Supporting Encryption in Postgresql

For our research project, I need to implement an encryption support for 
Postgressql. At this current phase, I need to at least support page 
level  encryption In other words, each page that belongs to a certain 
sensitive table will be stored encrypted on the harddisk.
Since we are trying to have a new design that can start the decryption 
before even we see the data, I need to have some kind of thread support.

My questions are in order to support page level encryption(i,e encrypt 
each page before writing back to disk and decrypt each page after we 
read from disk.) which parts of the code should be changed?
Our more simply, is /src/backend/storage/smgr/md.c the only code
that does the disk access?

Since our design requires thread support (we will do some of the 
decryption, before we see the data, therefore during disk access, we 
need to continue decryption) Can you suggest me a good thread lib you 
think will work fine with postgresql ?

Thanks for your help.
                                     Murat Kantarcioglu, PhD Candidate,                                     Computer
ScienceDepartment,                                     Purdue University