This package is unmaintained. I plan to remove it soon. If someone like it, please fork.
This package was originally provided by mentalis.org.
The maintainer of the project on github.com is greubelm, and the author has no participation on mentalis.org nor vice versa.
Here is the origin text from mentalis.org about the description of the package:
The cryptography classes of the .NET framework lack one crucial class: an implementation of the Diffie-Hellman key agreement algorithm. To fill this gap, we created a completely managed Diffie-Hellman implementation, based on mono's BigInteger class.