How to validate a JWT token

Question

I'm trying to use JWT tokens. I managed to generate a valid JWTTokenString and validated it on the JWT debugger but I'm having an impossible time validating the token in .Net. Here's the code I have so far:

class Program {

    static string key = "401b09eab3c013d4ca54922bb802bec8fd5318192b0a75f201d8b3727429090fb337591abd3e44453b954555b7a0812e1081c39b740293f765eae731f5a65ed1";

    static void Main(string[] args) {
        var stringToken = GenerateToken();
        ValidateToken(stringToken);
    }

    private static string GenerateToken() {
        var securityKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(key));

        var credentials = new SigningCredentials(securityKey, SecurityAlgorithms.HmacSha256);

        var header = new JwtHeader(credentials);

        var payload = new JwtPayload {
           { "some ", "hello "},
           { "scope", "world"},
        };

        var secToken = new JwtSecurityToken(header, payload);
        var handler = new JwtSecurityTokenHandler();

        return handler.WriteToken(secToken);

    }

    private static bool ValidateToken(string authToken) {
        var tokenHandler = new JwtSecurityTokenHandler();
        var validationParameters = GetValidationParameters();

        SecurityToken validatedToken;
        IPrincipal principal = tokenHandler.ValidateToken(authToken, validationParameters, out validatedToken);
        Thread.CurrentPrincipal = principal;
        return true;
    }

    private static TokenValidationParameters GetValidationParameters() {
        return new TokenValidationParameters() {
            //NOT A CLUE WHAT TO PLACE HERE
        };
    }
}

All I want is a function that receives a token and returns true or false based on its validity. From research I've seen people use IssuerSigningToken to assign the validation key. But when I try to use it, it doesn't seem to exist. Could anyone give me a hand on validating the token?

Answer 1

You must use the same key to validate the token as the one you use to generate it. Also you need to disable some validations such as expiration, issuer and audiance, because the token you generate doesn't have these information (or you can add these information). Here's a working example:

class Program
{
    static string key = "401b09eab3c013d4ca54922bb802bec8fd5318192b0a75f201d8b3727429090fb337591abd3e44453b954555b7a0812e1081c39b740293f765eae731f5a65ed1";

    static void Main(string[] args)
    {
        var stringToken = GenerateToken();
        ValidateToken(stringToken);
    }

    private static string GenerateToken()
    {
        var securityKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(key));
        var credentials = new SigningCredentials(securityKey, SecurityAlgorithms.HmacSha256);

        var secToken = new JwtSecurityToken(
            signingCredentials: credentials,
            issuer: "Sample",
            audience: "Sample",
            claims: new[]
            {
                new Claim(JwtRegisteredClaimNames.Sub, "meziantou")
            },
            expires: DateTime.UtcNow.AddDays(1));

        var handler = new JwtSecurityTokenHandler();
        return handler.WriteToken(secToken);
    }

    private static bool ValidateToken(string authToken)
    {
        var tokenHandler = new JwtSecurityTokenHandler();
        var validationParameters = GetValidationParameters();

        SecurityToken validatedToken;
        IPrincipal principal = tokenHandler.ValidateToken(authToken, validationParameters, out validatedToken);
        return true;
    }

    private static TokenValidationParameters GetValidationParameters()
    {
        return new TokenValidationParameters()
        {
            ValidateLifetime = false, // Because there is no expiration in the generated token
            ValidateAudience = false, // Because there is no audiance in the generated token
            ValidateIssuer = false,   // Because there is no issuer in the generated token
            ValidIssuer = "Sample",
            ValidAudience = "Sample",
            IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(key)) // The same key as the one that generate the token
        };
    }
}

Answer 2

Validate Token in Jwt Middleware Class that Invoke Method fire in every request for Authorization

JwtMiddleware
    {
        private readonly RequestDelegate _next;
        private readonly TokenValidationParameters _tokenValidationParams;
        public JwtMiddleware(RequestDelegate next, TokenValidationParameters 
        tokenValidationParams)
        {
            _next = next;
            _tokenValidationParams = tokenValidationParams;
        }

    

    public async Task Invoke(HttpContext context)
            {
            try{
                var token = context.Request.Headers["Authorization"].FirstOrDefault()?.Split(" ").Last();
    
                var jwtTokenHandler = new JwtSecurityTokenHandler();
                // Validation 1 - Validation JWT token format
                var tokenInVerification = jwtTokenHandler.ValidateToken(token, _tokenValidationParams, out var validatedToken);
    
                if (validatedToken is JwtSecurityToken jwtSecurityToken)
                {
                    var result = jwtSecurityToken.Header.Alg.Equals(SecurityAlgorithms.HmacSha256, StringComparison.InvariantCultureIgnoreCase);
    
                    if (result == false)
                    {
                        Error Invalid = new Error()
                        {
                            Success = false,
                            Errors = "Token is Invalid"
                        };
    
                        context.Items["Error"] = Invalid;
                    }
                }
           }
           catch (Exception ex)
            {
                Error Invalid = new Error()
                {
                    Success = false,
                    Errors = "Token does not match or may expired."
                };
                context.Items["Error"] = Invalid ; // userService.GetById(userId);
            }
                await _next(context);
        }
    }