Add base::UnguessableToken

base/nonce_token.h

Index: base/nonce_token.h
diff --git a/base/nonce_token.h b/base/nonce_token.h
new file mode 100644
index 0000000000000000000000000000000000000000..d40eda57ca1cb4cf5afd12f8dd61957632e005ff
--- /dev/null
+++ b/base/nonce_token.h
@@ -0,0 +1,94 @@
+// Copyright 2016 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 BASE_NONCE_TOKEN_H_
+#define BASE_NONCE_TOKEN_H_
+
+#include <stdint.h>
+#include <string.h>
+#include <tuple>
+
+#include "base/base_export.h"
+#include "base/hash.h"
+#include "base/logging.h"
+
+namespace base {
+
+struct NonceTokenHash;
+
+// A NonceToken is an unguessable 128-bit token generated from a
+// cryptographically strong random source.
+//
+// NonceToken should be used when a sensitive ID needs to be unguessable, and is
+// shared across processes. It can be used as part of a larger aggregate type,
+// or as an ID in and of itself.
+//
+// Use Create() for creating new unguessable nonce tokens.
+//
+// NOTE: A NonceToken is not semantically a "cryptographic nonce", since we
+// consider a zeroed out NonceToken to be a legal value. It is however illegal
+// to send empty NonceTokens across processes.
+class BASE_EXPORT NonceToken {
+ public:
+  // Create an unguessable and unique NonceToken.
+  // USE ONLY THIS FUNCTION TO GENERATE NEW NONCES.

[danakj, 2016/09/15 23:35:16]

I'd prefer less capslock in comments. The comment above already calls out this
method as the place to call too. I think you can drop this whole capslock part.

[tguilbert, 2016/09/16 01:03:01]

Done.

+  static NonceToken Create();
+
+  // Create a zeroed out NonceToken.
+  // NOTE: The NonceToken still needs to be initialized via Create().

[danakj, 2016/09/15 23:35:16]

How about // Creates an empty NonceToken. Assign to it with Create() before
using it.

[tguilbert, 2016/09/16 01:03:01]

Nicely worded, thank you! Done.

+  NonceToken() = default;
+
+  // Return |high_| and |low_| as out parameters.
+  // NOTE: Serializing an empty NonceToken is an illegal operation.
+  void Serialize(uint64_t* high_out, uint64_t* low_out) const;
+
+  // Return a NonceToken built from the high/low bytes provided.
+  // It should only be used in deserialization scenarios.
+  //
+  // NOTE: If |high| and |low| are both 0, the NonceToken we are trying to
+  // deserialize was never initialized, which is a security issue.
+  static NonceToken Deserialize(uint64_t high, uint64_t low);

[danakj, 2016/09/15 23:35:16]

It's easier to read if the static methods are grouped, can you put this with
Create()

[tguilbert, 2016/09/16 01:03:01]

Done.

+
+  // Helper function to check if the high/low bits passed are zeroed out.
+  static inline bool IsZeroData(uint64_t high, uint64_t low) {

[danakj, 2016/09/15 23:35:16]

Can be private? Why isn't it used for is_empty() as well?

[tguilbert, 2016/09/16 01:03:01]

I am using it in the IPC and Mojo deserialization code before actually calling
NonceToken::Deserialize, and therefore it can't be private.

And yes, I updated it to be used in is_empty(). (This was Leftover from a local
commit I had where I renamed is_empty() to is_initialized())

+    return !(high | low);
+  }
+
+  bool is_empty() const { return !(high_ | low_); }
+
+  std::string ToString() const;
+
+  explicit operator bool() const { return !is_empty(); }
+
+  bool operator<(const NonceToken& other) const {
+    return std::tie(high_, low_) < std::tie(other.high_, other.low_);
+  }
+
+  bool operator==(const NonceToken& other) const {
+    return !((high_ ^ other.high_) | (low_ ^ other.low_));

[danakj, 2016/09/15 23:35:16]

Either just do high_==other.high_ && low==other.low_ or explain this with a
comment (in a widely understandable way).

[tguilbert, 2016/09/16 01:03:01]

I think SecureMemEqual's comments give good context, so I added a  pointer to
those comments.

+  }
+
+  bool operator!=(const NonceToken& other) const { return !(*this == other); }
+
+ private:
+  friend struct NonceTokenHash;
+  NonceToken(uint64_t high, uint64_t low);
+
+  // Note: Two uint64_t are used instead of uint8_t[16], in order to have a
+  // simpler ToString() and is_empty().
+  uint64_t high_ = 0;
+  uint64_t low_ = 0;
+};
+
+// For use in std::unordered_map.
+struct NonceTokenHash {
+  size_t operator()(const base::NonceToken& nonce) const {
+    DCHECK(nonce);
+    return base::HashInts64(nonce.high_, nonce.low_);
+  }
+};
+
+}  // namespace base
+
+#endif  // BASE_NONCE_TOKEN_H_