User:Master-bob/MACS

The first step of Xbox Live authentication is connecting to the Machine Account Creation Service, located at MACS.XBOXLIVE.COM.

The Xbox would only connect to MACS once, the first time the console ever connected to Xbox Live. MACS was presumably used to keep banned consoles off of Xbox Live, and is still used to this day for Xbox 360s.

The MACS exchange, like the majority of Xbox Live authentication, is done using the Kerberos system.

AS-REQ

The first step of MACS authentication is sending an AS-REQ to MACS.XBOXLIVE.COM on UDP port 88. The AS-REQ is composed of several components.

• PVNO and MSG TYPE - These have default values of 5 (Kerberos version 5) and 10 (krb-as-req), respectively.

PADATA

Pre authentication data (PADATA) contains most of what makes this request unique from other Kerberos systems.

PA-ENC-TIMESTAMP

PA-DATA TYPE 2

An official part of the Kerberos standard, this is a Unix timestamp encrypted with the later defined encryption standard (in Xbox's case, RC4-HMAC-MD5). The server attempts to decrypt this to confirm that the client is using the correct password. For MACS, the password is the console's Online Key, which Microsoft stored server side on a very well guarded database.

Online Key

The online key is stored in an encrypted form on the Xbox's EEPROM. The key is decrypted using:

   // decrypt the EEPROM to get the correct hdd key
   rc4_key = HMAC-SHA1 of the hdd_key using a magical key
   decrypted_online_key = rc4 of online_key using rc4_key

PA-PAC-REQUEST-EX

PA-DATA TYPE 131

This is an Xbox modification to a custom Microsoft (PA-PAC-REQUEST) addition to Kerberos, used to define what the client expects in the response. This is encoded in ASN.1 (method of packing data structures into bytes, used by Kerberos). All AS-REQs contain the following data in this section:

   SEQUENCE (2 elem)
       [0] (1 elem)
           BOOLEAN true
       [1] (1 elem)
           SEQUENCE (2 elem)
               INTEGER 13
               INTEGER 14

13 and 14 correlate to PAC_CLIENT_IDENTITY and PAC_COMPOUND_IDENTITY, respectively, and are used to tell the server to include a PUID PAC (20) in the response.

PA-XBOX-PRE-PRE-AUTH

PA-DATA TYPE 204

This is a custom preauthentication header that was used by Microsoft to quickly look up Xbox information from their database and is probably not needed. Contains the following:

• currentTime - the current time, sent as a FILETIME
• PPA1 - SHA1-HMAC of the principal name (the Xbox serial number) using the principal key (the online key)
• SPPA2atT - computed as follows
  • SPPA1 - SHA1(PPA1)
  • PPA2 - First 8 bytes of SHA1-HMAC of PrincipalName appended to PrincipalName using the principal key
  • SPPA2atT - SHA1 of PPA2 appended to the current time

PA-XBOX-CLIENT-VERSION

PA-DATA TYPE 206

This is a custom preauthentication header and arguably the most important for MACS authentication from a homebrew perspective. Contains the following:

• signature - a 20 byte array used to verify the encryption key generated for the response
• version - null terminated string with information about the application sending the request
  • XboxVersion=1.00.5849.3 Title=0xFFFE0000 TitleVersion=408857856 - Xbox Live Dashboard 5849

REQ-BODY

The remainder of the Kerberos ticket. Bold indicates the data are used for response building.

• Padding - 0
• KDC-Options - 0b00010000 (canonicalize)
• cname - the serial number of the Xbox
• realm - MACS.XBOX.COM
• sname - krbtgt@MACS.XBOX.COM
• till - 2037-09-13 02:48:05 (UTC)
• nonce - a randomly generated number
• etype - RC4-HMAC-MD5

Building the Response

The server must generate a valid machine account and encrypt it using the correct key.

The Machine Account

The following is how the machine account is built for transfer. It is stored in a different fashion.

   ULONGLONG userID;
   CHAR gamertag[16];  // SN.XXXXXXXXXXXX (the console serial number)
   CHAR domain[20];    // xbox.com
   CHAR realm[24];     // passport.net
   BYTE key[16];

Nonce HMAC Key

The nonce HMAC key, used to encrypt the machine account, is computed as follows:

   temp_key = MD5-HMAC of the null terminated string "signaturekey" using the online key
   salted_nonce = MD5 of "02 04 00 00 00 00 00 00"
   nonce_hmac_key = MD5-HMAC of salted_nonce using temp_key

The key can be verified using the PA-XBOX-CLIENT-VERSION preauth:

   test_signature = SHA1-HMAC of the version_string using the nonce_hmac_key
   // if both signatures match, the key was generated correctly

Encryption

The machine account is encrypted with the nonce HMAC key and salt 1203 using RC4-HMAC-MD5. The encrypted bytes are packed as a Kerberos EncrpytedData object into a PA-DATA TYPE 203 preauthentication header in the AS-REP.