Verify JWT token signed with RS256 using the public key

2016-01-06 by _tasos 23 Comments

In this article we will see how we can verify a JWT token that has been signed with the RS256 algorithm. While there are more than a few libraries for .NET that provide functionality to decode and verify JWT tokens, none of them support the specific algorithm.

In short, a JWT token consists of three parts seperated by a .; the first part is the algorithm used to sign the token, the second part is the actual data and the third part is the signature we need to match so that the token can be verified.

If you are not very familiar with JWT tokens, here you can read a very good article about them and their anatomy.

The code provided in this article is for tokens that have the following header:

{
  "alg": "RS256"
}

Here is the Decode(...) function that can verify a RS256 signed JWT token. It makes use of the BouncyCastle library. It is available as a NuGet package with version 1.7.0 but on their website version 1.8.1 is available. The code is tested with both versions and works fine.

using System;
using System.Security.Cryptography;
using System.Text;
using Newtonsoft.Json.Linq;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Security;
 
public static string Decode(string token, string key, bool verify = true)
{
    string[] parts = token.Split('.');
    string header = parts[0];
    string payload = parts[1];
    byte[] crypto = Base64UrlDecode(parts[2]);
 
    string headerJson = Encoding.UTF8.GetString(Base64UrlDecode(header));
    JObject headerData = JObject.Parse(headerJson);
 
    string payloadJson = Encoding.UTF8.GetString(Base64UrlDecode(payload));
    JObject payloadData = JObject.Parse(payloadJson);
 
    if (verify)
    {
        var keyBytes = Convert.FromBase64String(key); // your key here
 
        AsymmetricKeyParameter asymmetricKeyParameter = PublicKeyFactory.CreateKey(keyBytes);
        RsaKeyParameters rsaKeyParameters = (RsaKeyParameters)asymmetricKeyParameter;
        RSAParameters rsaParameters = new RSAParameters();
        rsaParameters.Modulus = rsaKeyParameters.Modulus.ToByteArrayUnsigned();
        rsaParameters.Exponent = rsaKeyParameters.Exponent.ToByteArrayUnsigned();
        RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
        rsa.ImportParameters(rsaParameters);
 
        SHA256 sha256 = SHA256.Create();
        byte[] hash = sha256.ComputeHash(Encoding.UTF8.GetBytes(parts[0] + '.' + parts[1]));
 
        RSAPKCS1SignatureDeformatter rsaDeformatter = new RSAPKCS1SignatureDeformatter(rsa);
        rsaDeformatter.SetHashAlgorithm("SHA256");
        if (!rsaDeformatter.VerifySignature(hash, FromBase64Url(parts[2])))
            throw new ApplicationException(string.Format("Invalid signature"));
    }
 
    return payloadData.ToString();
}

Here are some helper functions used in the above snippet.

// from JWT spec
private static byte[] Base64UrlDecode(string input)
{
    var output = input;
    output = output.Replace('-', '+'); // 62nd char of encoding
    output = output.Replace('_', '/'); // 63rd char of encoding
    switch (output.Length % 4) // Pad with trailing '='s
    {
        case 0: break; // No pad chars in this case
        case 1: output += "==="; break; // Three pad chars
        case 2: output += "=="; break; // Two pad chars
        case 3: output += "="; break; // One pad char
        default: throw new System.Exception("Illegal base64url string!");
    }
    var converted = Convert.FromBase64String(output); // Standard base64 decoder
    return converted;
}

The above is a combination of code snippets found in the following links

http://stackoverflow.com/questions/34403823/verifying-jwt-signed-with-the-rs256-algorithm-using-public-key-in-c-sharp. This article contains part of the solution since it does not provide how to get the Modulus and the Exponent from the public key
http://stackoverflow.com/questions/29455404/net-equivalent-of-java-keyfactory-getinstance-rsa-rsa-ecb-pkcs1padding. This article shows how we can get the Modulus and the Exponent from the public key.
http://stackoverflow.com/questions/10055158/is-there-a-json-web-token-jwt-example-in-c. Although the code there is tweaked to support RS256 it actually supports the HS256 instead. I got the helper functions from this article though; the code there is pretty much based on John Sheehans JWT library.

Update: I have also written the Sign(...) function that complements this one, click here to see it.

Update 2: You can check this post here, where I have created a C# library that manages Jwt tokens.

Update 3: If you are having trouble making your keys work, have a look in my Check your RSA private and public keys post and make sure to check the Additional Resources section as well

Comments

Adam says

2016-03-22 at 16:51

Thank you! This was the best example I found regarding decoding a JWT token using a RS256 public key.

In case anyone else runs into this, the key variable should be the public key all on one line and removing the “—–BEGIN PUBLIC KEY—–” and “—–END PUBLIC KEY—–“. Otherwise, you will get an exception on this line “var keyBytes = Convert.FromBase64String(key);”.

