C# code needed for RSA-SHA256 decryption with public key

Question

I need help to implement EWP (https://developers.erasmuswithoutpaper.eu/). I have set up some API's and use an online validator tool by EWP to test them. Everything is working now, except for 1 thing: the signature verification.

I have a few methods to go through the request headers to check if they are valid. Only method remaining is VerifySignature. I have implemented logging and have a valid signature along with the public key I should use to decrypt it, but whatever I try, I don't get any useable value out of it.

The signature I receive is:

UiBgtTc4hIYeZehhP8RWKvPIvWFXeh7ERFEvJr43v87YE7I4dqAHbD8l5DwZW3jezVfIcBflBS7ezjFDOH0/43T21ZCxwa/0qKhQRTjXoWQSETap5fXI9MCtWcGcP5iNmnBang7zfIsr+DBqQU5N3vlCBLORGqGVM0eMEv8nwBanAM2J7ZjbIVg7gou22eHau9751M4OoQM5FCo3nBTRepf2XA0K4W00TJg55chjMW/s91rw2ryJSPLUlrhvl5kWUYozGO56SzRjNhW+/HawcNZRa+OioWvlLp4bYi8mFyjlWWNAXqKPbqROQILhJnlHjGeMMq2qZTbd3t8aGHpeDg==

The public key is found in an xml file managed by EWP, using the keyId I receive in the request. Since I can find the public key by the keyId, I'm confident it is correct:

MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAkH+X94XHM5RDyTNETNmt8vuu1Q5leSpb
EMIru1WE/vte5Co8k9lluffhvGWeyZRCKH4kzIBHf5n/kKjftTvvT/mPwArnJFZXcRbT4ebZ66RE
hi/+uJkJbIQ2md43MJBjM6fSXZAUiNw8VP29yVhbVPV5UUIA2ddJfygk/4ZNOmxdgnEJdc3aJTsK
99dEJ6BYqbcxA0Bk19Fv3/azV0jGZgrC4Y2hx3A+NvIYC05QYoqNkcfDExRVrKwduVWzqz6XDt7C
9ERl5Ss2bsgm4gbDouJC+k+WoCcxkUO2tnxrsKxQtZetZZSvrkst/5ELBaJAvKpcq12CvnB09dY1
MBV2XwIDAQAB

Now I need to decrypt the signature with only this public key (it was encrypted with the private key matching this public key) and the result should be a signing string, looking like this:

(request-target): get /api/institutions?hei_id_param=uhasselt.be
date: Tue, 08 Dec 2020 15:27:05 GMT
digest: SHA-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
host: staging-ewp.uhasselt.be
x-request-id: 0e9b58f8-f98e-41a2-ae30-c09f566f437c

EWP documentation states that the signature is formed like this: signature="Base64(RSA-SHA256(signing string))

Can anyone show me code to decrypt this signature with this public key, resulting in above signing string? (or at least something similar) I've been struggling with this for a long time now...

Edit: So I understand that it is not possible to get the original message from the signature and public key, but only a hash generated with the original message and I should reproduce that hash to see if it matches. Ok for me, I have all the data needed to do so, just no working code to get a match.

Some more documentation about standards used by EWP:

https://github.com/erasmus-without-paper/ewp-specs-sec-cliauth-httpsig/tree/v1.0.1 https://datatracker.ietf.org/doc/html/draft-cavage-http-signatures-07#section-2.5

About the test data I use: I implemented a lot of logging in my API for testing, and these are just values I receive from the official EWP online validator. The signature is directly coming from the request, the public key is found in the EWP registry xml. Only thing I created myself is the signing string, as explained here: https://datatracker.ietf.org/doc/html/draft-cavage-http-signatures-07#section-3.1.2

My code after testing some more (requestContext.ClientPublicKey being the complete string MIIBIj...IDAQAB):

requestContext.SigningString = GetSigningString(request, ref requestContext);
log.Info("Signing string: " + requestContext.SigningString);

string x509Pem = @"-----BEGIN PUBLIC KEY-----" + requestContext.ClientPublicKey + "-----END PUBLIC KEY-----";

byte[] message = Encoding.UTF8.GetBytes(requestContext.SigningString);
byte[] signature = Convert.FromBase64String(requestContext.Signature);

byte[] Sha256Message = SHA256.Create().ComputeHash(message);
string x = Convert.ToBase64String(Sha256Message);
log.Info("x = " + x);

PemReader pr = new PemReader(new StringReader(x509Pem));
AsymmetricKeyParameter publicKey = (AsymmetricKeyParameter)pr.ReadObject();

RSAParameters rsaParams = DotNetUtilities.ToRSAParameters((RsaKeyParameters)publicKey);
RSACng rsaCng = new RSACng();
rsaCng.ImportParameters(rsaParams);

bool verified = rsaCng.VerifyData(message, signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pss);
log.Info(verified);

log.Info(rsaCng.VerifyHash(Sha256Message, signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1));
log.Info(rsaCng.VerifyHash(Sha256Message, signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pss));
log.Info(rsaCng.VerifyHash(message, signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1));
log.Info(rsaCng.VerifyHash(message, signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pss));

string publicKeyXml = "" + requestContext.ClientPublicKey + "AQAB";
log.Info("public key: " + publicKeyXml);

