P2Pool/edwards25519
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scalar: replace wide reduction with limbed operations

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This commit is contained in:
Filippo Valsorda 2022-07-31 22:59:11 +02:00 committed by Filippo Valsorda
parent 467a92bcfd
commit 63e0935134
1 changed files with 39 additions and 358 deletions
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397
scalar.go
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@ -111,16 +111,47 @@ func (s *Scalar) SetUniformBytes(x []byte) (*Scalar, error) {
return nil, errors.New("edwards25519: invalid SetUniformBytes input length")
}
// TODO: replace scReduce with a limbed reduction.
var reduced [32]byte
scReduce(&reduced, (*[64]byte)(x))
// We have a value x of 512 bits, but our fiatScalarFromBytes function
// expects an input lower than l, which is a little over 252 bits.
//
// Instead of writing a reduction function that operates on wider inputs, we
// can interpret x as the sum of three shorter values a, b, and c.
//
// x = a + b * 2^168 + c * 2^336 mod l
//
// We then precompute 2^168 and 2^336 modulo l, and perform the reduction
// with two multiplications and two additions.
fiatScalarFromBytes((*[4]uint64)(&s.s), &reduced)
fiatScalarToMontgomery(&s.s, (*fiatScalarNonMontgomeryDomainFieldElement)(&s.s))
s.setShortBytes(x[:21])
t := new(Scalar).setShortBytes(x[21:42])
s.Add(s, t.Multiply(t, scalarTwo168))
t.setShortBytes(x[42:])
s.Add(s, t.Multiply(t, scalarTwo336))
return s, nil
}
// scalarTwo168 and scalarTwo336 are 2^168 and 2^336 modulo l, encoded as a
// fiatScalarMontgomeryDomainFieldElement, which is a little-endian 4-limb value
// in the 2^256 Montgomery domain.
var scalarTwo168 = &Scalar{s: [4]uint64{0x5b8ab432eac74798, 0x38afddd6de59d5d7,
0xa2c131b399411b7c, 0x6329a7ed9ce5a30}}
var scalarTwo336 = &Scalar{s: [4]uint64{0xbd3d108e2b35ecc5, 0x5c3a3718bdf9c90b,
0x63aa97a331b4f2ee, 0x3d217f5be65cb5c}}
// setShortBytes sets s = x mod l, where x is a little-endian integer shorter
// than 32 bytes.
func (s *Scalar) setShortBytes(x []byte) *Scalar {
if len(x) >= 32 {
panic("edwards25519: internal error: setShortBytes called with a long string")
}
var buf [32]byte
copy(buf[:], x)
fiatScalarFromBytes((*[4]uint64)(&s.s), &buf)
fiatScalarToMontgomery(&s.s, (*fiatScalarNonMontgomeryDomainFieldElement)(&s.s))
return s
}
// SetCanonicalBytes sets s = x, where x is a 32-byte little-endian encoding of
// s, and returns s. If x is not a canonical encoding of s, SetCanonicalBytes
// returns nil and an error, and the receiver is unchanged.
@ -179,20 +210,14 @@ func (s *Scalar) SetBytesWithClamping(x []byte) (*Scalar, error) {
return nil, errors.New("edwards25519: invalid SetBytesWithClamping input length")
}
// We need to use the wide reduction from SetUniformBytes, since clamping
// sets the 2^254 bit, making the value higher than the order.
var wideBytes [64]byte
copy(wideBytes[:], x[:])
wideBytes[0] &= 248
wideBytes[31] &= 63
wideBytes[31] |= 64
// TODO: replace scReduce with a limbed reduction.
var reduced [32]byte
scReduce(&reduced, &wideBytes)
fiatScalarFromBytes((*[4]uint64)(&s.s), &reduced)
fiatScalarToMontgomery(&s.s, (*fiatScalarNonMontgomeryDomainFieldElement)(&s.s))
return s, nil
return s.SetUniformBytes(wideBytes[:])
}
// Bytes returns the canonical 32-byte little-endian encoding of s.
