Update monet.

crypto/p224.cc

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// This is an implementation of the P224 elliptic curve group. It's written to
+// be short and simple rather than fast, although it's still constant-time.
+//
+// See http://www.imperialviolet.org/2010/12/04/ecc.html ([1]) for background.
+
+#include "crypto/p224.h"
+
+#include <string.h>
+
+#include "base/sys_byteorder.h"
+
+namespace {
+
+using base::HostToNet32;
+using base::NetToHost32;
+
+// 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.
+
+using crypto::p224::FieldElement;
+
+// kP is the P224 prime.
+const FieldElement kP = {
+  1, 0, 0, 268431360,
+  268435455, 268435455, 268435455, 268435455,
+};
+
+void Contract(FieldElement* inout);
+
+// IsZero returns 0xffffffff if a == 0 mod p and 0 otherwise.
+uint32 IsZero(const FieldElement& a) {
+  FieldElement minimal;
+  memcpy(&minimal, &a, sizeof(minimal));
+  Contract(&minimal);
+
+  uint32 is_zero = 0, is_p = 0;
+  for (unsigned i = 0; i < 8; i++) {
+    is_zero |= minimal[i];
+    is_p |= minimal[i] - kP[i];
+  }
+
+  // If either is_zero or is_p is 0, then we should return 1.
+  is_zero |= is_zero >> 16;
+  is_zero |= is_zero >> 8;
+  is_zero |= is_zero >> 4;
+  is_zero |= is_zero >> 2;
+  is_zero |= is_zero >> 1;
+
+  is_p |= is_p >> 16;
+  is_p |= is_p >> 8;
+  is_p |= is_p >> 4;
+  is_p |= is_p >> 2;
+  is_p |= is_p >> 1;
+
+  // For is_zero and is_p, the LSB is 0 iff all the bits are zero.
+  is_zero &= is_p & 1;
+  is_zero = (~is_zero) << 31;
+  is_zero = static_cast<int32>(is_zero) >> 31;
+  return is_zero;
+}
+
+// Add computes *out = a+b
+//
+// a[i] + b[i] < 2**32
+void Add(FieldElement* out, const FieldElement& a, const FieldElement& b) {
+  for (int i = 0; i < 8; i++) {
+    (*out)[i] = a[i] + b[i];
+  }
+}
+
+static const uint32 kTwo31p3 = (1u<<31) + (1u<<3);
+static const uint32 kTwo31m3 = (1u<<31) - (1u<<3);
+static const uint32 kTwo31m15m3 = (1u<<31) - (1u<<15) - (1u<<3);
+// kZero31ModP 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 why.
+static const FieldElement kZero31ModP = {
+  kTwo31p3, kTwo31m3, kTwo31m3, kTwo31m15m3,
+  kTwo31m3, kTwo31m3, kTwo31m3, kTwo31m3
+};
+
+// Subtract computes *out = a-b
+//
+// a[i], b[i] < 2**30
+// out[i] < 2**32
+void Subtract(FieldElement* out, const FieldElement& a, const FieldElement& b) {
+  for (int i = 0; i < 8; i++) {
+    // See the section on "Subtraction" in [1] for details.
+    (*out)[i] = a[i] + kZero31ModP[i] - b[i];
+  }
+}
+
+static const uint64 kTwo63p35 = (1ull<<63) + (1ull<<35);
+static const uint64 kTwo63m35 = (1ull<<63) - (1ull<<35);
+static const uint64 kTwo63m35m19 = (1ull<<63) - (1ull<<35) - (1ull<<19);
+// kZero63ModP is 0 mod p where bit 63 is set in all limbs. See the section
+// "Subtraction" in [1] for why.
+static const uint64 kZero63ModP[8] = {
+  kTwo63p35, kTwo63m35, kTwo63m35, kTwo63m35,
+  kTwo63m35m19, kTwo63m35, kTwo63m35, kTwo63m35,
+};
+
+static const uint32 kBottom28Bits = 0xfffffff;
+
+// 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.
