status-go/vendor/github.com/decred/dcrd/dcrec/secp256k1/v4/modnscalar.go

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// Copyright (c) 2020 The Decred developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.
package secp256k1
import (
"encoding/hex"
"math/big"
)
// References:
// [SECG]: Recommended Elliptic Curve Domain Parameters
// https://www.secg.org/sec2-v2.pdf
//
// [HAC]: Handbook of Applied Cryptography Menezes, van Oorschot, Vanstone.
// http://cacr.uwaterloo.ca/hac/
// Many elliptic curve operations require working with scalars in a finite field
// characterized by the order of the group underlying the secp256k1 curve.
// Given this precision is larger than the biggest available native type,
// obviously some form of bignum math is needed. This code implements
// specialized fixed-precision field arithmetic rather than relying on an
// arbitrary-precision arithmetic package such as math/big for dealing with the
// math modulo the group order since the size is known. As a result, rather
// large performance gains are achieved by taking advantage of many
// optimizations not available to arbitrary-precision arithmetic and generic
// modular arithmetic algorithms.
//
// There are various ways to internally represent each element. For example,
// the most obvious representation would be to use an array of 4 uint64s (64
// bits * 4 = 256 bits). However, that representation suffers from the fact
// that there is no native Go type large enough to handle the intermediate
// results while adding or multiplying two 64-bit numbers.
//
// Given the above, this implementation represents the field elements as 8
// uint32s with each word (array entry) treated as base 2^32. This was chosen
// because most systems at the current time are 64-bit (or at least have 64-bit
// registers available for specialized purposes such as MMX) so the intermediate
// results can typically be done using a native register (and using uint64s to
// avoid the need for additional half-word arithmetic)
const (
// These fields provide convenient access to each of the words of the
// secp256k1 curve group order N to improve code readability.
//
// The group order of the curve per [SECG] is:
// 0xffffffff ffffffff ffffffff fffffffe baaedce6 af48a03b bfd25e8c d0364141
orderWordZero uint32 = 0xd0364141
orderWordOne uint32 = 0xbfd25e8c
orderWordTwo uint32 = 0xaf48a03b
orderWordThree uint32 = 0xbaaedce6
orderWordFour uint32 = 0xfffffffe
orderWordFive uint32 = 0xffffffff
orderWordSix uint32 = 0xffffffff
orderWordSeven uint32 = 0xffffffff
// These fields provide convenient access to each of the words of the two's
// complement of the secp256k1 curve group order N to improve code
// readability.
//
// The two's complement of the group order is:
// 0x00000000 00000000 00000000 00000001 45512319 50b75fc4 402da173 2fc9bebf
orderComplementWordZero uint32 = (^orderWordZero) + 1
orderComplementWordOne uint32 = ^orderWordOne
orderComplementWordTwo uint32 = ^orderWordTwo
orderComplementWordThree uint32 = ^orderWordThree
//orderComplementWordFour uint32 = ^orderWordFour // unused
//orderComplementWordFive uint32 = ^orderWordFive // unused
//orderComplementWordSix uint32 = ^orderWordSix // unused
//orderComplementWordSeven uint32 = ^orderWordSeven // unused
// These fields provide convenient access to each of the words of the
// secp256k1 curve group order N / 2 to improve code readability and avoid
// the need to recalculate them.
//
// The half order of the secp256k1 curve group is:
// 0x7fffffff ffffffff ffffffff ffffffff 5d576e73 57a4501d dfe92f46 681b20a0
halfOrderWordZero uint32 = 0x681b20a0
halfOrderWordOne uint32 = 0xdfe92f46
halfOrderWordTwo uint32 = 0x57a4501d
halfOrderWordThree uint32 = 0x5d576e73
halfOrderWordFour uint32 = 0xffffffff
halfOrderWordFive uint32 = 0xffffffff
halfOrderWordSix uint32 = 0xffffffff
halfOrderWordSeven uint32 = 0x7fffffff
// uint32Mask is simply a mask with all bits set for a uint32 and is used to
// improve the readability of the code.
uint32Mask = 0xffffffff
)
var (
// zero32 is an array of 32 bytes used for the purposes of zeroing and is
// defined here to avoid extra allocations.
zero32 = [32]byte{}
)
// ModNScalar implements optimized 256-bit constant-time fixed-precision
// arithmetic over the secp256k1 group order. This means all arithmetic is
// performed modulo:
//
// 0xfffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141
//
// It only implements the arithmetic needed for elliptic curve operations,
// however, the operations that are not implemented can typically be worked
// around if absolutely needed. For example, subtraction can be performed by
// adding the negation.
//
// Should it be absolutely necessary, conversion to the standard library
// math/big.Int can be accomplished by using the Bytes method, slicing the
// resulting fixed-size array, and feeding it to big.Int.SetBytes. However,
// that should typically be avoided when possible as conversion to big.Ints
// requires allocations, is not constant time, and is slower when working modulo
// the group order.
type ModNScalar struct {
// The scalar is represented as 8 32-bit integers in base 2^32.
//
// The following depicts the internal representation:
// ---------------------------------------------------------
// | n[7] | n[6] | ... | n[0] |
// | 32 bits | 32 bits | ... | 32 bits |
// | Mult: 2^(32*7) | Mult: 2^(32*6) | ... | Mult: 2^(32*0) |
// ---------------------------------------------------------
//
// For example, consider the number 2^87 + 2^42 + 1. It would be
// represented as:
// n[0] = 1
// n[1] = 2^10
// n[2] = 2^23
// n[3..7] = 0
//
// The full 256-bit value is then calculated by looping i from 7..0 and
// doing sum(n[i] * 2^(32i)) like so:
// n[7] * 2^(32*7) = 0 * 2^224 = 0
// n[6] * 2^(32*6) = 0 * 2^192 = 0
// ...
