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// Copyright 2012 The Go Authors.  All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package elliptic

// This is a constant-time, 32-bit implementation of P224. See FIPS 186-3,
// section D.2.2.
//
// See http://www.imperialviolet.org/2010/12/04/ecc.html ([1]) for background.

import (
        "math/big"
)

var p224 p224Curve

type p224Curve struct {
        *CurveParams
        gx, gy, b p224FieldElement
}

func initP224() {
        // See FIPS 186-3, section D.2.2
        p224.CurveParams = new(CurveParams)
        p224.P, _ = new(big.Int).SetString("26959946667150639794667015087019630673557916260026308143510066298881", 10)
        p224.N, _ = new(big.Int).SetString("26959946667150639794667015087019625940457807714424391721682722368061", 10)
        p224.B, _ = new(big.Int).SetString("b4050a850c04b3abf54132565044b0b7d7bfd8ba270b39432355ffb4", 16)
        p224.Gx, _ = new(big.Int).SetString("b70e0cbd6bb4bf7f321390b94a03c1d356c21122343280d6115c1d21", 16)
        p224.Gy, _ = new(big.Int).SetString("bd376388b5f723fb4c22dfe6cd4375a05a07476444d5819985007e34", 16)
        p224.BitSize = 224

        p224FromBig(&p224.gx, p224.Gx)
        p224FromBig(&p224.gy, p224.Gy)
        p224FromBig(&p224.b, p224.B)
}

// P224 returns a Curve which implements P-224 (see FIPS 186-3, section D.2.2)
func P224() Curve {
        initonce.Do(initAll)
        return p224
}

func (curve p224Curve) Params() *CurveParams {
        return curve.CurveParams
}

func (curve p224Curve) IsOnCurve(bigX, bigY *big.Int) bool {
        var x, y p224FieldElement
        p224FromBig(&x, bigX)
        p224FromBig(&y, bigY)

        // y² = x³ - 3x + b
        var tmp p224LargeFieldElement
        var x3 p224FieldElement
        p224Square(&x3, &x, &tmp)
        p224Mul(&x3, &x3, &x, &tmp)

        for i := 0; i < 8; i++ {
                x[i] *= 3
        }
        p224Sub(&x3, &x3, &x)
        p224Reduce(&x3)
        p224Add(&x3, &x3, &curve.b)
        p224Contract(&x3, &x3)

        p224Square(&y, &y, &tmp)
        p224Contract(&y, &y)

        for i := 0; i < 8; i++ {
                if y[i] != x3[i] {
                        return false
                }
        }
        return true
}

func (p224Curve) Add(bigX1, bigY1, bigX2, bigY2 *big.Int) (x, y *big.Int) {
        var x1, y1, z1, x2, y2, z2, x3, y3, z3 p224FieldElement

        p224FromBig(&x1, bigX1)
        p224FromBig(&y1, bigY1)
        z1[0] = 1
        p224FromBig(&x2, bigX2)
        p224FromBig(&y2, bigY2)
        z2[0] = 1

        p224AddJacobian(&x3, &y3, &z3, &x1, &y1, &z1, &x2, &y2, &z2)
        return p224ToAffine(&x3, &y3, &z3)
}

func (p224Curve) Double(bigX1, bigY1 *big.Int) (x, y *big.Int) {
        var x1, y1, z1, x2, y2, z2 p224FieldElement

        p224FromBig(&x1, bigX1)
        p224FromBig(&y1, bigY1)
        z1[0] = 1

        p224DoubleJacobian(&x2, &y2, &z2, &x1, &y1, &z1)
        return p224ToAffine(&x2, &y2, &z2)
}

func (p224Curve) ScalarMult(bigX1, bigY1 *big.Int, scalar []byte) (x, y *big.Int) {
        var x1, y1, z1, x2, y2, z2 p224FieldElement

        p224FromBig(&x1, bigX1)
        p224FromBig(&y1, bigY1)
        z1[0] = 1

        p224ScalarMult(&x2, &y2, &z2, &x1, &y1, &z1, scalar)
        return p224ToAffine(&x2, &y2, &z2)
}

func (curve p224Curve) ScalarBaseMult(scalar []byte) (x, y *big.Int) {
        var z1, x2, y2, z2 p224FieldElement

        z1[0] = 1
        p224ScalarMult(&x2, &y2, &z2, &curve.gx, &curve.gy, &z1, scalar)
        return p224ToAffine(&x2, &y2, &z2)
}

