[dev-crypto-csharp] Question about how to export certificates for S/MIME

[Jeff Stedfast]

Hey all,

It's taken me longer than I had hoped to implement S/MIME in MimeKit using
Bouncy Castle, but I'm getting there...

I'm presently stuck on importing and exporting certificates from a
"certs-only" message.

rfc2633 says this about generating a certs-only application/pkcs7-mime part:

3.6 Creating a Certificates-only Message
>
>    The certificates only message or MIME entity is used to transport
>    certificates, such as in response to a registration request. This
>    format can also be used to convey CRLs.
>
>    Step 1. The certificates are made available to the CMS generating
>    process which creates a CMS object of type signedData. The signedData
>    encapContentInfo eContent field MUST be absent and signerInfos field
>    MUST be empty.
>
>    Step 2. The CMS signedData object is enclosed in an
>    application/pkcs7-mime MIME entity
>
>    The smime-type parameter for a certs-only message is "certs-only".
>    The file extension for this type of message is ".p7c".
>
>
As far as I understand it, I need to use a CmsSignedDataGenerator like this:

var cms = new CmsSignedDataGenerator ();
cms.AddCertificates (certificates);

var signedData = cms.Generate (new CmsProcessableByteArray (new byte[0]));
var rawData = signedData.GetEncoded ();

I can then stuff that raw data into an application/pkcs7-mime;
smime-type=certs-only mime part.

Assuming that is correct, I've got the following code to parse that raw
data:

var parser = new CmsSignedDataParser (rawData);
var certs = parser.GetCertificates ("Collection");
var store = parser.GetSignerInfos ();

foreach (SignerInformation signerInfo in store.GetSigners ()) {
var matches = certs.GetMatches (signerInfo.SignerID);

foreach (X509Certificate certificate in matches) {
certificates.Add (certificate);
}
}

The problem I'm running into is that I'm hitting the following exception in
the CmsSignedDataParser constructor:

Org.BouncyCastle.Cms.CmsException: no digests could be created for message.
   at Org.BouncyCastle.Cms.CmsSignedDataParser..ctor(CmsTypedStream
signedContent, Stream sigData) in
C:\Users\jeff\Documents\Projects\bc-csharp\crypto\src\cms\CMSSignedDataParser.cs:line
176
   at Org.BouncyCastle.Cms.CmsSignedDataParser..ctor(Stream sigData) in
C:\Users\jeff\Documents\Projects\bc-csharp\crypto\src\cms\CMSSignedDataParser.cs:line
92
   at Org.BouncyCastle.Cms.CmsSignedDataParser..ctor(Byte[] sigBlock) in
C:\Users\jeff\Documents\Projects\bc-csharp\crypto\src\cms\CMSSignedDataParser.cs:line
76
   at UnitTests.DummySecureMimeContext.ImportKeys(Byte[] rawData) in
c:\Users\jeff\Documents\Projects\MimeKit\UnitTests\DummySecureMimeContext.cs:line
174
   at MimeKit.Cryptography.ApplicationPkcs7Mime.Import(CryptographyContext
ctx) in
c:\Users\jeff\Documents\Projects\MimeKit\MimeKit\Cryptography\ApplicationPkcs7Mime.cs:line
214
   at UnitTests.SecureMimeTests.TestSecureMimeImportExport() in
c:\Users\jeff\Documents\Projects\MimeKit\UnitTests\SecureMimeTests.cs:line
247


Any ideas?

Thanks for any help you guys can offer me,

Jeff