From dc9f2b24d64b4865eba9ac01d96f61cd0703d639 Mon Sep 17 00:00:00 2001
From: Savely Krendelhoff <alicecandyom@proton.me>
Date: Fri, 22 Aug 2025 21:03:54 +0700
Subject: [PATCH] Implement codec

---
 internal/protocol/codec.go | 111 +++++++++++++++++++++++++++++++++++++
 1 file changed, 111 insertions(+)
 create mode 100644 internal/protocol/codec.go

diff --git a/internal/protocol/codec.go b/internal/protocol/codec.go
new file mode 100644
index 0000000..e5d83cd
--- /dev/null
+++ b/internal/protocol/codec.go
@@ -0,0 +1,111 @@
+package protocol
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+	"unicode/utf8"
+)
+
+// Codec handles encoding and decoding of protocol messages
+type Codec struct{}
+
+// NewCodec creates a new protocol codec
+func NewCodec() *Codec {
+	return &Codec{}
+}
+
+// Encode writes a message to the writer using the protocol format
+func (c *Codec) Encode(w io.Writer, msg *Message) error {
+	if msg == nil {
+		return fmt.Errorf("message cannot be nil")
+	}
+
+	// Validate payload size
+	if len(msg.Payload) > MaxPayloadSize {
+		return fmt.Errorf("payload size %d exceeds maximum %d", len(msg.Payload), MaxPayloadSize)
+	}
+
+	// Write message type (1 byte)
+	if err := binary.Write(w, binary.BigEndian, msg.Type); err != nil {
+		return fmt.Errorf("failed to write message type: %w", err)
+	}
+
+	// Write payload length (4 bytes, big-endian)
+	payloadLength := uint32(len(msg.Payload))
+	if err := binary.Write(w, binary.BigEndian, payloadLength); err != nil {
+		return fmt.Errorf("failed to write payload length: %w", err)
+	}
+
+	// Write payload if present
+	if len(msg.Payload) > 0 {
+		if _, err := w.Write(msg.Payload); err != nil {
+			return fmt.Errorf("failed to write payload: %w", err)
+		}
+	}
+
+	return nil
+}
+
+// Decode reads a message from the reader using the protocol format
+func (c *Codec) Decode(r io.Reader) (*Message, error) {
+	// Read message type (1 byte)
+	var msgType MessageType
+	if err := binary.Read(r, binary.BigEndian, &msgType); err != nil {
+		if err == io.EOF {
+			return nil, err
+		}
+		return nil, fmt.Errorf("failed to read message type: %w", err)
+	}
+
+	// Validate message type
+	if !isValidMessageType(msgType) {
+		return nil, fmt.Errorf("invalid message type: 0x%02x", msgType)
+	}
+
+	// Read payload length (4 bytes, big-endian)
+	var payloadLength uint32
+	if err := binary.Read(r, binary.BigEndian, &payloadLength); err != nil {
+		return nil, fmt.Errorf("failed to read payload length: %w", err)
+	}
+
+	// Validate payload length
+	if payloadLength > MaxPayloadSize {
+		return nil, fmt.Errorf("payload length %d exceeds maximum %d", payloadLength, MaxPayloadSize)
+	}
+
+	// Read payload if present
+	var payload []byte
+	if payloadLength > 0 {
+		payload = make([]byte, payloadLength)
+		// Use LimitReader to ensure we don't read more than payloadLength bytes
+		// even if the underlying reader has more data available
+		limitedReader := io.LimitReader(r, int64(payloadLength))
+		// Note: ReadFull may block waiting for data. The connection handler
+		// MUST set appropriate read deadlines to prevent slowloris attacks
+		if _, err := io.ReadFull(limitedReader, payload); err != nil {
+			return nil, fmt.Errorf("failed to read payload: %w", err)
+		}
+
+		// Validate payload is valid UTF-8
+		if !utf8.Valid(payload) {
+			return nil, fmt.Errorf("payload contains invalid UTF-8")
+		}
+	}
+
+	return &Message{
+		Type:    msgType,
+		Payload: payload,
+	}, nil
+}
+
+
+// isValidMessageType checks if the message type is defined in the protocol
+func isValidMessageType(msgType MessageType) bool {
+	switch msgType {
+	case ChallengeRequest, ChallengeResponse, SolutionRequest, QuoteResponse, ErrorResponse:
+		return true
+	default:
+		return false
+	}
+}