// Field element functions.
//
// The field that we're dealing with is ℤ/pℤ where p = 2**224 - 2**96 + 1.
//
// Field elements are represented by a FieldElement, which is a typedef to an
// array of 8 uint32's. The value of a FieldElement, a, is:
//   a[0] + 2**28·a[1] + 2**56·a[1] + ... + 2**196·a[7]
//
// Using 28-bit limbs means that there's only 4 bits of headroom, which is less
// than we would really like. But it has the useful feature that we hit 2**224
// exactly, making the reflections during a reduce much nicer.
type p224FieldElement [8]uint32

// p224Add computes *out = a+b
//
// a[i] + b[i] < 2**32
func p224Add(out, a, b *p224FieldElement) {
        for i := 0; i < 8; i++ {
                out[i] = a[i] + b[i]
        }
}

const two31p3 = 1<<31 + 1<<3
const two31m3 = 1<<31 - 1<<3
const two31m15m3 = 1<<31 - 1<<15 - 1<<3

// p224ZeroModP31 is 0 mod p where bit 31 is set in all limbs so that we can
// subtract smaller amounts without underflow. See the section "Subtraction" in
// [1] for reasoning.
var p224ZeroModP31 = []uint32{two31p3, two31m3, two31m3, two31m15m3, two31m3, two31m3, two31m3, two31m3}

// p224Sub computes *out = a-b
//
// a[i], b[i] < 2**30
// out[i] < 2**32
func p224Sub(out, a, b *p224FieldElement) {
        for i := 0; i < 8; i++ {
                out[i] = a[i] + p224ZeroModP31[i] - b[i]
        }
}

// LargeFieldElement also represents an element of the field. The limbs are
// still spaced 28-bits apart and in little-endian order. So the limbs are at
// 0, 28, 56, ..., 392 bits, each 64-bits wide.
type p224LargeFieldElement [15]uint64

const two63p35 = 1<<63 + 1<<35
const two63m35 = 1<<63 - 1<<35
const two63m35m19 = 1<<63 - 1<<35 - 1<<19

// p224ZeroModP63 is 0 mod p where bit 63 is set in all limbs. See the section
// "Subtraction" in [1] for why.
var p224ZeroModP63 = [8]uint64{two63p35, two63m35, two63m35, two63m35, two63m35m19, two63m35, two63m35, two63m35}

const bottom12Bits = 0xfff
const bottom28Bits = 0xfffffff

// p224Mul computes *out = a*b
//
// a[i] < 2**29, b[i] < 2**30 (or vice versa)
// out[i] < 2**29
func p224Mul(out, a, b *p224FieldElement, tmp *p224LargeFieldElement) {
        for i := 0; i < 15; i++ {
                tmp[i] = 0
        }

        for i := 0; i < 8; i++ {
                for j := 0; j < 8; j++ {
                        tmp[i+j] += uint64(a[i]) * uint64(b[j])
                }
        }

        p224ReduceLarge(out, tmp)
}

// Square computes *out = a*a
//
// a[i] < 2**29
// out[i] < 2**29
func p224Square(out, a *p224FieldElement, tmp *p224LargeFieldElement) {
        for i := 0; i < 15; i++ {
                tmp[i] = 0
        }

        for i := 0; i < 8; i++ {
                for j := 0; j <= i; j++ {
                        r := uint64(a[i]) * uint64(a[j])
                        if i == j {
                                tmp[i+j] += r
                        } else {
                                tmp[i+j] += r << 1
                        }
                }
        }

        p224ReduceLarge(out, tmp)
}

// ReduceLarge converts a p224LargeFieldElement to a p224FieldElement.
//
// in[i] < 2**62
func p224ReduceLarge(out *p224FieldElement, in *p224LargeFieldElement) {
        for i := 0; i < 8; i++ {
                in[i] += p224ZeroModP63[i]
        }