+typedef uint64 LargeFieldElement[15];
+
+// ReduceLarge converts a LargeFieldElement to a FieldElement.
+//
+// in[i] < 2**62
+
+// GCC 4.9 incorrectly vectorizes the first coefficient elimination loop, so
+// disable that optimization via pragma. Don't use the pragma under Clang, since
+// clang doesn't understand it.
+// TODO(wez): Remove this when crbug.com/439566 is fixed.
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC optimize("no-tree-vectorize")
+#endif
+
+void ReduceLarge(FieldElement* out, LargeFieldElement* inptr) {
+  LargeFieldElement& in(*inptr);
+
+  for (int i = 0; i < 8; i++) {
+    in[i] += kZero63ModP[i];
+  }
+
+  // Eliminate the coefficients at 2**224 and greater while maintaining the
+  // same value mod p.
+  for (int i = 14; i >= 8; i--) {
+    in[i-8] -= in[i];  // reflection off the "+1" term of p.
+    in[i-5] += (in[i] & 0xffff) << 12;  // part of the "-2**96" reflection.
+    in[i-4] += in[i] >> 16;  // the rest of the "-2**96" reflection.
+  }
+  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 (int i = 1; i < 8; i++) {
+    in[i+1] += in[i] >> 28;
+    (*out)[i] = static_cast<uint32>(in[i] & kBottom28Bits);
+  }
+  // Eliminate the term at 2*224 that we introduced while keeping the same
+  // value mod p.
+  in[0] -= in[8];  // reflection off the "+1" term of p.
+  (*out)[3] += static_cast<uint32>(in[8] & 0xffff) << 12;  // "-2**96" term
+  (*out)[4] += static_cast<uint32>(in[8] >> 16);  // rest of "-2**96" term
+  // in[0] < 2**64
+  // out[3] < 2**29
+  // out[4] < 2**29
+  // out[1,2,5..7] < 2**28
+
+  (*out)[0] = static_cast<uint32>(in[0] & kBottom28Bits);
+  (*out)[1] += static_cast<uint32>((in[0] >> 28) & kBottom28Bits);
+  (*out)[2] += static_cast<uint32>(in[0] >> 56);
+  // out[0] < 2**28
+  // out[1..4] < 2**29
+  // out[5..7] < 2**28
+}
+
+// TODO(wez): Remove this when crbug.com/439566 is fixed.
+#if defined(__GNUC__) && !defined(__clang__)
+// Reenable "tree-vectorize" optimization if it got disabled for ReduceLarge.
+#pragma GCC reset_options
+#endif
+
+// Mul computes *out = a*b
+//
+// a[i] < 2**29, b[i] < 2**30 (or vice versa)
+// out[i] < 2**29
+void Mul(FieldElement* out, const FieldElement& a, const FieldElement& b) {
+  LargeFieldElement tmp;
+  memset(&tmp, 0, sizeof(tmp));
+
+  for (int i = 0; i < 8; i++) {
+    for (int j = 0; j < 8; j++) {
+      tmp[i+j] += static_cast<uint64>(a[i]) * static_cast<uint64>(b[j]);
+    }
+  }
+
+  ReduceLarge(out, &tmp);
+}
+
+// Square computes *out = a*a
+//
+// a[i] < 2**29
+// out[i] < 2**29
+void Square(FieldElement* out, const FieldElement& a) {
+  LargeFieldElement tmp;
+  memset(&tmp, 0, sizeof(tmp));
+
+  for (int i = 0; i < 8; i++) {
+    for (int j = 0; j <= i; j++) {
+      uint64 r = static_cast<uint64>(a[i]) * static_cast<uint64>(a[j]);
+      if (i == j) {
+        tmp[i+j] += r;
+      } else {
+        tmp[i+j] += r << 1;
+      }
+    }
+  }
+
+  ReduceLarge(out, &tmp);
+}
+
+// Reduce reduces the coefficients of in_out to smaller bounds.
