Verify JWT using public key in string

Question

I have public key in following format: -----BEGIN RSA PUBLIC KEY-----MIIBCgKCAQEA1pjRK+cOnEsh5L1wrt1Tmx+FvyNRj4wuAotlqIWtHS8pIqRzsIOJg+tlbUtEXYju+KOcIohZnnmfj0cq28RcQ19HIohqUXypmUbNy9np/M+Aj9NcyKIaNyuEZ+UhcU/OIStHK4eTUJt+RWL+0Q1/rl49tg7h+toB9Y6Y3SeytXvZhMx3N5qqmHHNorpfb65bvvHsicGDB7vK5kn55o+C2LhRIxfnw87nKnhPBMRbZg+BKCTXD4svz4a2xiR/uM4rxebIBgB3Sm4X1w3yoRr0F3IDhAgXmEhSZ078wm3ohPfuuwymfVhvdzavm42NwzixZ7n52SowFE2v0DSJh3IbPwIDAQAB-----END RSA PUBLIC KEY-----

How can I implement C# to verify my JWT using that string? I have found similar topic at Verifying JWT signed with the RS256 algorithm using public key in C# but none of solutions suits my case.

There is my code:

        public static bool TransformKey(string fullKey)
        {
            try
            {
                var publicKey = fullKey.Replace("-----BEGIN RSA PUBLIC KEY-----", "");
                publicKey = publicKey.Replace("-----END RSA PUBLIC KEY-----", "");
                publicKey = publicKey.Replace("\n", "");

                var keyBytes = Convert.FromBase64String(publicKey); // your key here 
                AsymmetricKeyParameter asymmetricKeyParameter = PublicKeyFactory.CreateKey(keyBytes);
                RsaKeyParameters rsaKeyParameters = (RsaKeyParameters)asymmetricKeyParameter;
                RSAParameters rsaParameters = new RSAParameters
                {
                    Modulus = rsaKeyParameters.Modulus.ToByteArrayUnsigned(),
                    Exponent = rsaKeyParameters.Exponent.ToByteArrayUnsigned()
                };
                return true;
            }
            catch (Exception e)
            {
                Console.WriteLine(e);
                return false;
            }
        }
      
        static void Main(string[] args)
        {
            // The code provided will print ‘Hello World’ to the console.
            // Press Ctrl+F5 (or go to Debug > Start Without Debugging) to run your app.
            string key = "-----BEGIN RSA PUBLIC KEY-----MIIBCgKCAQEA1pjRK+cOnEsh5L1wrt1Tmx+FvyNRj4wuAotlqIWtHS8pIqRzsIOJg+tlbUtEXYju+KOcIohZnnmfj0cq28RcQ19HIohqUXypmUbNy9np/M+Aj9NcyKIaNyuEZ+UhcU/OIStHK4eTUJt+RWL+0Q1/rl49tg7h+toB9Y6Y3SeytXvZhMx3N5qqmHHNorpfb65bvvHsicGDB7vK5kn55o+C2LhRIxfnw87nKnhPBMRbZg+BKCTXD4svz4a2xiR/uM4rxebIBgB3Sm4X1w3yoRr0F3IDhAgXmEhSZ078wm3ohPfuuwymfVhvdzavm42NwzixZ7n52SowFE2v0DSJh3IbPwIDAQAB-----END RSA PUBLIC KEY-----";
            bool test = TransformKey(key);
            Console.ReadKey();
        }

It returns exception when I try to initialize asymmetricKeyParameter object:

System.ArgumentException: Unknown object in GetInstance: Org.BouncyCastle.Asn1.DerInteger
Parameter name: obj
   w Org.BouncyCastle.Asn1.Asn1Sequence.GetInstance(Object obj)
   w Org.BouncyCastle.Asn1.X509.AlgorithmIdentifier.GetInstance(Object obj)
   w Org.BouncyCastle.Asn1.X509.SubjectPublicKeyInfo..ctor(Asn1Sequence seq)
   w Org.BouncyCastle.Asn1.X509.SubjectPublicKeyInfo.GetInstance(Object obj)
   w Org.BouncyCastle.Security.PublicKeyFactory.CreateKey(Byte[] keyInfoData)