        // Eliminate the coefficients at 2**224 and greater.
        for i := 14; i >= 8; i-- {
                in[i-8] -= in[i]
                in[i-5] += (in[i] & 0xffff) << 12
                in[i-4] += in[i] >> 16
        }
        in[8] = 0
        // in[0..8] < 2**64

        // As the values become small enough, we start to store them in |out|
        // and use 32-bit operations.
        for i := 1; i < 8; i++ {
                in[i+1] += in[i] >> 28
                out[i] = uint32(in[i] & bottom28Bits)
        }
        in[0] -= in[8]
        out[3] += uint32(in[8]&0xffff) << 12
        out[4] += uint32(in[8] >> 16)
        // in[0] < 2**64
        // out[3] < 2**29
        // out[4] < 2**29
        // out[1,2,5..7] < 2**28

        out[0] = uint32(in[0] & bottom28Bits)
        out[1] += uint32((in[0] >> 28) & bottom28Bits)
        out[2] += uint32(in[0] >> 56)
        // out[0] < 2**28
        // out[1..4] < 2**29
        // out[5..7] < 2**28
}

// Reduce reduces the coefficients of a to smaller bounds.
//
// On entry: a[i] < 2**31 + 2**30
// On exit: a[i] < 2**29
func p224Reduce(a *p224FieldElement) {
        for i := 0; i < 7; i++ {
                a[i+1] += a[i] >> 28
                a[i] &= bottom28Bits
        }
        top := a[7] >> 28
        a[7] &= bottom28Bits

        // top < 2**4
        mask := top
        mask |= mask >> 2
        mask |= mask >> 1
        mask <<= 31
        mask = uint32(int32(mask) >> 31)
        // Mask is all ones if top != 0, all zero otherwise

        a[0] -= top
        a[3] += top << 12

        // We may have just made a[0] negative but, if we did, then we must
        // have added something to a[3], this it's > 2**12. Therefore we can
        // carry down to a[0].
        a[3] -= 1 & mask
        a[2] += mask & (1<<28 - 1)
        a[1] += mask & (1<<28 - 1)
        a[0] += mask & (1 << 28)
}

// p224Invert calculates *out = in**-1 by computing in**(2**224 - 2**96 - 1),
// i.e. Fermat's little theorem.
func p224Invert(out, in *p224FieldElement) {
        var f1, f2, f3, f4 p224FieldElement
        var c p224LargeFieldElement

        p224Square(&f1, in, &c)    // 2
        p224Mul(&f1, &f1, in, &c)  // 2**2 - 1
        p224Square(&f1, &f1, &c)   // 2**3 - 2
        p224Mul(&f1, &f1, in, &c)  // 2**3 - 1
        p224Square(&f2, &f1, &c)   // 2**4 - 2
        p224Square(&f2, &f2, &c)   // 2**5 - 4
        p224Square(&f2, &f2, &c)   // 2**6 - 8
        p224Mul(&f1, &f1, &f2, &c) // 2**6 - 1
        p224Square(&f2, &f1, &c)   // 2**7 - 2
        for i := 0; i < 5; i++ {   // 2**12 - 2**6
                p224Square(&f2, &f2, &c)
        }
        p224Mul(&f2, &f2, &f1, &c) // 2**12 - 1
        p224Square(&f3, &f2, &c)   // 2**13 - 2
        for i := 0; i < 11; i++ {  // 2**24 - 2**12
                p224Square(&f3, &f3, &c)
        }
        p224Mul(&f2, &f3, &f2, &c) // 2**24 - 1
        p224Square(&f3, &f2, &c)   // 2**25 - 2
        for i := 0; i < 23; i++ {  // 2**48 - 2**24
                p224Square(&f3, &f3, &c)
        }
        p224Mul(&f3, &f3, &f2, &c) // 2**48 - 1
        p224Square(&f4, &f3, &c)   // 2**49 - 2
        for i := 0; i < 47; i++ {  // 2**96 - 2**48
                p224Square(&f4, &f4, &c)
        }
        p224Mul(&f3, &f3, &f4, &c) // 2**96 - 1
        p224Square(&f4, &f3, &c)   // 2**97 - 2
        for i := 0; i < 23; i++ {  // 2**120 - 2**24
                p224Square(&f4, &f4, &c)
        }
        p224Mul(&f2, &f4, &f2, &c) // 2**120 - 1
        for i := 0; i < 6; i++ {   // 2**126 - 2**6
                p224Square(&f2, &f2, &c)
        }
        p224Mul(&f1, &f1, &f2, &c) // 2**126 - 1
        p224Square(&f1, &f1, &c)   // 2**127 - 2
        p224Mul(&f1, &f1, in, &c)  // 2**127 - 1
        for i := 0; i < 97; i++ {  // 2**224 - 2**97
                p224Square(&f1, &f1, &c)
        }
        p224Mul(out, &f1, &f3, &c) // 2**224 - 2**96 - 1
}

