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1 # WebSocket Demystified
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2
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3 WebSockets have been around for more than 10 years now—the [RFC 6455](https://www.rfc-editor.org/rfc/rfc6455) was dropped way back in 2011. This was inevitable. As apps got more complex, people wanted real bidirectional communication without resorting to hacky solutions like raw TCP connections with custom keys or the constant overhead of short/long polling.
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4
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5 Today, it’s the standard for everything from chat apps to LLM interfaces, where the model streams bytes back to you one token at a time as it predicts the next word.
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6
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7 Most developers just grab a library like [ws](https://github.com/websockets/ws) for Node.js or [websockets](https://websockets.readthedocs.io/) for Python and call it a day. But many don’t realize the underlying mechanism is actually pretty simple to implement yourself in a day or so. Let's look at how to build it from scratch.
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8
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9 ---
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10
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11 ## Requirements
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12
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13 * Ability to type
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14 * Half a brain
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15 * A computer
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16
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17 ---
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18
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19 ## The Lifecycle
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20
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21 To get a WebSocket up and running, you have to follow a specific dance. It’s not just "connecting to a port"; it's an evolution of an existing relationship.
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22
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23 1. **The Handshake:** A client sends a "pretty please" HTTP request asking to upgrade the connection.
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24 2. **The Response:** The server agrees (101 Switching Protocols) and sends back a specific hash.
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25 3. **The Switch:** Both sides stop talking "HTTP" and start talking "Frames."
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26 4. **The Interaction:** Bidirectional, binary-framed messaging until someone closes the door.
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27
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28 ---
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29
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30 ## Opening Handshakes
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31
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32 To start the upgrade from HTTP to WebSocket, the client sends a standard GET request but with some very specific headers.
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33
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34 <div class="center"> <img src="/public/white-noise-grass.png" /> </div>
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35
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36 I’m assuming you know how HTTP works. If not, you can open a developer tool by right clicking on your browser and seeing into network tab and refershign the page. The only interesting values here is the `Sec-WebSocket-Key`. This key is usually a 16-byte random value encoded in **Base64**.
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37
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38 **Note:** It’s not for security—it’s to prevent intermediate caches from accidentally serving a cached WebSocket response to a different client.
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39
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40 But before we jump into that, we need to construct that Base64 key.
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41
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42 ### What is Base64?
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43
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44 Let's ask Gemini:
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45
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46 > "Base64 is a binary-to-text encoding scheme that represents data in an ASCII string format by translating it into a radix-64 representation, using a specific set of 64 printable characters." — Gemini
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47
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48
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49 Nice, it didn't halluciante. Let's constracut these. Here they are 64 characters that are safe to print in ASCII.:
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50
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51 ```c
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52 static const char base64_chars[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
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53
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54 ```
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55
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56 In Python, this is a one-liner:
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57
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58 ```python
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59 import base64
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60 import os
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61 print(base64.b64encode(os.urandom(16)))
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62
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63 ```
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64
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65 But we are rewriting this from scratch in **C**, so we need to suffer a little. The logic is: take 3 bytes (24 bits) and split them into 4 chunks of 6 bits each. Each 6-bit chunk becomes an index into our `base64_chars` array.
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66
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67 #### Step 1: Generate Random Bytes
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68
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69 First, we grab 16 random bytes.
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70
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71 ```c
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72 srand((unsigned int)time(NULL));
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73 uint8 random_value[16];
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74
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75 for (int i = 0; i < 16; i++)
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76 random_value[i] = (uint8)(rand() % 256);
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77 ```
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78
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79 #### Step 2: The Bit-Shifting Magic
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80
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81 We loop through our 16 bytes in groups of 3. We pack them into a 32-bit integer, then carve that integer into 6-bit slices.
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82
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83 > **Note:** The length isn't strictly defined as 32, but many implementations land there.