// n[2] * 2^(32*2) = 2^23 * 2^64 = 2^87
// n[1] * 2^(32*1) = 2^10 * 2^32 = 2^42
// n[0] * 2^(32*0) = 1 * 2^0 = 1
// Sum: 0 + 0 + ... + 2^87 + 2^42 + 1 = 2^87 + 2^42 + 1
n [8]uint32
}
// String returns the scalar as a human-readable hex string.
//
// This is NOT constant time.
func (s ModNScalar) String() string {
b := s.Bytes()
return hex.EncodeToString(b[:])
}
// Set sets the scalar equal to a copy of the passed one in constant time.
//
// The scalar is returned to support chaining. This enables syntax like:
// s := new(ModNScalar).Set(s2).Add(1) so that s = s2 + 1 where s2 is not
// modified.
func (s *ModNScalar) Set(val *ModNScalar) *ModNScalar {
*s = *val
return s
}
// Zero sets the scalar to zero in constant time. A newly created scalar is
// already set to zero. This function can be useful to clear an existing scalar
// for reuse.
func (s *ModNScalar) Zero() {
s.n[0] = 0
s.n[1] = 0
s.n[2] = 0
s.n[3] = 0
s.n[4] = 0
s.n[5] = 0
s.n[6] = 0
s.n[7] = 0
}
// IsZero returns whether or not the scalar is equal to zero in constant time.
func (s *ModNScalar) IsZero() bool {
// The scalar can only be zero if no bits are set in any of the words.
bits := s.n[0] | s.n[1] | s.n[2] | s.n[3] | s.n[4] | s.n[5] | s.n[6] | s.n[7]
return bits == 0
}
// SetInt sets the scalar to the passed integer in constant time. This is a
// convenience function since it is fairly common to perform some arithmetic
// with small native integers.
//
// The scalar is returned to support chaining. This enables syntax like:
// s := new(ModNScalar).SetInt(2).Mul(s2) so that s = 2 * s2.
func (s *ModNScalar) SetInt(ui uint32) *ModNScalar {
s.Zero()
s.n[0] = ui
return s
}
// constantTimeEq returns 1 if a == b or 0 otherwise in constant time.
func constantTimeEq(a, b uint32) uint32 {
return uint32((uint64(a^b) - 1) >> 63)
}
// constantTimeNotEq returns 1 if a != b or 0 otherwise in constant time.
func constantTimeNotEq(a, b uint32) uint32 {
return ^uint32((uint64(a^b)-1)>>63) & 1
}
// constantTimeLess returns 1 if a < b or 0 otherwise in constant time.
func constantTimeLess(a, b uint32) uint32 {
return uint32((uint64(a) - uint64(b)) >> 63)
}
// constantTimeLessOrEq returns 1 if a <= b or 0 otherwise in constant time.
func constantTimeLessOrEq(a, b uint32) uint32 {
return uint32((uint64(a) - uint64(b) - 1) >> 63)
}
// constantTimeGreater returns 1 if a > b or 0 otherwise in constant time.
func constantTimeGreater(a, b uint32) uint32 {
return constantTimeLess(b, a)
}
// constantTimeGreaterOrEq returns 1 if a >= b or 0 otherwise in constant time.
func constantTimeGreaterOrEq(a, b uint32) uint32 {
return constantTimeLessOrEq(b, a)
}
// constantTimeMin returns min(a,b) in constant time.
func constantTimeMin(a, b uint32) uint32 {
return b ^ ((a ^ b) & -constantTimeLess(a, b))
}
// overflows determines if the current scalar is greater than or equal to the
// group order in constant time and returns 1 if it is or 0 otherwise.
func (s *ModNScalar) overflows() uint32 {
// The intuition here is that the scalar is greater than the group order if
// one of the higher individual words is greater than corresponding word of
// the group order and all higher words in the scalar are equal to their
// corresponding word of the group order. Since this type is modulo the
// group order, being equal is also an overflow back to 0.
//
// Note that the words 5, 6, and 7 are all the max uint32 value, so there is
// no need to test if those individual words of the scalar exceeds them,
// hence, only equality is checked for them.
highWordsEqual := constantTimeEq(s.n[7], orderWordSeven)
highWordsEqual &= constantTimeEq(s.n[6], orderWordSix)
highWordsEqual &= constantTimeEq(s.n[5], orderWordFive)
overflow := highWordsEqual & constantTimeGreater(s.n[4], orderWordFour)
highWordsEqual &= constantTimeEq(s.n[4], orderWordFour)
overflow |= highWordsEqual & constantTimeGreater(s.n[3], orderWordThree)
highWordsEqual &= constantTimeEq(s.n[3], orderWordThree)
overflow |= highWordsEqual & constantTimeGreater(s.n[2], orderWordTwo)
highWordsEqual &= constantTimeEq(s.n[2], orderWordTwo)
overflow |= highWordsEqual & constantTimeGreater(s.n[1], orderWordOne)
highWordsEqual &= constantTimeEq(s.n[1], orderWordOne)
overflow |= highWordsEqual & constantTimeGreaterOrEq(s.n[0], orderWordZero)
return overflow
}
// reduce256 reduces the current scalar modulo the group order in accordance
// with the overflows parameter in constant time. The overflows parameter
// specifies whether or not the scalar is known to be greater than the group
// order and MUST either be 1 in the case it is or 0 in the case it is not for a
// correct result.
func (s *ModNScalar) reduce256(overflows uint32) {
// Notice that since s < 2^256 < 2N (where N is the group order), the max
// possible number of reductions required is one. Therefore, in the case a
// reduction is needed, it can be performed with a single subtraction of N.
// Also, recall that subtraction is equivalent to addition by the two's
// complement while ignoring the carry.
//
// When s >= N, the overflows parameter will be 1. Conversely, it will be 0
// when s < N. Thus multiplying by the overflows parameter will either
// result in 0 or the multiplicand itself.
//
// Combining the above along with the fact that s + 0 = s, the following is
// a constant time implementation that works by either adding 0 or the two's
// complement of N as needed.