// p224Contract converts a FieldElement to its unique, minimal form.
//
// On entry, in[i] < 2**29
// On exit, in[i] < 2**28
func p224Contract(out, in *p224FieldElement) {
        copy(out[:], in[:])

        for i := 0; i < 7; i++ {
                out[i+1] += out[i] >> 28
                out[i] &= bottom28Bits
        }
        top := out[7] >> 28
        out[7] &= bottom28Bits

        out[0] -= top
        out[3] += top << 12

        // We may just have made out[i] negative. So we carry down. If we made
        // out[0] negative then we know that out[3] is sufficiently positive
        // because we just added to it.
        for i := 0; i < 3; i++ {
                mask := uint32(int32(out[i]) >> 31)
                out[i] += (1 << 28) & mask
                out[i+1] -= 1 & mask
        }

        // We might have pushed out[3] over 2**28 so we perform another, partial,
        // carry chain.
        for i := 3; i < 7; i++ {
                out[i+1] += out[i] >> 28
                out[i] &= bottom28Bits
        }
        top = out[7] >> 28
        out[7] &= bottom28Bits

        // Eliminate top while maintaining the same value mod p.
        out[0] -= top
        out[3] += top << 12

        // There are two cases to consider for out[3]:
        //   1) The first time that we eliminated top, we didn't push out[3] over
        //      2**28. In this case, the partial carry chain didn't change any values
        //      and top is zero.
        //   2) We did push out[3] over 2**28 the first time that we eliminated top.
        //      The first value of top was in [0..16), therefore, prior to eliminating
        //      the first top, 0xfff1000 <= out[3] <= 0xfffffff. Therefore, after
        //      overflowing and being reduced by the second carry chain, out[3] <=
        //      0xf000. Thus it cannot have overflowed when we eliminated top for the
        //      second time.

        // Again, we may just have made out[0] negative, so do the same carry down.
        // As before, if we made out[0] negative then we know that out[3] is
        // sufficiently positive.
        for i := 0; i < 3; i++ {
                mask := uint32(int32(out[i]) >> 31)
                out[i] += (1 << 28) & mask
                out[i+1] -= 1 & mask
        }

        // Now we see if the value is >= p and, if so, subtract p.

        // First we build a mask from the top four limbs, which must all be
        // equal to bottom28Bits if the whole value is >= p. If top4AllOnes
        // ends up with any zero bits in the bottom 28 bits, then this wasn't
        // true.
        top4AllOnes := uint32(0xffffffff)
        for i := 4; i < 8; i++ {
                top4AllOnes &= (out[i] & bottom28Bits) - 1
        }
        top4AllOnes |= 0xf0000000
        // Now we replicate any zero bits to all the bits in top4AllOnes.
        top4AllOnes &= top4AllOnes >> 16
        top4AllOnes &= top4AllOnes >> 8
        top4AllOnes &= top4AllOnes >> 4
        top4AllOnes &= top4AllOnes >> 2
        top4AllOnes &= top4AllOnes >> 1
        top4AllOnes = uint32(int32(top4AllOnes<<31) >> 31)