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84
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85 ```c
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86 char result[32] = {0};
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87 int32 result_index = 0;
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88
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89 for (int i = 0; i < 16; i += 3) {
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90 uint32 first_value = 0, second_value = 0, third_value = 0;
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91
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92 if (i < 15) {
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93 first_value = (uint32)random_value[i] << 16;
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94 second_value = (uint32)random_value[i+1] << 8; // Fixed logic from original draft
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95 third_value = (uint32)random_value[i+2];
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96 } else {
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97 // Handle the trailing bytes (padding logic usually goes here)
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98 first_value = (uint32)random_value[i] << 16;
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99 }
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100
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101 uint32 group_value = first_value | second_value | third_value;
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102
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103 // Map bits to characters: 0x3F is 0011 1111 (keeps only 6 bits)
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104 result[result_index++] = base64_chars[(group_value >> 18) & 0x3F];
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105 result[result_index++] = base64_chars[(group_value >> 12) & 0x3F];
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106 result[result_index++] = base64_chars[(group_value >> 6) & 0x3F];
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107 result[result_index++] = base64_chars[group_value & 0x3F];
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108 }
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109
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110 ```
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111
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112 Now you have a `Sec-WebSocket-Key`. When the server gets it, it appends a "Magic String" (`258EAFA5-E914-47DA-95CA-C5AB0DC85B11`), SHA1 hashes it, and Base64 encodes it back to you as `Sec-WebSocket-Accept`.
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113
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114 ---
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115
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116 ## Upgrading the Protocol
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117
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118 If you are the client, you just wait for that `101 Switching Protocols` response. Once you see it, you stop sending HTTP text and start sending frames.
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119
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120 If you are the **server**, you need to keep that connection alive. In my project, `Seobeo`, I create a separate connection object, throw away the HTTP request info to save memory, and start a fresh buffer for WebSocket frames.
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121
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122 ```c
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123 // Transitioning the state from HTTP to WebSocket
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124 Seobeo_WebSocket_Server_Connection *p_conn = malloc(sizeof(Seobeo_WebSocket_Server_Connection));
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125 memset(p_conn, 0, sizeof(Seobeo_WebSocket_Server_Connection));
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126
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127 p_conn->p_handle = p_handle;
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128 p_conn->is_active = TRUE;
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129 p_conn->fragment_capacity = 4096;
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130 p_conn->fragment_buffer = malloc(p_conn->fragment_capacity);
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131
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132 ```
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133
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134 ---
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135
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136 ## Frame-Based Protocols
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137
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138 This is where the logic gets "cancerous." WebSockets don't just send raw strings; they wrap everything in a **Frame**.
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139
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140 ### The Opcode Table
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123
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141
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142 The first byte contains the `FIN` bit (is this the end of the message?) and the `Opcode` (what kind of data is this?).
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143
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144 | Opcode (Hex) | Meaning | Description |
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145 | --- | --- | --- |
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146 | `0x0` | Continuation | Part of a multi-frame message |
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147 | `0x1` | Text | UTF-8 payload |
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148 | `0x2` | Binary | Raw binary data |
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149 | `0x8` | Close | Terminate the connection |
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150 | `0x9` | Ping | Heartbeat check |
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151 | `0xA` | Pong | Heartbeat response |
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152
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153 ### The Masking Rule
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154
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155 * **Client to Server:** MUST be masked.
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156 * **Server to Client:** MUST NOT be masked.
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157 If a client sends unmasked data, the server must close the connection. It’s the law.
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158
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159 ### Why the Bitwise Mess? (Endianness)
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160
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161 In the code below, you’ll see things like `payload_length >> 56`. This is because network protocol headers use **Big-Endian** (most significant byte first). If your computer is Little-Endian (most are), you have to manually shift bits into the right order so the wire sees them correctly.
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162
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163 ---
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164
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165 ## Sending a Frame (Client Side)
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166
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167 Here is how we construct a frame to send data to the server.