//
// The final result will be in the range 0 <= s < N as expected.
overflows64 := uint64(overflows)
c := uint64(s.n[0]) + overflows64*uint64(orderComplementWordZero)
s.n[0] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(s.n[1]) + overflows64*uint64(orderComplementWordOne)
s.n[1] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(s.n[2]) + overflows64*uint64(orderComplementWordTwo)
s.n[2] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(s.n[3]) + overflows64*uint64(orderComplementWordThree)
s.n[3] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(s.n[4]) + overflows64 // * 1
s.n[4] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(s.n[5]) // + overflows64 * 0
s.n[5] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(s.n[6]) // + overflows64 * 0
s.n[6] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(s.n[7]) // + overflows64 * 0
s.n[7] = uint32(c & uint32Mask)
}
// SetBytes interprets the provided array as a 256-bit big-endian unsigned
// integer, reduces it modulo the group order, sets the scalar to the result,
// and returns either 1 if it was reduced (aka it overflowed) or 0 otherwise in
// constant time.
//
// Note that a bool is not used here because it is not possible in Go to convert
// from a bool to numeric value in constant time and many constant-time
// operations require a numeric value.
func (s *ModNScalar) SetBytes(b *[32]byte) uint32 {
// Pack the 256 total bits across the 8 uint32 words. This could be done
// with a for loop, but benchmarks show this unrolled version is about 2
// times faster than the variant that uses a loop.
s.n[0] = uint32(b[31]) | uint32(b[30])<<8 | uint32(b[29])<<16 | uint32(b[28])<<24
s.n[1] = uint32(b[27]) | uint32(b[26])<<8 | uint32(b[25])<<16 | uint32(b[24])<<24
s.n[2] = uint32(b[23]) | uint32(b[22])<<8 | uint32(b[21])<<16 | uint32(b[20])<<24
s.n[3] = uint32(b[19]) | uint32(b[18])<<8 | uint32(b[17])<<16 | uint32(b[16])<<24
s.n[4] = uint32(b[15]) | uint32(b[14])<<8 | uint32(b[13])<<16 | uint32(b[12])<<24
s.n[5] = uint32(b[11]) | uint32(b[10])<<8 | uint32(b[9])<<16 | uint32(b[8])<<24
s.n[6] = uint32(b[7]) | uint32(b[6])<<8 | uint32(b[5])<<16 | uint32(b[4])<<24
s.n[7] = uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24
// The value might be >= N, so reduce it as required and return whether or
// not it was reduced.
needsReduce := s.overflows()
s.reduce256(needsReduce)
return needsReduce
}
// zeroArray32 zeroes the provided 32-byte buffer.
func zeroArray32(b *[32]byte) {
copy(b[:], zero32[:])
}
// SetByteSlice interprets the provided slice as a 256-bit big-endian unsigned
// integer (meaning it is truncated to the first 32 bytes), reduces it modulo
// the group order, sets the scalar to the result, and returns whether or not
// the resulting truncated 256-bit integer overflowed in constant time.
//
// Note that since passing a slice with more than 32 bytes is truncated, it is
// possible that the truncated value is less than the order of the curve and
// hence it will not be reported as having overflowed in that case. It is up to
// the caller to decide whether it needs to provide numbers of the appropriate
// size or it is acceptable to use this function with the described truncation
// and overflow behavior.
func (s *ModNScalar) SetByteSlice(b []byte) bool {
var b32 [32]byte
b = b[:constantTimeMin(uint32(len(b)), 32)]
copy(b32[:], b32[:32-len(b)])
copy(b32[32-len(b):], b)
result := s.SetBytes(&b32)
zeroArray32(&b32)
return result != 0
}
// PutBytesUnchecked unpacks the scalar to a 32-byte big-endian value directly
// into the passed byte slice in constant time. The target slice must must have
// at least 32 bytes available or it will panic.
//
// There is a similar function, PutBytes, which unpacks the scalar into a
// 32-byte array directly. This version is provided since it can be useful to
// write directly into part of a larger buffer without needing a separate
// allocation.
//
// Preconditions:
// - The target slice MUST have at least 32 bytes available
func (s *ModNScalar) PutBytesUnchecked(b []byte) {
// Unpack the 256 total bits from the 8 uint32 words. This could be done
// with a for loop, but benchmarks show this unrolled version is about 2
// times faster than the variant which uses a loop.
b[31] = byte(s.n[0])
b[30] = byte(s.n[0] >> 8)
b[29] = byte(s.n[0] >> 16)
b[28] = byte(s.n[0] >> 24)
b[27] = byte(s.n[1])
b[26] = byte(s.n[1] >> 8)
b[25] = byte(s.n[1] >> 16)
b[24] = byte(s.n[1] >> 24)
b[23] = byte(s.n[2])
b[22] = byte(s.n[2] >> 8)
b[21] = byte(s.n[2] >> 16)
b[20] = byte(s.n[2] >> 24)
b[19] = byte(s.n[3])
b[18] = byte(s.n[3] >> 8)
b[17] = byte(s.n[3] >> 16)
b[16] = byte(s.n[3] >> 24)
b[15] = byte(s.n[4])
b[14] = byte(s.n[4] >> 8)
b[13] = byte(s.n[4] >> 16)
b[12] = byte(s.n[4] >> 24)
b[11] = byte(s.n[5])
b[10] = byte(s.n[5] >> 8)
b[9] = byte(s.n[5] >> 16)
b[8] = byte(s.n[5] >> 24)
b[7] = byte(s.n[6])
b[6] = byte(s.n[6] >> 8)
b[5] = byte(s.n[6] >> 16)
b[4] = byte(s.n[6] >> 24)
b[3] = byte(s.n[7])
b[2] = byte(s.n[7] >> 8)
b[1] = byte(s.n[7] >> 16)
b[0] = byte(s.n[7] >> 24)
}
// PutBytes unpacks the scalar to a 32-byte big-endian value using the passed
// byte array in constant time.
//
// There is a similar function, PutBytesUnchecked, which unpacks the scalar into
// a slice that must have at least 32 bytes available. This version is provided
// since it can be useful to write directly into an array that is type checked.
