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// Copyright 2016 The Chromium Authors
// 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 <compare>
#include <iosfwd>
#include <tuple>
#include "base/base_export.h"
#include "base/check.h"
#include "base/containers/span.h"
#include "base/strings/string_piece.h"
#include "base/token.h"
namespace base {
struct UnguessableTokenHash;
// UnguessableToken is, like Token, a randomly chosen 128-bit value. Unlike
// Token, a new UnguessableToken is always generated at runtime from a
// cryptographically strong random source (or copied or serialized and
// deserialized from another such UnguessableToken). Also unlike Token, the ==
// and != operators are constant time. It can be used as part of a larger
// aggregate type, or as an ID in and of itself.
// An UnguessableToken is a strong *bearer token*. Bearer tokens are like HTTP
// cookies: if a caller has the token, the callee thereby considers the caller
// authorized to request the operation the callee performs.
// 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 on 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, use
// std::optional instead of an empty token.
class BASE_EXPORT UnguessableToken {
// Create a unique UnguessableToken. It's guaranteed to be nonempty.
static UnguessableToken Create();
// Returns a reference to a global null UnguessableToken. This should only be
// used for functions that need to return a reference to an UnguessableToken,
// and should not be used as a general-purpose substitute for invoking the
// default constructor.
static const UnguessableToken& Null();
// Return an UnguessableToken built from the high/low bytes provided.
// It should only be used in deserialization scenarios.
// NOTE: If the returned `std::optional` does not have a value, it means that
// `high` and `low` correspond to an `UnguesssableToken` that was never
// initialized via Create(). This is a security issue, and should be handled.
static std::optional<UnguessableToken> Deserialize(uint64_t high,
uint64_t low);
// Returns an `UnguessableToken` built from its string representation. It
// should only be used in deserialization scenarios.
// NOTE: If the returned `std::optional` does not have a value, it means that
// the given string does not represent a valid serialized `UnguessableToken`.
// This should be handled as a security issue.
static std::optional<UnguessableToken> DeserializeFromString(
StringPiece string_representation);
// Creates an empty UnguessableToken.
// Assign to it with Create() before using it.
constexpr UnguessableToken() = default;
constexpr UnguessableToken(const UnguessableToken&) = default;
constexpr UnguessableToken& operator=(const UnguessableToken&) = default;
constexpr UnguessableToken(UnguessableToken&&) noexcept = default;
constexpr UnguessableToken& operator=(UnguessableToken&&) = default;
// NOTE: Serializing an empty UnguessableToken is an illegal operation.
uint64_t GetHighForSerialization() const {
return token_.high();
// NOTE: Serializing an empty UnguessableToken is an illegal operation.
uint64_t GetLowForSerialization() const {
return token_.low();
constexpr bool is_empty() const { return token_.is_zero(); }
// Hex representation of the unguessable token.
std::string ToString() const { return token_.ToString(); }
explicit constexpr operator bool() const { return !is_empty(); }
span<const uint8_t, 16> AsBytes() const { return token_.AsBytes(); }
friend constexpr auto operator<=>(const UnguessableToken& lhs,
const UnguessableToken& rhs) = default;
// operator== uses constant-time comparison for security where available.
friend BASE_EXPORT bool operator==(const UnguessableToken& lhs,
const UnguessableToken& rhs);
#if defined(UNIT_TEST)
static UnguessableToken CreateForTesting(uint64_t high, uint64_t low) {
std::optional<UnguessableToken> token = Deserialize(high, low);
return token.value();
friend struct UnguessableTokenHash;
explicit UnguessableToken(const Token& token);
base::Token token_;
BASE_EXPORT bool operator==(const UnguessableToken& lhs,
const UnguessableToken& rhs);
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 TokenHash()(token.token_);
} // namespace base