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Decode a Websocket mask

0

I'm trying to write a function to decode a masked buffer sent to a node server via a WebSocket connection. Using a combination of RFC6455 and a few old SO answers, this is what I've managed so far:

function decodeMessage(buffer) 

 const opCode = buffer.readUInt8(0) & 0xF;

 if(opCode === 0x1) 
 const data_length = buffer.readUInt8(1) & 0x7F;
 let decoded = Buffer.alloc(data_length);

 const mask_key = buffer.readUInt32BE(2);

 for (let i = 0; i < data_length; i++) 
 decoded[i] = buffer.readUInt8(i) ^ mask_key[i % 4];
 

 buffer.copy(decoded, 0, 2);

 return decoded.toString('utf8');

 else 
 return null;
 

There's two issues with this code:

• It doesn't unmask the message correctly and just outputs garbled Unicode




• It outputs a different decoded message every time, even if the input is constant

(The payload length will never be longer than 126, so it doesn't need to worry about handling additional bytes)

javascript node.js websocket

share|improve this question

edited Mar 7 at 18:10

Jacob Barrow

asked Mar 7 at 10:35

Jacob BarrowJacob Barrow

1341316

add a comment | 

0

I'm trying to write a function to decode a masked buffer sent to a node server via a WebSocket connection. Using a combination of RFC6455 and a few old SO answers, this is what I've managed so far:

function decodeMessage(buffer) 

 const opCode = buffer.readUInt8(0) & 0xF;

 if(opCode === 0x1) 
 const data_length = buffer.readUInt8(1) & 0x7F;
 let decoded = Buffer.alloc(data_length);

 const mask_key = buffer.readUInt32BE(2);

 for (let i = 0; i < data_length; i++) 
 decoded[i] = buffer.readUInt8(i) ^ mask_key[i % 4];
 

 buffer.copy(decoded, 0, 2);

 return decoded.toString('utf8');

 else 
 return null;
 

There's two issues with this code:

• It doesn't unmask the message correctly and just outputs garbled Unicode




• It outputs a different decoded message every time, even if the input is constant

(The payload length will never be longer than 126, so it doesn't need to worry about handling additional bytes)

javascript node.js websocket

share|improve this question

edited Mar 7 at 18:10

Jacob Barrow

asked Mar 7 at 10:35

Jacob BarrowJacob Barrow

1341316

add a comment | 

I'm trying to write a function to decode a masked buffer sent to a node server via a WebSocket connection. Using a combination of RFC6455 and a few old SO answers, this is what I've managed so far:

function decodeMessage(buffer) 

 const opCode = buffer.readUInt8(0) & 0xF;

 if(opCode === 0x1) 
 const data_length = buffer.readUInt8(1) & 0x7F;
 let decoded = Buffer.alloc(data_length);

 const mask_key = buffer.readUInt32BE(2);

 for (let i = 0; i < data_length; i++) 
 decoded[i] = buffer.readUInt8(i) ^ mask_key[i % 4];
 

 buffer.copy(decoded, 0, 2);

 return decoded.toString('utf8');

 else 
 return null;
 

There's two issues with this code:

• It doesn't unmask the message correctly and just outputs garbled Unicode




• It outputs a different decoded message every time, even if the input is constant

(The payload length will never be longer than 126, so it doesn't need to worry about handling additional bytes)

javascript node.js websocket

share|improve this question

edited Mar 7 at 18:10

Jacob Barrow

asked Mar 7 at 10:35

Jacob BarrowJacob Barrow

1341316

function decodeMessage(buffer) 

 const opCode = buffer.readUInt8(0) & 0xF;

 if(opCode === 0x1) 
 const data_length = buffer.readUInt8(1) & 0x7F;
 let decoded = Buffer.alloc(data_length);

 const mask_key = buffer.readUInt32BE(2);

 for (let i = 0; i < data_length; i++) 
 decoded[i] = buffer.readUInt8(i) ^ mask_key[i % 4];
 

 buffer.copy(decoded, 0, 2);

 return decoded.toString('utf8');

 else 
 return null;
 

share|improve this question

edited Mar 7 at 18:10

Jacob Barrow

asked Mar 7 at 10:35

Jacob BarrowJacob Barrow

1341316

share|improve this question

edited Mar 7 at 18:10

Jacob Barrow

asked Mar 7 at 10:35

Jacob BarrowJacob Barrow

1341316

asked Mar 7 at 10:35

Jacob BarrowJacob Barrow

1341316

asked Mar 7 at 10:35

Jacob BarrowJacob Barrow

1341316

1 Answer
1

active

oldest

votes

0

By modifying a function from this post, I was able to get this working:

