title: how to define AuthorizationCodeTokenRequest with ClientAssertion
body: <p>i'm trying to figure out how the define my AuthorizationCodeTokenRequest for Code Flow
  for the JWT scenario , using the IdentityModole freamwork</p>
  <p>let say i have a defined client on my OP Server</p>
  <pre><code>
  new Client
  {
  ClientId = &quot;myClientId&quot;
  ClientSecrets = {
  new Secret(&quot;MyVerySpecialSecret&quot;.Sha256())
  }
  
  </code></pre>
  <p>on the client side
  i would like to get an AuthorizationCode using <strong>JWT</strong></p>
  <pre><code>        var securityToken = tokenHandler.CreateJwtSecurityToken(
              issuer: clientID,
              audience: opEndPoint.TokenEndpoint,
              subject: new ClaimsIdentity(new List&lt;Claim&gt;()
              {
                          new Claim(JwtClaimTypes.JwtId, Guid.NewGuid().ToString()),
                          new Claim(JwtClaimTypes.Subject, clientID),
                          new Claim(JwtClaimTypes.IssuedAt, new DateTimeOffset(now).ToEpochTime().ToString(),
                              ClaimValueTypes.Integer64)
              }),
              expires:now.AddMinutes(5),
              signingCredentials: new SigningCredentials(new SymmetricSecurityKey(Encoding.UTF8.GetBytes(&quot;MyVerySpecialSecret&quot;)), SecurityAlgorithms.HmacSha256Signature)
          );
  
          var clientAuthJwt = tokenHandler.WriteToken(securityToken);
  var request = new AuthorizationCodeTokenRequest()
  {
  Address = opEndPoint.TokenEndpoint,
  ClientId = clientID,
  Code = code,
  ClientAssertion = new ClientAssertion()
  {
  Type = OidcConstants.ClientAssertionTypes.JwtBearer,
  Value = clientAuthJwt
  },
  RedirectUri = opEndPoint.RedirectUri,
  GrantType = OidcConstants.GrantTypes.AuthorizationCode
  };
  
      var response = client.RequestAuthorizationCodeTokenAsync(request).Result;
  </code></pre>
  <p>i'm getting &quot;invalid_client&quot; , so clearly the SigningCredentials i am using is not correct
  could not find anywhere a working code example.</p>