SignedXml signedXml = new SignedXml(xmldocument);
        signedXml.SigningKey = certificado.PrivateKey;
        Signature XMLSignature = signedXml.Signature;
        Reference reference = new Reference();
        reference.Uri = referenciaUri;
        XMLSignature.SignedInfo.AddReference(reference);
        KeyInfo keyInfo = new KeyInfo();
        keyInfo.AddClause(new RSAKeyValue((RSA)certificado.PrivateKey));
        keyInfo.AddClause(new KeyInfoX509Data(certificado));
        XMLSignature.KeyInfo = keyInfo;
        signedXml.ComputeSignature();
        XmlElement xmlDigitalSignature = signedXml.GetXml();
        xmldocument.DocumentElement.SelectSingleNode(para).AppendChild(xmldocument.ImportNode(xmlDigitalSignature, true));

// Base64 encoding
$SignatureValue = rtrim(chunk_split(base64_encode($SignatureValue_bruto), 76));

public static string base64_encode(string str)
    {
        return System.Convert.ToBase64String(Encoding.UTF8.GetBytes(str));
    }

    public static string chunk_split(string str, int len)
    {
        var strings = new List<string>();
        var div = str.Length % len;
        var remainder = len - div * len;

        if (div == 1)
        {
            return string.Join("", str.Take(len));
        }

        for (var j = 0; j < div; j++)
        {
            strings.Add(string.Join("", str.Skip(j * len).Take(len)));
        }

        if (remainder > 0)
        {
            strings.Add(string.Join("", str.Skip(div * len).Take(remainder)));
        }

        return string.Join(Environment.NewLine, strings.ToArray());
    }

    public static string rtrim(string input)
    {
        return input.TrimEnd(' ');
    }

chunk_split(base64_encode(strstr), 76);