fix: issue 4682 and add solver for unconstrained bigintegers (#4729)

# Description

## Problem\*

Managing bigintegers as ID makes it difficult to handle them in
conditionals and also when passing them around unconstrained functions.
As a matter of fact these two use cases do not work, the first one being
documented in issue #4682, while the second one was never implemented.

## Summary\*

In order to solve this two issues, I modified the bigint representation
so that it stores its byte array instead of an id, allowing it to work
directly with conditional and unconstrained functions.


## Additional Context

For simplicity, I also made the byte arrays of hardcoded length 32.
Since the moduli we support are of this length and since custom
bigintegers will be implemented differently, I do not expect this length
to change.
The barrentenberg interface has not changed and we still use id for it.
The Id is constructed when required by doing a call to from_bytes.

## Documentation\*
The traits have not changed, I have simply added a from_byte_32 method
which takes a 32-bytes array as input.

Check one:
- [] No documentation needed.
- [X] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [X] I have tested the changes locally.
- [X] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

---------

Co-authored-by: Tom French <tom@tomfren.ch>

acvm-repo/acvm/src/pwg/blackbox/bigint.rs

@@ -1,53 +1,23 @@
-use std::collections::HashMap;
-
 use acir::{
     circuit::opcodes::FunctionInput,
     native_types::{Witness, WitnessMap},
     BlackBoxFunc, FieldElement,
 };
 
-use num_bigint::BigUint;
+use acvm_blackbox_solver::BigIntSolver;
 
 use crate::pwg::OpcodeResolutionError;
 
-/// Resolve BigInt opcodes by storing BigInt values (and their moduli) by their ID in a HashMap:
+/// Resolve BigInt opcodes by storing BigInt values (and their moduli) by their ID in the BigIntSolver
 /// - When it encounters a bigint operation opcode, it performs the operation on the stored values
 /// and store the result using the provided ID.
 /// - When it gets a to_bytes opcode, it simply looks up the value and resolves the output witness accordingly.
 #[derive(Default)]
-pub(crate) struct BigIntSolver {
-    bigint_id_to_value: HashMap<u32, BigUint>,
-    bigint_id_to_modulus: HashMap<u32, BigUint>,
+pub(crate) struct AcvmBigIntSolver {
+    bigint_solver: BigIntSolver,
 }
 
-impl BigIntSolver {
-    pub(crate) fn get_bigint(
-        &self,
-        id: u32,
-        func: BlackBoxFunc,
-    ) -> Result<BigUint, OpcodeResolutionError> {
-        self.bigint_id_to_value
-            .get(&id)
-            .ok_or(OpcodeResolutionError::BlackBoxFunctionFailed(
-                func,
-                format!("could not find bigint of id {id}"),
-            ))
-            .cloned()
-    }
-
-    pub(crate) fn get_modulus(
-        &self,
-        id: u32,
-        func: BlackBoxFunc,
-    ) -> Result<BigUint, OpcodeResolutionError> {
-        self.bigint_id_to_modulus
-            .get(&id)
-            .ok_or(OpcodeResolutionError::BlackBoxFunctionFailed(
-                func,
-                format!("could not find bigint of id {id}"),
-            ))
-            .cloned()
-    }
+impl AcvmBigIntSolver {
     pub(crate) fn bigint_from_bytes(
         &mut self,
         inputs: &[FunctionInput],
@@ -59,10 +29,7 @@ impl BigIntSolver {
             .iter()
             .map(|input| initial_witness.get(&input.witness).unwrap().to_u128() as u8)
             .collect::<Vec<u8>>();
-        let bigint = BigUint::from_bytes_le(&bytes);
-        self.bigint_id_to_value.insert(output, bigint);
-        let modulus = BigUint::from_bytes_le(modulus);
-        self.bigint_id_to_modulus.insert(output, modulus);
+        self.bigint_solver.bigint_from_bytes(&bytes, modulus, output)?;
         Ok(())
     }
 