@ -225,350 +250,6 @@ func (s *Scalar) Equal(t *Scalar) int {
return int(^nonzero) & 1
}
// scReduce is ported from the public domain, “ref10”
// implementation of ed25519 from SUPERCOP.
func load3(in []byte) int64 {
r := int64(in[0])
r |= int64(in[1]) << 8
r |= int64(in[2]) << 16
return r
}
func load4(in []byte) int64 {
r := int64(in[0])
r |= int64(in[1]) << 8
r |= int64(in[2]) << 16
r |= int64(in[3]) << 24
return r
}
// Input:
//
// s[0]+256*s[1]+...+256^63*s[63] = s
//
// Output:
//
// s[0]+256*s[1]+...+256^31*s[31] = s mod l
// where l = 2^252 + 27742317777372353535851937790883648493.
func scReduce(out *[32]byte, s *[64]byte) {
s0 := 2097151 & load3(s[:])
s1 := 2097151 & (load4(s[2:]) >> 5)
s2 := 2097151 & (load3(s[5:]) >> 2)
s3 := 2097151 & (load4(s[7:]) >> 7)
s4 := 2097151 & (load4(s[10:]) >> 4)
s5 := 2097151 & (load3(s[13:]) >> 1)
s6 := 2097151 & (load4(s[15:]) >> 6)
s7 := 2097151 & (load3(s[18:]) >> 3)
s8 := 2097151 & load3(s[21:])
s9 := 2097151 & (load4(s[23:]) >> 5)
s10 := 2097151 & (load3(s[26:]) >> 2)
s11 := 2097151 & (load4(s[28:]) >> 7)
s12 := 2097151 & (load4(s[31:]) >> 4)
s13 := 2097151 & (load3(s[34:]) >> 1)
s14 := 2097151 & (load4(s[36:]) >> 6)
s15 := 2097151 & (load3(s[39:]) >> 3)
s16 := 2097151 & load3(s[42:])
s17 := 2097151 & (load4(s[44:]) >> 5)
s18 := 2097151 & (load3(s[47:]) >> 2)
s19 := 2097151 & (load4(s[49:]) >> 7)
s20 := 2097151 & (load4(s[52:]) >> 4)
s21 := 2097151 & (load3(s[55:]) >> 1)
s22 := 2097151 & (load4(s[57:]) >> 6)
s23 := (load4(s[60:]) >> 3)
s11 += s23 * 666643
s12 += s23 * 470296
s13 += s23 * 654183
s14 -= s23 * 997805
s15 += s23 * 136657
s16 -= s23 * 683901
s23 = 0
s10 += s22 * 666643
s11 += s22 * 470296
s12 += s22 * 654183
s13 -= s22 * 997805
s14 += s22 * 136657
s15 -= s22 * 683901
s22 = 0
s9 += s21 * 666643
s10 += s21 * 470296
s11 += s21 * 654183
s12 -= s21 * 997805
s13 += s21 * 136657
s14 -= s21 * 683901
s21 = 0
s8 += s20 * 666643
s9 += s20 * 470296
s10 += s20 * 654183
s11 -= s20 * 997805
s12 += s20 * 136657
s13 -= s20 * 683901
s20 = 0
s7 += s19 * 666643
s8 += s19 * 470296
s9 += s19 * 654183
s10 -= s19 * 997805
s11 += s19 * 136657
s12 -= s19 * 683901
s19 = 0
s6 += s18 * 666643
s7 += s18 * 470296
s8 += s18 * 654183
s9 -= s18 * 997805
s10 += s18 * 136657
s11 -= s18 * 683901
s18 = 0
var carry [17]int64
carry[6] = (s6 + (1 << 20)) >> 21
s7 += carry[6]
s6 -= carry[6] << 21
carry[8] = (s8 + (1 << 20)) >> 21
s9 += carry[8]
s8 -= carry[8] << 21
