If I get a JWT and I can decode the payload, how is that secure? Couldn't I just grab the token out of the header, decode and change the user information in the payload, and send it back with the same correct encoded secret?
I know they must be secure, but I just would really like to understand the technologies. What am I missing?
JWTs can be either signed, encrypted or both. If a token is signed, but not encrypted, everyone can read its contents, but when you don't know the private key, you can't change it. Otherwise, the receiver will notice that the signature won't match anymore.
Answer to your comment: I'm not sure if I understand your comment the right way. Just to be sure: do you know and understand digital signatures? I'll just briefly explain one variant (HMAC, which is symmetrical, but there are many others).
Let's assume Alice wants to send a JWT to Bob. They both know some shared secret. Mallory doesn't know that secret, but wants to interfere and change the JWT. To prevent that, Alice calculates Hash(payload + secret) and appends this as signature.
When receiving the message, Bob can also calculate Hash(payload + secret) to check whether the signature matches.
If however, Mallory changes something in the content, she isn't able to calculate the matching signature (which would be Hash(newContent + secret)). She doesn't know the secret and has no way of finding it out.
This means if she changes something, the signature won't match anymore, and Bob will simply not accept the JWT anymore.
Let's suppose, I send another person the message {"id":1} and sign it with Hash(content + secret). (+ is just concatenation here). I use the SHA256 Hash function, and the signature I get is: 330e7b0775561c6e95797d4dd306a150046e239986f0a1373230fda0235bda8c. Now it's your turn: play the role of Mallory and try to sign the message {"id":2}. You can't because you don't know which secret I used. If I suppose that the recipient knows the secret, he CAN calculate the signature of any message and check if it's correct.
You can go to jwt.io, paste your token and read the contents. This is jarring for a lot of people initially.
The short answer is that JWT doesn't concern itself with encryption. It cares about validation. That is to say, it can always get the answer for "Have the contents of this token been manipulated"? This means user manipulation of the JWT token is futile because the server will know and disregard the token. The server adds a signature based on the payload when issuing a token to the client. Later on it verifies the payload and matching signature.
The logical question is what is the motivation for not concerning itself with encrypted contents?
The simplest reason is because it assumes this is a solved problem for the most part. If dealing with a client like the web browser for example, you can store the JWT tokens in a cookie that is secure (is not transmitted via HTTP, only via HTTPS) and httpOnly (can't be read by Javascript) and talks to the server over an encrypted channel (HTTPS). Once you know you have a secure channel between the server and client you can securely exchange JWT or whatever else you want.
This keeps thing simple. A simple implementation makes adoption easier but it also lets each layer do what it does best (let HTTPS handle encryption).
JWT isn't meant to store sensitive data. Once the server receives the JWT token and validates it, it is free to lookup the user ID in its own database for additional information for that user (like permissions, postal address, etc). This keeps JWT small in size and avoids inadvertent information leakage because everyone knows not to keep sensitive data in JWT.
It's not too different from how cookies themselves work. Cookies often contain unencrypted payloads. If you are using HTTPS then everything is good. If you aren't then it's advisable to encrypt sensitive cookies themselves. Not doing so will mean that a man-in-the-middle attack is possible--a proxy server or ISP reads the cookies and then replays them later on pretending to be you. For similar reasons, JWT should always be exchanged over a secure layer like HTTPS.
Let's discuss from the very beginning:
JWT is a very modern, simple and secure approach which extends for Json Web Tokens. Json Web Tokens are a stateless solution for authentication. So there is no need to store any session state on the server, which of course is perfect for restful APIs.
Restful APIs should always be stateless, and the most widely used alternative to authentication with JWTs is to just store the user's log-in state on the server using sessions. But then of course does not follow the principle that says that restful APIs should be stateless and that's why solutions like JWT became popular and effective.
So now let's know how authentication actually works with Json Web Tokens. Assuming we already have a registered user in our database. So the user's client starts by making a post request with the username and the password, the application then checks if the user exists and if the password is correct, then the application will generate a unique Json Web Token for only that user.
The token is created using a secret string that is stored on a server. Next, the server then sends that JWT back to the client which will store it either in a cookie or in local storage.
Just like this, the user is authenticated and basically logged into our application without leaving any state on the server.
So the server does in fact not know which user is actually logged in, but of course, the user knows that he's logged in because he has a valid Json Web Token which is a bit like a passport to access protected parts of the application.
So again, just to make sure you got the idea. A user is logged in as soon as he gets back his unique valid Json Web Token which is not saved anywhere on the server. And so this process is therefore completely stateless.
