add check for limbs not being less than 2^128

blackbox_solver/src/barretenberg/wasm/mod.rs

@@ -34,8 +34,10 @@ pub(crate) enum FeatureError {
     NoValue,
     #[error("Value expected to be i32")]
     InvalidI32,
-    #[error("Value is not a valid grumpkin scalar")]
+    #[error("Value {scalar_as_hex} is not a valid grumpkin scalar")]
     InvalidGrumpkinScalar { scalar_as_hex: String },
+    #[error("Limb {limb_as_hex} is not less than 2^128")]
+    InvalidGrumpkinScalarLimb { limb_as_hex: String },
     #[error("Could not convert value {value} from i32 to u32")]
     InvalidU32 { value: i32, source: std::num::TryFromIntError },
     #[error("Could not convert value {value} from i32 to usize")]

blackbox_solver/src/barretenberg/wasm/scalar_mul.rs

@@ -31,6 +31,18 @@ impl ScalarMul for Barretenberg {
         let mut bytes = high_16_bytes.to_vec();
         bytes.extend_from_slice(&low_16_bytes);
 
+        let two_pow_128 = BigUint::from(2u128).pow(128);
+        if BigUint::from_bytes_be(&low_16_bytes) >= two_pow_128 {
+            return Err(Error::FromFeature(FeatureError::InvalidGrumpkinScalarLimb {
+                limb_as_hex: hex::encode(low_16_bytes),
+            }));
+        }
+        if BigUint::from_bytes_be(&high_16_bytes) >= two_pow_128 {
+            return Err(Error::FromFeature(FeatureError::InvalidGrumpkinScalarLimb {
+                limb_as_hex: hex::encode(high_16_bytes),
+            }));
+        }
+
         // Check if this is smaller than the grumpkin modulus
         let grumpkin_integer = BigUint::from_bytes_be(&bytes);
         let grumpkin_modulus = BigUint::from_bytes_be(&[