Gost3410CryptoServiceProvider gost = (Gost3410CryptoServiceProvider)CertificateForSigning.PrivateKey;

using (FileStream fs = new FileStream("abcdef.docx", FileMode.Open))
            {
                Gost3411 hasher=new Gost3411CryptoServiceProvider();
                return gost.SignData(fs, hasher);
            }

File.WriteAllBytes("result.docx", signedData);

Gost3410CryptoServiceProvider verifier = (Gost3410CryptoServiceProvider) certificateForSign.PrivateKey;
            using (FileStream fs = new FileStream("abcdef.docx", FileMode.Open))
            {
                Gost3411 hasher = new Gost3411CryptoServiceProvider();
                bool res = verifier.VerifyData(fs, hasher, signedData);
                Console.WriteLine(res);
            }