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// 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.
#include <stdint.h>
#include <string.h>
#include <iosfwd>
#include <tuple>
#include "base/base_export.h"
#include "base/hash.h"
#include "base/logging.h"
namespace base {
struct UnguessableTokenHash;
// A UnguessableToken is an 128-bit token generated from a cryptographically
// strong random source. It can be used as part of a larger aggregate type,
// or as an ID in and of itself.
// UnguessableToken can be used to implement "Capability-Based Security".
// In other words, UnguessableToken can be used when the resource associated
// with the ID needs to be protected against manipulation by other untrusted
// agents in the system, and there is no other convenient way to verify the
// authority of the agent to do so (because the resource is part of a table
// shared across processes, for instance). In such a scheme, knowledge of the
// token value in and of itself is sufficient proof of authority to carry out
// an operation against the associated resource.
// Use Create() for creating new UnguessableTokens.
// NOTE: It is illegal to send empty UnguessableTokens across processes, and
// sending/receiving empty tokens should be treated as a security issue.
// If there is a valid scenario for sending "no token" across processes,
// base::Optional should be used instead of an empty token.
class BASE_EXPORT UnguessableToken {
// Create a unique UnguessableToken.
static UnguessableToken Create();
// Return a UnguessableToken built from the high/low bytes provided.
// It should only be used in deserialization scenarios.
// NOTE: If the deserialized token is empty, it means that it was never
// initialized via Create(). This is a security issue, and should be handled.
static UnguessableToken Deserialize(uint64_t high, uint64_t low);
// Creates an empty UnguessableToken.
// Assign to it with Create() before using it.
constexpr UnguessableToken() = default;
// NOTE: Serializing an empty UnguessableToken is an illegal operation.
uint64_t GetHighForSerialization() const {
return high_;
// NOTE: Serializing an empty UnguessableToken is an illegal operation.
uint64_t GetLowForSerialization() const {
return low_;
bool is_empty() const { return high_ == 0 && low_ == 0; }
// Hex representation of the unguessable token.
std::string ToString() const;
explicit operator bool() const { return !is_empty(); }
bool operator<(const UnguessableToken& other) const {
return std::tie(high_, low_) < std::tie(other.high_, other.low_);
bool operator==(const UnguessableToken& other) const {
return high_ == other.high_ && low_ == other.low_;
bool operator!=(const UnguessableToken& other) const {
return !(*this == other);
friend struct UnguessableTokenHash;
UnguessableToken(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;
BASE_EXPORT std::ostream& operator<<(std::ostream& out,
const UnguessableToken& token);
// For use in std::unordered_map.
struct UnguessableTokenHash {
size_t operator()(const base::UnguessableToken& token) const {
return base::HashInts64(token.high_, token.low_);
} // namespace base