RSACryptoServiceProvider rsaCryptoServiceProvider = new RSACryptoServiceProvider();
rsaCryptoServiceProvider.FromXmlString(publicKeyXml);

log.Info(rsaCryptoServiceProvider.VerifyHash(Sha256Message, signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1));
log.Info(rsaCryptoServiceProvider.VerifyHash(Sha256Message, signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pss));
log.Info(rsaCryptoServiceProvider.VerifyHash(message, signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1));
log.Info(rsaCryptoServiceProvider.VerifyHash(message, signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pss));

Edit2: I found a working test in my logs! But, strange as it is, it only worked once... I can reproduce the successful verification with these same data:

private void test()
{
    string SigningString = "";
    string keyId = "769c7a9c8bac71a238ea9e78dda3e49bcabbaf695f56fbc99dd4d6e8f777756a";

    SigningString += "(request-target): get /api/institutions?hei_id=tul.edu
";
    SigningString += "original-date: Wed, 09 Dec 2020 08:36:58 GMT
";
    SigningString += "x-request-id: bc8da180-1217-4fe6-ac22-fc4919e96718
";
    SigningString += "digest: SHA-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
";
    SigningString += "host: staging-ewp.uhasselt.be";

    string Signature = "iqd5ZLHbzshwm3uClkKPpTniLqdQvodZAexEnywDEir9mfytcf8NDWBYi5bLrwf7jiZ29GO5F1CtlhstcqgO2Ia3LvLFbl4Dru58TzYasspsZtEr6r0HDFNNzfuVhWXSa349yJADGCDYbvAo3rBS7SM2lMZ9dpf8QZDOUL9wTUgpzt2X9IO83nmUxUQWYC9LQZJ5x/95RQsbXN4bY8lDFjfUAC2nW30g2SVtMKlpVXAZwLv7MoYlVXpFl56DuoOIGPmeH6LDumA6krU28EwK4vs1hbDr+GKpzQRvSKiabraUkTq3ITGeXhZgZy+0Av5KxCKNQOVjYiK3Hf0k8sAgpQ==";
    string ClientPublicKey = LookUpTheKey(keyId);

    Console.WriteLine("ClientPublicKey: " + ClientPublicKey);
    Console.WriteLine("Signing string: " + SigningString);

    string x509Pem = @"-----BEGIN PUBLIC KEY-----" + ClientPublicKey + "-----END PUBLIC KEY-----";

    byte[] message = Encoding.UTF8.GetBytes(SigningString);
    byte[] signature = Convert.FromBase64String(Signature);

    byte[] Sha256Message = SHA256.Create().ComputeHash(message);
    string x = Convert.ToBase64String(Sha256Message);
    Console.WriteLine("x = " + x);

    PemReader pr = new PemReader(new StringReader(x509Pem));
    AsymmetricKeyParameter publicKey = (AsymmetricKeyParameter)pr.ReadObject();

    RSAParameters rsaParams = DotNetUtilities.ToRSAParameters((RsaKeyParameters)publicKey);
    RSACng rsaCng = new RSACng();
    rsaCng.ImportParameters(rsaParams);

    Console.WriteLine(rsaCng.VerifyHash(Sha256Message, signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1));
}

protected string LookUpTheKey(string keyId)
{
    string clientPublicKey = "";
    string registryPath = "registry.xml";
    if ((!File.Exists(registryPath)) || (Math.Abs((File.GetLastWriteTime(registryPath) - DateTime.Now).TotalMinutes) > minutesToChacheRegistry))
    {
        using (var handler = new HttpClientHandler())
        using (var client = new HttpClient(handler))
        {
            var request = new HttpRequestMessage(HttpMethod.Get, "https://dev-  registry.erasmuswithoutpaper.eu/catalogue-v1.xml");
            request.Headers.Add(HttpRequestHeader.Accept.ToString(), "application/xml");
            var response = client.SendAsync(request).Result;
            var returnedXml = response.Content.ReadAsStringAsync().Result;
            File.WriteAllText(registryPath, returnedXml);
        }
    }

    try
    {
        XmlDocument doc = new XmlDocument();
        doc.Load(registryPath);
        var namespaceManager = new XmlNamespaceManager(doc.NameTable);
        namespaceManager.AddNamespace("r", "https://github.com/erasmus-without- paper/ewp-specs-api-registry/tree/stable-v1");
        XmlNode root = doc.DocumentElement;
        var XPathExpr = @"//r:binaries/r:rsa-public-key[@sha-256=""" + keyId + @"""]";
        XmlNode node = root.SelectSingleNode(XPathExpr, namespaceManager);
        if (node == null) { }
        else { clientPublicKey = node.InnerText; }
    }
    catch
    {
    }
    return clientPublicKey;
}

But in my API, with new data, other keyId, public key, signing string and signature, it is failing again... how is this possible?

C# code needed for RSA-SHA256 decryption with public key

Answers (1)

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