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brook-server-protocol.md

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brook server protocol

Terminology

  • DST Address: The address that the application actually wants to request, address contains IP/domain and port

    ATYP + IP/Domain + PORT
    • ATYP: 1 byte
      • 0x01: IPv4
      • 0x03: Domain
      • 0x04: IPv6
    • IP/Domain: 4/n/16 bytes
      • If ATYP is 0x01, then this is IPv4, 4 bytes
      • If ATYP is 0x03, then this is domain, n bytes, and the first byte is the domain length
      • If ATYP is 0x04, then this is IPv6, 16 bytes
    • Port: 2 bytes
      • Big Endian 16-bit unsigned integer

  • KEY: AES key, 32 bytes

    • KEY: HKDF_SHA256(Password, Nonce, Info)
      • Password: User-defined password
      • Nonce: 12 bytes
      • Info: [0x62, 0x72, 0x6f, 0x6f, 0x6b]. Note that this can be overwrite by brook link --clientHKDFInfo and brook link --serverHKDFInfo

  • HKDF: Defined in RFC 5869

  • SHA256: Defined in FIPS 180-4

  • AES: Defined in U.S. Federal Information Processing Standards Publication 197

  • AES-GCM: Defined in RFC 5246, 5869

Client --TCP--> Server

Client Nonce + [AES_GCM(Fragment Length) + AES_GCM(Fragment)]...

The maximum length of AES_GCM(Fragment Length) + AES_GCM(Fragment) is 2048 bytes

  • Client Nonce: 12 bytes, randomly generated
    • The nonce should be recalculated when it is not used for the first time, the calculation method: add 1 to the first 8 bytes according to the Little Endian 64-bit unsigned integer
  • Fragment Length: Big Endian 16-bit unsigned integer
  • Fragment: Actual data being proxied
    • The first Fragment should be: 
      Unix Timestamp + DST Address
      • Unix Timestamp: If it is not even, it should be increased by 1. Big Endian 32-bit unsigned integer

Server --TCP--> Client

Server Nonce + [AES_GCM(Fragment Length) + AES_GCM(Fragment)]...

The maximum length of AES_GCM(Fragment Length) + AES_GCM(Fragment) is 2048 bytes

  • Server Nonce: 12 bytes, randomly generated
    • The nonce should be recalculated when it is not used for the first time, the calculation method: add 1 to the first 8 bytes according to the Little Endian 64-bit unsigned integer
  • Fragment Length: Big Endian 16-bit unsigned integer
  • Fragment: Actual data being proxied

Client --UDP--> Server

Client Nonce + AES_GCM(Fragment)

The maximum length of Client Nonce + AES_GCM(Fragment) is 65507 bytes

  • Client Nonce: 12 bytes, randomly generated each time
  • Fragment: 
    Unix Timestamp + DST Address + Data
    • Unix Timestamp: Big Endian 32-bit unsigned integer
    • Data: Actual data being proxied

Server --UDP--> Client

Server Nonce + AES_GCM(Fragment)

The maximum length of Server Nonce + AES_GCM(Fragment) is 65507 bytes

  • Server Nonce: 12 bytes, randomly generated each time
  • Fragment: 
    DST Address + Data
    • Data: Actual data being proxied

Client --UDP(UDP over TCP)--> Server

Client Nonce + [AES_GCM(Fragment Length) + AES_GCM(Fragment)]...

The maximum length of AES_GCM(Fragment Length) + AES_GCM(Fragment) is 65507 bytes, but the maximum length if the first one is 2048 bytes

  • Client Nonce: 12 bytes, randomly generated
    • The nonce should be recalculated when it is not used for the first time, the calculation method: add 1 to the first 8 bytes according to the Little Endian 64-bit unsigned integer
  • Fragment Length: Big Endian 16-bit unsigned integer
  • Fragment: Actual data being proxied
    • The first Fragment should be: 
      Unix Timestamp + DST Address
      • Unix Timestamp: If it is not odd, it should be increased by 1. Big Endian 32-bit unsigned integer

Server --UDP(UDP over TCP)--> Client

Server Nonce + [AES_GCM(Fragment Length) + AES_GCM(Fragment)]...

The maximum length of AES_GCM(Fragment Length) + AES_GCM(Fragment) is 65507 bytes

  • Server Nonce: 12 bytes, randomly generated
    • The nonce should be recalculated when it is not used for the first time, the calculation method: add 1 to the first 8 bytes according to the Little Endian 64-bit unsigned integer
  • Fragment Length: Big Endian 16-bit unsigned integer
  • Fragment: Actual data being proxied