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Emman 2022-02-10 14:34:39 +08:00
parent 2fa225552f
commit 69aa220c2f
1 changed files with 7 additions and 5 deletions
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12
sm2/p256.go
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@ -10,6 +10,7 @@ import (
)
// See https://www.imperialviolet.org/2010/12/04/ecc.html ([1]) for background.
// Group Level Optimizations, "Efficient and Secure Elliptic Curve Cryptography Implementation of Curve P-256"
// SM2 P256 parameters reference GB/T 32918.5-2017 part 5.
type p256Curve struct {
@ -1156,6 +1157,7 @@ func p256CopyConditional(out, in *[p256Limbs]uint32, mask uint32) {
// p256SelectAffinePoint sets {out_x,out_y} to the index'th entry of table.
// On entry: index < 16, table[0] must be zero.
// Constant time table access, safe select.
func p256SelectAffinePoint(xOut, yOut *[p256Limbs]uint32, table []uint32, index uint32) {
for i := range xOut {
xOut[i] = 0
@ -1165,11 +1167,11 @@ func p256SelectAffinePoint(xOut, yOut *[p256Limbs]uint32, table []uint32, index
}
for i := uint32(1); i < 16; i++ {
mask := i ^ index
mask |= mask >> 2
mask |= mask >> 1
mask &= 1
mask--
mask := i ^ index // mask is zero when i equals index, otherwise non-zero. mask = {b3, b2, b1, b0}, ignore unused bits.
mask |= mask >> 2 // mask = {b3, b2, b1 | b3, b0 | b2}
mask |= mask >> 1 // mask = {b3, b2 | b3, b1 | b2 | b3, b0 | b1 | b2 | b3}
mask &= 1 // mask = {0, 0, 0, b0 | b1 | b2 | b3}
mask-- // mask = 0xffffffff when i equals index, otherwise 0x00000000
for j := range xOut {
xOut[j] |= table[0] & mask
table = table[1:]