+//
+// On entry: a[i] < 2**31 + 2**30
+// On exit: a[i] < 2**29
+void Reduce(FieldElement* in_out) {
+  FieldElement& a = *in_out;
+
+  for (int i = 0; i < 7; i++) {
+    a[i+1] += a[i] >> 28;
+    a[i] &= kBottom28Bits;
+  }
+  uint32 top = a[7] >> 28;
+  a[7] &= kBottom28Bits;
+
+  // top < 2**4
+  // Constant-time: mask = (top != 0) ? 0xffffffff : 0
+  uint32 mask = top;
+  mask |= mask >> 2;
+  mask |= mask >> 1;
+  mask <<= 31;
+  mask = static_cast<uint32>(static_cast<int32>(mask) >> 31);
+
+  // Eliminate top while maintaining the same value mod p.
+  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], thus 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);
+}
+
+// Invert calcuates *out = in**-1 by computing in**(2**224 - 2**96 - 1), i.e.
+// Fermat's little theorem.
+void Invert(FieldElement* out, const FieldElement& in) {
+  FieldElement f1, f2, f3, f4;
+
+  Square(&f1, in);                        // 2
+  Mul(&f1, f1, in);                       // 2**2 - 1
+  Square(&f1, f1);                        // 2**3 - 2
+  Mul(&f1, f1, in);                       // 2**3 - 1
+  Square(&f2, f1);                        // 2**4 - 2
+  Square(&f2, f2);                        // 2**5 - 4
+  Square(&f2, f2);                        // 2**6 - 8
+  Mul(&f1, f1, f2);                       // 2**6 - 1
+  Square(&f2, f1);                        // 2**7 - 2
+  for (int i = 0; i < 5; i++) {           // 2**12 - 2**6
+    Square(&f2, f2);
+  }
+  Mul(&f2, f2, f1);                       // 2**12 - 1
+  Square(&f3, f2);                        // 2**13 - 2
+  for (int i = 0; i < 11; i++) {          // 2**24 - 2**12
+    Square(&f3, f3);
+  }
+  Mul(&f2, f3, f2);                       // 2**24 - 1
+  Square(&f3, f2);                        // 2**25 - 2
+  for (int i = 0; i < 23; i++) {          // 2**48 - 2**24
+    Square(&f3, f3);
+  }
+  Mul(&f3, f3, f2);                       // 2**48 - 1
+  Square(&f4, f3);                        // 2**49 - 2
+  for (int i = 0; i < 47; i++) {          // 2**96 - 2**48
+    Square(&f4, f4);
+  }
+  Mul(&f3, f3, f4);                       // 2**96 - 1
+  Square(&f4, f3);                        // 2**97 - 2
+  for (int i = 0; i < 23; i++) {          // 2**120 - 2**24
+    Square(&f4, f4);
+  }
+  Mul(&f2, f4, f2);                       // 2**120 - 1
+  for (int i = 0; i < 6; i++) {           // 2**126 - 2**6
+    Square(&f2, f2);
+  }
+  Mul(&f1, f1, f2);                       // 2**126 - 1
+  Square(&f1, f1);                        // 2**127 - 2
+  Mul(&f1, f1, in);                       // 2**127 - 1
+  for (int i = 0; i < 97; i++) {          // 2**224 - 2**97
+    Square(&f1, f1);
+  }
+  Mul(out, f1, f3);                       // 2**224 - 2**96 - 1
+}
+
+// Contract converts a FieldElement to its minimal, distinguished form.
+//
+// On entry, in[i] < 2**29
+// On exit, in[i] < 2**28
+void Contract(FieldElement* inout) {
+  FieldElement& out = *inout;
+
+  // Reduce the coefficients to < 2**28.
+  for (int i = 0; i < 7; i++) {
+    out[i+1] += out[i] >> 28;
+    out[i] &= kBottom28Bits;
+  }
+  uint32 top = out[7] >> 28;
+  out[7] &= kBottom28Bits;
+
+  // Eliminate top while maintaining the same value mod p.
