WebSocket Frame Masking Key & Opcode Hex Inspector
Inspect and parse RFC 6455 WebSocket frames, disassemble raw hexadecimal streams, reverse 4-byte XOR client masking keys, and synthesize custom binary packets.
Raw Hex Stream Input
Spaces, newlines, colons, and commas are automatically stripped during ingestion.
RFC 6455 Header Field Matrix
Unmasked Payload (4-Byte XOR Reversed)
Hello
7f 9f 4d 51 58
48 65 6c 6c 6f
Anatomical Architecture of the WebSocket Framing Protocol (RFC 6455)
Unlike traditional HTTP request-response transactions wrapped within bulky textual headers, the WebSocket protocol relies on a micro-framing layer designed for high-frequency, low-latency duplex transmission over persistent TCP connections. Every frame starts with a compact 2-byte header, extending dynamically up to 14 bytes depending on payload dimensions and client masking keys. When dissecting individual bit flags and nibbles in raw packet buffers, developers often use our Binary Converter to translate hexadecimal bytes into 8-bit sequences and decimal byte counts.
Byte 0: Control & Opcodes
Houses the 1-bit FIN flag, 3 extension-reserved bits (RSV1-RSV3), and the 4-bit Opcode determining whether the frame contains UTF-8 text (0x1), raw binary buffers (0x2), fragmented streams (0x0), or control signals like Ping/Pong (0x9, 0xA) and Close (0x8).
Byte 1: Masking & Length
The high-order bit dictates the Mask flag (must be 1 for client-to-server frames). The lower 7 bits define the payload magnitude: 0–125 bytes directly, 126 for 16-bit extended lengths (up to 64 KB), and 127 for massive 64-bit payloads.
4-Byte XOR Key
Client frames append a cryptographic 32-bit (4-byte) random value immediately before payload bytes. Payload bytes are transformed cyclically across this 4-byte key using symmetric bitwise XOR operations.
Wire Diagram: RFC 6455 Binary Frame Structure
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-------+-+-------------+-------------------------------+ |F|R|R|R| opcode|M| Payload len | Extended payload length | |I|S|S|S| (4) |A| (7) | (16/64) | |N|V|V|V| |S| | (if payload len==126/127) | | |1|2|3| |K| | | +-+-+-+-+-------+-+-------------+ - - - - - - - - - - - - - - - + | Extended payload length continued, if payload len == 127 | + - - - - - - - - - - - - - - - +-------------------------------+ | |Masking-key, if MASK set to 1 | +-------------------------------+-------------------------------+ | Masking-key (continued) | Payload Data | +-------------------------------- - - - - - - - - - - - - - - - + : Payload Data continued ... : +---------------------------------------------------------------+
WebSocket Opcodes & Frame Categories Comparison
RFC 6455 categorizes frames strictly into Data Frames and Control Frames. Control frames manage state transitions and connection health, and are constrained by strict payload limitations:
| Opcode | Designation | Frame Class | Max Payload Size | Can Fragment? | Intermediary Injection |
|---|---|---|---|---|---|
| 0x0 | Continuation | Data Stream | Up to 64-bit max | Yes | Sequenced after 0x1 or 0x2 |
| 0x1 | Text Frame | Data Stream | Up to 64-bit max | Yes (FIN=0) | Must contain valid UTF-8 |
| 0x2 | Binary Frame | Data Stream | Up to 64-bit max | Yes (FIN=0) | Arbitrary byte buffer |
| 0x8 | Connection Close | Control | ≤ 125 Bytes | No (Must FIN=1) | Carries 2-byte status code |
| 0x9 | Ping (Heartbeat) | Control | ≤ 125 Bytes | No (Must FIN=1) | Can interrupt data sequences |
| 0xA | Pong (Echo) | Control | ≤ 125 Bytes | No (Must FIN=1) | Must mirror Ping payload |
The Security Mechanics of Client Masking & Cache Poisoning
A frequent question among network engineers is why the WebSocket specification mandates client frame masking if the 4-byte key is transmitted in plain text alongside the payload without asymmetric encryption or secrecy:
The Threat: Transparent Proxy Poisoning
When WebSockets emerged, millions of corporate firewalls and ISP proxies parsed traffic transparently without recognizing the `101 Switching Protocols` handshake. If an attacker ran malicious client-side JavaScript on a browser, they could craft an unmasked WebSocket frame containing an identical raw HTTP request:
GET /script.js HTTP/1.1\r\nHost: victim-corp.com
The blind proxy, misinterpreting the TCP payload as a new HTTP request, would fetch the attacker's script and cache it for every internal corporate user across the enterprise network.
The Defense: 4-Byte Entropy Scrambling
By mandating that browser user agents generate a cryptographically unpredictable 4-byte key for every single outbound frame and XOR the payload against it, the data transmitted across the wire is completely randomized entropy.
Because the attacker in JavaScript cannot predict the browser's internal random masking key before the frame hits the wire, they cannot construct a pre-xor'd payload that would decrypt into a recognizable HTTP verb inside intermediary caches.
Algorithmic Reference: High-Performance Buffer Masking
In production socket gateways, XOR unmasking represents a significant CPU hotspot under high packet volume. Below is the optimized 32-bit word transformation pattern commonly implemented in native engines:
Frequently Asked Questions (FAQ)
Why does the WebSocket RFC 6455 protocol require client masking keys?
WebSocket client-to-server frames must be masked with a random 4-byte key to prevent cache poisoning attacks on intermediary HTTP transparent proxies. If frames were unmasked, a malicious browser script could inject valid HTTP GET requests into the raw TCP stream, causing misconfigured proxies to poison cache entries for other users.
How is the 4-byte XOR masking algorithm applied to frame payloads?
The masking algorithm calculates transformed byte j as: transformed_byte[j] = original_byte[j] XOR mask_key[j % 4]. Because XOR is a symmetric, self-inverting mathematical operation, applying the exact same XOR formula with the same 4-byte key unmasks the payload back to its original plain text or binary buffer without performance penalty.
What do the FIN bit and RSV1-3 reservation flags signify?
The FIN (Final) bit indicates whether the current frame is the terminal segment of a message. A FIN value of 1 marks the final frame, while 0 indicates subsequent continuation frames (opcode 0x0) will follow. RSV1, RSV2, and RSV3 flags are reserved for protocol extensions, such as per-message deflate compression (RFC 7692), and must otherwise evaluate to 0.
How does WebSocket handle extended payload lengths beyond 125 bytes?
Byte 1 uses a 7-bit length indicator. If the payload length is between 0 and 125 bytes, that value is used directly. If the length is 126, the following 2 bytes (16-bit unsigned integer) define lengths up to 65,535 bytes. If the length is 127, the following 8 bytes (64-bit unsigned integer) define massive frames up to 9 quintillion bytes.
Why must servers never mask frames sent to clients?
RFC 6455 explicitly forbids servers from masking frames transmitted downstream to clients. Clients cannot initiate cache poisoning attacks against upstream proxies with incoming server traffic. In fact, if a client receives a masked frame from a WebSocket server, RFC 6455 mandates that the client must immediately terminate the connection.
Does this tool process binary network payloads entirely within the browser?
Yes. All bitwise operations, UTF-8 string decodings, and 4-byte XOR transformations execute locally inside client-side JavaScript. No packet captures, security tokens, or network payload bytes are ever transmitted across external servers.
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