@@ -72,9 +39,7 @@ impl BigIntSolver {
         outputs: &[Witness],
         initial_witness: &mut WitnessMap,
     ) -> Result<(), OpcodeResolutionError> {
-        let bigint = self.get_bigint(input, BlackBoxFunc::BigIntToLeBytes)?;
-
-        let mut bytes = bigint.to_bytes_le();
+        let mut bytes = self.bigint_solver.bigint_to_bytes(input)?;
         while bytes.len() < outputs.len() {
             bytes.push(0);
         }
@@ -91,30 +56,7 @@ impl BigIntSolver {
         output: u32,
         func: BlackBoxFunc,
     ) -> Result<(), OpcodeResolutionError> {
-        let modulus = self.get_modulus(lhs, func)?;
-        let lhs = self.get_bigint(lhs, func)?;
-        let rhs = self.get_bigint(rhs, func)?;
-        let mut result = match func {
-            BlackBoxFunc::BigIntAdd => lhs + rhs,
-            BlackBoxFunc::BigIntSub => {
-                if lhs >= rhs {
-                    &lhs - &rhs
-                } else {
-                    &lhs + &modulus - &rhs
-                }
-            }
-            BlackBoxFunc::BigIntMul => lhs * rhs,
-            BlackBoxFunc::BigIntDiv => {
-                lhs * rhs.modpow(&(&modulus - BigUint::from(2_u32)), &modulus)
-            } //TODO ensure that modulus is prime
-            _ => unreachable!("ICE - bigint_op must be called for an operation"),
-        };
-        if result > modulus {
-            let q = &result / &modulus;
-            result -= q * &modulus;
-        }
-        self.bigint_id_to_value.insert(output, result);
-        self.bigint_id_to_modulus.insert(output, modulus);
+        self.bigint_solver.bigint_op(lhs, rhs, output, func)?;
         Ok(())
     }
 }

acvm-repo/acvm/src/pwg/blackbox/mod.rs

@@ -6,7 +6,7 @@ use acir::{
 use acvm_blackbox_solver::{blake2s, blake3, keccak256, keccakf1600, sha256};
 
 use self::{
-    bigint::BigIntSolver, hash::solve_poseidon2_permutation_opcode, pedersen::pedersen_hash,
+    bigint::AcvmBigIntSolver, hash::solve_poseidon2_permutation_opcode, pedersen::pedersen_hash,
 };
 
 use super::{insert_value, OpcodeNotSolvable, OpcodeResolutionError};
@@ -56,7 +56,7 @@ pub(crate) fn solve(
     backend: &impl BlackBoxFunctionSolver,
     initial_witness: &mut WitnessMap,
     bb_func: &BlackBoxFuncCall,
-    bigint_solver: &mut BigIntSolver,
+    bigint_solver: &mut AcvmBigIntSolver,
 ) -> Result<(), OpcodeResolutionError> {
     let inputs = bb_func.get_inputs_vec();
     if !contains_all_inputs(initial_witness, &inputs) {

acvm-repo/acvm/src/pwg/mod.rs

@@ -11,7 +11,7 @@ use acir::{
 use acvm_blackbox_solver::BlackBoxResolutionError;
 
 use self::{
-    arithmetic::ExpressionSolver, blackbox::bigint::BigIntSolver, directives::solve_directives,
+    arithmetic::ExpressionSolver, blackbox::bigint::AcvmBigIntSolver, directives::solve_directives,
     memory_op::MemoryOpSolver,
 };
 use crate::BlackBoxFunctionSolver;
@@ -148,7 +148,7 @@ pub struct ACVM<'a, B: BlackBoxFunctionSolver> {
     /// Stores the solver for memory operations acting on blocks of memory disambiguated by [block][`BlockId`].
     block_solvers: HashMap<BlockId, MemoryOpSolver>,
 
-    bigint_solver: BigIntSolver,
+    bigint_solver: AcvmBigIntSolver,
 
     /// A list of opcodes which are to be executed by the ACVM.
     opcodes: &'a [Opcode],
@@ -174,7 +174,7 @@ impl<'a, B: BlackBoxFunctionSolver> ACVM<'a, B> {
             status,
             backend,
             block_solvers: HashMap::default(),
-            bigint_solver: BigIntSolver::default(),
+            bigint_solver: AcvmBigIntSolver::default(),
             opcodes,
             instruction_pointer: 0,
             witness_map: initial_witness,

acvm-repo/blackbox_solver/Cargo.toml

@@ -15,6 +15,7 @@ repository.workspace = true
 [dependencies]
 acir.workspace = true
 thiserror.workspace = true
+num-bigint = "0.4"
 
 blake2 = "0.10.6"
 blake3 = "1.5.0"