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168
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169 ```c
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170 uint8 frame[14];
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171 size_t frame_len = 0;
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172
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173 // Byte 0: FIN bit (0x80) and Opcode
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174 frame[0] = (fin ? 0x80 : 0x00) | (opcode & 0x0F);
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175 frame_len++;
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176
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177 // Generate a 4-byte mask key
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178 uint8 mask_key[4];
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179 for (int i = 0; i < 4; i++)
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180 mask_key[i] = (uint8)(rand() % 256);
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181
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182 // Byte 1+: Payload Length logic
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183 if (payload_length < 126) {
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184 frame[1] = 0x80 | (uint8)payload_length; // 0x80 sets the MASK bit
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185 frame_len++;
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186 } else if (payload_length <= 65535) {
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187 frame[1] = 0x80 | 126;
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188 frame[2] = (uint8)((payload_length >> 8) & 0xFF);
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189 frame[3] = (uint8)(payload_length & 0xFF);
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190 frame_len += 3;
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191 } else {
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192 frame[1] = 0x80 | 127;
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193 for (int i = 0; i < 8; i++)
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194 frame[2 + i] = (uint8)((payload_length >> (56 - i * 8)) & 0xFF);
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195 frame_len += 9;
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196 }
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197
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198 // Attach the mask key
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199 memcpy(frame + frame_len, mask_key, 4);
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200 frame_len += 4;
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201
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202 ```
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203
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204 To actually send the data, you XOR every byte with the mask:
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205
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206 ```c
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207 for (size_t i = 0; i < length; i++)
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208 data[i] ^= mask_key[i % 4];
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123
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209
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130
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210 ```
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211
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212 ---
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213
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214 ## Receiving a Frame (Server Side)
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215
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216 On the server side, we have to do the reverse. We peel the onion layer by layer.
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217
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218 #### 1. Parse the Header
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219
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220 We check the first two bytes to see how big the payload is and if it's masked.
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221
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222 ```c
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223 uint8 *buf = p_conn->p_handle->read_buffer;
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224 uint8 byte1 = buf[0];
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225 uint8 byte2 = buf[1];
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226
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227 boolean fin = (byte1 & 0x80) != 0;
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228 Seobeo_WebSocket_Opcode opcode = (Seobeo_WebSocket_Opcode)(byte1 & 0x0F);
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229 boolean masked = (byte2 & 0x80) != 0;
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230 uint64 payload_len = byte2 & 0x7F;
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231
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232 size_t header_len = 2;
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233
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234 ```
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235
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236 #### 2. Handle Extended Lengths
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237
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238 If the length is 126 or 127, it means the actual size is hidden in the next 2 or 8 bytes.
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239
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240 ```c
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241 if (payload_len == 126) {
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242 payload_len = (buf[2] << 8) | buf[3];
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243 header_len += 2;
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244 } else if (payload_len == 127) {
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245 payload_len = 0;
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246 for (int i = 0; i < 8; i++)
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247 payload_len = (payload_len << 8) | buf[2 + i];
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248 header_len += 8;
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249 }
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250
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251 ```
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252
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253 #### 3. Unmask the Payload
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254
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255 If the data is masked (and it should be if it's from a client), we use that 4-byte key to flip the bits back to normal.
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256
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257 ```c
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258 uint8 mask_key[4] = {0};
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259 if (masked) {
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260 memcpy(mask_key, buf + header_len, 4);
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261 header_len += 4;
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262 }
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263
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264 uint8 *payload = malloc(payload_len);
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265 memcpy(payload, buf + header_len, payload_len);
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266
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267 if (masked)
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268 Seobeo_WebSocket_Unmask_Data(payload, payload_len, mask_key);
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269
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270 ```
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271
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272 ---
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273
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274 ## Conclusion
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275
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276 That’s it. That is WebSockets in a nutshell. Once you handle the bit-shifting for the length and the XOR masking, you’re just reading and writing to a socket like any other protocol.
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277
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278 You can test my implementation at `mrjunejune.com/talk`. Open two tabs and talk to yourself—it’s a great way to verify your frames are flying correctly.
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