One potential approach might be using Java encryption software
like http://www.cryptix.org or http://jcewww.iaik.tu-graz.ac.at
as Java programs have a high chance to work on the 3000 and other
platforms (like NT) without porting or recompilation.

If you have the HP Web Wise secure web server for MPE/iX you might
also consider trying to use the OpenSSL program for encrypting or
decrypting files. According to the www.openssl.org documentation,
this might work. (If you don't own HP Web Wise, you might still try
porting the OpenSSL program yourself).

Haven't tried either of the above ideas myself so far.

Lars.