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Use BigUint::modpow for ModExp precompile #11772

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vorot93 merged 3 commits into from
Jun 12, 2020
Merged

Use BigUint::modpow for ModExp precompile #11772

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001cae9
Use BigUint::modpow
vorot93 Jun 8, 2020
e6f6ca5
Fixes
vorot93 Jun 8, 2020
08b2171
Fix unused import
vorot93 Jun 12, 2020
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95 changes: 7 additions & 88 deletions ethcore/builtin/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -738,54 +738,6 @@ impl Implementation for Ripemd160 {
}
}

// 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;

// n^m % 0 || n^m % 1
if modulus <= BigUint::one() {
return BigUint::zero();
}

// normalize exponent
let mut exp = exp.into_iter().skip_while(|d| *d == 0).peekable();

// n^0 % m
if exp.peek().is_none() {
return BigUint::one();
}

// 0^n % m, n > 0
if base.is_zero() {
return BigUint::zero();
}

base %= &modulus;

// Fast path for base divisible by modulus.
if base.is_zero() { return BigUint::zero() }

// 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
}

impl Implementation for Modexp {
fn execute(&self, input: &[u8], output: &mut BytesRef) -> Result<(), &'static str> {
let mut reader = input.chain(io::repeat(0));
Expand Down Expand Up @@ -817,13 +769,14 @@ impl Implementation for Modexp {
};

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 exponent = read_num(&mut reader, exp_len);
let modulus = read_num(&mut reader, mod_len);

modexp(base, exp_buf, modulus)
if modulus.is_zero() || modulus.is_one() {
BigUint::zero()
} else {
base.modpow(&exponent, &modulus)
}
};

// write output to given memory, left padded and same length as the modulus.
Expand Down Expand Up @@ -1168,11 +1121,10 @@ mod tests {
};
use hex_literal::hex;
use maplit::btreemap;
use num::{BigUint, Zero, One};
use parity_bytes::BytesRef;
use super::{
Builtin, EthereumBuiltin, FromStr, Implementation, Linear,
ModexpPricer, modexp as me, Pricing,
ModexpPricer, Pricing,
Comment thread
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Bls12ConstOperations,
Bls12PairingPrice,Bls12PairingPricer
};
Expand Down Expand Up @@ -1296,39 +1248,6 @@ mod tests {
assert_eq!(&out[..], &expected[..]);
}

#[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.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.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.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.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.to_bytes_be(), modulus), BigUint::parse_bytes(b"55", 10).unwrap());
}

#[test]
fn identity() {
let f = EthereumBuiltin::from_str("identity").unwrap();
Expand Down