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// Copyright 2020 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 "third_party/blink/public/common/privacy_budget/identifiable_token_builder.h"
#include <algorithm>
#include <iterator>
#include <type_traits>
#include "base/check.h"
#include "base/check_op.h"
#include "base/hash/legacy_hash.h"
namespace blink {
namespace {
// A big random prime. It's also the digest returned for an empty block.
constexpr uint64_t kChainingValueSeed = UINT64_C(6544625333304541877);
} // namespace
const size_t IdentifiableTokenBuilder::kBlockSizeInBytes;
IdentifiableTokenBuilder::IdentifiableTokenBuilder()
: chaining_value_(kChainingValueSeed) {
// Ensures that BlockBuffer iterators are random-access on all platforms.
static_assert(
std::is_same<std::random_access_iterator_tag,
std::iterator_traits<
BlockBuffer::iterator>::iterator_category>::value,
"Iterator operations may not be constant time.");
}
IdentifiableTokenBuilder::IdentifiableTokenBuilder(
const IdentifiableTokenBuilder& other) {
partial_ = other.partial_;
position_ = partial_.begin();
std::advance(position_, other.PartialSize());
chaining_value_ = other.chaining_value_;
}
IdentifiableTokenBuilder::IdentifiableTokenBuilder(ByteSpan buffer)
: IdentifiableTokenBuilder() {
AddBytes(buffer);
}
IdentifiableTokenBuilder& IdentifiableTokenBuilder::AddBytes(ByteSpan message) {
DCHECK_NE(position_, partial_.end());
// Phase 1:
// Slurp in as much of the message as necessary if there's a partial block
// already assembled. Copying is expensive, so |partial_| is only involved
// when there's some left over bytes from a prior round.
if (partial_.begin() != position_ && !message.empty())
message = SkimIntoPartial(message);
if (message.empty())
return *this;
// Phase 2:
// Consume as many full blocks as possible from |message|.
DCHECK_EQ(position_, partial_.begin());
while (message.size() >= kBlockSizeInBytes) {
DigestBlock(message.first<kBlockSizeInBytes>());
message = message.subspan(kBlockSizeInBytes);
}
if (message.empty())
return *this;
// Phase 3:
// Whatever remains is stuffed into the partial buffer.
message = SkimIntoPartial(message);
DCHECK(message.empty());
return *this;
}
IdentifiableTokenBuilder& IdentifiableTokenBuilder::AddAtomic(ByteSpan buffer) {
AlignPartialBuffer();
AddValue(buffer.size_bytes());
AddBytes(buffer);
AlignPartialBuffer();
return *this;
}
IdentifiableTokenBuilder::operator IdentifiableToken() const {
return GetToken();
}
IdentifiableToken IdentifiableTokenBuilder::GetToken() const {
if (position_ == partial_.begin())
return chaining_value_;
return IdentifiableToken(
base::legacy::CityHash64WithSeed(GetPartialBlock(), chaining_value_));
}
IdentifiableTokenBuilder::ByteSpan IdentifiableTokenBuilder::SkimIntoPartial(
ByteSpan message) {
DCHECK(!message.empty() && position_ != partial_.end());
const auto to_copy = std::min<size_t>(
std::distance(position_, partial_.end()), message.size());
position_ = std::copy_n(message.begin(), to_copy, position_);
if (position_ == partial_.end())
DigestBlock(TakeCompletedBlock());
return message.subspan(to_copy);
}
void IdentifiableTokenBuilder::AlignPartialBuffer() {
const auto padding_to_add =
kBlockAlignment - (PartialSize() % kBlockAlignment);
if (padding_to_add == kBlockAlignment)
return;
position_ = std::fill_n(position_, padding_to_add, 0);
if (position_ == partial_.end())
DigestBlock(TakeCompletedBlock());
DCHECK_NE(position_, partial_.end());
DCHECK(IsAligned());
}
void IdentifiableTokenBuilder::DigestBlock(ConstFullBlockSpan block) {
// partial_ should've been flushed before calling this.
DCHECK_EQ(position_, partial_.begin());
// The chaining value (initialized with the initialization vector
// kChainingValueSeed) is only used for diffusion. There's no length padding
// being done here since we aren't interested in second-preimage issues.
//
// There is a concern over hash flooding, but that's something the entire
// study has more-or-less accepted for some metrics and is dealt with during
// the analysis phase.
chaining_value_ =
base::legacy::CityHash64WithSeed(base::make_span(block), chaining_value_);
}
size_t IdentifiableTokenBuilder::PartialSize() const {
return std::distance<BlockBuffer::const_iterator>(partial_.begin(),
position_);
}
IdentifiableTokenBuilder::ConstFullBlockSpan
IdentifiableTokenBuilder::TakeCompletedBlock() {
DCHECK_EQ(position_, partial_.end());
auto buffer = base::make_span(partial_);
position_ = partial_.begin();
return buffer;
}
bool IdentifiableTokenBuilder::IsAligned() const {
return PartialSize() % kBlockAlignment == 0;
}
IdentifiableTokenBuilder::ByteSpan IdentifiableTokenBuilder::GetPartialBlock()
const {
return ByteSpan(partial_.begin(), position_);
}
} // namespace blink
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