Hold on tight: the (*) and when it comes to user authentication this screams loudly. HTTP protocol is terribly flawed problem For a long time we, as developers, fought with it: sometimes with good results, sometimes not, but we thought we were happy. Unfortunately, the web goes fast and many of these solutions was getting old too quickly. He who hesitates is lost… Later on, a group of people realised that it was time to stop fighting with the and try to embrace it. The result of that epiphany is called JSON Web Token (JWT for short) and here we will try to tell its story… “problem” Once upon a time Suppose you have a (e.g. ) and you want to restrict access to the authorized users only. In the most naïve approach, the API would ask for a username and password; then it will be searched into a database if those credentials really exists. We check for . Finally, it will be checked if the user is also to perform that request. If both checks passes the real API will be executed. It seems logical. REST API GET /orders authenticity authenticated authorized A problem of state , that means a new request (e.g. ) won’t know anything about the previous one, (fig.1). The HTTP protocol is stateless GET /order/42 so we need to reauthenticate for each new request Fig. 1 — Due to the stateless nature of HTTP protocol, every new API request needs a complete authentication. In this scenario, we first check for username and password; if they are authentic the server will save a in memory and return it to the client. From now on, client will just need to send its to be recognized (fig.2). The traditional way of dealing with this is the use of Server Side Sessions (SSS). session id session id Fig. 2–Using SSS, we reduce the number of authentications towards the Credentials database. This solution will fix a problem but it will create another one. Probably bigger. A problem of scale In the IT world, the time goes fast and a solution that yesterday was commonly used might be outdated now. Server Side Sessions are one of those. In the APIs era, our endpoints can face a huge amount of requests, so our infrastructures needs to scale. There are two types of scaling: scaling up your infrastructure means merely add more resources to a server. This is an expensive solution with a low upper limit (i.e. the server’s max resources allocation); • vertical scaling – • scaling out your infrastructure is simpler and cost-effective as add a new server behind a ; horizontal scaling – load balancer Now it’s seems pretty clear that the second approach will be far most beneficial; but let’s take a look at what may happens. In the initial scenario, behind the load balancer, there’s just one server. When a client will perform a request, using session id xyz, its record will be surely found in server’s memory (fig.3). So far, so good. Fig. 3–One single server behind the load balancer. The session id of the request will be found in memory. Now imagine that the above infrastructure needs to scale. A new server (i.e. ) will be added behind the load balancer and this brand new server will handle the next request issued by client… Server 2:2 xyz Fig.4–A new server is behind the LB, it knows nothing about previous session so the user won’t be recognized. Unauthenticated! The brand new server, has no sessions in its memory so the authentication process will fail. To fix this we have mainly three that can be used: xyz workarounds s — tricky and error-prone; • Synchronize sessions between server • — Good solutions but it will add another infrastructure’s component; Use an external in-memory database Third: and search for a better solution! embrace the stateless nature of HTTP The better solution JSON Web Token (JWT) is an open standard ( ) that defines a way for transmitting information –like authentication and authorization facts– between two parties: an and an . Communication is safe because each token issued is digitally signed, so the consumer can verify if the token is authentic or has been forged. RFC 7519 issuer audience Each token is , that means it contains every information needed to allow or deny any given requests to an API. To understand how we can verify a token and how authorization happens we need to take a step back and look into a JWT. self-contained Anatomy of a JWT A JWT token is essentially a long encoded text string. This string is composed of three smaller parts, separated by a dot sign. These parts are: • the header; • a payload or body; • a signature; Therefore, our tokens will look like this: header.payload.signature The header section contains information about the token itself. Header { : , : } "kid" "ywdoAL4WL...rV4InvRo=" "alg" "RS256" The following JSON explains what algorithm has been used to sign the token (alg) and what is the key (kid) that we need to use to validate it. One moment of patience, please, we will look into this soon. :) The JSON is finally encoded as Base64URL: eyJraWQiOiJ -TRUNCATED- JTMjU2In0 The payload is the most important part of a JWT token. It contains information (claims in JWT jargon) about the client: Payload or body {