+  out[0] -= top;
+  out[3] += top << 12;
+
+  // We may just have made out[0] 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 (int i = 0; i < 3; i++) {
+    uint32 mask = static_cast<uint32>(static_cast<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 (int i = 3; i < 7; i++) {
+    out[i+1] += out[i] >> 28;
+    out[i] &= kBottom28Bits;
+  }
+  top = out[7] >> 28;
+  out[7] &= kBottom28Bits;
+
+  // 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 (int i = 0; i < 3; i++) {
+    uint32 mask = static_cast<uint32>(static_cast<int32>(out[i]) >> 31);
+    out[i] += (1 << 28) & mask;
+    out[i+1] -= 1 & mask;
+  }
+
+  // The value is < 2**224, but maybe greater than p. In order to reduce to a
+  // unique, minimal value 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 top_4_all_ones
+  // ends up with any zero bits in the bottom 28 bits, then this wasn't
+  // true.
+  uint32 top_4_all_ones = 0xffffffffu;
+  for (int i = 4; i < 8; i++) {
+    top_4_all_ones &= out[i];
+  }
+  top_4_all_ones |= 0xf0000000;
+  // Now we replicate any zero bits to all the bits in top_4_all_ones.
+  top_4_all_ones &= top_4_all_ones >> 16;
+  top_4_all_ones &= top_4_all_ones >> 8;
+  top_4_all_ones &= top_4_all_ones >> 4;
+  top_4_all_ones &= top_4_all_ones >> 2;
+  top_4_all_ones &= top_4_all_ones >> 1;
+  top_4_all_ones =
+      static_cast<uint32>(static_cast<int32>(top_4_all_ones << 31) >> 31);
+
+  // Now we test whether the bottom three limbs are non-zero.
+  uint32 bottom_3_non_zero = out[0] | out[1] | out[2];
+  bottom_3_non_zero |= bottom_3_non_zero >> 16;
+  bottom_3_non_zero |= bottom_3_non_zero >> 8;
+  bottom_3_non_zero |= bottom_3_non_zero >> 4;
+  bottom_3_non_zero |= bottom_3_non_zero >> 2;
+  bottom_3_non_zero |= bottom_3_non_zero >> 1;
+  bottom_3_non_zero =
+      static_cast<uint32>(static_cast<int32>(bottom_3_non_zero) >> 31);
+
+  // Everything depends on the value of out[3].
+  //    If it's > 0xffff000 and top_4_all_ones != 0 then the whole value is >= p
+  //    If it's = 0xffff000 and top_4_all_ones != 0 and bottom_3_non_zero != 0,
+  //      then the whole value is >= p
+  //    If it's < 0xffff000, then the whole value is < p
+  uint32 n = out[3] - 0xffff000;
+  uint32 out_3_equal = n;
+  out_3_equal |= out_3_equal >> 16;
+  out_3_equal |= out_3_equal >> 8;
+  out_3_equal |= out_3_equal >> 4;
+  out_3_equal |= out_3_equal >> 2;
+  out_3_equal |= out_3_equal >> 1;
+  out_3_equal =
+      ~static_cast<uint32>(static_cast<int32>(out_3_equal << 31) >> 31);
+
+  // If out[3] > 0xffff000 then n's MSB will be zero.
+  uint32 out_3_gt = ~static_cast<uint32>(static_cast<int32>(n << 31) >> 31);
+
+  uint32 mask = top_4_all_ones & ((out_3_equal & bottom_3_non_zero) | out_3_gt);
+  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.
+
+using crypto::p224::Point;
+
+// kB is parameter of the elliptic curve.
+const FieldElement kB = {
+  55967668, 11768882, 265861671, 185302395,
+  39211076, 180311059, 84673715, 188764328,
+};
+
+void CopyConditional(Point* out, const Point& a, uint32 mask);
+void DoubleJacobian(Point* out, const Point& a);
+
+// AddJacobian computes *out = a+b where a != b.
