The default methods for OpenPGP signing and one pass sign and encrypt provided byDidiSoft OpenPGP Library for Java and OpenPGP Library for .NET produce version 4 OpenPGP digital signatures (for more information, please refer to the OpenPGP specification RFC 4880 ).
Unfortunately PGP version 6.5 and below recognize only version 3 digital signatures. In order to solve this issue, please use the provided for backward compatibility with PGP 6.5 methods:
One pass sign and encrypt
These modified versions of the sign and encrypt methods do not have the last parameter of the standard versions, which specifies if integrity check information will be appended to the encrypted content. This is because in PGP 6.5 and below this feature is not supported.