function decodeMessage(buffer) 

 if((buffer.readUInt8(0) & 0xF) === 0x1) 
 const length = (buffer.readUInt8(1) & 0x7F)+4;

 let currentOffset = 2;
 const mask_key = buffer.readUInt32BE(2);

 const data = Buffer.alloc(length);

 for (let i = 0, j = 0; i < length; ++i, j = i % 4) 
 const shift = j === 3 ? 0 : (3 - j) << 3;

 const mask = (shift === 0 ? mask_key : (mask_key >>> shift)) & 0xFF;
 const source = buffer.readUInt8(currentOffset++);
 data.writeUInt8(mask ^ source, i);
 

 return data.toString('utf8');

 else 
 return null;
 

share|improve this answer

answered Mar 8 at 16:06

Jacob BarrowJacob Barrow

1341316

add a comment | 

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0

By modifying a function from this post, I was able to get this working:

function decodeMessage(buffer) 

 if((buffer.readUInt8(0) & 0xF) === 0x1) 
 const length = (buffer.readUInt8(1) & 0x7F)+4;

 let currentOffset = 2;
 const mask_key = buffer.readUInt32BE(2);

 const data = Buffer.alloc(length);

 for (let i = 0, j = 0; i < length; ++i, j = i % 4) 
 const shift = j === 3 ? 0 : (3 - j) << 3;

 const mask = (shift === 0 ? mask_key : (mask_key >>> shift)) & 0xFF;
 const source = buffer.readUInt8(currentOffset++);
 data.writeUInt8(mask ^ source, i);
 

 return data.toString('utf8');

 else 
 return null;
 

share|improve this answer

answered Mar 8 at 16:06

Jacob BarrowJacob Barrow

1341316

add a comment | 

0

By modifying a function from this post, I was able to get this working:

function decodeMessage(buffer) 

 if((buffer.readUInt8(0) & 0xF) === 0x1) 
 const length = (buffer.readUInt8(1) & 0x7F)+4;

 let currentOffset = 2;
 const mask_key = buffer.readUInt32BE(2);

 const data = Buffer.alloc(length);

 for (let i = 0, j = 0; i < length; ++i, j = i % 4) 
 const shift = j === 3 ? 0 : (3 - j) << 3;

 const mask = (shift === 0 ? mask_key : (mask_key >>> shift)) & 0xFF;
 const source = buffer.readUInt8(currentOffset++);
 data.writeUInt8(mask ^ source, i);
 

 return data.toString('utf8');

 else 
 return null;
 

share|improve this answer

answered Mar 8 at 16:06

Jacob BarrowJacob Barrow

1341316

add a comment | 

By modifying a function from this post, I was able to get this working:

function decodeMessage(buffer) 

 if((buffer.readUInt8(0) & 0xF) === 0x1) 
 const length = (buffer.readUInt8(1) & 0x7F)+4;

 let currentOffset = 2;
 const mask_key = buffer.readUInt32BE(2);

 const data = Buffer.alloc(length);

 for (let i = 0, j = 0; i < length; ++i, j = i % 4) 
 const shift = j === 3 ? 0 : (3 - j) << 3;

 const mask = (shift === 0 ? mask_key : (mask_key >>> shift)) & 0xFF;
 const source = buffer.readUInt8(currentOffset++);
 data.writeUInt8(mask ^ source, i);
 

 return data.toString('utf8');

 else 
 return null;
 

share|improve this answer

answered Mar 8 at 16:06

Jacob BarrowJacob Barrow

1341316

function decodeMessage(buffer) 

 if((buffer.readUInt8(0) & 0xF) === 0x1) 
 const length = (buffer.readUInt8(1) & 0x7F)+4;

 let currentOffset = 2;
 const mask_key = buffer.readUInt32BE(2);

 const data = Buffer.alloc(length);

 for (let i = 0, j = 0; i < length; ++i, j = i % 4) 
 const shift = j === 3 ? 0 : (3 - j) << 3;

 const mask = (shift === 0 ? mask_key : (mask_key >>> shift)) & 0xFF;
 const source = buffer.readUInt8(currentOffset++);
 data.writeUInt8(mask ^ source, i);
 

 return data.toString('utf8');

 else 
 return null;
 

share|improve this answer

answered Mar 8 at 16:06

Jacob BarrowJacob Barrow

1341316

answered Mar 8 at 16:06

Jacob BarrowJacob Barrow

1341316

answered Mar 8 at 16:06

Jacob BarrowJacob Barrow

1341316