runtime: CmsSigner.ComputeSignature() creates a signature with incorrect SignerInfo

I’m currently porting over some code to .Net Core and using the recently added CmsSigner from System.Security.Cryptography.Pkcs (I’m using the 4.5.0-rc1) The certificate I’m using has a serial number of FD319CB1514B06AF49E00522277E43C8, it also happens to be self signed cert.

Here is what I’m doing to create the signature.

X509Certificate2 cert = new X509Certificate2(certificate, password);
ContentInfo contentInfo = new ContentInfo(message);
SignedCms signedCms = new SignedCms(contentInfo, true); 
CmsSigner cmsSigner = new CmsSigner(cert);
cmsSigner.SignerIdentifierType = SubjectIdentifierType.IssuerAndSerialNumber;

signedCms.ComputeSignature(cmsSigner);
signature = signedCms.Encode();

After calling ComputeSignature the signedCms object created has the wrong serial number in the SignerInfos list, with “00” prefix being added. Calling signedCms.SignerInfos[0].SignerIdentifier.Value

Produces the result below, where you can see the extra “00” being added {System.Security.Cryptography.Xml.X509IssuerSerial} IssuerName: “CN=myorg” SerialNumber: “00FD319CB1514B06AF49E00522277E43C8”

In addition, if I check the signedCms.Certificates[0].SerialNumber object I can see the correct serial number with no “00” prefix.

Can you tell me what is causing this “00” to be added and if its a bug or something I can correct with config.