Validate a simple token in the request

The following viewer request function validates a JSON web token (JWT) in the query string of a request. If the token is valid, the function returns the original, unmodified request to CloudFront. If the token is not valid, the function generates an error response. This function uses the crypto module. For more information, see Built-in modules.

This function assumes that requests contain a JWT value in a query string parameter named jwt. The CloudFront KeyValueStore ID is optional in the following code.

See this example on GitHub.

JavaScript runtime 2.0

import crypto from 'crypto';
import cf from 'cloudfront';

//Response when JWT is not valid.
const response401 = {
    statusCode: 401,
    statusDescription: 'Unauthorized'
};

// Remember to associate the KVS with your function before calling the const kvsKey = 'jwt.secret'. 
// https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/kvs-with-functions-associate.html
const kvsKey = 'jwt.secret';
// set to true to enable console logging
const loggingEnabled = false;


function jwt_decode(token, key, noVerify, algorithm) {
    // check token
    if (!token) {
        throw new Error('No token supplied');
    }
    // check segments
    const segments = token.split('.');
    if (segments.length !== 3) {
        throw new Error('Not enough or too many segments');
    }

    // All segment should be base64
    const headerSeg = segments[0];
    const payloadSeg = segments[1];
    const signatureSeg = segments[2];

    // base64 decode and parse JSON
    const payload = JSON.parse(_base64urlDecode(payloadSeg));

    if (!noVerify) {
        const signingMethod = 'sha256';
        const signingType = 'hmac';

        // Verify signature. `sign` will return base64 string.
        const signingInput = [headerSeg, payloadSeg].join('.');

        if (!_verify(signingInput, key, signingMethod, signingType, signatureSeg)) {
            throw new Error('Signature verification failed');
        }

        // Support for nbf and exp claims.
        // According to the RFC, they should be in seconds.
        if (payload.nbf && Date.now() < payload.nbf*1000) {
            throw new Error('Token not yet active');
        }

        if (payload.exp && Date.now() > payload.exp*1000) {
            throw new Error('Token expired');
        }
    }

    return payload;
}

//Function to ensure a constant time comparison to prevent
//timing side channels.
function _constantTimeEquals(a, b) {
    if (a.length != b.length) {
        return false;
    }

    let xor = 0;
    for (let i = 0; i < a.length; i++) {
    xor |= (a.charCodeAt(i) ^ b.charCodeAt(i));
    }

    return 0 === xor;
}

function _verify(input, key, method, type, signature) {
    if(type === "hmac") {
        return _constantTimeEquals(signature, _sign(input, key, method));
    }
    else {
        throw new Error('Algorithm type not recognized');
    }
}

function _sign(input, key, method) {
    return crypto.createHmac(method, key).update(input).digest('base64url');
}

function _base64urlDecode(str) {
    return Buffer.from(str, 'base64url')
}

async function handler(event) {
    let request = event.request;

    //Secret key used to verify JWT token.
    //Update with your own key.
    const secret_key = await getSecret()

    if(!secret_key) {
        return response401;
    }

    // If no JWT token, then generate HTTP redirect 401 response.
    if(!request.querystring.jwt) {
        log("Error: No JWT in the querystring");
        return response401;
    }

    const jwtToken = request.querystring.jwt.value;

    try{ 
        jwt_decode(jwtToken, secret_key);
    }
    catch(e) {
        log(e);
        return response401;
    }

    //Remove the JWT from the query string if valid and return.
    delete request.querystring.jwt;
    log("Valid JWT token");
    return request;
}

// get secret from key value store 
async function getSecret() {
    // initialize cloudfront kv store and get the key value 
    try {
        const kvsHandle = cf.kvs();
        return await kvsHandle.get(kvsKey);
    } catch (err) {
        log(`Error reading value for key: ${kvsKey}, error: ${err}`);
        return null;
    }

}

function log(message) {
    if (loggingEnabled) {
        console.log(message);
    }
}