Signing a json document or string with x509 certificate

Question

How to signing a json document or string with x509 certificate?

public static void fund()
{
    string filePath = @"C:\Users\VIKAS\Desktop\Data.xml";
    //Read the file    

    XmlDocument xmlDoc = new XmlDocument();
    XElement ele = XElement.Load(filePath);
    String Xml = ele.ToString();
    xmlDoc.LoadXml(Xml);
    string signature = SignedXMLCert(xmlDoc);
    bool verified = ValidateSignature(signature);
}

public static string SignedXMLCert(XmlDocument xmlDoc)
{
    string startupPath = AppDomain.CurrentDomain.BaseDirectory + @"Certificates\unidesk.p12";
    //  startupPath = AppDomain.CurrentDomain.BaseDirectory + @"\Certificates\BBPS_enc.cer";

    //X509Certificate2 cert = new X509Certificate2(@"D:\Sonal\AXISOU_TEST.P12", "axisbank", X509KeyStorageFlags.Exportable);
    X509Certificate2 cert = new X509Certificate2(startupPath, "axisbank", X509KeyStorageFlags.Exportable);
    //  string PrivateKey = GetRSAPrivateKeyBase64(cert);

    var privateKey = cert.PrivateKey as RSACryptoServiceProvider;
    SignedXml signedXml = new SignedXml(xmlDoc);
    signedXml.SigningKey = privateKey;

    // Create a reference to be signed.
    Reference reference = new Reference();
    reference.Uri = "";

    KeyInfo keyInfo = new KeyInfo();
    //startupPath = AppDomain.CurrentDomain.BaseDirectory + @"\Certificates\BBPS_enc.cer";
    X509Certificate MSCert = new X509Certificate(startupPath, "axisbank", X509KeyStorageFlags.Exportable);
    // X509Certificate MSCert = X509Certificate.CreateFromCertFile(startupPath);

    keyInfo.AddClause(new KeyInfoX509Data(MSCert));
    signedXml.KeyInfo = keyInfo;


    // Add an enveloped transformation to the reference.
    XmlDsigEnvelopedSignatureTransform env = new XmlDsigEnvelopedSignatureTransform();
    reference.AddTransform(env);

    // Add the reference to the SignedXml object.
    signedXml.AddReference(reference);

    // Compute the signature.
    signedXml.ComputeSignature();

    // Get the XML representation of the signature and save
    // it to an XmlElement object.
    XmlElement xmlDigitalSignature = signedXml.GetXml();

    // Append the element to the XML document.
  xmlDoc.DocumentElement.AppendChild(xmlDoc.ImportNode(xmlDigitalSignature, true));

    return xmlDoc.InnerXml.ToString();
}

public static bool ValidateSignature(String signedServiceMetadataContent)
{
    bool result = false;

    X509Certificate2 cert = GetCertificate();

    //Load the key
    CspParameters csp = new CspParameters();
    csp.KeyContainerName = cert.PublicKey.Key.ToString();

    RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(csp);

    //Load XML document
    XmlDocument xmlDocument = new XmlDocument();
    xmlDocument.PreserveWhitespace = true;
    xmlDocument.LoadXml(signedServiceMetadataContent);

    //create a SignedXml and load the xml document
    SignedXml signedXml = new SignedXml(xmlDocument);

    //find signature and create signature node list
    XmlNodeList xmlNodeList = xmlDocument.GetElementsByTagName("Signature");

    if (xmlNodeList.Count <= 0)
    {
        throw new CryptographicException("Verification failed: No Signature was found in the document.");
    }
    // if more than one signature was found.
    if (xmlNodeList.Count >= 2)
    {
        throw new CryptographicException("Verification failed: More that one signature was found for the document.");
    }

    //Load signature into SignedXml
    signedXml.LoadXml((XmlElement)xmlNodeList[0]);

    //check the signature
    result = signedXml.CheckSignature(cert, true);
    //result = signedXml.CheckSignature(rsa);


    return result;
}
private static X509Certificate2 GetCertificate()
{
    string startupPath = AppDomain.CurrentDomain.BaseDirectory + @"Certificates\unidesk.p12";
    X509Certificate2 cert = new X509Certificate2(startupPath, "axisbank", X509KeyStorageFlags.Exportable);
    return new X509Certificate2(cert);
}

Answer 1

XMLDsig signature format is only suitable for XML documents. You could apply JSON Web Signature (JWS) which is designed for JSON documents.