acvm-repo/blackbox_solver/src/bigint.rs

@@ -0,0 +1,99 @@
+use std::collections::HashMap;
+
+use acir::BlackBoxFunc;
+
+use num_bigint::BigUint;
+
+use crate::BlackBoxResolutionError;
+
+/// Resolve BigInt opcodes by storing BigInt values (and their moduli) by their ID in a HashMap:
+/// - When it encounters a bigint operation opcode, it performs the operation on the stored values
+/// and store the result using the provided ID.
+/// - When it gets a to_bytes opcode, it simply looks up the value and resolves the output witness accordingly.
+#[derive(Default, Debug, Clone, PartialEq, Eq)]
+
+pub struct BigIntSolver {
+    bigint_id_to_value: HashMap<u32, BigUint>,
+    bigint_id_to_modulus: HashMap<u32, BigUint>,
+}
+
+impl BigIntSolver {
+    pub(crate) fn get_bigint(
+        &self,
+        id: u32,
+        func: BlackBoxFunc,
+    ) -> Result<BigUint, BlackBoxResolutionError> {
+        self.bigint_id_to_value
+            .get(&id)
+            .ok_or(BlackBoxResolutionError::Failed(
+                func,
+                format!("could not find bigint of id {id}"),
+            ))
+            .cloned()
+    }
+
+    pub(crate) fn get_modulus(
+        &self,
+        id: u32,
+        func: BlackBoxFunc,
+    ) -> Result<BigUint, BlackBoxResolutionError> {
+        self.bigint_id_to_modulus
+            .get(&id)
+            .ok_or(BlackBoxResolutionError::Failed(
+                func,
+                format!("could not find bigint of id {id}"),
+            ))
+            .cloned()
+    }
+    pub fn bigint_from_bytes(
+        &mut self,
+        inputs: &[u8],
+        modulus: &[u8],
+        output: u32,
+    ) -> Result<(), BlackBoxResolutionError> {
+        let bigint = BigUint::from_bytes_le(inputs);
+        self.bigint_id_to_value.insert(output, bigint);
+        let modulus = BigUint::from_bytes_le(modulus);
+        self.bigint_id_to_modulus.insert(output, modulus);
+        Ok(())
+    }
+
+    pub fn bigint_to_bytes(&self, input: u32) -> Result<Vec<u8>, BlackBoxResolutionError> {
+        let bigint = self.get_bigint(input, BlackBoxFunc::BigIntToLeBytes)?;
+        Ok(bigint.to_bytes_le())
+    }
+
+    pub fn bigint_op(
+        &mut self,
+        lhs: u32,
+        rhs: u32,
+        output: u32,
+        func: BlackBoxFunc,
+    ) -> Result<(), BlackBoxResolutionError> {
+        let modulus = self.get_modulus(lhs, func)?;
+        let lhs = self.get_bigint(lhs, func)?;
+        let rhs = self.get_bigint(rhs, func)?;
+        let mut result = match func {
+            BlackBoxFunc::BigIntAdd => lhs + rhs,
+            BlackBoxFunc::BigIntSub => {
+                if lhs >= rhs {
+                    &lhs - &rhs
+                } else {
+                    &lhs + &modulus - &rhs
+                }
+            }
+            BlackBoxFunc::BigIntMul => lhs * rhs,
+            BlackBoxFunc::BigIntDiv => {
+                lhs * rhs.modpow(&(&modulus - BigUint::from(2_u32)), &modulus)
+            } //TODO ensure that modulus is prime
+            _ => unreachable!("ICE - bigint_op must be called for an operation"),
+        };
+        if result > modulus {
+            let q = &result / &modulus;
+            result -= q * &modulus;
+        }
+        self.bigint_id_to_value.insert(output, result);
+        self.bigint_id_to_modulus.insert(output, modulus);
+        Ok(())
+    }
+}

acvm-repo/blackbox_solver/src/lib.rs

@@ -10,10 +10,12 @@
 use acir::BlackBoxFunc;
 use thiserror::Error;
 
+mod bigint;
 mod curve_specific_solver;
 mod ecdsa;
 mod hash;
 
+pub use bigint::BigIntSolver;
 pub use curve_specific_solver::{BlackBoxFunctionSolver, StubbedBlackBoxSolver};
 pub use ecdsa::{ecdsa_secp256k1_verify, ecdsa_secp256r1_verify};
 pub use hash::{blake2s, blake3, keccak256, keccakf1600, sha256, sha256compression};