//
// Alternatively, there is also Bytes, which unpacks the scalar into a new array
// and returns that which can sometimes be more ergonomic in applications that
// aren't concerned about an additional copy.
func (s *ModNScalar) PutBytes(b *[32]byte) {
s.PutBytesUnchecked(b[:])
}
// Bytes unpacks the scalar to a 32-byte big-endian value in constant time.
//
// See PutBytes and PutBytesUnchecked for variants that allow an array or slice
// to be passed which can be useful to cut down on the number of allocations
// by allowing the caller to reuse a buffer or write directly into part of a
// larger buffer.
func (s *ModNScalar) Bytes() [32]byte {
var b [32]byte
s.PutBytesUnchecked(b[:])
return b
}
// IsOdd returns whether or not the scalar is an odd number in constant time.
func (s *ModNScalar) IsOdd() bool {
// Only odd numbers have the bottom bit set.
return s.n[0]&1 == 1
}
// Equals returns whether or not the two scalars are the same in constant time.
func (s *ModNScalar) Equals(val *ModNScalar) bool {
// Xor only sets bits when they are different, so the two scalars can only
// be the same if no bits are set after xoring each word.
bits := (s.n[0] ^ val.n[0]) | (s.n[1] ^ val.n[1]) | (s.n[2] ^ val.n[2]) |
(s.n[3] ^ val.n[3]) | (s.n[4] ^ val.n[4]) | (s.n[5] ^ val.n[5]) |
(s.n[6] ^ val.n[6]) | (s.n[7] ^ val.n[7])
return bits == 0
}
// Add2 adds the passed two scalars together modulo the group order in constant
// time and stores the result in s.
//
// The scalar is returned to support chaining. This enables syntax like:
// s3.Add2(s, s2).AddInt(1) so that s3 = s + s2 + 1.
func (s *ModNScalar) Add2(val1, val2 *ModNScalar) *ModNScalar {
c := uint64(val1.n[0]) + uint64(val2.n[0])
s.n[0] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(val1.n[1]) + uint64(val2.n[1])
s.n[1] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(val1.n[2]) + uint64(val2.n[2])
s.n[2] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(val1.n[3]) + uint64(val2.n[3])
s.n[3] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(val1.n[4]) + uint64(val2.n[4])
s.n[4] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(val1.n[5]) + uint64(val2.n[5])
s.n[5] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(val1.n[6]) + uint64(val2.n[6])
s.n[6] = uint32(c & uint32Mask)
c = (c >> 32) + uint64(val1.n[7]) + uint64(val2.n[7])
s.n[7] = uint32(c & uint32Mask)
// The result is now 256 bits, but it might still be >= N, so use the
// existing normal reduce method for 256-bit values.
s.reduce256(uint32(c>>32) + s.overflows())
return s
}
// Add adds the passed scalar to the existing one modulo the group order in
// constant time and stores the result in s.
//
// The scalar is returned to support chaining. This enables syntax like:
// s.Add(s2).AddInt(1) so that s = s + s2 + 1.
func (s *ModNScalar) Add(val *ModNScalar) *ModNScalar {
return s.Add2(s, val)
}
// accumulator96 provides a 96-bit accumulator for use in the intermediate
// calculations requiring more than 64-bits.
type accumulator96 struct {
n [3]uint32
}
// Add adds the passed unsigned 64-bit value to the accumulator.
func (a *accumulator96) Add(v uint64) {
low := uint32(v & uint32Mask)
hi := uint32(v >> 32)
a.n[0] += low
hi += constantTimeLess(a.n[0], low) // Carry if overflow in n[0].
a.n[1] += hi
a.n[2] += constantTimeLess(a.n[1], hi) // Carry if overflow in n[1].
}
// Rsh32 right shifts the accumulator by 32 bits.
func (a *accumulator96) Rsh32() {
a.n[0] = a.n[1]
a.n[1] = a.n[2]
a.n[2] = 0
}
// reduce385 reduces the 385-bit intermediate result in the passed terms modulo
// the group order in constant time and stores the result in s.
func (s *ModNScalar) reduce385(t0, t1, t2, t3, t4, t5, t6, t7, t8, t9, t10, t11, t12 uint64) {
// At this point, the intermediate result in the passed terms has been
// reduced to fit within 385 bits, so reduce it again using the same method
// described in reduce512. As before, the intermediate result will end up
// being reduced by another 127 bits to 258 bits, thus 9 32-bit terms are
// needed for this iteration. The reduced terms are assigned back to t0
// through t8.
//
// Note that several of the intermediate calculations require adding 64-bit
// products together which would overflow a uint64, so a 96-bit accumulator
// is used instead until the value is reduced enough to use native uint64s.
// Terms for 2^(32*0).
var acc accumulator96
acc.n[0] = uint32(t0) // == acc.Add(t0) because acc is guaranteed to be 0.
acc.Add(t8 * uint64(orderComplementWordZero))
t0 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*1).
acc.Add(t1)
acc.Add(t8 * uint64(orderComplementWordOne))
acc.Add(t9 * uint64(orderComplementWordZero))
t1 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*2).
acc.Add(t2)
acc.Add(t8 * uint64(orderComplementWordTwo))
acc.Add(t9 * uint64(orderComplementWordOne))
acc.Add(t10 * uint64(orderComplementWordZero))
t2 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*3).
acc.Add(t3)
acc.Add(t8 * uint64(orderComplementWordThree))
acc.Add(t9 * uint64(orderComplementWordTwo))
acc.Add(t10 * uint64(orderComplementWordOne))
acc.Add(t11 * uint64(orderComplementWordZero))
t3 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*4).
acc.Add(t4)
acc.Add(t8) // * uint64(orderComplementWordFour) // * 1
acc.Add(t9 * uint64(orderComplementWordThree))
acc.Add(t10 * uint64(orderComplementWordTwo))
acc.Add(t11 * uint64(orderComplementWordOne))
acc.Add(t12 * uint64(orderComplementWordZero))
t4 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*5).