        // Now we test whether the bottom three limbs are non-zero.
        bottom3NonZero := out[0] | out[1] | out[2]
        bottom3NonZero |= bottom3NonZero >> 16
        bottom3NonZero |= bottom3NonZero >> 8
        bottom3NonZero |= bottom3NonZero >> 4
        bottom3NonZero |= bottom3NonZero >> 2
        bottom3NonZero |= bottom3NonZero >> 1
        bottom3NonZero = uint32(int32(bottom3NonZero<<31) >> 31)

        // Everything depends on the value of out[3].
        //    If it's > 0xffff000 and top4AllOnes != 0 then the whole value is >= p
        //    If it's = 0xffff000 and top4AllOnes != 0 and bottom3NonZero != 0,
        //      then the whole value is >= p
        //    If it's < 0xffff000, then the whole value is < p
        n := out[3] - 0xffff000
        out3Equal := n
        out3Equal |= out3Equal >> 16
        out3Equal |= out3Equal >> 8
        out3Equal |= out3Equal >> 4
        out3Equal |= out3Equal >> 2
        out3Equal |= out3Equal >> 1
        out3Equal = ^uint32(int32(out3Equal<<31) >> 31)

        // If out[3] > 0xffff000 then n's MSB will be zero.
        out3GT := ^uint32(int32(n<<31) >> 31)

        mask := top4AllOnes & ((out3Equal & bottom3NonZero) | out3GT)
        out[0] -= 1 & mask
        out[3] -= 0xffff000 & mask
        out[4] -= 0xfffffff & mask
        out[5] -= 0xfffffff & mask
        out[6] -= 0xfffffff & mask
        out[7] -= 0xfffffff & mask
}

// Group element functions.
//
// These functions deal with group elements. The group is an elliptic curve
// group with a = -3 defined in FIPS 186-3, section D.2.2.

// p224AddJacobian computes *out = a+b where a != b.
func p224AddJacobian(x3, y3, z3, x1, y1, z1, x2, y2, z2 *p224FieldElement) {
        // See http://hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-3.html#addition-p224Add-2007-bl
        var z1z1, z2z2, u1, u2, s1, s2, h, i, j, r, v p224FieldElement
        var c p224LargeFieldElement

        // Z1Z1 = Z1²
        p224Square(&z1z1, z1, &c)
        // Z2Z2 = Z2²
        p224Square(&z2z2, z2, &c)
        // U1 = X1*Z2Z2
        p224Mul(&u1, x1, &z2z2, &c)
        // U2 = X2*Z1Z1
        p224Mul(&u2, x2, &z1z1, &c)
        // S1 = Y1*Z2*Z2Z2
        p224Mul(&s1, z2, &z2z2, &c)
        p224Mul(&s1, y1, &s1, &c)
        // S2 = Y2*Z1*Z1Z1
        p224Mul(&s2, z1, &z1z1, &c)
        p224Mul(&s2, y2, &s2, &c)
        // H = U2-U1
        p224Sub(&h, &u2, &u1)
        p224Reduce(&h)
        // I = (2*H)²
        for j := 0; j < 8; j++ {
                i[j] = h[j] << 1
        }
        p224Reduce(&i)
        p224Square(&i, &i, &c)
        // J = H*I
        p224Mul(&j, &h, &i, &c)
        // r = 2*(S2-S1)
        p224Sub(&r, &s2, &s1)
        p224Reduce(&r)
        for i := 0; i < 8; i++ {
                r[i] <<= 1
        }
        p224Reduce(&r)
        // V = U1*I
        p224Mul(&v, &u1, &i, &c)
        // Z3 = ((Z1+Z2)²-Z1Z1-Z2Z2)*H
        p224Add(&z1z1, &z1z1, &z2z2)
        p224Add(&z2z2, z1, z2)
        p224Reduce(&z2z2)
        p224Square(&z2z2, &z2z2, &c)
        p224Sub(z3, &z2z2, &z1z1)
        p224Reduce(z3)
        p224Mul(z3, z3, &h, &c)
        // X3 = r²-J-2*V
        for i := 0; i < 8; i++ {
                z1z1[i] = v[i] << 1
        }
        p224Add(&z1z1, &j, &z1z1)
        p224Reduce(&z1z1)
        p224Square(x3, &r, &c)
        p224Sub(x3, x3, &z1z1)
        p224Reduce(x3)
        // Y3 = r*(V-X3)-2*S1*J
        for i := 0; i < 8; i++ {
                s1[i] <<= 1
        }
        p224Mul(&s1, &s1, &j, &c)
        p224Sub(&z1z1, &v, x3)
        p224Reduce(&z1z1)
        p224Mul(&z1z1, &z1z1, &r, &c)
        p224Sub(y3, &z1z1, &s1)
        p224Reduce(y3)
}