Code snippet with Modulus and Exponent in string type that can be useful:

RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
            rsa.ImportParameters(
              new RSAParameters()
              {
                  Modulus = FromBase64Url("w7Zdfmece8iaB0kiTY8pCtiBtzbptJmP28nSWwtdjRu0f2GFpajvWE4VhfJAjEsOcwYzay7XGN0b-X84BfC8hmCTOj2b2eHT7NsZegFPKRUQzJ9wW8ipn_aDJWMGDuB1XyqT1E7DYqjUCEOD1b4FLpy_xPn6oV_TYOfQ9fZdbE5HGxJUzekuGcOKqOQ8M7wfYHhHHLxGpQVgL0apWuP2gDDOdTtpuld4D2LK1MZK99s9gaSjRHE8JDb1Z4IGhEcEyzkxswVdPndUWzfvWBBWXWxtSUvQGBRkuy1BHOa4sP6FKjWEeeF7gm7UMs2Nm2QUgNZw6xvEDGaLk4KASdIxRQ"),
                  Exponent = FromBase64Url("AQAB")
              });

This is a public key in PKCS#1 format. It can be read with an ASN.1 parser, e.g. online, so you can get an impression how it is structured. You can easily extract exponent and modulus manually. Depending on your .NET version (which you haven't posted), the format may also be processed directly (however in DER encoding), e.g. here. — Topaco
I have added my code snipped with exception in first post. Have you got any idea why it is not working? — Vaurine
Topaco, can you provide me more details how to extract exponent and modulus manually? — Vaurine
.Net Framework 4.7.2. I have also included code that can be interesting from my point of view in first post. — Vaurine

Topaco Topaco · Accepted Answer · 2020-08-21T16:30:53

In the case of the .NET Framework (e.g. 4.7.2), a public PKCS#1 key can be imported for example using BouncyCastle (e.g. v1.8.6.1):

using System.IO;
using System.Security.Cryptography;

using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.OpenSsl;
using Org.BouncyCastle.Security;

...

string publicPKCS1 = @"-----BEGIN RSA PUBLIC KEY-----
                       MIIBCgKCAQEA1pjRK+cOnEsh5L1wrt1Tmx+FvyNRj4wuAotlqIWtHS8pIqRzsIOJ
                       g+tlbUtEXYju+KOcIohZnnmfj0cq28RcQ19HIohqUXypmUbNy9np/M+Aj9NcyKIa
                       NyuEZ+UhcU/OIStHK4eTUJt+RWL+0Q1/rl49tg7h+toB9Y6Y3SeytXvZhMx3N5qq
                       mHHNorpfb65bvvHsicGDB7vK5kn55o+C2LhRIxfnw87nKnhPBMRbZg+BKCTXD4sv
                       z4a2xiR/uM4rxebIBgB3Sm4X1w3yoRr0F3IDhAgXmEhSZ078wm3ohPfuuwymfVhv
                       dzavm42NwzixZ7n52SowFE2v0DSJh3IbPwIDAQAB
                       -----END RSA PUBLIC KEY-----";

PemReader pemReader = new PemReader(new StringReader(publicPKCS1));
AsymmetricKeyParameter asymmetricKeyParameter = (AsymmetricKeyParameter)pemReader.ReadObject();
RSAParameters rsaParameters = DotNetUtilities.ToRSAParameters((RsaKeyParameters)asymmetricKeyParameter);

RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
rsa.ImportParameters(rsaParameters);

...

The validation of the JWT can be done with a JWT library (e.g. System.IdentityModel.Tokens.Jwt) or with a pure cryptography API. Both are described in detail in the answers to the linked question. Regarding the former, see also here, 2nd paragraph.

Verify JWT using public key in string

1 Answers