acc.Add(t5)
// acc.Add(t8 * uint64(orderComplementWordFive)) // 0
acc.Add(t9) // * uint64(orderComplementWordFour) // * 1
acc.Add(t10 * uint64(orderComplementWordThree))
acc.Add(t11 * uint64(orderComplementWordTwo))
acc.Add(t12 * uint64(orderComplementWordOne))
t5 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*6).
acc.Add(t6)
// acc.Add(t8 * uint64(orderComplementWordSix)) // 0
// acc.Add(t9 * uint64(orderComplementWordFive)) // 0
acc.Add(t10) // * uint64(orderComplementWordFour) // * 1
acc.Add(t11 * uint64(orderComplementWordThree))
acc.Add(t12 * uint64(orderComplementWordTwo))
t6 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*7).
acc.Add(t7)
// acc.Add(t8 * uint64(orderComplementWordSeven)) // 0
// acc.Add(t9 * uint64(orderComplementWordSix)) // 0
// acc.Add(t10 * uint64(orderComplementWordFive)) // 0
acc.Add(t11) // * uint64(orderComplementWordFour) // * 1
acc.Add(t12 * uint64(orderComplementWordThree))
t7 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*8).
// acc.Add(t9 * uint64(orderComplementWordSeven)) // 0
// acc.Add(t10 * uint64(orderComplementWordSix)) // 0
// acc.Add(t11 * uint64(orderComplementWordFive)) // 0
acc.Add(t12) // * uint64(orderComplementWordFour) // * 1
t8 = uint64(acc.n[0])
// acc.Rsh32() // No need since not used after this. Guaranteed to be 0.
// NOTE: All of the remaining multiplications for this iteration result in 0
// as they all involve multiplying by combinations of the fifth, sixth, and
// seventh words of the two's complement of N, which are 0, so skip them.
// At this point, the result is reduced to fit within 258 bits, so reduce it
// again using a slightly modified version of the same method. The maximum
// value in t8 is 2 at this point and therefore multiplying it by each word
// of the two's complement of N and adding it to a 32-bit term will result
// in a maximum requirement of 33 bits, so it is safe to use native uint64s
// here for the intermediate term carry propagation.
//
// Also, since the maximum value in t8 is 2, this ends up reducing by
// another 2 bits to 256 bits.
c := t0 + t8*uint64(orderComplementWordZero)
s.n[0] = uint32(c & uint32Mask)
c = (c >> 32) + t1 + t8*uint64(orderComplementWordOne)
s.n[1] = uint32(c & uint32Mask)
c = (c >> 32) + t2 + t8*uint64(orderComplementWordTwo)
s.n[2] = uint32(c & uint32Mask)
c = (c >> 32) + t3 + t8*uint64(orderComplementWordThree)
s.n[3] = uint32(c & uint32Mask)
c = (c >> 32) + t4 + t8 // * uint64(orderComplementWordFour) == * 1
s.n[4] = uint32(c & uint32Mask)
c = (c >> 32) + t5 // + t8*uint64(orderComplementWordFive) == 0
s.n[5] = uint32(c & uint32Mask)
c = (c >> 32) + t6 // + t8*uint64(orderComplementWordSix) == 0
s.n[6] = uint32(c & uint32Mask)
c = (c >> 32) + t7 // + t8*uint64(orderComplementWordSeven) == 0
s.n[7] = uint32(c & uint32Mask)
// The result is now 256 bits, but it might still be >= N, so use the
// existing normal reduce method for 256-bit values.
s.reduce256(uint32(c>>32) + s.overflows())
}
// reduce512 reduces the 512-bit intermediate result in the passed terms modulo
// the group order down to 385 bits in constant time and stores the result in s.
func (s *ModNScalar) reduce512(t0, t1, t2, t3, t4, t5, t6, t7, t8, t9, t10, t11, t12, t13, t14, t15 uint64) {
// At this point, the intermediate result in the passed terms is grouped
// into the respective bases.
//
// Per [HAC] section 14.3.4: Reduction method of moduli of special form,
// when the modulus is of the special form m = b^t - c, where log_2(c) < t,
// highly efficient reduction can be achieved per the provided algorithm.
//
// The secp256k1 group order fits this criteria since it is:
// 2^256 - 432420386565659656852420866394968145599
//
// Technically the max possible value here is (N-1)^2 since the two scalars
// being multiplied are always mod N. Nevertheless, it is safer to consider
// it to be (2^256-1)^2 = 2^512 - 2^256 + 1 since it is the product of two
// 256-bit values.
//
// The algorithm is to reduce the result modulo the prime by subtracting
// multiples of the group order N. However, in order simplify carry
// propagation, this adds with the two's complement of N to achieve the same
// result.
//
// Since the two's complement of N has 127 leading zero bits, this will end
// up reducing the intermediate result from 512 bits to 385 bits, resulting
// in 13 32-bit terms. The reduced terms are assigned back to t0 through
// t12.
//
// Note that several of the intermediate calculations require adding 64-bit
// products together which would overflow a uint64, so a 96-bit accumulator
// is used instead.
