.net - Decrypting PKCS#7 encrypted data in C#

Question

I am working on a solution that needs to decrypt PKCS#7 encrypted data, preferably in C#. As far as I can see, the .NET api has support for this through the System.Security.Cryptography.Pkcs namespace. However it seems that the implementation can only work on byte arrays. So what do I do when I have a large encrypted file that does not fit into the memory?

Am I missing something here, or is there another way to do this on a Stream level instead of using bytearrays?

Answer 1

The .NET framework has only rudimentary support for PKCS#7/CMS, so it does not support streaming the data.

You need a 3rd party library. As others have mentioned, BouncyCastle is an excellent choice. It does support PKCS#7 (PKCS#7 is also known as CMS).

Their tests are probably a good place to look for sample code: EnvelopedDataStreamTest.cs.