// p224DoubleJacobian computes *out = a+a.
func p224DoubleJacobian(x3, y3, z3, x1, y1, z1 *p224FieldElement) {
        var delta, gamma, beta, alpha, t p224FieldElement
        var c p224LargeFieldElement

        p224Square(&delta, z1, &c)
        p224Square(&gamma, y1, &c)
        p224Mul(&beta, x1, &gamma, &c)

        // alpha = 3*(X1-delta)*(X1+delta)
        p224Add(&t, x1, &delta)
        for i := 0; i < 8; i++ {
                t[i] += t[i] << 1
        }
        p224Reduce(&t)
        p224Sub(&alpha, x1, &delta)
        p224Reduce(&alpha)
        p224Mul(&alpha, &alpha, &t, &c)

        // Z3 = (Y1+Z1)²-gamma-delta
        p224Add(z3, y1, z1)
        p224Reduce(z3)
        p224Square(z3, z3, &c)
        p224Sub(z3, z3, &gamma)
        p224Reduce(z3)
        p224Sub(z3, z3, &delta)
        p224Reduce(z3)

        // X3 = alpha²-8*beta
        for i := 0; i < 8; i++ {
                delta[i] = beta[i] << 3
        }
        p224Reduce(&delta)
        p224Square(x3, &alpha, &c)
        p224Sub(x3, x3, &delta)
        p224Reduce(x3)

        // Y3 = alpha*(4*beta-X3)-8*gamma²
        for i := 0; i < 8; i++ {
                beta[i] <<= 2
        }
        p224Sub(&beta, &beta, x3)
        p224Reduce(&beta)
        p224Square(&gamma, &gamma, &c)
        for i := 0; i < 8; i++ {
                gamma[i] <<= 3
        }
        p224Reduce(&gamma)
        p224Mul(y3, &alpha, &beta, &c)
        p224Sub(y3, y3, &gamma)
        p224Reduce(y3)
}

// p224CopyConditional sets *out = *in iff the least-significant-bit of control
// is true, and it runs in constant time.
func p224CopyConditional(out, in *p224FieldElement, control uint32) {
        control <<= 31
        control = uint32(int32(control) >> 31)

        for i := 0; i < 8; i++ {
                out[i] ^= (out[i] ^ in[i]) & control
        }
}

func p224ScalarMult(outX, outY, outZ, inX, inY, inZ *p224FieldElement, scalar []byte) {
        var xx, yy, zz p224FieldElement
        for i := 0; i < 8; i++ {
                outZ[i] = 0
        }

        firstBit := uint32(1)
        for _, byte := range scalar {
                for bitNum := uint(0); bitNum < 8; bitNum++ {
                        p224DoubleJacobian(outX, outY, outZ, outX, outY, outZ)
                        bit := uint32((byte >> (7 - bitNum)) & 1)
                        p224AddJacobian(&xx, &yy, &zz, inX, inY, inZ, outX, outY, outZ)
                        p224CopyConditional(outX, inX, firstBit&bit)
                        p224CopyConditional(outY, inY, firstBit&bit)
                        p224CopyConditional(outZ, inZ, firstBit&bit)
                        p224CopyConditional(outX, &xx, ^firstBit&bit)
                        p224CopyConditional(outY, &yy, ^firstBit&bit)
                        p224CopyConditional(outZ, &zz, ^firstBit&bit)
                        firstBit = firstBit & ^bit
                }
        }
}