// Terms for 2^(32*0).
var acc accumulator96
acc.n[0] = uint32(t0) // == acc.Add(t0) because acc is guaranteed to be 0.
acc.Add(t8 * uint64(orderComplementWordZero))
t0 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*1).
acc.Add(t1)
acc.Add(t8 * uint64(orderComplementWordOne))
acc.Add(t9 * uint64(orderComplementWordZero))
t1 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*2).
acc.Add(t2)
acc.Add(t8 * uint64(orderComplementWordTwo))
acc.Add(t9 * uint64(orderComplementWordOne))
acc.Add(t10 * uint64(orderComplementWordZero))
t2 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*3).
acc.Add(t3)
acc.Add(t8 * uint64(orderComplementWordThree))
acc.Add(t9 * uint64(orderComplementWordTwo))
acc.Add(t10 * uint64(orderComplementWordOne))
acc.Add(t11 * uint64(orderComplementWordZero))
t3 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*4).
acc.Add(t4)
acc.Add(t8) // * uint64(orderComplementWordFour) // * 1
acc.Add(t9 * uint64(orderComplementWordThree))
acc.Add(t10 * uint64(orderComplementWordTwo))
acc.Add(t11 * uint64(orderComplementWordOne))
acc.Add(t12 * uint64(orderComplementWordZero))
t4 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*5).
acc.Add(t5)
// acc.Add(t8 * uint64(orderComplementWordFive)) // 0
acc.Add(t9) // * uint64(orderComplementWordFour) // * 1
acc.Add(t10 * uint64(orderComplementWordThree))
acc.Add(t11 * uint64(orderComplementWordTwo))
acc.Add(t12 * uint64(orderComplementWordOne))
acc.Add(t13 * uint64(orderComplementWordZero))
t5 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*6).
acc.Add(t6)
// acc.Add(t8 * uint64(orderComplementWordSix)) // 0
// acc.Add(t9 * uint64(orderComplementWordFive)) // 0
acc.Add(t10) // * uint64(orderComplementWordFour)) // * 1
acc.Add(t11 * uint64(orderComplementWordThree))
acc.Add(t12 * uint64(orderComplementWordTwo))
acc.Add(t13 * uint64(orderComplementWordOne))
acc.Add(t14 * uint64(orderComplementWordZero))
t6 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*7).
acc.Add(t7)
// acc.Add(t8 * uint64(orderComplementWordSeven)) // 0
// acc.Add(t9 * uint64(orderComplementWordSix)) // 0
// acc.Add(t10 * uint64(orderComplementWordFive)) // 0
acc.Add(t11) // * uint64(orderComplementWordFour) // * 1
acc.Add(t12 * uint64(orderComplementWordThree))
acc.Add(t13 * uint64(orderComplementWordTwo))
acc.Add(t14 * uint64(orderComplementWordOne))
acc.Add(t15 * uint64(orderComplementWordZero))
t7 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*8).
// acc.Add(t9 * uint64(orderComplementWordSeven)) // 0
// acc.Add(t10 * uint64(orderComplementWordSix)) // 0
// acc.Add(t11 * uint64(orderComplementWordFive)) // 0
acc.Add(t12) // * uint64(orderComplementWordFour) // * 1
acc.Add(t13 * uint64(orderComplementWordThree))
acc.Add(t14 * uint64(orderComplementWordTwo))
acc.Add(t15 * uint64(orderComplementWordOne))
t8 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*9).
// acc.Add(t10 * uint64(orderComplementWordSeven)) // 0
// acc.Add(t11 * uint64(orderComplementWordSix)) // 0
// acc.Add(t12 * uint64(orderComplementWordFive)) // 0
acc.Add(t13) // * uint64(orderComplementWordFour) // * 1
acc.Add(t14 * uint64(orderComplementWordThree))
acc.Add(t15 * uint64(orderComplementWordTwo))
t9 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*10).
// acc.Add(t11 * uint64(orderComplementWordSeven)) // 0
// acc.Add(t12 * uint64(orderComplementWordSix)) // 0
// acc.Add(t13 * uint64(orderComplementWordFive)) // 0
acc.Add(t14) // * uint64(orderComplementWordFour) // * 1
acc.Add(t15 * uint64(orderComplementWordThree))
t10 = uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*11).
// acc.Add(t12 * uint64(orderComplementWordSeven)) // 0
// acc.Add(t13 * uint64(orderComplementWordSix)) // 0
// acc.Add(t14 * uint64(orderComplementWordFive)) // 0
acc.Add(t15) // * uint64(orderComplementWordFour) // * 1
t11 = uint64(acc.n[0])
acc.Rsh32()
// NOTE: All of the remaining multiplications for this iteration result in 0
// as they all involve multiplying by combinations of the fifth, sixth, and
// seventh words of the two's complement of N, which are 0, so skip them.
// Terms for 2^(32*12).
t12 = uint64(acc.n[0])
// acc.Rsh32() // No need since not used after this. Guaranteed to be 0.
// At this point, the result is reduced to fit within 385 bits, so reduce it
// again using the same method accordingly.
s.reduce385(t0, t1, t2, t3, t4, t5, t6, t7, t8, t9, t10, t11, t12)
}
// Mul2 multiplies the passed two scalars together modulo the group order in
// constant time and stores the result in s.
//
// The scalar is returned to support chaining. This enables syntax like:
// s3.Mul2(s, s2).AddInt(1) so that s3 = (s * s2) + 1.
func (s *ModNScalar) Mul2(val, val2 *ModNScalar) *ModNScalar {
// This could be done with for loops and an array to store the intermediate
// terms, but this unrolled version is significantly faster.
// The overall strategy employed here is:
// 1) Calculate the 512-bit product of the two scalars using the standard
// pencil-and-paper method.
// 2) Reduce the result modulo the prime by effectively subtracting
// multiples of the group order N (actually performed by adding multiples
// of the two's complement of N to avoid implementing subtraction).
// 3) Repeat step 2 noting that each iteration reduces the required number
// of bits by 127 because the two's complement of N has 127 leading zero
// bits.
// 4) Once reduced to 256 bits, call the existing reduce method to perform
// a final reduction as needed.
//
// Note that several of the intermediate calculations require adding 64-bit
// products together which would overflow a uint64, so a 96-bit accumulator
// is used instead.
