Underfire..why do you need encryption when-
1- The message length is 0.0002 seconds long and
2- It contains a unique 23bit identifier.
That 23bit identifier is worldwide unique. To be even seen by TAAATS your identifier has to be coded into the system...and this has exactly what to do with exemptions from mandatory carriage in Australia ABV FL290?
EDIT- Underfire, all the docs attributed, on your link, are dated around 2006...pretty up to date
