 96;
-        let base_end = base_start + base_len;
-        let exp_end = base_end + exp_len;
-        let exp_highp_end = base_end + min(32, exp_len);
-        let mod_end = exp_end + mod_len;
-
-        let exp_highp = {
-            let mut out = [0; 32];
-            let from = min(base_end, len);
-            let to = min(exp_highp_end, len);
-            let target_from = 32 - (exp_highp_end - base_end); // 32 - exp length
-            let target_to = target_from + (to - from); // beginning + size to copy
-            out[target_from..target_to].copy_from_slice(&input[from..to]);
-            BigUint::from_bytes_be(&out)
-        };
-
-        let gas_cost = calc_gas(base_len as u64, exp_len as u64, mod_len as u64, &exp_highp);
-        if gas_cost > gas_limit {
-            return Err(Error::OutOfGas);
-        }
+    // cast exponent length to usize, it does not make sense to handle larger values.
+    let Ok(exp_len) = usize::try_from(exp_len) else {
+        return Err(Error::ModexpModOverflow);
+    };
 
-        let read_big = |from: usize, to: usize| {
-            let mut out = vec![0; to - from];
-            let from = min(from, len);
-            let to = min(to, len);
-            out[..to - from].copy_from_slice(&input[from..to]);
-            BigUint::from_bytes_be(&out)
-        };
+    // Used to extract ADJUSTED_EXPONENT_LENGTH.
+    let exp_highp_len = min(exp_len, 32);
 
-        let base = read_big(base_start, base_end);
-        let exponent = read_big(base_end, exp_end);
-        let modulus = read_big(exp_end, mod_end);
+    // throw away the header data as we already extracted lengths.
+    let input = if input.len() >= 96 {
+        &input[HEADER_LENGTH..]
+    } else {
+        // or set input to zero if there is no more data
+        &[]
+    };
 
-        if modulus.is_zero() || modulus.is_one() {
-            (BigUint::zero(), gas_cost)
-        } else {
-            (base.modpow(&exponent, &modulus), gas_cost)
-        }
+    let exp_highp = {
+        // get right padded bytes so if data.len is less then exp_len we will get right padded zeroes.
+        let right_padded_highp = get_right_padded::<32>(input, base_len);
+        // If exp_len is less then 32 bytes get only exp_len bytes and do left padding.
+        let out = left_padding::<32>(&right_padded_highp[..exp_highp_len]);
+        U256::from_be_bytes(out)
     };
 
-    // write output to given memory, left padded and same length as the modulus.
-    let bytes = r.to_bytes_be();
-    // always true except in the case of zero-length modulus, which leads to
-    // output of length and value 1.
-    match bytes.len().cmp(&mod_len) {
-        Ordering::Equal => Ok((gas_cost, bytes)),
-        Ordering::Less => {
-            let mut ret = Vec::with_capacity(mod_len);
-            ret.extend(core::iter::repeat(0).take(mod_len - bytes.len()));
-            ret.extend_from_slice(&bytes[..]);
-            Ok((gas_cost, ret))
-        }
-        Ordering::Greater => Ok((gas_cost, Vec::new())),
+    // calculate gas spent.
+    let gas_cost = calc_gas(base_len as u64, exp_len as u64, mod_len as u64, &exp_highp);
+    // check if we have enough gas.
+    if gas_cost > gas_limit {
+        return Err(Error::OutOfGas);
     }
+
+    // Padding is needed if the input does not contain all 3 values.
+    let base = get_right_padded_vec(input, 0, base_len);
+    let exponent = get_right_padded_vec(input, base_len, exp_len);
+    let modulus = get_right_padded_vec(input, base_len.saturating_add(exp_len), mod_len);
+
+    // Call the modexp.
+    let output = modexp(&base, &exponent, &modulus);
+
+    // left pad the result to modulus length. bytes will always by less or equal to modulus length.
+    Ok((gas_cost, left_padding_vec(&output, mod_len)))
 }
 
-fn byzantium_gas_calc(base_len: u64, exp_len: u64, mod_len: u64, exp_highp: &BigUint) -> u64 {
+fn byzantium_gas_calc(base_len: u64, exp_len: u64, mod_len: u64, exp_highp: &U256) -> u64 {
     // ouput of this function is bounded by 2^128
     fn mul_complexity(x: u64) -> U256 {
         if x <= 64 {
@@ -175,7 +149,7 @@ fn byzantium_gas_calc(base_len: u64, exp_len: u64, mod_len: u64, exp_highp: &Big
 
 // Calculate gas cost according to EIP 2565:
 // https://eips.ethereum.org/EIPS/eip-2565
-fn berlin_gas_calc(base_length: u64, exp_length: u64, mod_length: u64, exp_highp: &BigUint) -> u64 {
+fn berlin_gas_calc(base_length: u64, exp_length: u64, mod_length: u64, exp_highp: &U256) -> u64 {
     fn calculate_multiplication_complexity(base_length: u64, mod_length: u64) -> U256 {
         let max_length = max(base_length, mod_length);
         let mut words = max_length / 8;

crates/precompile/src/utilities.rs

@@ -0,0 +1,43 @@
+use core::cmp::min;
+
+use alloc::vec::Vec;
+
+/// Get an array from the data, if data does not contain `start` to `len` bytes, add right padding with
+/// zeroes
+#[inline(always)]
+pub fn get_right_padded<const S: usize>(data: &[u8], offset: usize) -> [u8; S] {
+    let mut padded = [0; S];
+    let start = min(offset, data.len());
+    let end = min(start.saturating_add(S), data.len());
+    padded[..end - start].copy_from_slice(&data[start..end]);
+    padded
+}
+
+/// Get a vector of the data, if data does not contain the slice of `start` to `len`, right pad missing
+/// part with zeroes
+#[inline(always)]
+pub fn get_right_padded_vec(data: &[u8], offset: usize, len: usize) -> Vec<u8> {
+    let mut padded = vec![0; len];
+    let start = min(offset, data.len());
+    let end = min(start.saturating_add(len), data.len());
+    padded[..end - start].copy_from_slice(&data[start..end]);
+    padded
+}
+
+/// Left padding until `len`. If data is more then len, truncate the right most bytes.
+#[inline(always)]
+pub fn left_padding<const S: usize>(data: &[u8]) -> [u8; S] {
+    let mut padded = [0; S];
+    let end = min(S, data.len());
+    padded[S - end..].copy_from_slice(&data[..end]);
+    padded
+}
+
+/// Left padding until `len`. If data is more then len, truncate the right most bytes.
+#[inline(always)]
+pub fn left_padding_vec(data: &[u8], len: usize) -> Vec<u8> {
+    let mut padded = vec![0; len];
+    let end = min(len, data.len());
+    padded[len - end..].copy_from_slice(&data[..end]);
+    padded
+}