// Terms for 2^(32*0).
var acc accumulator96
acc.Add(uint64(val.n[0]) * uint64(val2.n[0]))
t0 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*1).
acc.Add(uint64(val.n[0]) * uint64(val2.n[1]))
acc.Add(uint64(val.n[1]) * uint64(val2.n[0]))
t1 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*2).
acc.Add(uint64(val.n[0]) * uint64(val2.n[2]))
acc.Add(uint64(val.n[1]) * uint64(val2.n[1]))
acc.Add(uint64(val.n[2]) * uint64(val2.n[0]))
t2 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*3).
acc.Add(uint64(val.n[0]) * uint64(val2.n[3]))
acc.Add(uint64(val.n[1]) * uint64(val2.n[2]))
acc.Add(uint64(val.n[2]) * uint64(val2.n[1]))
acc.Add(uint64(val.n[3]) * uint64(val2.n[0]))
t3 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*4).
acc.Add(uint64(val.n[0]) * uint64(val2.n[4]))
acc.Add(uint64(val.n[1]) * uint64(val2.n[3]))
acc.Add(uint64(val.n[2]) * uint64(val2.n[2]))
acc.Add(uint64(val.n[3]) * uint64(val2.n[1]))
acc.Add(uint64(val.n[4]) * uint64(val2.n[0]))
t4 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*5).
acc.Add(uint64(val.n[0]) * uint64(val2.n[5]))
acc.Add(uint64(val.n[1]) * uint64(val2.n[4]))
acc.Add(uint64(val.n[2]) * uint64(val2.n[3]))
acc.Add(uint64(val.n[3]) * uint64(val2.n[2]))
acc.Add(uint64(val.n[4]) * uint64(val2.n[1]))
acc.Add(uint64(val.n[5]) * uint64(val2.n[0]))
t5 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*6).
acc.Add(uint64(val.n[0]) * uint64(val2.n[6]))
acc.Add(uint64(val.n[1]) * uint64(val2.n[5]))
acc.Add(uint64(val.n[2]) * uint64(val2.n[4]))
acc.Add(uint64(val.n[3]) * uint64(val2.n[3]))
acc.Add(uint64(val.n[4]) * uint64(val2.n[2]))
acc.Add(uint64(val.n[5]) * uint64(val2.n[1]))
acc.Add(uint64(val.n[6]) * uint64(val2.n[0]))
t6 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*7).
acc.Add(uint64(val.n[0]) * uint64(val2.n[7]))
acc.Add(uint64(val.n[1]) * uint64(val2.n[6]))
acc.Add(uint64(val.n[2]) * uint64(val2.n[5]))
acc.Add(uint64(val.n[3]) * uint64(val2.n[4]))
acc.Add(uint64(val.n[4]) * uint64(val2.n[3]))
acc.Add(uint64(val.n[5]) * uint64(val2.n[2]))
acc.Add(uint64(val.n[6]) * uint64(val2.n[1]))
acc.Add(uint64(val.n[7]) * uint64(val2.n[0]))
t7 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*8).
acc.Add(uint64(val.n[1]) * uint64(val2.n[7]))
acc.Add(uint64(val.n[2]) * uint64(val2.n[6]))
acc.Add(uint64(val.n[3]) * uint64(val2.n[5]))
acc.Add(uint64(val.n[4]) * uint64(val2.n[4]))
acc.Add(uint64(val.n[5]) * uint64(val2.n[3]))
acc.Add(uint64(val.n[6]) * uint64(val2.n[2]))
acc.Add(uint64(val.n[7]) * uint64(val2.n[1]))
t8 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*9).
acc.Add(uint64(val.n[2]) * uint64(val2.n[7]))
acc.Add(uint64(val.n[3]) * uint64(val2.n[6]))
acc.Add(uint64(val.n[4]) * uint64(val2.n[5]))
acc.Add(uint64(val.n[5]) * uint64(val2.n[4]))
acc.Add(uint64(val.n[6]) * uint64(val2.n[3]))
acc.Add(uint64(val.n[7]) * uint64(val2.n[2]))
t9 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*10).
acc.Add(uint64(val.n[3]) * uint64(val2.n[7]))
acc.Add(uint64(val.n[4]) * uint64(val2.n[6]))
acc.Add(uint64(val.n[5]) * uint64(val2.n[5]))
acc.Add(uint64(val.n[6]) * uint64(val2.n[4]))
acc.Add(uint64(val.n[7]) * uint64(val2.n[3]))
t10 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*11).
acc.Add(uint64(val.n[4]) * uint64(val2.n[7]))
acc.Add(uint64(val.n[5]) * uint64(val2.n[6]))
acc.Add(uint64(val.n[6]) * uint64(val2.n[5]))
acc.Add(uint64(val.n[7]) * uint64(val2.n[4]))
t11 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*12).
acc.Add(uint64(val.n[5]) * uint64(val2.n[7]))
acc.Add(uint64(val.n[6]) * uint64(val2.n[6]))
acc.Add(uint64(val.n[7]) * uint64(val2.n[5]))
t12 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*13).
acc.Add(uint64(val.n[6]) * uint64(val2.n[7]))
acc.Add(uint64(val.n[7]) * uint64(val2.n[6]))
t13 := uint64(acc.n[0])
acc.Rsh32()
// Terms for 2^(32*14).
acc.Add(uint64(val.n[7]) * uint64(val2.n[7]))
t14 := uint64(acc.n[0])
acc.Rsh32()
// What's left is for 2^(32*15).
t15 := uint64(acc.n[0])
// acc.Rsh32() // No need since not used after this. Guaranteed to be 0.
// At this point, all of the terms are grouped into their respective base
// and occupy up to 512 bits. Reduce the result accordingly.
s.reduce512(t0, t1, t2, t3, t4, t5, t6, t7, t8, t9, t10, t11, t12, t13, t14,
t15)
return s
}
// Mul multiplies the passed scalar with the existing one modulo the group order
// in constant time and stores the result in s.
//
// The scalar is returned to support chaining. This enables syntax like:
// s.Mul(s2).AddInt(1) so that s = (s * s2) + 1.
func (s *ModNScalar) Mul(val *ModNScalar) *ModNScalar {
return s.Mul2(s, val)
}
// SquareVal squares the passed scalar modulo the group order in constant time
// and stores the result in s.