Then, each time a user wants to access a protected route like his user profile data, for example. He sends his Json Web Token along with a request, so it's a bit like showing his passport to get access to that route.
Once the request hits the server, our app will then verify if the Json Web Token is actually valid and if the user is really who he says he is, well then the requested data will be sent to the client and if not, then there will be an error telling the user that he's not allowed to access that resource.
All this communication must happen over https, so secure encrypted Http in order to prevent that anyone can get access to passwords or Json Web Tokens. Only then we have a really secure system.
So a Json Web Token looks like left part of this screenshot which was taken from the JWT debugger at jwt.io. So essentially, it's an encoding string made up of three parts. The header, the payload and the signature Now the header is just some metadata about the token itself and the payload is the data that we can encode into the token, any data really that we want. So the more data we want to encode here the bigger the JWT. Anyway, these two parts are just plain text that will get encoded, but not encrypted.
So anyone will be able to decode them and to read them, we cannot store any sensitive data in here. But that's not a problem at all because in the third part, so in the signature, is where things really get interesting. The signature is created using the header, the payload, and the secret that is saved on the server.
And this whole process is then called signing the Json Web Token. The signing algorithm takes the header, the payload, and the secret to create a unique signature. So only this data plus the secret can create this signature, all right?
Then together with the header and the payload, these signature forms the JWT,
which then gets sent to the client.
Once the server receives a JWT to grant access to a protected route, it needs to verify it in order to determine if the user really is who he claims to be. In other words, it will verify if no one changed the header and the payload data of the token. So again, this verification step will check if no third party actually altered either the header or the payload of the Json Web Token.
So, how does this verification actually work? Well, it is actually quite straightforward. Once the JWT is received, the verification will take its header and payload, and together with the secret that is still saved on the server, basically create a test signature.
But the original signature that was generated when the JWT was first created is still in the token, right? And that's the key to this verification. Because now all we have to do is to compare the test signature with the original signature.
And if the test signature is the same as the original signature, then it means that the payload and the header have not been modified.
Because if they had been modified, then the test signature would have to be different. Therefore in this case where there has been no alteration of the data, we can then authenticate the user. And of course, if the two signatures
are actually different, well, then it means that someone tampered with the data.
Usually by trying to change the payload. But that third party manipulating the payload does of course not have access to the secret, so they cannot sign the JWT.
So the original signature will never correspond to the manipulated data.
And therefore, the verification will always fail in this case. And that's the key to making this whole system work. It's the magic that makes JWT so simple,
but also extremely powerful.
The contents in a json web token (JWT) are not inherently secure, but there is a built-in feature for verifying token authenticity. A JWT is three hashes separated by periods. The third is the signature. In a public/private key system, the issuer signs the token signature with a private key which can only be verified by its corresponding public key.
It is important to understand the distinction between issuer and verifier. The recipient of the token is responsible for verifying it.
There are two critical steps in using JWT securely in a web application: 1) send them over an encrypted channel, and 2) verify the signature immediately upon receiving it. The asymmetric nature of public key cryptography makes JWT signature verification possible. A public key verifies a JWT was signed by its matching private key. No other combination of keys can do this verification, thus preventing impersonation attempts. Follow these two steps and we can guarantee with mathematical certainty the authenticity of a JWT.
More reading: How does a public key verify a signature?
I would explain this with an example.
Say I borrowed $10 from you, then I gave you an IOU with my signature on it. I will pay you back whenever you or someone else bring this IOU back to me, I will check the signature to make sure that is mine.
I can't make sure you don't show the content of this IOU to anyone or even give it to a third person, all I care is that this IOU is signed by me, when someone shows this IOU to me and ask me to pay it.
The way how JWT works is quite the same, the server can only make sure that the token received was issued by itself.
You need other measures to make it secure, like encryption in transfer with HTTPS, making sure that the local storage storing the token is secured, setting up origins.
Ref - JWT Structure and Security
It is important to note that JWT are used for authorization and not authentication.
So a JWT will be created for you only after you have been authenticated by the server by may be specifying the credentials. Once JWT has been created for all future interactions with server JWT can be used. So JWT tells that server that this user has been authenticated, let him access the particular resource if he has the role.
Information in the payload of the JWT is visible to everyone. There can be a "Man in the Middle" attack and the contents of the JWT can be changed. So we should not pass any sensitive information like passwords in the payload. We can encrypt the payload data if we want to make it more secure. If Payload is tampered with server will recognize it.