+void AddJacobian(Point *out,
+                 const Point& a,
+                 const Point& b) {
+  // See http://hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-3.html#addition-add-2007-bl
+  FieldElement z1z1, z2z2, u1, u2, s1, s2, h, i, j, r, v;
+
+  uint32 z1_is_zero = IsZero(a.z);
+  uint32 z2_is_zero = IsZero(b.z);
+
+  // Z1Z1 = Z1²
+  Square(&z1z1, a.z);
+
+  // Z2Z2 = Z2²
+  Square(&z2z2, b.z);
+
+  // U1 = X1*Z2Z2
+  Mul(&u1, a.x, z2z2);
+
+  // U2 = X2*Z1Z1
+  Mul(&u2, b.x, z1z1);
+
+  // S1 = Y1*Z2*Z2Z2
+  Mul(&s1, b.z, z2z2);
+  Mul(&s1, a.y, s1);
+
+  // S2 = Y2*Z1*Z1Z1
+  Mul(&s2, a.z, z1z1);
+  Mul(&s2, b.y, s2);
+
+  // H = U2-U1
+  Subtract(&h, u2, u1);
+  Reduce(&h);
+  uint32 x_equal = IsZero(h);
+
+  // I = (2*H)²
+  for (int k = 0; k < 8; k++) {
+    i[k] = h[k] << 1;
+  }
+  Reduce(&i);
+  Square(&i, i);
+
+  // J = H*I
+  Mul(&j, h, i);
+  // r = 2*(S2-S1)
+  Subtract(&r, s2, s1);
+  Reduce(&r);
+  uint32 y_equal = IsZero(r);
+
+  if (x_equal && y_equal && !z1_is_zero && !z2_is_zero) {
+    // The two input points are the same therefore we must use the dedicated
+    // doubling function as the slope of the line is undefined.
+    DoubleJacobian(out, a);
+    return;
+  }
+
+  for (int k = 0; k < 8; k++) {
+    r[k] <<= 1;
+  }
+  Reduce(&r);
+
+  // V = U1*I
+  Mul(&v, u1, i);
+
+  // Z3 = ((Z1+Z2)²-Z1Z1-Z2Z2)*H
+  Add(&z1z1, z1z1, z2z2);
+  Add(&z2z2, a.z, b.z);
+  Reduce(&z2z2);
+  Square(&z2z2, z2z2);
+  Subtract(&out->z, z2z2, z1z1);
+  Reduce(&out->z);
+  Mul(&out->z, out->z, h);
+
+  // X3 = r²-J-2*V
+  for (int k = 0; k < 8; k++) {
+    z1z1[k] = v[k] << 1;
+  }
+  Add(&z1z1, j, z1z1);
+  Reduce(&z1z1);
+  Square(&out->x, r);
+  Subtract(&out->x, out->x, z1z1);
+  Reduce(&out->x);
+
+  // Y3 = r*(V-X3)-2*S1*J
+  for (int k = 0; k < 8; k++) {
+    s1[k] <<= 1;
+  }
+  Mul(&s1, s1, j);
+  Subtract(&z1z1, v, out->x);
+  Reduce(&z1z1);
+  Mul(&z1z1, z1z1, r);
+  Subtract(&out->y, z1z1, s1);
+  Reduce(&out->y);
+
+  CopyConditional(out, a, z2_is_zero);
+  CopyConditional(out, b, z1_is_zero);
+}
+
+// DoubleJacobian computes *out = a+a.
