Editing WoW:Battle.net Mobile Authenticator Specification (section)

==Code Calculation==
Starting point of the calculation are the secret key (returned by the server within the authenticator initialization response) and the current code interval number. Code interval length is 30 seconds (every 30 seconds a new code is generated). To calculate the current code interval number the current server time in milliseconds (as returned from the time synchronization request) has to be known and is simply divided by 30,000 (midnight, January 1, 1970 UTC started with code interval 0 and every 30 seconds the code interval counter is incremented by 1).

If there is a local clock with enough accuracy available, it is not necessary to request the server time for every code calculation. It is enough to store the time difference between server and local clock when doing the initialization of the authenticator and calculate the current server time from the local time and the stored time difference. Only if the divergence between server and local clock has significantly changed, a new synchronization has to be done.

The code interval number is stored in 8 bytes (big endian order) and is used as message for HMAC-SHA1 (the key is the secret key from the initialization response). The result is a 20 bytes MAC of the code interval number. From this 20 bytes MAC 4 bytes are selected as the current code. The last 4 bit of the MAC determine the starting byte of the 4 selected bytes.

Finally the last 8 digits (radix 10) of the 4 selected bytes (big endian order) are displayed by the authenticator as the current code.

'''Short form:'''

 // calculate current interval number
 long intervalNumber = (CLIENT_TIME_IN_MILLISECONDS + TIME_DIFFERENCE_TO_SERVER) / 30000
 
 // calculate HMAC-SHA1 from secret key and interval number
 byte[20] mac = HMAC-SHA1(SECRET_KEY, intervalNumber)
 
 // determine which 4 bytes of the MAC are taken as the current code
 // last 4 bit of the MAC points to the starting byte
 int startPos = mac[19] & 0x0F
 
 // select the byte at starting position and the following 3 bytes
 int selectedInt = mac[startPos .. startPos + 3]
 
 // use the lowest 8 decimal digits from the selected integer as the
 // current authenticator code
 return selectedInt % 100000000