var signedCms = new SignedCms(new ContentInfo(contentBytes), false);
                    signedCms.ComputeSignature(new CmsSigner(_certificate), true);
                    var cmsBytes = signedCms.Encode();
                    var cmsStr = Convert.ToBase64String(cmsBytes);  

import java.security.Signature;

import java.security.cert.*;
....
private byte[] signMessage(String algoName, PrivateKey privateKey, String msg) throws NoSuchProviderException, NoSuchAlgorithmException, InvalidKeyException, SignatureException {
        String algoName = JCP.GOST_DH_2012_256_NAME;
        Signature signer = Signature.getInstance(algoName, JCP.PROVIDER_NAME);
        signer.initSign(privateKey);
        signer.update(msg.getBytes());
        return signer.sign();
}