+void DoubleJacobian(Point* out, const Point& a) {
+  // See http://hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-3.html#doubling-dbl-2001-b
+  FieldElement delta, gamma, beta, alpha, t;
+
+  Square(&delta, a.z);
+  Square(&gamma, a.y);
+  Mul(&beta, a.x, gamma);
+
+  // alpha = 3*(X1-delta)*(X1+delta)
+  Add(&t, a.x, delta);
+  for (int i = 0; i < 8; i++) {
+          t[i] += t[i] << 1;
+  }
+  Reduce(&t);
+  Subtract(&alpha, a.x, delta);
+  Reduce(&alpha);
+  Mul(&alpha, alpha, t);
+
+  // Z3 = (Y1+Z1)²-gamma-delta
+  Add(&out->z, a.y, a.z);
+  Reduce(&out->z);
+  Square(&out->z, out->z);
+  Subtract(&out->z, out->z, gamma);
+  Reduce(&out->z);
+  Subtract(&out->z, out->z, delta);
+  Reduce(&out->z);
+
+  // X3 = alpha²-8*beta
+  for (int i = 0; i < 8; i++) {
+          delta[i] = beta[i] << 3;
+  }
+  Reduce(&delta);
+  Square(&out->x, alpha);
+  Subtract(&out->x, out->x, delta);
+  Reduce(&out->x);
+
+  // Y3 = alpha*(4*beta-X3)-8*gamma²
+  for (int i = 0; i < 8; i++) {
+          beta[i] <<= 2;
+  }
+  Reduce(&beta);
+  Subtract(&beta, beta, out->x);
+  Reduce(&beta);
+  Square(&gamma, gamma);
+  for (int i = 0; i < 8; i++) {
+          gamma[i] <<= 3;
+  }
+  Reduce(&gamma);
+  Mul(&out->y, alpha, beta);
+  Subtract(&out->y, out->y, gamma);
+  Reduce(&out->y);
+}
+
+// CopyConditional sets *out=a if mask is 0xffffffff. mask must be either 0 of
+// 0xffffffff.
+void CopyConditional(Point* out,
+                     const Point& a,
+                     uint32 mask) {
+  for (int i = 0; i < 8; i++) {
+    out->x[i] ^= mask & (a.x[i] ^ out->x[i]);
+    out->y[i] ^= mask & (a.y[i] ^ out->y[i]);
+    out->z[i] ^= mask & (a.z[i] ^ out->z[i]);
+  }
+}
+
+// ScalarMult calculates *out = a*scalar where scalar is a big-endian number of
+// length scalar_len and != 0.
+void ScalarMult(Point* out, const Point& a,
+                const uint8* scalar, size_t scalar_len) {
+  memset(out, 0, sizeof(*out));
+  Point tmp;
+
+  for (size_t i = 0; i < scalar_len; i++) {
+    for (unsigned int bit_num = 0; bit_num < 8; bit_num++) {
+      DoubleJacobian(out, *out);
+      uint32 bit = static_cast<uint32>(static_cast<int32>(
+          (((scalar[i] >> (7 - bit_num)) & 1) << 31) >> 31));
+      AddJacobian(&tmp, a, *out);
+      CopyConditional(out, tmp, bit);
+    }
+  }
+}
+
+// Get224Bits reads 7 words from in and scatters their contents in
+// little-endian form into 8 words at out, 28 bits per output word.
+void Get224Bits(uint32* out, const uint32* in) {
+  out[0] = NetToHost32(in[6]) & kBottom28Bits;
+  out[1] = ((NetToHost32(in[5]) << 4) |
+            (NetToHost32(in[6]) >> 28)) & kBottom28Bits;
+  out[2] = ((NetToHost32(in[4]) << 8) |
+            (NetToHost32(in[5]) >> 24)) & kBottom28Bits;
+  out[3] = ((NetToHost32(in[3]) << 12) |
+            (NetToHost32(in[4]) >> 20)) & kBottom28Bits;
+  out[4] = ((NetToHost32(in[2]) << 16) |
+            (NetToHost32(in[3]) >> 16)) & kBottom28Bits;
+  out[5] = ((NetToHost32(in[1]) << 20) |
+            (NetToHost32(in[2]) >> 12)) & kBottom28Bits;
+  out[6] = ((NetToHost32(in[0]) << 24) |
+            (NetToHost32(in[1]) >> 8)) & kBottom28Bits;
+  out[7] = (NetToHost32(in[0]) >> 4) & kBottom28Bits;
+}
+
+// Put224Bits performs the inverse operation to Get224Bits: taking 28 bits from
+// each of 8 input words and writing them in big-endian order to 7 words at
+// out.
