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// Copyright 2019 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "chrome/browser/ash/child_accounts/parent_access_code/authenticator.h"
#include <utility>
#include <vector>
#include "base/logging.h"
#include "base/numerics/byte_conversions.h"
#include "base/strings/stringprintf.h"
#include "crypto/hash.h"
#include "crypto/hmac.h"
namespace ash {
namespace parent_access {
namespace {
// Value ranges for access code config data.
constexpr base::TimeDelta kMinCodeValidity = base::Seconds(60);
constexpr base::TimeDelta kMaxCodeValidity = base::Minutes(60);
constexpr base::TimeDelta kMinClockDriftTolerance = base::Minutes(0);
constexpr base::TimeDelta kMaxClockDriftTolerance = base::Minutes(30);
// Dictionary keys used to serialize access code config data.
constexpr char kSharedSecretDictKey[] = "shared_secret";
constexpr char kCodeValidityDictKey[] = "access_code_ttl";
constexpr char kClockDriftDictKey[] = "clock_drift_tolerance";
} // namespace
// static
std::optional<AccessCodeConfig> AccessCodeConfig::FromDictionary(
const base::Value::Dict& dict) {
const std::string* secret = dict.FindString(kSharedSecretDictKey);
if (!secret || secret->empty())
return std::nullopt;
std::optional<int> validity = dict.FindInt(kCodeValidityDictKey);
if (!(validity.has_value() && *validity >= kMinCodeValidity.InSeconds() &&
*validity <= kMaxCodeValidity.InSeconds())) {
return std::nullopt;
}
std::optional<int> clock_drift = dict.FindInt(kClockDriftDictKey);
if (!(clock_drift.has_value() &&
*clock_drift >= kMinClockDriftTolerance.InSeconds() &&
*clock_drift <= kMaxClockDriftTolerance.InSeconds())) {
return std::nullopt;
}
return AccessCodeConfig(*secret, base::Seconds(*validity),
base::Seconds(*clock_drift));
}
AccessCodeConfig::AccessCodeConfig(const std::string& shared_secret,
base::TimeDelta code_validity,
base::TimeDelta clock_drift_tolerance)
: shared_secret_(shared_secret),
code_validity_(code_validity),
clock_drift_tolerance_(clock_drift_tolerance) {
DCHECK(!shared_secret_.empty());
DCHECK(code_validity_ >= kMinCodeValidity);
DCHECK(code_validity_ <= kMaxCodeValidity);
DCHECK(clock_drift_tolerance_ >= kMinClockDriftTolerance);
DCHECK(clock_drift_tolerance_ <= kMaxClockDriftTolerance);
}
AccessCodeConfig::AccessCodeConfig(AccessCodeConfig&&) = default;
AccessCodeConfig& AccessCodeConfig::operator=(AccessCodeConfig&&) = default;
AccessCodeConfig::~AccessCodeConfig() = default;
base::Value::Dict AccessCodeConfig::ToDictionary() const {
base::Value::Dict config;
config.Set(kSharedSecretDictKey, base::Value(shared_secret_));
config.Set(kCodeValidityDictKey,
base::Value(static_cast<int>(code_validity_.InSeconds())));
config.Set(kClockDriftDictKey,
base::Value(static_cast<int>(clock_drift_tolerance_.InSeconds())));
return config;
}
AccessCode::AccessCode(const std::string& code,
base::Time valid_from,
base::Time valid_to)
: code_(code), valid_from_(valid_from), valid_to_(valid_to) {
DCHECK_EQ(6u, code_.length());
DCHECK_GT(valid_to_, valid_from_);
}
AccessCode::AccessCode(const AccessCode&) = default;
AccessCode& AccessCode::operator=(const AccessCode&) = default;
AccessCode::~AccessCode() = default;
std::ostream& operator<<(std::ostream& out, const AccessCode& code) {
return out << code.code() << " [" << code.valid_from() << " - "
<< code.valid_to() << "]";
}
// static
constexpr base::TimeDelta Authenticator::kAccessCodeGranularity;
Authenticator::Authenticator(AccessCodeConfig config)
: config_(std::move(config)) {}
Authenticator::~Authenticator() = default;
std::optional<AccessCode> Authenticator::Generate(base::Time timestamp) const {
DCHECK_LE(base::Time::UnixEpoch(), timestamp);
// We find the beginning of the interval for the given timestamp and adjust by
// the granularity.
const int64_t interval = timestamp.InMillisecondsSinceUnixEpoch() /
config_.code_validity().InMilliseconds();
const int64_t interval_beginning_timestamp =
interval * config_.code_validity().InMilliseconds();
const int64_t adjusted_timestamp =
interval_beginning_timestamp / kAccessCodeGranularity.InMilliseconds();
std::array<uint8_t, sizeof(uint64_t)> big_endian_timestamp =
base::U64ToBigEndian(
// NOTE: This will convert negative numbers to large positive ones.
static_cast<uint64_t>(adjusted_timestamp));
auto digest = crypto::hmac::SignSha1(
base::as_byte_span(config_.shared_secret()), big_endian_timestamp);
// Read 4 bytes in Big-endian order starting from |offset|.
const size_t offset = digest.back() & 0xf;
int32_t result =
base::U32FromBigEndian(base::span(digest).subspan(offset).first<4>());
// Clear sign bit.
result &= 0x7fffffff;
const base::Time valid_from =
base::Time::FromMillisecondsSinceUnixEpoch(interval_beginning_timestamp);
return AccessCode(base::StringPrintf("%06d", result % 1000000), valid_from,
valid_from + config_.code_validity());
}
std::optional<AccessCode> Authenticator::Validate(const std::string& code,
base::Time timestamp) const {
DCHECK_LE(base::Time::UnixEpoch(), timestamp);
base::Time valid_from = timestamp - config_.clock_drift_tolerance();
if (valid_from < base::Time::UnixEpoch())
valid_from = base::Time::UnixEpoch();
return ValidateInRange(code, valid_from,
timestamp + config_.clock_drift_tolerance());
}
std::optional<AccessCode> Authenticator::ValidateInRange(
const std::string& code,
base::Time valid_from,
base::Time valid_to) const {
DCHECK_LE(base::Time::UnixEpoch(), valid_from);
DCHECK_GE(valid_to, valid_from);
const int64_t start_interval = valid_from.InMillisecondsSinceUnixEpoch() /
kAccessCodeGranularity.InMilliseconds();
const int64_t end_interval = valid_to.InMillisecondsSinceUnixEpoch() /
kAccessCodeGranularity.InMilliseconds();
for (int i = start_interval; i <= end_interval; ++i) {
const base::Time generation_timestamp =
base::Time::FromMillisecondsSinceUnixEpoch(
i * kAccessCodeGranularity.InMilliseconds());
std::optional<AccessCode> pac = Generate(generation_timestamp);
if (pac.has_value() && pac->code() == code)
return pac;
}
return std::nullopt;
}
} // namespace parent_access
} // namespace ash
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