From 7687d0abb65af8c2335ac39a0f90313c2788d0e9 Mon Sep 17 00:00:00 2001
From: Ben Schwartz <bemasc@google.com>
Date: Mon, 6 Jul 2020 20:57:10 -0400
Subject: [PATCH 1/2] Coalesce the salt and the first message

This change saves one packet in each direction by coalescing the
writes for the salt and the first message.
---
 shadowsocks/stream.go | 60 ++++++++++++++++++++++++++++---------------
 1 file changed, 39 insertions(+), 21 deletions(-)

diff --git a/shadowsocks/stream.go b/shadowsocks/stream.go
index faeb1846..97432633 100644
--- a/shadowsocks/stream.go
+++ b/shadowsocks/stream.go
@@ -61,25 +61,28 @@ func (sw *shadowsocksWriter) init() (err error) {
 		if _, err := io.ReadFull(rand.Reader, salt); err != nil {
 			return fmt.Errorf("failed to generate salt: %v", err)
 		}
-		_, err := sw.writer.Write(salt)
-		if err != nil {
-			return fmt.Errorf("failed to write salt: %v", err)
-		}
 		sw.aead, err = sw.ssCipher.Encrypter(salt)
 		if err != nil {
 			return fmt.Errorf("failed to create AEAD: %v", err)
 		}
 		sw.counter = make([]byte, sw.aead.NonceSize())
-		sw.buf = make([]byte, 2+sw.aead.Overhead()+payloadSizeMask+sw.aead.Overhead())
+		// The maximum length message is the salt (first message only), length, length tag,
+		// payload, and payload tag.
+		sizeBufSize := 2 + sw.aead.Overhead()
+		maxPayloadBufSize := payloadSizeMask + sw.aead.Overhead()
+		sw.buf = make([]byte, len(salt)+sizeBufSize+maxPayloadBufSize)
+		// Store the salt at the start of sw.buf.
+		copy(sw.buf, salt)
 	}
 	return nil
 }
 
-// WriteBlock encrypts and writes the input buffer as one signed block.
-func (sw *shadowsocksWriter) encryptBlock(ciphertext []byte, plaintext []byte) ([]byte, error) {
-	out := sw.aead.Seal(ciphertext, sw.counter, plaintext, nil)
+// encryptBlock encrypts `plaintext` in-place.  The slice must have enough capacity
+// for the tag. Returns the total ciphertext length.
+func (sw *shadowsocksWriter) encryptBlock(plaintext []byte) int {
+	out := sw.aead.Seal(plaintext[:0], sw.counter, plaintext, nil)
 	increment(sw.counter)
-	return out, nil
+	return len(out)
 }
 
 func (sw *shadowsocksWriter) Write(p []byte) (int, error) {
@@ -88,28 +91,43 @@ func (sw *shadowsocksWriter) Write(p []byte) (int, error) {
 	return int(n), err
 }
 
+func isZero(b []byte) bool {
+	for _, v := range b {
+		if v != 0 {
+			return false
+		}
+	}
+	return true
+}
+
 func (sw *shadowsocksWriter) ReadFrom(r io.Reader) (int64, error) {
 	if err := sw.init(); err != nil {
 		return 0, err
 	}
 	var written int64
-	sizeBuf := sw.buf[:2+sw.aead.Overhead()]
-	payloadBuf := sw.buf[len(sizeBuf):]
+
+	// sw.buf starts with the salt.
+	saltSize := sw.ssCipher.SaltSize()
+	// Normally we ignore the salt at the beginning of sw.buf.
+	start := saltSize
+	if isZero(sw.counter) {
+		// For the first message, include the salt.
+		start = 0
+	}
+
+	// Each Shadowsocks-TCP message consists of a fixed-length size block, followed by
+	// a variable-length payload block.
+	sizeBuf := sw.buf[saltSize : saltSize+2+sw.aead.Overhead()]
+	payloadBuf := sw.buf[saltSize+len(sizeBuf):]
 	for {
 		plaintextSize, err := r.Read(payloadBuf[:payloadSizeMask])
 		if plaintextSize > 0 {
 			binary.BigEndian.PutUint16(sizeBuf, uint16(plaintextSize))
-			_, err = sw.encryptBlock(sizeBuf[:0], sizeBuf[:2])
-			if err != nil {
-				return written, fmt.Errorf("failed to encypt payload size: %v", err)
-			}
-			_, err := sw.encryptBlock(payloadBuf[:0], payloadBuf[:plaintextSize])
-			if err != nil {
-				return written, fmt.Errorf("failed to encrypt payload: %v", err)
-			}
-			payloadSize := plaintextSize + sw.aead.Overhead()
-			_, err = sw.writer.Write(sw.buf[:len(sizeBuf)+payloadSize])
+			sw.encryptBlock(sizeBuf[:2])
+			payloadSize := sw.encryptBlock(payloadBuf[:plaintextSize])
+			_, err = sw.writer.Write(sw.buf[start : saltSize+len(sizeBuf)+payloadSize])
 			written += int64(plaintextSize)
+			start = saltSize // Skip the salt for all writes except the first.
 		}
 		if err != nil {
 			if err == io.EOF { // ignore EOF as per io.ReaderFrom contract

From de20fbee5d8d2e063f9050ca3e6e08baad059cbe Mon Sep 17 00:00:00 2001
From: Ben Schwartz <bemasc@google.com>
Date: Mon, 6 Jul 2020 21:22:02 -0400
Subject: [PATCH 2/2] Expand comment

---
 shadowsocks/stream.go | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/shadowsocks/stream.go b/shadowsocks/stream.go
index 97432633..5dd30fb4 100644
--- a/shadowsocks/stream.go
+++ b/shadowsocks/stream.go
@@ -111,7 +111,9 @@ func (sw *shadowsocksWriter) ReadFrom(r io.Reader) (int64, error) {
 	// Normally we ignore the salt at the beginning of sw.buf.
 	start := saltSize
 	if isZero(sw.counter) {
-		// For the first message, include the salt.
+		// For the first message, include the salt.  Compared to writing the salt
+		// separately, this saves one packet during TCP slow-start and potentially
+		// avoids having a distinctive size for the first packet.
 		start = 0
 	}