saymrwulf/curve25519-dalek-source
1
0
Fork 0
mirror of https://github.com/saymrwulf/curve25519-dalek-source.git synced 2026-09-04 20:24:10 +00:00
Code Issues Projects Releases Packages Wiki Activity Actions

avoid 128-bit multiplications

Browse source
This commit is contained in:
Henry de Valence 2017-07-20 21:17:32 -07:00
parent 2e24b251df
commit 513ce26942
1 changed files with 11 additions and 5 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

16
src/field.rs
View file

@ -202,12 +202,18 @@ impl<'a, 'b> Mul<&'b FieldElement> for &'a FieldElement {
let a: &[u64; 5] = &self.0;
let b: &[u64; 5] = &_rhs.0;
// 64-bit precomputations
let b1_19 = b[1] * 19;
let b2_19 = b[2] * 19;
let b3_19 = b[3] * 19;
let b4_19 = b[4] * 19;
// Multiply to get 128-bit coefficients of output
let c0: u128 = m(a[0],b[0]) + ( m(a[4],b[1]) + m(a[3],b[2]) + m(a[2],b[3]) + m(a[1],b[4]) )*19;
let mut c1: u128 = m(a[1],b[0]) + m(a[0],b[1]) + ( m(a[4],b[2]) + m(a[3],b[3]) + m(a[2],b[4]) )*19;
let mut c2: u128 = m(a[2],b[0]) + m(a[1],b[1]) + m(a[0],b[2]) + ( m(a[4],b[3]) + m(a[3],b[4]) )*19;
let mut c3: u128 = m(a[3],b[0]) + m(a[2],b[1]) + m(a[1],b[2]) + m(a[0],b[3]) + ( m(a[4],b[4]) )*19;
let mut c4: u128 = m(a[4],b[0]) + m(a[3],b[1]) + m(a[2],b[2]) + m(a[1],b[3]) + m(a[0],b[4]);
let c0: u128 = m(a[0],b[0]) + m(a[4],b1_19) + m(a[3],b2_19) + m(a[2],b3_19) + m(a[1],b4_19);
let mut c1: u128 = m(a[1],b[0]) + m(a[0],b[1]) + m(a[4],b2_19) + m(a[3],b3_19) + m(a[2],b4_19);
let mut c2: u128 = m(a[2],b[0]) + m(a[1],b[1]) + m(a[0],b[2]) + m(a[4],b3_19) + m(a[3],b4_19);
let mut c3: u128 = m(a[3],b[0]) + m(a[2],b[1]) + m(a[1],b[2]) + m(a[0],b[3]) + m(a[4],b4_19);
let mut c4: u128 = m(a[4],b[0]) + m(a[3],b[1]) + m(a[2],b[2]) + m(a[1],b[3]) + m(a[0],b[4]);
// Now c[i] < 2^2b * (1+i + (4-i)*19) < 2^(2b + lg(1+4*19)) < 2^(2b + 6.27)
// where b is the bitlength of the input limbs.