Yes, this is possible using the following code:

C#:

	// load the content of a file whose signature we wish to check
	byte[] content = File.ReadAllBytes(@"c:\temp\file.pdf");
	ContentInfo contentInfo = new ContentInfo(content);

	// create a PKCS #7 SignedData object base on this,
	// the second argument specifies "detached" style
	SignedData p7s = new SignedData(contentInfo, true);

	// load the detached signature file
	using (Stream input = File.OpenRead(@"c:\temp\file.p7s"))
	{
		p7s.Load(input);
	}

	// validate the signature and the certificates
	SignatureValidationResult result = p7s.Validate();

	// display the validation result
	if (result.Valid)
	{
		Console.WriteLine("Both signature and certificates are valid.");
	}
	else
	{
		Console.WriteLine("Signature or certificates are NOT valid.");
		Console.WriteLine("Signature problems: {0}", result.Status);
		Console.WriteLine("Certificate problems: {0}", result.CertificateValidationStatus);
	}

In addition to Rebex.Security.Cryptography.Pkcs namespace, this also needs Rebex.Security.Certificates from the same DLL.

For completeness, this is how you would create a detached signature (the file.p7s file) for file.pdf:

C#:

	// get a certificate with a private key
	Certificate certificate = ...

	// load the content of a file we need to sign
	byte[] content = File.ReadAllBytes(@"c:\temp\file.pdf");
	ContentInfo contentInfo = new ContentInfo(content);

	// create a PKCS #7 SignedDat object based on this,
	// the second argument specifies "detached" style
	SignedData p7s = new SignedData(contentInfo, true);

	// add signers
	SignerInfo signer = new SignerInfo(certificate);
	p7s.SignerInfos.Add(signer);

	// create the signature
	p7s.Sign();

	// save the signature into a detached signature file
	using (Stream output = File.Create(@"c:\temp\file.p7s"))
	{
		p7s.Save(output);
	}

If you would like a VB.NET version of this code, just let me know!