diff --git a/ethcore/src/builtin.rs b/ethcore/src/builtin.rs
index b6d7064eb..a752064cd 100644
--- a/ethcore/src/builtin.rs
+++ b/ethcore/src/builtin.rs
@@ -267,6 +267,34 @@ impl Impl for Ripemd160 {
 	}
 }
 
+// calculate modexp: exponentiation by squaring. the `num` crate has pow, but not modular.
+fn modexp(mut base: BigUint, mut exp: BigUint, modulus: BigUint) -> BigUint {
+	use num::Integer;
+
+	match (base.is_zero(), exp.is_zero()) {
+		(_, true) => return BigUint::one(), // n^0 % m
+		(true, false) => return BigUint::zero(), // 0^n % m, n>0
+		(false, false) if modulus <= BigUint::one() => return BigUint::zero(), // a^b % 1 = 0.
+		_ => {}
+	}
+
+	let mut result = BigUint::one();
+	base = base % &modulus;
+
+	// fast path for base divisible by modulus.
+	if base.is_zero() { return BigUint::zero() }
+	while !exp.is_zero() {
+		if exp.is_odd() {
+			result = (result * &base) % &modulus;
+		}
+
+		exp = exp >> 1;
+		base = (base.clone() * base) % &modulus;
+	}
+
+	result
+}
+
 impl Impl for ModexpImpl {
 	fn execute(&self, input: &[u8], output: &mut BytesRef) -> Result<(), Error> {
 		let mut reader = input.chain(io::repeat(0));
@@ -295,34 +323,6 @@ impl Impl for ModexpImpl {
 		let exp = read_num(exp_len);
 		let modulus = read_num(mod_len);
 
-		// calculate modexp: exponentiation by squaring. the `num` crate has pow, but not modular.
-		fn modexp(mut base: BigUint, mut exp: BigUint, modulus: BigUint) -> BigUint {
-			use num::Integer;
-
-			match (base.is_zero(), exp.is_zero()) {
-				(_, true) => return BigUint::one(), // n^0 % m
-				(true, false) => return BigUint::zero(), // 0^n % m, n>0
-				(false, false) if modulus <= BigUint::one() => return BigUint::zero(), // a^b % 1 = 0.
-				_ => {}
-			}
-
-			let mut result = BigUint::one();
-			base = base % &modulus;
-
-			// fast path for base divisible by modulus.
-			if base.is_zero() { return result }
-			while !exp.is_zero() {
-				if exp.is_odd() {
-					result = (result * &base) % &modulus;
-				}
-
-				exp = exp >> 1;
-				base = (base.clone() * base) % &modulus;
-			}
-
-			result
-		}
-
 		// write output to given memory, left padded and same length as the modulus.
 		let bytes = modexp(base, exp, modulus).to_bytes_be();
 
@@ -504,10 +504,44 @@ impl Impl for Bn128PairingImpl {
 
 #[cfg(test)]
 mod tests {
-	use super::{Builtin, Linear, ethereum_builtin, Pricer, Modexp};
+	use super::{Builtin, Linear, ethereum_builtin, Pricer, Modexp, modexp as me};
 	use ethjson;
 	use util::{U256, BytesRef};
 	use rustc_hex::FromHex;
+	use num::{BigUint, Zero, One};
+
+	#[test]
+	fn modexp_func() {
+		// n^0 % m == 1
+		let mut base = BigUint::parse_bytes(b"12345", 10).unwrap();
+		let mut exp = BigUint::zero();
+		let mut modulus = BigUint::parse_bytes(b"789", 10).unwrap();
+		assert_eq!(me(base, exp, modulus), BigUint::one());
+
+		// 0^n % m == 0
+		base = BigUint::zero();
+		exp = BigUint::parse_bytes(b"12345", 10).unwrap();
+		modulus = BigUint::parse_bytes(b"789", 10).unwrap();
+		assert_eq!(me(base, exp, modulus), BigUint::zero());
+
+		// n^m % 1 == 0
+		base = BigUint::parse_bytes(b"12345", 10).unwrap();
+		exp = BigUint::parse_bytes(b"789", 10).unwrap();
+		modulus = BigUint::one();
+		assert_eq!(me(base, exp, modulus), BigUint::zero());
+
+		// if n % d == 0, then n^m % d == 0
+		base = BigUint::parse_bytes(b"12345", 10).unwrap();
+		exp = BigUint::parse_bytes(b"789", 10).unwrap();
+		modulus = BigUint::parse_bytes(b"15", 10).unwrap();
+		assert_eq!(me(base, exp, modulus), BigUint::zero());
+
+		// others
+		base = BigUint::parse_bytes(b"12345", 10).unwrap();
+		exp = BigUint::parse_bytes(b"789", 10).unwrap();
+		modulus = BigUint::parse_bytes(b"97", 10).unwrap();
+		assert_eq!(me(base, exp, modulus), BigUint::parse_bytes(b"55", 10).unwrap());
+	}
 
 	#[test]
 	fn identity() {