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How to verify that a string is JWT token?

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.

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Answer

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;
    }

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