Uninformed: Informative Information for the Uninformed

Vol 8» 2007.Sep

Choose a Bit-pad Offset

To help protect against some forms of statistical analysis that have proved effective against XOR obfuscation using repeated static keying information, it was decided against beginning every XOR loop at the same position within keyhash. To avoid this, a new offset into keyhash for each message must be chosen. The method that the SteganRTP reference implementation employs to determine this offset is to use the hashword[26] function to create a 32-bit hash of keyhash and the sum of the RTP packet being embedded into's Seq and Timestamp header fields. The resultant hash is then interpreted as a 32-bit integer. The integer modulus 20 is the chosen offset into keyhash.

The integer which is the result of the offset choosing operation and is within the range of 0 through 19 is defined here as keyhash_offset and described by Equation [*] below.

$\displaystyle keyhash\_offset = hashword( keyhash, ( RTP\_Seq + RTP\_TS ) ) \bmod 20$ (3.3)

The keyhash_offset equation incorporates keyhash so as to not be entirely computable from observable information in the RTP packet header.