Add a parser for messages with a Web Push Protocol-based payload

components/gcm_driver/crypto/message_payload_parser.cc

+// Copyright 2017 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.
+
+#include "components/gcm_driver/crypto/message_payload_parser.h"
+
+#include "base/big_endian.h"
+
+namespace gcm {
+
+namespace {
+
+// Size, in bytes, of the salt included in the message header.
+constexpr size_t kSaltSize = 16;
+
+// Size, in bytes, of the uncompressed point included in the message header.
+constexpr size_t kUncompressedPointSize = 65;

[johnme, 2017/05/22 16:18:39]

Nit: perhaps stick to "keyid" for consistency with the spec and class comment?

[Peter Beverloo, 2017/05/22 17:25:12]

I've updated the header instead.

+
+// Size, in bytes, of the smallest allowable record_size value.
+constexpr size_t kMinimumRecordSize = 18;
+
+// Size, in bytes, of an empty message with the minimum amount of padding.
+constexpr size_t kMinimumMessageSize =
+    kSaltSize + sizeof(uint32_t) + sizeof(uint8_t) + kUncompressedPointSize +

[johnme, 2017/05/22 16:18:39]

Nit: perhaps use sizeof(record_size_) instead of sizeof(uint32_t) for
consistency with below?

[Peter Beverloo, 2017/05/22 17:25:12]

That's a private class member, so it'd be a bit awkward to..

+    kMinimumRecordSize;
+
+}  // namespace
+
+MessagePayloadParser::MessagePayloadParser(base::StringPiece message) {
+  if (message.size() < kMinimumMessageSize)
+    return;
+
+  salt_ = message.substr(0, kSaltSize);
+  message.remove_prefix(kSaltSize);
+
+  base::ReadBigEndian(message.data(), &record_size_);

[johnme, 2017/05/22 16:18:39]

Do we need to worry about the "This would cause SIGBUS on ARMv5 or earlier and
ARMv6-M." comment?

[Peter Beverloo, 2017/05/22 17:25:12]

Looking at code search, it doesn't look like we support ARMv6 anymore? Other
uses of base::ReadBigEndian aren't worried about this.

+  message.remove_prefix(sizeof(record_size_));
+
+  if (record_size_ < kMinimumRecordSize)
+    return;
+
+  uint8_t public_key_length;
+  base::ReadBigEndian(message.data(), &public_key_length);
+  message.remove_prefix(sizeof(public_key_length));
+
+  if (public_key_length != kUncompressedPointSize)
+    return;
+
+  public_key_ = message.substr(0, kUncompressedPointSize);

[johnme, 2017/05/22 16:18:39]

It's a little surprising that you mention in the class comment that this must
start with 0x04, but then don't actually check that when validating the header.

[Peter Beverloo, 2017/05/22 17:25:12]

Done.

+  message.remove_prefix(kUncompressedPointSize);
+
+  ciphertext_ = message;
+
+  if (ciphertext_.size() < kMinimumRecordSize)

[johnme, 2017/05/22 16:18:39]

Shouldn't this be `(ciphertext_.size() != record_size_)`?

[Peter Beverloo, 2017/05/22 17:25:12]

No, because record_size_ can be larger too. (We default to 4096.)

+    return;
+
+  is_valid_ = true;
+}
+
+MessagePayloadParser::~MessagePayloadParser() = default;
+
+}  // namespace gcm