ASCIIEncoding ByteConverter = new ASCIIEncoding();
byte[] dataBytes = ByteConverter.GetBytes("Подписать эту строку.");
byte[] buffer = new byte[dataBytes.Length];

CmsSigner cmsSigner = new CmsSigner(cert);
ContentInfo contentInfo = new ContentInfo(dataBytes );
SignedCms signedCms = new SignedCms(contentInfo);

signedCms.ComputeSignature(cmsSigner, false);
byte[] signed = signedCms.Encode();

string signedFilename = filename + ".sig";
File.WriteAllBytes(signedFilename, signed);