var cert = InstanceTemplate.Invoke("ChooseSertificate", null) as X509Certificate2;
if (cert == null)
    return null;

//------------------------

//считаем хэш файла
byte[] hashValue;
using (var gostHashAlg = CryptoPro.Sharpei.Gost3411_2012_256.Create())
using (var fs = new FileStream(filePath, FileMode.Open, FileAccess.Read))
    hashValue = gostHashAlg.ComputeHash(fs);

//------------------------

//формируем ASN строку
var res = new byte[2 + hashValue.Length];
res[0] = 0x04;
res[1] = (byte)hashValue.Length;
Buffer.BlockCopy(hashValue, 0, res, 2, hashValue.Length);

//Создаем подпись
var contentInfo = new ContentInfo(new Oid("1.2.840.113549.1.7.1"), res);
var signer = new SignedCms(contentInfo, true);

var cmsSigner = new CmsSigner(SubjectIdentifierType.IssuerAndSerialNumber, cert)
{
    IncludeOption = X509IncludeOption.WholeChain,
    DigestAlgorithm = new Oid("1.2.643.7.1.1.2.2")
};

//Добавляем Digest
var messageDigestOID = new Oid("1.2.840.113549.1.9.4");
var asnHashData = new AsnEncodedData(messageDigestOID, res);

var asnCollection = new AsnEncodedDataCollection();
asnCollection.Add(asnHashData);

var messageDigestAttr = new CryptographicAttributeObject(messageDigestOID, asnCollection);
cmsSigner.SignedAttributes.Add(messageDigestAttr);

//------------------------

//Установка времени
var signedDate = DateTime.Now;
var pkcs9_time = new Pkcs9SigningTime(signedDate);
cmsSigner.SignedAttributes.Add(pkcs9_time);

signer.ComputeSignature(cmsSigner, true);
var signedCmsEncode = signer.Encode();
File.WriteAllBytes(filePath + ".sig", signedCmsEncode);