Thanks again,
Adam

Reply
Sammy says

2016-06-09 at 20:49

I’m using Google API from an app to access my own account at Google. Per their doc, I created a “service account” and downloaded the file that has PRIVATE key, client_email, auth uri, token uri etc. There is no public key.
Using RS256, I created JWT based on the given private key.
Now I need to validate that JWT. Your “Decode” function will not work for me since I do not have the public key. Is there a way to decode using the same private key or am I missing something?
Thanks!

Reply
- _tasos says
  
  2016-06-09 at 23:17
  
  Hey Sammy.
  
  Since you have the private key, it is easy to generate the public key and then use the function in the post.
  
  I haven’t tried it but there is this SO post that you might find helpful http://stackoverflow.com/questions/5244129/use-rsa-private-key-to-generate-public-key.
  
  Reply
Wei says

2016-08-11 at 22:35

Hey tasos,

Thank you! I couldn’t find better examples out there regarding verifying a Jwt token using RSA256 public key. Many of other examples are simply showing how to do it using HS256 algorithm.

I also tried to find examples of how to generate a signed Jwt token using RSA256 private keys and I don’t get any luck so far. I would appreciate if you could write an article or just a sample code showing how to generate a Jwt using the Bouncy Castle lib in C# with RSA256 private key?

Thanks!

Reply
Raju says

2017-01-20 at 23:07

Hello,

I am looking for similar article to verify RS256 signature. Would you please share me the link if anyone has it?

Thanks, in advance.

Reply
- _tasos says
  
  2018-02-24 at 01:31
  
  Hello Raju, I have written a post with the Sign(…) function, you can have a look here.
  
  Reply
Cristian says

2017-12-05 at 18:07

Dude, you save my day!
One billion thanks for that, great article and solution!!!

Reply
Rohan says

2018-11-19 at 13:16

Hi Tasos,
This article is great. I could accomplish the JWT signature verification with the BouncyCastle.
But for this we need to have public key in(pem generated) I have a issuer key and i want to generate PEM key using C#.
How do I do that?
And also is BouncyCastle is openSource?
–Rohan

Reply
Rushal Maksane says

2018-12-24 at 13:11

Can you please provide Base64URlDecode function?

Reply

ozan says

2019-03-21 at 17:41

Hi I have been trying to validate my JWT with the public key. I tried on JWT.io and it verified successfully. When I try on this code, I keep getting the exception “Bad sequence size: 3
Parameter name: seq” when the code executes the following line:

AsymmetricKeyParameter asymmetricKeyParameter = PublicKeyFactory.CreateKey(keyBytes)

Any advice?

Thank you!

_tasos says

2019-03-25 at 16:44

Hi Ozan,

It could be the issue on how you read the file. After loading the .pem file, pass the input via this helper function.

private static string RemoveHeaderFooterFromKey(string key)
{
    string tmp = key;
    tmp = tmp.Replace("\n", "");
    tmp = tmp.Replace("-----BEGIN PRIVATE KEY-----", "");
    tmp = tmp.Replace("-----END PRIVATE KEY-----", "");
    tmp = tmp.Replace("-----BEGIN RSA PRIVATE KEY-----", "");
    tmp = tmp.Replace("-----END RSA PRIVATE KEY-----", "");
    tmp = tmp.Replace("-----BEGIN PUBLIC KEY-----", "");
    tmp = tmp.Replace("-----END PUBLIC KEY-----", "");
    return tmp;
}

string fullPpublicKey = File.ReadAllText(publicKeyPath);
string publicKey = RemoveHeaderFooterFromKey(fullPpublicKey);

Also, have a look in this GitHub repo, where there is the whole code with tests to use as examples.

Jr says

2019-06-03 at 06:56

I am getting error for FromBase64Url() function. Getting error at FromBase64Url(parts[2])

Reply
- _tasos says
  
  2019-06-03 at 23:02
  
  Hi there. An error there indicates that the signature doesn’t match. Consider posting a question in SO with your code so that I can have a look.
  
  Reply
jr says

2019-06-04 at 16:39

First of all Thank you _Tasos, your code is very helpful.

I should rephrase, I am getting compile error @

if (!rsaDeformatter.VerifySignature(hash, FromBase64Url(parts[2])))
throw new ApplicationException(string.Format(“Invalid signature”));

where FromBase64Url function is missing. I am looking for how to get byte from Base64Url string.

Reply

Jr says

2019-06-04 at 16:59

I was able to find the FromBase64Url Function and was able to verify the signature as will. Thanks again for the nice post!

