crypto: add simple P224 implementation.

crypto/p224.h

+// Copyright (c) 2011 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.
+
+#ifndef CRYPTO_P224_H_
+#define CRYPTO_P224_H_
+#pragma once
+
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/string_piece.h"
+
+namespace crypto {
+
+// P224 implements an elliptic curve group, commonly known as P224 and defined
+// in FIPS 186-3, section D.2.2.
+namespace p224 {
+
+// An element of the field (ℤ/pℤ) is represented with 8, 28-bit limbs in
+// little endian order.
+typedef uint32 FieldElement[8];
+
+struct Point {
+  // Sets the value of the point from the 56 byte, external representation. The
+  // external point representation is an (x, y) pair of a point on the curve.
+  // Each field element is represented as a big-endian number < p.
+  bool Set(const base::StringPiece& in);

[Wez, 2011/11/04 22:13:46]

nit: The external representation is what this accepts, and what ToString
generates, so it seems logical to name them FromString() and ToString() (or even
FromExternal() and ToExternal(), or Import() and Export()).

[agl, 2011/11/07 15:15:12]

Went with SetFromString because FromString sounds like it should be a static
that returns a new object.

+
+  // ToString returns an external representation of the Point.

[Wez, 2011/11/04 22:13:46]

nit: Consider clarifying that it's a byte-string, not a printable string.

[agl, 2011/11/07 15:15:12]

Done.

+  std::string ToString() const;
+
+  // An Point is represented in Jacobian form (x/z², y/z³).
+  FieldElement x, y, z;
+};
+
+// ScalarMult computes *out = in*scalar where scalar is a 28-byte, big-endian
+// number.
+void ScalarMult(const Point& in, const uint8* scalar, Point* out);
+
+// ScalarBaseMult computes *out = g*scalar where g is the base point of the
+// curve and scalar is a 28-byte, big-endian number.
+void ScalarBaseMult(const uint8* scalar, Point* out);
+
+// Add computes *out = a+b.
+void Add(const Point& a, const Point& b, Point* out);
+
+// Negate calculates out = -a;
+void Negate(const Point& a, Point* out);
+
+}  // namespace p224
+
+}  // namespace crypto
+
+#endif  // CRYPTO_P224_H_