From edcddd00dd4a21ec7f40ba1e9e06a8e2cfb7ff5f Mon Sep 17 00:00:00 2001
From: Charles Kerr <charles@charleskerr.com>
Date: Sun, 14 May 2023 15:01:04 -0500
Subject: [PATCH] perf: avoid extra work in read_vc() (#5527)

---
 libtransmission/handshake.cc | 21 +++++++++++++--------
 libtransmission/handshake.h  |  5 +++++
 2 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/libtransmission/handshake.cc b/libtransmission/handshake.cc
index a09a2f0fc..6be04586c 100644
--- a/libtransmission/handshake.cc
+++ b/libtransmission/handshake.cc
@@ -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;
         }
diff --git a/libtransmission/handshake.h b/libtransmission/handshake.h
index dd935ddcb..c9b6582c4 100644
--- a/libtransmission/handshake.h
+++ b/libtransmission/handshake.h
@@ -249,6 +249,11 @@ private:
     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 };