So suppose a user has been authenticated and provided with a JWT. Generated JWT has a claim specifying role of Admin. Also the Signature is generated with
This JWT is now tampered with and suppose the
role is changed to Super Admin
Then when the server receives this token it will again generate the signature using the secret key(which only the server has) and the payload. It will not match the signature
in the JWT. So the server will know that the JWT has been tampered with.
Only JWT's privateKey, which is on your server will decrypt the encrypted JWT. Those who know the privateKey will be able to decrypt the encrypted JWT.
Hide the privateKey in a secure location in your server and never tell anyone the privateKey.
I am not a cryptography specialist and hence (I hope) my answer can help somebody who is neither.
There are two possible ways of using cryptography in programming:
Signing / verifying
Encryption / decryption
We use Signing when we want to ensure that data comes from a trusted source.
We use Encryption when we want to protect the data.
Signing / verifying uses asymmetrical algorithms i.e. we sign with one key (private) and the data receiver uses the other (public) key to verify.
A symmetric algorithm uses the same key to encrypt and decrypt data.
The encryption can be done using both symmetric and asymmetric algorithms.
relatively simple article on subject
The above is common knowledge below is my opinion.
When JWT is used for simple client-to-server identification there is no need for signing or asymmetric encryption. JWT can be encrypted with AES which is fast and supersecure. If the server can decrypt it, it means the server is the one who encrypted it.
Summary: non-encrypted JWT is not secure. Symmetric encryption can be used instead of signing in case no third party is involved.
I am new to JWT and hence trying to understand the intricacies and expectations when using JWT. In my case, I own the micro-service generating the JWT tokens. JWT supports RS256 and HS256 mechanisms. From what I understand, in case of RS256, I distribute the public key to my client. In case of HS256, I distribute the secret to my client.
Assuming I give access to either of these to my client, what is the expectation from the client? Should the client treat the token I passed back to them as immutable and they just send me the exact token back in the subsequent API calls? Or is it recommended/ okay for them to mutate the token and create a new one, for e.g. by changing things like audience in the token payload, and then for my server to verify the newly passed token? My server should be able to verify both tokens, but I don't know which is the recommended approach. If the recommended approach is for my client to not mutate the token, why do I need to distribute the keys to the client in the first place?
Do not distribute the key to the client. The client should not be able to modify the JWT access token. It should treat the token as opaque.
Only the resource server (exposing the API) that accepts the token should verify the signature.
In OAuth2, the client is the application getting the access token and using it to call an API server. It should not care about the token as long as it works to call the API.
The authorization server authenticates the user, gets the user's consent and issues the token to the client.
The client then uses the token to call the resource server (API) with the token in the Authorization header.
In your case, the authorization server and resource server may be the same, but they should not share signing keys with the client.
While I understand how jwt works for authentication, I'm trying to build registration.
Registration has a step that requires verification.
User enters the phone number
Code is sent via sms to user
User enters the code and device is verified
Since it's an API for mobile app there is no session/cookie built in.
I'm wondering if I can not implement cookie like system for mobile. And just make it work with JWT. But I don't have much experience with this.
This is my current flow:
User makes a POST request with Phone #
I respond with JWT (Time:Number)
I also send code via SMS
User sends the code via POST and JWT
Problem:
I don't know if code belongs to user or not, as I didn't save the code in DB.
I can't put into payload as it's just encoded not encrypted. (why is it not encrypted, what's the point of sending plain payload, what's even the point of JWT & didn't signed cookies already do that? they had session string encrypted so you couldn't change session string without invalidating the cookie)
JWT is overcome authentication/authorization mostly in APIs. JWT Access Token/Refresh Token is nothing but a JSON data in an encrypted form. JWTs can be signed using a secret (with the HMAC algorithm) or a public/private key pair using RSA or ECDSA. Key pair using RSA or ECDSA is more preferable as a security point of view. You can check authentication/authorization by decryption JWTs.
JWT token itself contains expiry date, so you can just configure expiration time to it. Access tokens are to check that user is authorized or not. Refresh tokens is necessary to get a new access token, generally used when the old access tokens are expired. Refresh token and Access token both expire but refresh token is long-lived compare to access tokens.
Generally developers use access token only but it is preferable to use access and refresh token both.
I am currently building out a micro service architecture and started with the auth server and client. I also wanted to confirm the best flow of authenticating a user with a token.
In the above image. Step 3 is were I start getting confused.
I thought of 2 solutions to the problem.