+void Put224Bits(uint32* out, const uint32* in) {
+  out[6] = HostToNet32((in[0] >> 0) | (in[1] << 28));
+  out[5] = HostToNet32((in[1] >> 4) | (in[2] << 24));
+  out[4] = HostToNet32((in[2] >> 8) | (in[3] << 20));
+  out[3] = HostToNet32((in[3] >> 12) | (in[4] << 16));
+  out[2] = HostToNet32((in[4] >> 16) | (in[5] << 12));
+  out[1] = HostToNet32((in[5] >> 20) | (in[6] << 8));
+  out[0] = HostToNet32((in[6] >> 24) | (in[7] << 4));
+}
+
+}  // anonymous namespace
+
+namespace crypto {
+
+namespace p224 {
+
+bool Point::SetFromString(const base::StringPiece& in) {
+  if (in.size() != 2*28)
+    return false;
+  const uint32* inwords = reinterpret_cast<const uint32*>(in.data());
+  Get224Bits(x, inwords);
+  Get224Bits(y, inwords + 7);
+  memset(&z, 0, sizeof(z));
+  z[0] = 1;
+
+  // Check that the point is on the curve, i.e. that y² = x³ - 3x + b.
+  FieldElement lhs;
+  Square(&lhs, y);
+  Contract(&lhs);
+
+  FieldElement rhs;
+  Square(&rhs, x);
+  Mul(&rhs, x, rhs);
+
+  FieldElement three_x;
+  for (int i = 0; i < 8; i++) {
+    three_x[i] = x[i] * 3;
+  }
+  Reduce(&three_x);
+  Subtract(&rhs, rhs, three_x);
+  Reduce(&rhs);
+
+  ::Add(&rhs, rhs, kB);
+  Contract(&rhs);
+  return memcmp(&lhs, &rhs, sizeof(lhs)) == 0;
+}
+
+std::string Point::ToString() const {
+  FieldElement zinv, zinv_sq, xx, yy;
+
+  // If this is the point at infinity we return a string of all zeros.
+  if (IsZero(this->z)) {
+    static const char zeros[56] = {0};
+    return std::string(zeros, sizeof(zeros));
+  }
+
+  Invert(&zinv, this->z);
+  Square(&zinv_sq, zinv);
+  Mul(&xx, x, zinv_sq);
+  Mul(&zinv_sq, zinv_sq, zinv);
+  Mul(&yy, y, zinv_sq);
+
+  Contract(&xx);
+  Contract(&yy);
+
+  uint32 outwords[14];
+  Put224Bits(outwords, xx);
+  Put224Bits(outwords + 7, yy);
+  return std::string(reinterpret_cast<const char*>(outwords), sizeof(outwords));
+}
+
+void ScalarMult(const Point& in, const uint8* scalar, Point* out) {
+  ::ScalarMult(out, in, scalar, 28);
+}
+
+// kBasePoint is the base point (generator) of the elliptic curve group.
+static const Point kBasePoint = {
+  {22813985, 52956513, 34677300, 203240812,
+   12143107, 133374265, 225162431, 191946955},
+  {83918388, 223877528, 122119236, 123340192,
+   266784067, 263504429, 146143011, 198407736},
+  {1, 0, 0, 0, 0, 0, 0, 0},
+};
+
+void ScalarBaseMult(const uint8* scalar, Point* out) {
+  ::ScalarMult(out, kBasePoint, scalar, 28);
+}
+
+void Add(const Point& a, const Point& b, Point* out) {
+  AddJacobian(out, a, b);
+}
+
+void Negate(const Point& in, Point* out) {
+  // Guide to elliptic curve cryptography, page 89 suggests that (X : X+Y : Z)
+  // is the negative in Jacobian coordinates, but it doesn't actually appear to
+  // be true in testing so this performs the negation in affine coordinates.
+  FieldElement zinv, zinv_sq, y;
+  Invert(&zinv, in.z);
+  Square(&zinv_sq, zinv);
+  Mul(&out->x, in.x, zinv_sq);
+  Mul(&zinv_sq, zinv_sq, zinv);
+  Mul(&y, in.y, zinv_sq);
+
+  Subtract(&out->y, kP, y);
+  Reduce(&out->y);
+
+  memset(&out->z, 0, sizeof(out->z));
+  out->z[0] = 1;
+}
+
+}  // namespace p224
+
+}  // namespace crypto