如何验证字符串是 JWT 令牌？

Question

In Java How can we verify that i given String is a JWT Token without using Signature?

I am using

try {
     return (new JwtConsumerBuilder()).setVerificationKey(SECRET_KEY).build().processToClaims(token);
} catch (InvalidJwtException var4) {
     throw new IOException("Failed to parse");
}  

This works fine but I want to verify this without SECRET_KEY .

I Just want to verify whether it is a JWT token or not.

Answer 1

Here is an example to check the structure of the JWT. You only need to add the validations of the data that the JWT should carry

boolean isJWT(String jwt) {
        String[] jwtSplitted = jwt.split("\\.");
        if (jwtSplitted.length != 3) // The JWT is composed of three parts
            return false;
        try {
            String jsonFirstPart = new String(Base64.getDecoder().decode(jwtSplitted[0]));
            JSONObject firstPart = new JSONObject(jsonFirstPart); // The first part of the JWT is a JSON
            if (!firstPart.has("alg")) // The first part has the attribute "alg"
                return false;
            String jsonSecondPart = new String(Base64.getDecoder().decode(jwtSplitted[1]));
            JSONObject secondPart = new JSONObject(jsonSecondPart); // The first part of the JWT is a JSON
            //Put the validations you think are necessary for the data the JWT should take to validate
        }catch (JSONException err){
            return false;
        }
        return true;
    }

Answer 2

You can decode JWT from base64 and check necessarily field. Or simply check token, why you want verify it, It's not faster then simpy check it.

Answer 3

Try this one.

  public static JSONObject getPayload(String jwt) {
        try {           
            Base64.Decoder dec = Base64.getDecoder();
            final String payload = jwt.split(SecurityConstants.SPLIT_SLASH)[PAYLOAD];
            final byte[] sectionDecoded = dec.decode(payload);
            final String jwtSection = new String(sectionDecoded, SecurityConstants.CODE_PAGE);
            return new JSONObject(jwtSection);
        } catch (final UnsupportedEncodingException e) {
            throw new InvalidParameterException(e.getMessage());
        } catch (final Exception e) {
            throw new InvalidParameterException(SecurityConstants.ERROR_IN_PARSING_JSON);
        }
    }

Next You need to get from jwt body this part which is important for You eg.

JSONObject body = JWTUtils.getPayload(token);
        String username = body.getString("username");