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perf: avoid extra work in read_vc() #5527

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merged 1 commit into from May 14, 2023
Merged

perf: avoid extra work in read_vc() #5527

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21 changes: 13 additions & 8 deletions libtransmission/handshake.cc
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Expand Up @@ -240,27 +240,32 @@ ReadState tr_handshake::read_vc(tr_peerIo* peer_io)
auto const info_hash = peer_io->torrent_hash();
TR_ASSERT_MSG(info_hash != tr_sha1_digest_t{}, "readVC requires an info_hash");

// find the end of PadB by looking for `ENCRYPT(VC)`
auto needle = VC;
auto filter = tr_message_stream_encryption::Filter{};
filter.encryptInit(true, dh_, info_hash);
filter.encrypt(std::size(needle), std::data(needle));
// We need to find the end of PadB by looking for `ENCRYPT(VC)`,
// so calculate and cache the value of `ENCRYPT(VC)`.
if (!encrypted_vc_)
{
auto needle = VC;
auto filter = tr_message_stream_encryption::Filter{};
filter.encryptInit(true, dh_, info_hash);
filter.encrypt(std::size(needle), std::data(needle));
encrypted_vc_ = needle;
}

for (size_t i = 0; i < PadbMaxlen; ++i)
{
if (peer_io->read_buffer_size() < std::size(needle))
if (peer_io->read_buffer_size() < std::size(*encrypted_vc_))
{
tr_logAddTraceHand(this, "not enough bytes... returning read_more");
return READ_LATER;
}

if (peer_io->read_buffer_starts_with(needle))
if (peer_io->read_buffer_starts_with(*encrypted_vc_))
{
tr_logAddTraceHand(this, "got it!");
// We already know it's a match; now we just need to
// consume it from the read buffer.
peer_io->decrypt_init(peer_io->is_incoming(), dh_, info_hash);
peer_io->read_bytes(std::data(needle), std::size(needle));
peer_io->read_bytes(std::data(*encrypted_vc_), std::size(*encrypted_vc_));
set_state(tr_handshake::State::AwaitingCryptoSelect);
return READ_NOW;
}
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5 changes: 5 additions & 0 deletions libtransmission/handshake.h
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Expand Up @@ -249,6 +249,11 @@ class tr_handshake
using vc_t = std::array<std::byte, 8>;
static auto constexpr VC = vc_t{};

// Used when resynchronizing in read_vc(). This value is cached to avoid
// the cost of recomputing it. MSE spec: "Since the length of [PadB is]
// unknown, A will be able to resynchronize on ENCRYPT(VC)".
std::optional<vc_t> encrypted_vc_;

///

static constexpr auto DhPoolMaxSize = size_t{ 32 };
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