JWS Signature

A JWS with compact serialization is represented by (see RFC7515)

BASE64URL(UTF8(JWS Protected Header)) || '.' ||

BASE64URL(JWS Payload) || '.' ||
BASE64URL(JWS Signature)

JWS protected header

The simplest header is composed by alg. RS256 means algorithm RSA with SHA-256

{"alg":"RS256"}

You can add other parameters such as x5c (X.509 Certificate Chain) or cty (Content Type)

JWS Payload

The payload is your JSON object encoded as base64url

eyJ1cGRhdGVTUlJlcSI6IHsgImluY2lkZW50SUQiOiAiIiwgImNyZWF0ZWRCeSI6ICIwMzcwMjIwMDAwNDIwNDgiLCAiZGVzY3JpcHRpb24iOiAiMDM3MDIyMDAwMDQyMDQ4IiwgImlzVmlzaWJsZVRvQ3VzdG9tZXIiOiAiMyIsICJ1cGRhdGVUeXBlIjogIjIiLCAiYWN0aXZpdHlUeXBlIjogIjIiLCAiY3JlYXRlZE9uIjogIjIwMTYtMDktMDggMTc6NTciLCAibGFzdFVwZGF0ZSI6ICIyMDE2LTA5LTA4IDE3OjU3IiwgInN0YXR1cyI6ICIyIiwgImNsb3NlZFRpbWUiOiAiIiB9LCAic3ViSGVhZGVyIjogeyAidmFsdWUiOiB7ICJyZXF1ZXN0VVVJRCI6ICIxMjMiLCAiU2VydmljZVJlcXVlc3RJZCI6ICJBRS5NQVBTLlVESy5TU1RQIiwgIlNlcnZpY2VSZXF1ZXN0VmVyc2lvbiI6ICIxLjAiLCAiQ2hhbm5lbElkIjogIk1BUFMifX19

JWS Signature

The JWS signature is computed on

BASE64URL(UTF8(JWS Protected Header)) || '.' || BASE64URL(JWS Payload))

Build the following string and apply the RSA digital signature algorithm with the private key of your certificate

eyJhbGciOiJSUzI1NiJ9.eyJ1cGRhdGVTUlJlcSI6IHsgImluY2lkZW50SUQiOiAiIiwgImNyZWF0ZWRCeSI6ICIwMzcwMjIwMDAwNDIwNDgiLCAiZGVzY3JpcHRpb24iOiAiMDM3MDIyMDAwMDQyMDQ4IiwgImlzVmlzaWJsZVRvQ3VzdG9tZXIiOiAiMyIsICJ1cGRhdGVUeXBlIjogIjIiLCAiYWN0aXZpdHlUeXBlIjogIjIiLCAiY3JlYXRlZE9uIjogIjIwMTYtMDktMDggMTc6NTciLCAibGFzdFVwZGF0ZSI6ICIyMDE2LTA5LTA4IDE3OjU3IiwgInN0YXR1cyI6ICIyIiwgImNsb3NlZFRpbWUiOiAiIiB9LCAic3ViSGVhZGVyIjogeyAidmFsdWUiOiB7ICJyZXF1ZXN0VVVJRCI6ICIxMjMiLCAiU2VydmljZVJlcXVlc3RJZCI6ICJBRS5NQVBTLlVESy5TU1RQIiwgIlNlcnZpY2VSZXF1ZXN0VmVyc2lvbiI6ICIxLjAiLCAiQ2hhbm5lbElkIjogIk1BUFMifX19 

Finally encode the signature as base64url and append the result to the previous data to sign. You will get a JWS like this hhhhh.ppppp.sssss where hhhhh is the header ppppp the payload and sssss the signature

Use the following links to

• Base64url encoding: Code for decoding/encoding a modified base64 URL
• Sign and verify signature with .net and a certificate (C#) : https://blogs.msdn.microsoft.com/alejacma/2008/06/25/how-to-sign-and-verify-the-signature-with-net-and-a-certificate-c/

JWS verification

To verify a signature from the compact format hhhhh.ppppp.sssss, base64url decode the signature sssss, and verify the signature with the signed data hhhhh.ppppp and the used certificate