//
// The scalar is returned to support chaining. This enables syntax like:
// s3.SquareVal(s).Mul(s) so that s3 = s^2 * s = s^3.
func (s *ModNScalar) SquareVal(val *ModNScalar) *ModNScalar {
// This could technically be optimized slightly to take advantage of the
// fact that many of the intermediate calculations in squaring are just
// doubling, however, benchmarking has shown that due to the need to use a
// 96-bit accumulator, any savings are essentially offset by that and
// consequently there is no real difference in performance over just
// multiplying the value by itself to justify the extra code for now. This
// can be revisited in the future if it becomes a bottleneck in practice.
return s.Mul2(val, val)
}
// Square squares the scalar modulo the group order in constant time. The
// existing scalar is modified.
//
// The scalar is returned to support chaining. This enables syntax like:
// s.Square().Mul(s2) so that s = s^2 * s2.
func (s *ModNScalar) Square() *ModNScalar {
return s.SquareVal(s)
}
// NegateVal negates the passed scalar modulo the group order and stores the
// result in s in constant time.
//
// The scalar is returned to support chaining. This enables syntax like:
// s.NegateVal(s2).AddInt(1) so that s = -s2 + 1.
func (s *ModNScalar) NegateVal(val *ModNScalar) *ModNScalar {
// Since the scalar is already in the range 0 <= val < N, where N is the
// group order, negation modulo the group order is just the group order
// minus the value. This implies that the result will always be in the
// desired range with the sole exception of 0 because N - 0 = N itself.
//
// Therefore, in order to avoid the need to reduce the result for every
// other case in order to achieve constant time, this creates a mask that is
// all 0s in the case of the scalar being negated is 0 and all 1s otherwise
// and bitwise ands that mask with each word.
//
// Finally, to simplify the carry propagation, this adds the two's
// complement of the scalar to N in order to achieve the same result.
bits := val.n[0] | val.n[1] | val.n[2] | val.n[3] | val.n[4] | val.n[5] |
val.n[6] | val.n[7]
mask := uint64(uint32Mask * constantTimeNotEq(bits, 0))
c := uint64(orderWordZero) + (uint64(^val.n[0]) + 1)
s.n[0] = uint32(c & mask)
c = (c >> 32) + uint64(orderWordOne) + uint64(^val.n[1])
s.n[1] = uint32(c & mask)
c = (c >> 32) + uint64(orderWordTwo) + uint64(^val.n[2])
s.n[2] = uint32(c & mask)
c = (c >> 32) + uint64(orderWordThree) + uint64(^val.n[3])
s.n[3] = uint32(c & mask)
c = (c >> 32) + uint64(orderWordFour) + uint64(^val.n[4])
s.n[4] = uint32(c & mask)
c = (c >> 32) + uint64(orderWordFive) + uint64(^val.n[5])
s.n[5] = uint32(c & mask)
c = (c >> 32) + uint64(orderWordSix) + uint64(^val.n[6])
s.n[6] = uint32(c & mask)
c = (c >> 32) + uint64(orderWordSeven) + uint64(^val.n[7])
s.n[7] = uint32(c & mask)
return s
}
// Negate negates the scalar modulo the group order in constant time. The
// existing scalar is modified.
//
// The scalar is returned to support chaining. This enables syntax like:
// s.Negate().AddInt(1) so that s = -s + 1.
func (s *ModNScalar) Negate() *ModNScalar {
return s.NegateVal(s)
}
// InverseValNonConst finds the modular multiplicative inverse of the passed
// scalar and stores result in s in *non-constant* time.
//
// The scalar is returned to support chaining. This enables syntax like:
// s3.InverseVal(s1).Mul(s2) so that s3 = s1^-1 * s2.
func (s *ModNScalar) InverseValNonConst(val *ModNScalar) *ModNScalar {
// This is making use of big integers for now. Ideally it will be replaced
// with an implementation that does not depend on big integers.
valBytes := val.Bytes()
bigVal := new(big.Int).SetBytes(valBytes[:])
bigVal.ModInverse(bigVal, curveParams.N)
s.SetByteSlice(bigVal.Bytes())
return s
}
// InverseNonConst finds the modular multiplicative inverse of the scalar in
// *non-constant* time. The existing scalar is modified.
//
// The scalar is returned to support chaining. This enables syntax like:
// s.Inverse().Mul(s2) so that s = s^-1 * s2.
func (s *ModNScalar) InverseNonConst() *ModNScalar {
return s.InverseValNonConst(s)
}
// IsOverHalfOrder returns whether or not the scalar exceeds the group order
// divided by 2 in constant time.
func (s *ModNScalar) IsOverHalfOrder() bool {
// The intuition here is that the scalar is greater than half of the group
// order if one of the higher individual words is greater than the
// corresponding word of the half group order and all higher words in the
// scalar are equal to their corresponding word of the half group order.
//
// Note that the words 4, 5, and 6 are all the max uint32 value, so there is
// no need to test if those individual words of the scalar exceeds them,
// hence, only equality is checked for them.
result := constantTimeGreater(s.n[7], halfOrderWordSeven)
highWordsEqual := constantTimeEq(s.n[7], halfOrderWordSeven)
highWordsEqual &= constantTimeEq(s.n[6], halfOrderWordSix)
highWordsEqual &= constantTimeEq(s.n[5], halfOrderWordFive)
highWordsEqual &= constantTimeEq(s.n[4], halfOrderWordFour)
result |= highWordsEqual & constantTimeGreater(s.n[3], halfOrderWordThree)
highWordsEqual &= constantTimeEq(s.n[3], halfOrderWordThree)
result |= highWordsEqual & constantTimeGreater(s.n[2], halfOrderWordTwo)
highWordsEqual &= constantTimeEq(s.n[2], halfOrderWordTwo)
result |= highWordsEqual & constantTimeGreater(s.n[1], halfOrderWordOne)
highWordsEqual &= constantTimeEq(s.n[1], halfOrderWordOne)
result |= highWordsEqual & constantTimeGreater(s.n[0], halfOrderWordZero)
return result != 0
}