One every api passes the token to the auth server and waits to get approval that the token stored inside matches the db and it is still valid.
Two is to include a secret phrase in the JWT token and just have the API service parse and check for itself if the token is valid.(The secret phrase would be so that if a hacker tried to fake a token and it parsed to a valid id somehow the phrase would be off without the secret code used to encrypt the token. Which I don't even know if it is possible. If not then I guess 2 would be the best course of action)
A hacker cannot create a valid JWT token if he does not know the the signing key. If he somehow manages to get that signing key it is reasonable to assume that he is able to get your "secret phrase" also.
About the checking: JWT tokens can be checked by the API service as they contain all the information needed (except the signing key that must be known by the API service). The expiration can be checked here also. Anyway, you also need the information stored inside the token, like user ID. You should do this if you want better scalability.
The only reason why you would need to check a JWT token against a third Auth service is to see if it has been invalidated; for this you need a central service although you could replicate the list of invalid tokens to all the API services for better resilience.
You really don't have to forward the request to Auth-server to validate the JWT token. A JWT token is like a bill note, once it's signed it can be validated by anyone who is sharing the key.
I would recommend you to have an edge service in front of all your API-services. The edge service either shares the key by which JWT token is signed by Auth service or has the public key to verify the signature.
Once the signature is verified, the edge service can extract the required information from the token and add it to request header. Your downstream services can consume this information according to their need.
You can use Https to enforce that your request isn't intercepted by anyone over the network. In case, even if someone tries to mess up with the JWT token, the signature won't match and you can detect that. Please go through JWT/KONG: Cannot create JWTs with a shared secret to know more about creating-parsing the JWT token with public-private keys.
I am a complete noob when it comes to security, authentication strategies. So I was reading this article about "Token Based Authentication":
https://scotch.io/tutorials/the-ins-and-outs-of-token-based-authentication
I have 2 questions:
I don't understand why a middleman(or a hacker) would not be able to see the token being sent by the client and use the same to impersonate as that client/person to retrieve resources? What makes JSON Web Tokens / OAuth2 based authentications safer in that sense? If we use a onetime-use-only token every time, I would understand that even if the hacker can read the token he will not be able to use it for another request. But as the token stays the same until it expires, how is that a safer authentication strategy?
How does the server know that the token sent by the client is valid i.e something that the server exchanged with the client during login. Does the server store the token generated in a database or somewhere and keep updating the "last accessed timestamp" or something and keeps removing the tokens where last_accessed_time is > 1hour ago, to keep expiring it after 1 hour of inactivity?
I don't understand why a middleman (or a hacker) would not be able to see the token being sent by the client and use the same to impersonate as that client/person to retrieve resources?
JWT does not protect you to a man-in-the-middle (MITM) attack. If an attacker gets a valid token, can effectively impersonate. Even if the content is encrypted.
JWT should be used with a SSL/TLS connection to avoid MITM
What makes JSON Web Tokens / OAuth2 based authentications safer in that sense?
JWT is a token format, and oauth2 is a protocol. oauth2 can use jwt. Oauth2 is safer to the user using a third party site because credentials are only sent from the user to the main site, then the site issues a token that can be used by the third party site to authenticate user. The third party site never see the user credentials
But as the token stays the same until it expires, how is that a safer authentication strategy?
Read above. You need to protect your tokens to not be stolen: Mainly use HTTPS, or mitigate its effects: store in cookies with HttpOnly (if you do not need to access JWT content in client side), set expiration time short, rotate tokens...
How does the server know that the token sent by the client is valid i.e something that the server exchanged with the client during login.
The third part of a JWT like hhhh.pppp.ssss is the signature. The signature is performed with server private key over the header and payload (hhhh.pppp), an protects the content. If an attacker alters the content or the signature, the server will detect it verifying the signature and will reject the authentication.
Does the server store the token generated in a database or somewhere and keep updating the "last accessed timestamp" or something and keeps removing the tokens where last_accessed_time is > 1 hour ago, to keep expiring it after 1 hour of inactivity?
It is not needed. The signature is packed in the token itself (ssss), therefore it is said that JWT is self-contained
The server has a cryptographic secret key or a key pair, public and private. The token is signed and verified with the secret key (for HMAC symmetric keys), or signed with the private key and verified with the corresponding public key (for RSA asymmetric keys).
It is all about signing the token not encrypting the token. The server just verifies the signature, JWT is not encrypted (unless you implement it).
Dont store sensitive data in the token, cause it is not encrypted by default.