  [...] : , : , : } "iss" "https://cognito-idp.eu-west-1.amazonaws.com/XXX" "name" "Mariano Calandra" "admin" false The property is a , it represent the identity provider that issued the token — in this case . Finally, we can add further claims based to our needs (e.g. claim). iss registered claim Amazon Cognito admin The payload is then encoded as Base64URL: eyJzdWIiOiJkZGU5N2Y0ZC0wNmQyLTQwZjEtYWJkNi0xZWRhODM1YzExM2UiLCJhdWQiOiI3c2Jzamh -TRUNCATED- hbnRfaWQiOiJ4cGVwcGVycy5jb20iLCJleHAiOjE1N jY4MzQwMDgsImlhdCI6MTU2NjgzMDQwOH0 The third part of token is an hash that is computed following these steps: Signature • join with a dot the encoded header and the encoded payload; • hash the result using the encryption algorithm specified in property of the header (in this case RS256) and a private ; alg key • encode the result as Base64URL; Here we can look at it as pseudo-code: data = base64UrlEncode(header) + + base64UrlEncode(payload);
hash = RS256(data, private_key);
signature = base64UrlEncode(hash); "." And here it is the computed signature: POstGetfAytaZS82wHcjoTyoqhMyxXiWdR7Nn7A29DNSl0EiXLdwJ6xC6AfgZWF1bOsS_TuYI3OG85 -TRUNCATED- FfEbLxtF2pZS6YC1aSfLQxeNe8djT9YjpvRZA Once we have the encoded header, the encoded payload and the encoded signature we can join everything together simply merging every piece with a dot: Put everything together eyJzdWIiOiJkZGU5N2Y0ZC0wNmQyLTQwZjEtYWJkNi0xZWRhODM1YzExM2UiLCJhdWQiOiI3c2Jzamh -TRUNCATED- hbnRfaWQiOiJ4cGVwcGVycy5jb20iLCJleHAiOjE1N jY4MzQwMDgsImlhdCI6MTU2NjgzMDQwOH0.eyJzdWIiOiJkZGU5N2Y0ZC0wNmQyLTQwZjEtYWJkNi0xZWRhODM1YzExM2UiLCJhdWQiOiI3c2Jzamh -TRUNCATED- hbnRfaWQiOiJ4cGVwcGVycy5jb20iLCJleHAiOjE1N jY4MzQwMDgsImlhdCI6MTU2NjgzMDQwOH0.POstGetfAytaZS82wHcjoTyoqhMyxXiWdR7Nn7A29DNSl0EiXLdwJ6xC6AfgZWF1bOsS_TuYI3OG85 -TRUNCATED- FfEbLxtF2pZS6YC1aSfLQxeNe8djT9YjpvRZA Even if the above token seems encrypted it isn’t! Unlike RS256, , so mind your payload! Note: Base64URL is not an encryption algorithm JWT validation Since token is self-contained, we own all the information needed for its validation. For example, we know that token has been signed using RS256 ( property of the header) and a private key. Now we need to know . Yes, the public key! alg how to get the right public key to perform the validation Note: In the asymmetric encryption, we all know that public key is used to encrypt a message, whereas private key is used to decrypt it. In a signing algorithm this process is completely switched! Here the message (the data in the pseudo-code above) is signed using the private key and the public key is used to verify that the signature is valid. The property of the body represent the endpoint of the issuer (Amazon Cognito in our case, but it should be no great differences with other providers), copy that URI and prepend it to the string . It should look something like: iss /.well-known/jwks.json https: //cognito-idp.eu-west-1.amazonaws.com/XXX/.well-known/jwks.json Following this URL, we will find a JSON: { : [
    { : , : , : , : , : , : },
    {...}
  ]
} "keys" "alg" "RS256" "e" "AQAB" "kid" "ywdoAL4WL...rV4InvRo=" "kty" "RSA" "n" "m7uImGR -TRUNCATED AhaabmiCq5WMQ" "use" "sig" In the array, search for the element that has the same of the token’s header. The properties and are the and that compute the public key. keys kid e n public exponent modulus Once we get the it, we can verify the signature. If it’s valid, we can be sure that information contained in the token are trusted. Note: The process of public key calculation or sign verification is not easy and will be out of scope for this post. A real case scenario At the first access, a client needs to contact the (Amazon Cognito here, but Microsoft, Salesforce or any other provider should be pretty similar), sending username and password to it. If credentials are valid, a JWT token will be returned to the client that will use it to request an API (in this example Amazon API Gateway endpoint?). authorization server Fig.5 – The complete flow of a real case scenario. In the above scenario (fig.5), API itself is the only responsible for token validation and it’s able to reject the request if signature seems forged. Suppose a client wants to invoke a protected API to delete an order (e.g. ) and this action should be only performed by administrators. Going further DELETE /order/42 With a JWT in place, this operation is hard as add a custom claim to the payload body (i.e. the claim of the payload above). When invoked, the API will first verify the signature authenticity and afterward it’ll check if claim is . admin: true admin true Summary That’s all for now. We have seen many things about JWT, but something else still misses: • How do we configure Amazon Cognito to get a JWT token? • How do we configure Amazon Cognito to add a custom claim? • How do we programmatically verify the JWT for authenticity? Don’t worry, we have room for answering this questions in a later story. For now, let’s summarise some key points: • HTTP protocol is stateless, that means a new request won’t know anything about the previous one; • Server Side Sessions was a solution to statelessness of HTTP, but these, on the long run, were a threat to our scaling abilities; • JWT is , that means it contains every information needed to allow or deny any given requests to an API; self-contained • JWT is stateless by design, so we don’t have to fight with stateless design of HTTP; • JWT is encoded, not encrypted have it in mind; (*) Stateless nature of HTTP is clearly not a flaw. Just a provocation :) If you liked this story and want to support my work, please buy me a coffee .

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