x509Certificate - Save a binary signature in pkcs#7 format (.net c#)

Question

I am signing some data with a self signed X509Certificate in c#. Signature results as a binary byte[]. I want to save this signature to a pkcs#7 format file with extenstion .p7b. When I save this using FileStream as required. It generates an invalid p7b file.

Can anyone help save this signature as a valid external file? Or point out if there is something wrong with the approach?

// EDIT: Adding the code as asked by Eugene Mayevski

// Open Store Location & fetch certificate
X509Store store = new X509Store(StoreName.My, StoreLocation.CurrentUser);
store.Open(OpenFlags.ReadWrite);
X509Certificate2Collection certifcates = store.Certificates;
X509Certificate2 certificate = certifcates[0];`

// fetch private key
string publicKeyString = certificate.GetPublicKeyString();
RSACryptoServiceProvider privateKey = certificate.PrivateKey as RSACryptoServiceProvider;

// get binary of data & sign it.
byte[] buffer = Encoding.Default.GetBytes("Sample data to sign. Although it would be a document.");
byte[] signature = privateKey.SignData(buffer, new SHA1Managed());

Answer 1

You are not generating a Cryptographic Message Syntax (CMS) message format which is defined in PKCS#7. You generate just the signature, which can be placed in the container format.

To create a CMS formatted message yourself, please take a look at the Microsoft documentation or use the C# version of Bouncy Castle.