The "reserved" filed is written fter the version number major,
minor which is used to identify the encryption as agile. The
"reserved" field must always have the value 0x00000040. This
change writes the reserved filed correctly and when encryption and
when decrypting it checks the value an potentially bails out if
it desn't contain the expected value.
Reviewed-by: Tomaž Vajngerl <email@example.com>