ethcore: minor optimization of modexp by using LR exponentiation

ethcore/benches/builtin.rs

@@ -25,12 +25,10 @@ extern crate ethereum_types;
 extern crate parity_bytes as bytes;
 extern crate rustc_hex;
 
-use std::collections::BTreeMap;
-
 use bytes::BytesRef;
 use ethcore::builtin::Builtin;
 use ethcore::machine::EthereumMachine;
-use ethereum_types::{Address, U256};
+use ethereum_types::U256;
 use ethcore::ethereum::new_byzantium_test_machine;
 use rustc_hex::FromHex;
 use self::test::Bencher;

ethcore/src/builtin.rs

@@ -311,35 +311,51 @@ 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;
+// calculate modexp: left-to-right binary exponentiation to keep multiplicands lower
+fn modexp(mut base: BigUint, exp: Vec<u8>, modulus: BigUint) -> BigUint {
+	const BITS_PER_DIGIT: usize = 8;
 
-	if modulus <= BigUint::one() { // n^m % 0 || n^m % 1
+	// n^m % 0 || n^m % 1
+	if modulus <= BigUint::one() {
 		return BigUint::zero();
 	}
 
-	if exp.is_zero() { // n^0 % m
+	// normalize exponent
+	let mut exp = exp.into_iter().skip_while(|d| *d == 0).peekable();
+
+	// n^0 % m
+	if let None = exp.peek() {
 		return BigUint::one();
 	}
 
-	if base.is_zero() { // 0^n % m, n>0
+	// 0^n % m, n > 0
+	if base.is_zero() {
 		return BigUint::zero();
 	}
 
-	let mut result = BigUint::one();
 	base = base % &modulus;
 
-	// fast path for base divisible by 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;
+    // Left-to-right binary exponentiation (Handbook of Applied Cryptography - Algorithm 14.79).
+	// http://www.cacr.math.uwaterloo.ca/hac/about/chap14.pdf
+	let mut result = BigUint::one();
+
+	for digit in exp {
+		let mut mask = 1 << (BITS_PER_DIGIT - 1);
+
+		for _ in 0..BITS_PER_DIGIT {
+			result = &result * &result % &modulus;
+
+			if digit & mask > 0 {
+				result = result * &base % &modulus;
+			}
+
+			mask >>= 1;
+		}
 	}
+
 	result
 }
 
@@ -366,15 +382,19 @@ impl Impl for ModexpImpl {
 		} else {
 			// read the numbers themselves.
 			let mut buf = vec![0; max(mod_len, max(base_len, exp_len))];
-			let mut read_num = |len| {
+			let mut read_num = |reader: &mut io::Chain<&[u8], io::Repeat>, len: usize| {
 				reader.read_exact(&mut buf[..len]).expect("reading from zero-extended memory cannot fail; qed");
 				BigUint::from_bytes_be(&buf[..len])
 			};
 
-			let base = read_num(base_len);
-			let exp = read_num(exp_len);
-			let modulus = read_num(mod_len);
-			modexp(base, exp, modulus)
+			let base = read_num(&mut reader, base_len);
+
+			let mut exp_buf = vec![0; exp_len];
+			reader.read_exact(&mut exp_buf[..exp_len]).expect("reading from zero-extended memory cannot fail; qed");
+
+			let modulus = read_num(&mut reader, mod_len);
+
+			modexp(base, exp_buf, modulus)
 		};
 
 		// write output to given memory, left padded and same length as the modulus.
@@ -551,31 +571,31 @@ mod tests {
 		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());
+		assert_eq!(me(base, exp.to_bytes_be(), 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());
+		assert_eq!(me(base, exp.to_bytes_be(), 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());
+		assert_eq!(me(base, exp.to_bytes_be(), 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());
+		assert_eq!(me(base, exp.to_bytes_be(), 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());
+		assert_eq!(me(base, exp.to_bytes_be(), modulus), BigUint::parse_bytes(b"55", 10).unwrap());
 	}
 
 	#[test]