Unofficial Steam Friends Protocol Specification

Introduction

Steam Friends is an instant messaging protocol that is built into Steam, a game content delivery system developed by Valve. This page details the protocol specification for Steam Friends as was ascertained though reverse-engineering.

Connection

Steam Friends uses UDP on port 27017. Connections have been made to the following servers:

• 68.142.64.165
• 68.142.64.164

Layers

On top of UDP, another layer is implemented to keep track of sequencing and splitting of packets. This layers prepends every packet with the following 36-byte structure:

Offset	0	4	8	C
00:	"VS01"	type	destination	source
10:	sequence #	last rcv. #	split count	seq. # of 1st packet
20:	data length

Note: These values are little-endian, which is not network order.

Field	Type	Length (bytes)	Description
"VS01"	string (non-null terminated)	4	the 4 characters 'V', 'S', '0', '1'
type	bit-field	4	a bit field describing the type of packet (needs further analysis)
destination	integer	4	the destination ID of the packet
source	integer	4	the source ID of the packet
sequence #	integer	4	the packet's sequence number. server and client keep track of own numbers
last recv. #	integer	4	the sequence number of the last packet received
split count	integer	4	the number of packets the current message was split in to
seq. # of 1st packet	integer	4	the sequence number of the first packet for current message
data length	integer	4	the length of the data that follows

Login sequence

1. Client initiates the login by sending a type '0x00020004' packet to the server with no data, and sequence # 1

   00:   56 53 30 31 00 00 01 04 00 02 00 00 00 00 00 00
   10:   01 00 00 00 00 00 00 00 01 00 00 00 01 00 00 00
   20:   00 00 00 00
   

2. Server responds with type '0x04020004' (first one ORed with 0x04000000) with 4 bytes of data attached (although the data length field is 0), sequence: 1, last: 1, src: 0x00000200

   00:   56 53 30 31 04 00 02 00 00 00 00 00 00 02 00 00
   10:   01 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00
   20:   00 00 00 00 f9 8d 22 25
   

3. Client responds with type '0x04030004' (previous ORed with 0x00010000) with 4 bytes of data attached (with 4 as data size). That 4 bytes corresponds to the 4 bytes that the server sent XORed with 0xA426DF2B. sequence: 2, last: 1, dest: 0x00000200

   00:   56 53 30 31 04 00 03 04 00 02 00 00 00 00 00 00
   10:   02 00 00 00 01 00 00 00 01 00 00 00 02 00 00 00
   20:   04 00 00 00 d2 52 04 81
   

4. Server responds with type '0x04040000' (possibly as some sort of ACK). sequence: 2, response: 2, dest: this becomes the source for all subsequent packets from client

   00:   56 53 30 31 00 00 04 04 00 34 7e fe 00 02 00 00
   10:   02 00 00 00 02 00 00 00 01 00 00 00 02 00 00 00
   20:   00 00 00 00
   

5. Server sends type '0x0406001c' with the following data stream: "17 05 00 00 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff 01 00 00 00 01 00 00 00". sequence: 3, last: 2

   00:   56 53 30 31 1c 00 06 04 00 34 7e fe 00 02 00 00
   10:   03 00 00 00 02 00 00 00 01 00 00 00 03 00 00 00
   20:   1c 00 00 00 17 05 00 00 ff ff ff ff ff ff ff ff
   30:   ff ff ff ff ff ff ff ff 01 00 00 00 01 00 00 00
   

6. Client responds type '0x040600a4' with the following data stream: "18 05 00 00 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff 01 00 00 00 80 00 00 00 [128 byte random string (1024-bit encryption key, RSA?)] [CRC32 of preceding 128 bytes] 00 00 00 00".

   00:   56 53 30 31 a4 00 06 04 00 02 00 00 00 34 7e fe
   10:   03 00 00 00 03 00 00 00 01 00 00 00 03 00 00 00
   20:   a4 00 00 00 18 05 00 00 ff ff ff ff ff ff ff ff
   30:   ff ff ff ff ff ff ff ff 01 00 00 00 80 00 00 00
   40:   xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
   50:   xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
   60:   xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
   70:   xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
   80:   xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
   90:   xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
   a0:   xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
   b0:   xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
   c0:   xx xx xx xx 00 00 00 00
   

7. Server responds with type '0x04060018' with the following data: "19 05 00 00 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff 01 00 00 00"

   00:   56 53 30 31 18 00 06 04 00 34 7e fe 00 02 00 00
   10:   04 00 00 00 03 00 00 00 01 00 00 00 04 00 00 00
   20:   18 00 00 00 19 05 00 00 ff ff ff ff ff ff ff ff
   30:   ff ff ff ff ff ff ff ff 01 00 00 00
   

Encryption