acvm-repo/brillig_vm/src/black_box.rs

@@ -1,5 +1,6 @@
 use acir::brillig::{BlackBoxOp, HeapArray, HeapVector};
 use acir::{BlackBoxFunc, FieldElement};
+use acvm_blackbox_solver::BigIntSolver;
 use acvm_blackbox_solver::{
     blake2s, blake3, ecdsa_secp256k1_verify, ecdsa_secp256r1_verify, keccak256, keccakf1600,
     sha256, sha256compression, BlackBoxFunctionSolver, BlackBoxResolutionError,
@@ -34,6 +35,7 @@ pub(crate) fn evaluate_black_box<Solver: BlackBoxFunctionSolver>(
     op: &BlackBoxOp,
     solver: &Solver,
     memory: &mut Memory,
+    bigint_solver: &mut BigIntSolver,
 ) -> Result<(), BlackBoxResolutionError> {
     match op {
         BlackBoxOp::Sha256 { message, output } => {
@@ -177,12 +179,57 @@ pub(crate) fn evaluate_black_box<Solver: BlackBoxFunctionSolver>(
             memory.write(*output, hash.into());
             Ok(())
         }
-        BlackBoxOp::BigIntAdd { .. } => todo!(),
-        BlackBoxOp::BigIntSub { .. } => todo!(),
-        BlackBoxOp::BigIntMul { .. } => todo!(),
-        BlackBoxOp::BigIntDiv { .. } => todo!(),
-        BlackBoxOp::BigIntFromLeBytes { .. } => todo!(),
-        BlackBoxOp::BigIntToLeBytes { .. } => todo!(),
+        BlackBoxOp::BigIntAdd { lhs, rhs, output } => {
+            let lhs = memory.read(*lhs).try_into().unwrap();
+            let rhs = memory.read(*rhs).try_into().unwrap();
+            let output = memory.read(*output).try_into().unwrap();
+            bigint_solver.bigint_op(lhs, rhs, output, BlackBoxFunc::BigIntAdd)?;
+            Ok(())
+        }
+        BlackBoxOp::BigIntSub { lhs, rhs, output } => {
+            let lhs = memory.read(*lhs).try_into().unwrap();
+            let rhs = memory.read(*rhs).try_into().unwrap();
+            let output = memory.read(*output).try_into().unwrap();
+            bigint_solver.bigint_op(lhs, rhs, output, BlackBoxFunc::BigIntSub)?;
+            Ok(())
+        }
+        BlackBoxOp::BigIntMul { lhs, rhs, output } => {
+            let lhs = memory.read(*lhs).try_into().unwrap();
+            let rhs = memory.read(*rhs).try_into().unwrap();
+            let output = memory.read(*output).try_into().unwrap();
+            bigint_solver.bigint_op(lhs, rhs, output, BlackBoxFunc::BigIntMul)?;
+            Ok(())
+        }
+        BlackBoxOp::BigIntDiv { lhs, rhs, output } => {
+            let lhs = memory.read(*lhs).try_into().unwrap();
+            let rhs = memory.read(*rhs).try_into().unwrap();
+            let output = memory.read(*output).try_into().unwrap();
+            bigint_solver.bigint_op(lhs, rhs, output, BlackBoxFunc::BigIntDiv)?;
+            Ok(())
+        }
+        BlackBoxOp::BigIntFromLeBytes { inputs, modulus, output } => {
+            let input = read_heap_vector(memory, inputs);
+            let input: Vec<u8> = input.iter().map(|x| x.try_into().unwrap()).collect();
+            let modulus = read_heap_vector(memory, modulus);
+            let modulus: Vec<u8> = modulus.iter().map(|x| x.try_into().unwrap()).collect();
+            let output = memory.read(*output).try_into().unwrap();
+            bigint_solver.bigint_from_bytes(&input, &modulus, output)?;
+            Ok(())
+        }
+        BlackBoxOp::BigIntToLeBytes { input, output } => {
+            let input: u32 = memory.read(*input).try_into().unwrap();
+            let bytes = bigint_solver.bigint_to_bytes(input)?;
+            let mut values = Vec::new();
+            for i in 0..32 {
+                if i < bytes.len() {
+                    values.push(bytes[i].into());
+                } else {
+                    values.push(0_u8.into());
+                }
+            }
+            memory.write_slice(memory.read_ref(output.pointer), &values);
+            Ok(())
+        }
         BlackBoxOp::Poseidon2Permutation { message, output, len } => {
             let input = read_heap_vector(memory, message);
             let input: Vec<FieldElement> = input.iter().map(|x| x.try_into().unwrap()).collect();
@@ -256,7 +303,7 @@ fn black_box_function_from_op(op: &BlackBoxOp) -> BlackBoxFunc {
 #[cfg(test)]
 mod test {
     use acir::brillig::{BlackBoxOp, MemoryAddress};
-    use acvm_blackbox_solver::StubbedBlackBoxSolver;
+    use acvm_blackbox_solver::{BigIntSolver, StubbedBlackBoxSolver};
 
     use crate::{
         black_box::{evaluate_black_box, to_u8_vec, to_value_vec},
@@ -281,7 +328,8 @@ mod test {
             output: HeapArray { pointer: 2.into(), size: 32 },
         };
 
-        evaluate_black_box(&op, &StubbedBlackBoxSolver, &mut memory).unwrap();
+        evaluate_black_box(&op, &StubbedBlackBoxSolver, &mut memory, &mut BigIntSolver::default())
+            .unwrap();
 
         let result = memory.read_slice(MemoryAddress(result_pointer), 32);