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51 lines
1.7 KiB
Go
51 lines
1.7 KiB
Go
// Copyright (c) 2021 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package field
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import "errors"
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// This file contains additional functionality that is not included in the
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// upstream crypto/ed25519/edwards25519/field package.
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// SetWideBytes sets v to x, where x is a 64-byte little-endian encoding, which
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// is reduced modulo the field order. If x is not of the right length,
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// SetWideBytes returns nil and an error, and the receiver is unchanged.
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//
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// SetWideBytes is not necessary to select a uniformly distributed value, and is
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// only provided for compatibility: SetBytes can be used instead as the chance
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// of bias is less than 2⁻²⁵⁰.
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func (v *Element) SetWideBytes(x []byte) (*Element, error) {
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if len(x) != 64 {
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return nil, errors.New("edwards25519: invalid SetWideBytes input size")
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}
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// Split the 64 bytes into two elements, and extract the most significant
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// bit of each, which is ignored by SetBytes.
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lo, _ := new(Element).SetBytes(x[:32])
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loMSB := uint64(x[31] >> 7)
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hi, _ := new(Element).SetBytes(x[32:])
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hiMSB := uint64(x[63] >> 7)
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// The output we want is
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//
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// v = lo + loMSB * 2²⁵⁵ + hi * 2²⁵⁶ + hiMSB * 2⁵¹¹
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//
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// which applying the reduction identity comes out to
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//
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// v = lo + loMSB * 19 + hi * 2 * 19 + hiMSB * 2 * 19²
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//
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// l0 will be the sum of a 52 bits value (lo.l0), plus a 5 bits value
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// (loMSB * 19), a 6 bits value (hi.l0 * 2 * 19), and a 10 bits value
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// (hiMSB * 2 * 19²), so it fits in a uint64.
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v.l0 = lo.l0 + loMSB*19 + hi.l0*2*19 + hiMSB*2*19*19
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v.l1 = lo.l1 + hi.l1*2*19
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v.l2 = lo.l2 + hi.l2*2*19
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v.l3 = lo.l3 + hi.l3*2*19
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v.l4 = lo.l4 + hi.l4*2*19
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return v.carryPropagate(), nil
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}
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