carry[10] = (s10 + (1 << 20)) >> 21
s11 += carry[10]
s10 -= carry[10] << 21
carry[12] = (s12 + (1 << 20)) >> 21
s13 += carry[12]
s12 -= carry[12] << 21
carry[14] = (s14 + (1 << 20)) >> 21
s15 += carry[14]
s14 -= carry[14] << 21
carry[16] = (s16 + (1 << 20)) >> 21
s17 += carry[16]
s16 -= carry[16] << 21
carry[7] = (s7 + (1 << 20)) >> 21
s8 += carry[7]
s7 -= carry[7] << 21
carry[9] = (s9 + (1 << 20)) >> 21
s10 += carry[9]
s9 -= carry[9] << 21
carry[11] = (s11 + (1 << 20)) >> 21
s12 += carry[11]
s11 -= carry[11] << 21
carry[13] = (s13 + (1 << 20)) >> 21
s14 += carry[13]
s13 -= carry[13] << 21
carry[15] = (s15 + (1 << 20)) >> 21
s16 += carry[15]
s15 -= carry[15] << 21
s5 += s17 * 666643
s6 += s17 * 470296
s7 += s17 * 654183
s8 -= s17 * 997805
s9 += s17 * 136657
s10 -= s17 * 683901
s17 = 0
s4 += s16 * 666643
s5 += s16 * 470296
s6 += s16 * 654183
s7 -= s16 * 997805
s8 += s16 * 136657
s9 -= s16 * 683901
s16 = 0
s3 += s15 * 666643
s4 += s15 * 470296
s5 += s15 * 654183
s6 -= s15 * 997805
s7 += s15 * 136657
s8 -= s15 * 683901
s15 = 0
s2 += s14 * 666643
s3 += s14 * 470296
s4 += s14 * 654183
s5 -= s14 * 997805
s6 += s14 * 136657
s7 -= s14 * 683901
s14 = 0
s1 += s13 * 666643
s2 += s13 * 470296
s3 += s13 * 654183
s4 -= s13 * 997805
s5 += s13 * 136657
s6 -= s13 * 683901
s13 = 0
s0 += s12 * 666643
s1 += s12 * 470296
s2 += s12 * 654183
s3 -= s12 * 997805
s4 += s12 * 136657
s5 -= s12 * 683901
s12 = 0
carry[0] = (s0 + (1 << 20)) >> 21
s1 += carry[0]
s0 -= carry[0] << 21
carry[2] = (s2 + (1 << 20)) >> 21
s3 += carry[2]
s2 -= carry[2] << 21
carry[4] = (s4 + (1 << 20)) >> 21
s5 += carry[4]
s4 -= carry[4] << 21
carry[6] = (s6 + (1 << 20)) >> 21
s7 += carry[6]
s6 -= carry[6] << 21
carry[8] = (s8 + (1 << 20)) >> 21
s9 += carry[8]
s8 -= carry[8] << 21
carry[10] = (s10 + (1 << 20)) >> 21
s11 += carry[10]
s10 -= carry[10] << 21
carry[1] = (s1 + (1 << 20)) >> 21
s2 += carry[1]
s1 -= carry[1] << 21
carry[3] = (s3 + (1 << 20)) >> 21
s4 += carry[3]
s3 -= carry[3] << 21
carry[5] = (s5 + (1 << 20)) >> 21
s6 += carry[5]
s5 -= carry[5] << 21
carry[7] = (s7 + (1 << 20)) >> 21
s8 += carry[7]
s7 -= carry[7] << 21
carry[9] = (s9 + (1 << 20)) >> 21
s10 += carry[9]
s9 -= carry[9] << 21
carry[11] = (s11 + (1 << 20)) >> 21
s12 += carry[11]
s11 -= carry[11] << 21
s0 += s12 * 666643
s1 += s12 * 470296
s2 += s12 * 654183
s3 -= s12 * 997805
s4 += s12 * 136657
s5 -= s12 * 683901
s12 = 0
carry[0] = s0 >> 21
s1 += carry[0]
s0 -= carry[0] << 21
