Unfortunately there is a weird bug in BouncyCastle C# version 2.5 that prevents session keys based on Elliptic Curve Curve-25519 in OpenPGP messages to be decrypted. Trying to decrypt the session key an InvalidCypherTextException is thrown.
We have released version 1.9.5 of OpenPGP Library for .NET based on BouncyCastle for C# version 2.5 and this bug leaked in our code.
If you have upgraded to version 1.9.5 of our library, then please upgrade to 1.9.5.23 where this is fixed (there instead of using the BouncyCastle Curve-25519 implementation for decrypting, we have reverted to our old code from version 1.9.4 which is based on NaCl
