diff --git a/evm/src/cpu/kernel/prover_input.rs b/evm/src/cpu/kernel/prover_input.rs
index 1251f7d2..79a8483d 100644
--- a/evm/src/cpu/kernel/prover_input.rs
+++ b/evm/src/cpu/kernel/prover_input.rs
@@ -1,3 +1,12 @@
+use std::str::FromStr;
+
+use ethereum_types::U256;
+
+use crate::cpu::kernel::prover_input::Field::{
+    Bn254Base, Bn254Scalar, Secp256k1Base, Secp256k1Scalar,
+};
+use crate::cpu::kernel::prover_input::FieldOp::{Inverse, Sqrt};
+
 #[derive(PartialEq, Eq, Debug, Clone)]
 pub(crate) struct ProverInputFn(Vec<String>);
 
@@ -6,3 +15,98 @@ impl From<Vec<String>> for ProverInputFn {
         Self(v)
     }
 }
+
+impl ProverInputFn {
+    pub(crate) fn run(&self, mut stack: Vec<U256>) -> U256 {
+        match self.0[0].as_str() {
+            "ff" => self.run_ff(stack),
+            "storage" => todo!(),
+            _ => panic!("Unrecognized prover input function."),
+        }
+    }
+
+    fn run_ff(&self, mut stack: Vec<U256>) -> U256 {
+        let field = Field::from_str(self.0[1].as_str()).unwrap();
+        let op = FieldOp::from_str(self.0[2].as_str()).unwrap();
+        let x = stack.pop().expect("Empty stack");
+        field.op(op, x)
+    }
+}
+
+enum Field {
+    Bn254Base,
+    Bn254Scalar,
+    Secp256k1Base,
+    Secp256k1Scalar,
+}
+
+enum FieldOp {
+    Inverse,
+    Sqrt,
+}
+
+impl FromStr for Field {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(match s {
+            "bn254_base" => Bn254Base,
+            "bn254_scalar" => Bn254Scalar,
+            "secp256k1_base" => Secp256k1Base,
+            "secp256k1_scalar" => Secp256k1Scalar,
+            _ => panic!("Unrecognized field."),
+        })
+    }
+}
+
+impl FromStr for FieldOp {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(match s {
+            "inverse" => Inverse,
+            "sqrt" => Sqrt,
+            _ => panic!("Unrecognized field operation."),
+        })
+    }
+}
+
+impl Field {
+    fn order(&self) -> U256 {
+        match self {
+            Field::Bn254Base => {
+                U256::from_str("0x30644e72e131a029b85045b68181585d97816a916871ca8d3c208c16d87cfd47")
+                    .unwrap()
+            }
+            Field::Bn254Scalar => todo!(),
+            Field::Secp256k1Base => todo!(),
+            Field::Secp256k1Scalar => todo!(),
+        }
+    }
+
+    fn op(&self, op: FieldOp, x: U256) -> U256 {
+        match op {
+            FieldOp::Inverse => self.inverse(x),
+            FieldOp::Sqrt => todo!(),
+        }
+    }
+
+    fn inverse(&self, x: U256) -> U256 {
+        let n = self.order();
+        assert!(x < n);
+        modexp(x, n - 2, n)
+    }
+}
+
+fn modexp(x: U256, e: U256, n: U256) -> U256 {
+    let mut current = x;
+    let mut product = U256::one();
+
+    for j in 0..256 {
+        if !(e >> j & U256::one()).is_zero() {
+            product = U256::try_from(product.full_mul(current) % n).unwrap();
+        }
+        current = U256::try_from(current.full_mul(current) % n).unwrap();
+    }
+    product
+}