Pasting the function,

// https://stackoverflow.com/questions/34284810/decoding-base64urluint-encoded-value
static byte[] FromBase64Url(string base64Url)
{
    string padded = base64Url.Length % 4 == 0
        ? base64Url : base64Url + "====".Substring(base64Url.Length % 4);
    string base64 = padded.Replace("_", "/")
        .Replace("-", "+");
    return Convert.FromBase64String(base64);
}

Bin says

2019-09-12 at 15:21

Hi , Thanks for the post. I am getting error

‘Bad sequence size: 3
Parameter name: seq’.

I am using RemoveHeaderFooterFromKey menthod as you mentioned . But still I am getting same error. Can you please help me.?

Reply
- _tasos says
  
  2019-09-14 at 19:40
  
  Hi Bin,
  
  It could be that there is a mixup with the key. Make sure that your key contains the lines the RemoveHeaderFooterFromKey tries to remove
  
  Reply

George says

2020-09-08 at 22:59

Hi _tasos,

This is a GREAT example! I have been struggling getting my public key. Please help me.

I am using Azure AD to authenticate users. I passed IdToken over to my server side for the validation.

I noticed that Azure AD changes public key in a period of times. By reading this artical, https://nicksnettravels.builttoroam.com/post-2017-01-24-verifying-azure-active-directory-jwt-tokens-aspx/, I used HttpClient (c#) to abtain the public key from the url in Azure.

https://login.microsoftonline.com/{TenantId}/discovery/v2.0/keys?appid={AppId}

I did get the result, which is an string array in x5c key. Below is my code:

 using (var client = new HttpClient())
            {
                HttpResponseMessage response = client.GetAsync("https://login.microsoftonline.com/{tenantId}/discovery/v2.0/keys?appid={appId}").Result;
 
                if (response.IsSuccessStatusCode)
                {
                    var responseContent = response.Content;
 
                    // by calling .Result you are synchronously reading the result
                    string responseBody = responseContent.ReadAsStringAsync().Result;
 
                    JObject json = JObject.Parse(responseBody);
                    string c = json["keys"].ToString();
 
                    List allPublicKeys = JsonConvert.DeserializeObject&lt;List&gt;(json["keys"].ToString());
 
                    foreach (ADPublic key in allPublicKeys)
                    {
                        if (key.kid == kid)
                        {
                            string certificateString = key.x5c[0];
                            var certificate = new X509Certificate2(Convert.FromBase64String(certificateString));
 
                            string pkey = Convert.ToBase64String(certificate.PublicKey.EncodedKeyValue.RawData);
 
                          //  var x509SecurityKey = new X509SecurityKey(certificate)
                           // {
                              //  KeyId = key.kid
                           // };
                            return pkey;
                        }
                    }
 
                }
                return null;
            }

When I pass my public key over to your method. I always get an error at :

// call my get public key function..
 string key = GetPublicKeyAsync(headerData["kid"].ToString());
 
var keyBytes = Convert.FromBase64String(key); // your key here
 
        AsymmetricKeyParameter asymmetricKeyParameter = PublicKeyFactory.CreateKey(keyBytes);
 
Unknown object in GetInstance: Org.BouncyCastle.Asn1.DerInteger Parameter name: obj

_tasos says

2020-09-09 at 07:20

Hi George,

Thank you for your interest in my article. I would suggest to try the most recent version of the library from my Github repo.

Also, check this post, where I have a command that can check the public key. You can use it to check if the key you are trying is indeed RS256.

Reply

Kannan says

2020-09-25 at 19:00

Its throwing error in AsymmetricKeyParameter asymmetricKeyParameter = PublicKeyFactory.CreateKey(keyBytes), if we pass CERTIFICATE String

Reply
- _tasos says
  
  2020-10-03 at 09:48
  
  Hi Kannan,
  
  Thank you for your comment. I would suggest to try the most recent version of the library from my Github repo.
  
  Also, check the previous comments, you might find them helpful.
  
  Reply
Aleti says

2022-09-19 at 11:45

I am getting this error at this line
RsaKeyParameters rsaKeyParameters = (RsaKeyParameters)asymmetricKeyParameter;

Unhandled exception. System.InvalidCastException: Unable to cast object of type ‘Org.BouncyCastle.Crypto.Parameters.ECPublicKeyParameters’ to type ‘Org.BouncyCastle.Crypto.Parameters.RsaKeyParameters’.
at JwtDecode.Program.VerifyToken(String token, String key) in C:\Users\jagad\OneDrive\Desktop\Jwt Decode\Jwt Decode\Program.cs:line 69
at JwtDecode.Program.Main(String[] args) in C:\Users\jagad\OneDrive\Desktop\Jwt Decode\Jwt Decode\Program.cs:line 167

Reply
- _tasos says
  
  2022-10-14 at 10:00
  
  Hi Aleti,
  
  Check the library, I have a repo with it in GitHub. It has the latest version and a few bugs sorted out.
  
  Alternatively, you can post your question on StackOverflow with a code snippet. Then, post the link here so I can follow up.
  
  Reply

Verify JWT token signed with RS256 using the public key

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