// p224ToAffine converts from Jacobian to affine form.
func p224ToAffine(x, y, z *p224FieldElement) (*big.Int, *big.Int) {
        var zinv, zinvsq, outx, outy p224FieldElement
        var tmp p224LargeFieldElement

        isPointAtInfinity := true
        for i := 0; i < 8; i++ {
                if z[i] != 0 {
                        isPointAtInfinity = false
                        break
                }
        }

        if isPointAtInfinity {
                return nil, nil
        }

        p224Invert(&zinv, z)
        p224Square(&zinvsq, &zinv, &tmp)
        p224Mul(x, x, &zinvsq, &tmp)
        p224Mul(&zinvsq, &zinvsq, &zinv, &tmp)
        p224Mul(y, y, &zinvsq, &tmp)

        p224Contract(&outx, x)
        p224Contract(&outy, y)
        return p224ToBig(&outx), p224ToBig(&outy)
}

// get28BitsFromEnd returns the least-significant 28 bits from buf>>shift,
// where buf is interpreted as a big-endian number.
func get28BitsFromEnd(buf []byte, shift uint) (uint32, []byte) {
        var ret uint32

        for i := uint(0); i < 4; i++ {
                var b byte
                if l := len(buf); l > 0 {
                        b = buf[l-1]
                        // We don't remove the byte if we're about to return and we're not
                        // reading all of it.
                        if i != 3 || shift == 4 {
                                buf = buf[:l-1]
                        }
                }
                ret |= uint32(b) << (8 * i) >> shift
        }
        ret &= bottom28Bits
        return ret, buf
}

// p224FromBig sets *out = *in.
func p224FromBig(out *p224FieldElement, in *big.Int) {
        bytes := in.Bytes()
        out[0], bytes = get28BitsFromEnd(bytes, 0)
        out[1], bytes = get28BitsFromEnd(bytes, 4)
        out[2], bytes = get28BitsFromEnd(bytes, 0)
        out[3], bytes = get28BitsFromEnd(bytes, 4)
        out[4], bytes = get28BitsFromEnd(bytes, 0)
        out[5], bytes = get28BitsFromEnd(bytes, 4)
        out[6], bytes = get28BitsFromEnd(bytes, 0)
        out[7], bytes = get28BitsFromEnd(bytes, 4)
}

// p224ToBig returns in as a big.Int.
func p224ToBig(in *p224FieldElement) *big.Int {
        var buf [28]byte
        buf[27] = byte(in[0])
        buf[26] = byte(in[0] >> 8)
        buf[25] = byte(in[0] >> 16)
        buf[24] = byte(((in[0] >> 24) & 0x0f) | (in[1]<<4)&0xf0)

        buf[23] = byte(in[1] >> 4)
        buf[22] = byte(in[1] >> 12)
        buf[21] = byte(in[1] >> 20)

        buf[20] = byte(in[2])
        buf[19] = byte(in[2] >> 8)
        buf[18] = byte(in[2] >> 16)
        buf[17] = byte(((in[2] >> 24) & 0x0f) | (in[3]<<4)&0xf0)

        buf[16] = byte(in[3] >> 4)
        buf[15] = byte(in[3] >> 12)
        buf[14] = byte(in[3] >> 20)

        buf[13] = byte(in[4])
        buf[12] = byte(in[4] >> 8)
        buf[11] = byte(in[4] >> 16)
        buf[10] = byte(((in[4] >> 24) & 0x0f) | (in[5]<<4)&0xf0)

        buf[9] = byte(in[5] >> 4)
        buf[8] = byte(in[5] >> 12)
        buf[7] = byte(in[5] >> 20)

        buf[6] = byte(in[6])
        buf[5] = byte(in[6] >> 8)
        buf[4] = byte(in[6] >> 16)
        buf[3] = byte(((in[6] >> 24) & 0x0f) | (in[7]<<4)&0xf0)

        buf[2] = byte(in[7] >> 4)
        buf[1] = byte(in[7] >> 12)
        buf[0] = byte(in[7] >> 20)

        return new(big.Int).SetBytes(buf[:])
}