carry[1] = s1 >> 21
s2 += carry[1]
s1 -= carry[1] << 21
carry[2] = s2 >> 21
s3 += carry[2]
s2 -= carry[2] << 21
carry[3] = s3 >> 21
s4 += carry[3]
s3 -= carry[3] << 21
carry[4] = s4 >> 21
s5 += carry[4]
s4 -= carry[4] << 21
carry[5] = s5 >> 21
s6 += carry[5]
s5 -= carry[5] << 21
carry[6] = s6 >> 21
s7 += carry[6]
s6 -= carry[6] << 21
carry[7] = s7 >> 21
s8 += carry[7]
s7 -= carry[7] << 21
carry[8] = s8 >> 21
s9 += carry[8]
s8 -= carry[8] << 21
carry[9] = s9 >> 21
s10 += carry[9]
s9 -= carry[9] << 21
carry[10] = s10 >> 21
s11 += carry[10]
s10 -= carry[10] << 21
carry[11] = s11 >> 21
s12 += carry[11]
s11 -= carry[11] << 21
s0 += s12 * 666643
s1 += s12 * 470296
s2 += s12 * 654183
s3 -= s12 * 997805
s4 += s12 * 136657
s5 -= s12 * 683901
s12 = 0
carry[0] = s0 >> 21
s1 += carry[0]
s0 -= carry[0] << 21
carry[1] = s1 >> 21
s2 += carry[1]
s1 -= carry[1] << 21
carry[2] = s2 >> 21
s3 += carry[2]
s2 -= carry[2] << 21
carry[3] = s3 >> 21
s4 += carry[3]
s3 -= carry[3] << 21
carry[4] = s4 >> 21
s5 += carry[4]
s4 -= carry[4] << 21
carry[5] = s5 >> 21
s6 += carry[5]
s5 -= carry[5] << 21
carry[6] = s6 >> 21
s7 += carry[6]
s6 -= carry[6] << 21
carry[7] = s7 >> 21
s8 += carry[7]
s7 -= carry[7] << 21
carry[8] = s8 >> 21
s9 += carry[8]
s8 -= carry[8] << 21
carry[9] = s9 >> 21
s10 += carry[9]
s9 -= carry[9] << 21
carry[10] = s10 >> 21
s11 += carry[10]
s10 -= carry[10] << 21
out[0] = byte(s0 >> 0)
out[1] = byte(s0 >> 8)
out[2] = byte((s0 >> 16) | (s1 << 5))
out[3] = byte(s1 >> 3)
out[4] = byte(s1 >> 11)
out[5] = byte((s1 >> 19) | (s2 << 2))
out[6] = byte(s2 >> 6)
out[7] = byte((s2 >> 14) | (s3 << 7))
out[8] = byte(s3 >> 1)
out[9] = byte(s3 >> 9)
out[10] = byte((s3 >> 17) | (s4 << 4))
out[11] = byte(s4 >> 4)
out[12] = byte(s4 >> 12)
out[13] = byte((s4 >> 20) | (s5 << 1))
out[14] = byte(s5 >> 7)
out[15] = byte((s5 >> 15) | (s6 << 6))
out[16] = byte(s6 >> 2)
out[17] = byte(s6 >> 10)
out[18] = byte((s6 >> 18) | (s7 << 3))
out[19] = byte(s7 >> 5)
out[20] = byte(s7 >> 13)
out[21] = byte(s8 >> 0)
out[22] = byte(s8 >> 8)
out[23] = byte((s8 >> 16) | (s9 << 5))
out[24] = byte(s9 >> 3)
out[25] = byte(s9 >> 11)
out[26] = byte((s9 >> 19) | (s10 << 2))
out[27] = byte(s10 >> 6)
out[28] = byte((s10 >> 14) | (s11 << 7))
out[29] = byte(s11 >> 1)
out[30] = byte(s11 >> 9)
out[31] = byte(s11 >> 17)
}
// nonAdjacentForm computes a width-w non-adjacent form for this scalar.
//
// w must be between 2 and 8, or nonAdjacentForm will panic.