When is JWTSecurityTokenHandler.ValidateToken() actually valid? - authentication

Question

I am attempting to create a token validation method that returns true if a JWT token is valid based on the signature. I don't think I really need to validate everything in the token but what actually signifies a token is valid after calling ValidateToken()? The existence of a principle? The out referenced token contains certain values? Not sure when to return true from this method.
public bool ValidateToken(string tokenString)
{
var validationParameters = new TokenValidationParameters()
{
ValidIssuer = "My Company",
ValidAudience = ApplicationId,
IssuerSigningKey = JsonWebTokenSecretKey
};
SecurityToken token = new JwtSecurityToken();
var tokenHandler = new JwtSecurityTokenHandler();
var principal = tokenHandler.ValidateToken(tokenString, validationParameters, out token);
return principal != null;
}

Answer 1

I check all of the claims values manually. I've been searching for a definitive answer to your same question but the only thing I have seen is that the ValidateToken function will throw an Exception if something is wrong, so I begin by wrapping the call in a try-catch and return false from the catch.
That's just my "first-pass" at validating the token, though. Afterwards I do a little more heavy lifting to check certain values manually. For example, I make sure that the unique_name value in the claims section actually exists as a user in my database, that the user has not been deactivated, and other proprietary system stuff like that.
public static bool VerifyToken(string token)
{
var validationParameters = new TokenValidationParameters()
{
IssuerSigningToken = new BinarySecretSecurityToken(_key),
ValidAudience = _audience,
ValidIssuer = _issuer,
ValidateLifetime = true,
ValidateAudience = true,
ValidateIssuer = true,
ValidateIssuerSigningKey = true
};
var tokenHandler = new JwtSecurityTokenHandler();
SecurityToken validatedToken = null;
try
{
tokenHandler.ValidateToken(token, validationParameters, out validatedToken);
}
catch(SecurityTokenException)
{
return false;
}
catch(Exception e)
{
log(e.ToString()); //something else happened
throw;
}
//... manual validations return false if anything untoward is discovered
return validatedToken != null;
}
The last line, return validatedToken != null, is purely superstition on my part. I've never seen the validatedToken be null.