Optimizations to P256 operations (#5181)

Co-authored-by: Hadrien Croubois <[email protected]>
Co-authored-by: Ernesto García <[email protected]>

Author: 3 people

contracts/utils/cryptography/P256.sol

@@ -125,11 +125,12 @@ library P256 {
             return (0, 0);
         }
 
+        uint256 p = P; // cache P on the stack
         uint256 rx = uint256(r);
-        uint256 ry2 = addmod(mulmod(addmod(mulmod(rx, rx, P), A, P), rx, P), B, P); // weierstrass equation y² = x³ + a.x + b
-        uint256 ry = Math.modExp(ry2, P1DIV4, P); // This formula for sqrt work because P ≡ 3 (mod 4)
-        if (mulmod(ry, ry, P) != ry2) return (0, 0); // Sanity check
-        if (ry % 2 != v % 2) ry = P - ry;
+        uint256 ry2 = addmod(mulmod(addmod(mulmod(rx, rx, p), A, p), rx, p), B, p); // weierstrass equation y² = x³ + a.x + b
+        uint256 ry = Math.modExp(ry2, P1DIV4, p); // This formula for sqrt work because P ≡ 3 (mod 4)
+        if (mulmod(ry, ry, p) != ry2) return (0, 0); // Sanity check
+        if (ry % 2 != v % 2) ry = p - ry;
 
         JPoint[16] memory points = _preComputeJacobianPoints(rx, ry);
         uint256 w = Math.invModPrime(uint256(r), N);
@@ -170,11 +171,13 @@ library P256 {
      */
     function _affineFromJacobian(uint256 jx, uint256 jy, uint256 jz) private view returns (uint256 ax, uint256 ay) {
         if (jz == 0) return (0, 0);
-        uint256 zinv = Math.invModPrime(jz, P);
-        uint256 zzinv = mulmod(zinv, zinv, P);
-        uint256 zzzinv = mulmod(zzinv, zinv, P);
-        ax = mulmod(jx, zzinv, P);
-        ay = mulmod(jy, zzzinv, P);
+        uint256 p = P; // cache P on the stack
+        uint256 zinv = Math.invModPrime(jz, p);
+        assembly ("memory-safe") {
+            let zzinv := mulmod(zinv, zinv, p)
+            ax := mulmod(jx, zzinv, p)
+            ay := mulmod(jy, mulmod(zzinv, zinv, p), p)
+        }
     }
 
     /**
@@ -190,12 +193,11 @@ library P256 {
         assembly ("memory-safe") {
             let p := P
             let z1 := mload(add(p1, 0x40))
+            let zz1 := mulmod(z1, z1, p) // zz1 = z1²
             let s1 := mulmod(mload(add(p1, 0x20)), mulmod(mulmod(z2, z2, p), z2, p), p) // s1 = y1*z2³
-            let s2 := mulmod(y2, mulmod(mulmod(z1, z1, p), z1, p), p) // s2 = y2*z1³
-            let r := addmod(s2, sub(p, s1), p) // r = s2-s1
+            let r := addmod(mulmod(y2, mulmod(zz1, z1, p), p), sub(p, s1), p) // r = s2-s1 = y2*z1³-s1
             let u1 := mulmod(mload(p1), mulmod(z2, z2, p), p) // u1 = x1*z2²
-            let u2 := mulmod(x2, mulmod(z1, z1, p), p) // u2 = x2*z1²
-            let h := addmod(u2, sub(p, u1), p) // h = u2-u1
+            let h := addmod(mulmod(x2, zz1, p), sub(p, u1), p) // h = u2-u1 = x2*z1²-u1
             let hh := mulmod(h, h, p) // h²
 
             // x' = r²-h³-2*u1*h²
@@ -226,12 +228,11 @@ library P256 {
             let zz := mulmod(z, z, p)
             let s := mulmod(4, mulmod(x, yy, p), p) // s = 4*x*y²
             let m := addmod(mulmod(3, mulmod(x, x, p), p), mulmod(A, mulmod(zz, zz, p), p), p) // m = 3*x²+a*z⁴
-            let t := addmod(mulmod(m, m, p), sub(p, mulmod(2, s, p)), p) // t = m²-2*s
 
-            // x' = t
-            rx := t
-            // y' = m*(s-t)-8*y⁴
-            ry := addmod(mulmod(m, addmod(s, sub(p, t), p), p), sub(p, mulmod(8, mulmod(yy, yy, p), p)), p)
+            // x' = t = m²-2*s
+            rx := addmod(mulmod(m, m, p), sub(p, mulmod(2, s, p)), p)
+            // y' = m*(s-t)-8*y⁴ = m*(s-x')-8*y⁴
+            ry := addmod(mulmod(m, addmod(s, sub(p, rx), p), p), sub(p, mulmod(8, mulmod(yy, yy, p), p)), p)
             // z' = 2*y*z
             rz := mulmod(2, mulmod(y, z, p), p)
         }