Hello David. I tell you: the standard defines only two algorithms in order to decrypt pdf streams and strings.
In the GNU PDF Library we will want to provide the possibility of using more advanced encryption algorithms from libgcrypt. On the one hand we must implement the AES algorithm, just like it already is in the pdf knowledge database [1]. Do libgcrypt implment the AES algorithm? If not we may need to write it ourselves. On the other hand, the second algorithm is known as RC4, which unfortunately is, according to the reference, a proprietary algorithm copyrighted by RSA Security, Inf. I hope it doesn't arise any legal impediment. We discussed the legal issue of RC4 some time ago and the conclusion was that it is not a problem to implement the 4arcs algorithm. Finally we will need md5 message-digest functions too. I am not sure but I think libcrypt doesn't provide this. Have we work to do here? Could you please check?
