Securing REST Web Service using token (Java)

Question

This question is in some way related to the below linked question. However, I need a little more clarity on some aspects and some additional information. Refer: REST Web Service authentication token implementation

Background:

• I need to implement security for a REST Web Service using token
• The webservice is intended for use with Java client. Hence, form authentication and popups for credentials are not useful.
• I'm new to REST security and encryption

This is what I have understood till now:

For first request:

1. User establishes https connection (or container ensures https using 301)
2. User POSTs username and password to login service
3. If credentials are valid we:
   • Generate a random temporary token
   • Store the random token on server mapping it to actual username
   • Encrypt the token using a symmetric key only known to server
   • Hash the encrypted token
   • Send the encrypted token and the hash to the client

For subsequent requests:

1. Client sends this encrypted token and hash combination (using username field of basic?)
2. We make sure the encrypted token is not tampered using the hash and then decrypt it
3. We check the decrypted token in the session-tracking-table for a not-expired entry and get the actual username (expiry to be managed by code?)
4. If the username is found, based on allowed roles, allowed operations are configured

More details:

1. Since client is a java client, the first request can be a POST containing the credentials. However, this looks like it may expose the credentials before the https gets established. Hence should there be a dummy GET to a secured resource so that https is established first?
2. Assuming above is required, the second request is a LoginAction POST with credentials. This request is handled manually (not using container's authorisation). Is this right?
3. The above LoginAction returns the user the combination of encrypted token + hash
4. User sets it to the header that is used by BASIC authentication mechanism (field username)
5. We implement a JAASRealm to decrypt and validate the token, and find the roles allowed
6. The rest of authorisation process is taken care of by the container with the WebResourceCollection defined in the web.xml

Is this the correct approach?

Answer 1

Why not simplify it to the following?

For first request:

1. User establishes HTTPS connection to server (service does not listen on any other ports) and POSTs credentials to login service.
2. Server replies with HSTS header to ensure all further communication is HTTPS.
3. If credentials are valid we:
   • Generate a random temporary token which is securely generated using a CSPRNG. Make this long enough to be secure (128 bit).
   • Store the random token on server mapping it to actual username.
   • Send the random token to the client

For subsequent requests:

1. Client sends token in a custom HTTP header over HTTPS.
2. Token is located in the DB and mapped to the username. If found access is configured based on allowed roles and allowed operations.
3. If not found user is considered unauthenticated and will have to authenticate with the login service again to get a new token.

On the server side the token will be stored with an expiry date. On each access to the service this date will be updated to create a sliding expiration. There will be a job that will run every few minutes to delete expired tokens and the query that checks the token for a valid session will only check those that have not deemed to have expired (to prevent permanent sessions if the scheduled job fails for any reason).

There is no need to hash and encrypt the tokens within the database - it adds no real value apart from a touch of security through obscurity. You could just hash though. This would prevent an attacker that managed to get at the session data table from hijacking existing user sessions.

Answer 2

The approach looks ok. Not very secure. Let me highlight some of the attacks possible with the request.

1. Man-In-the-middle attack in a POST request, the user can tamper with the request and server does not have any way to ensure the data is not tampered.

2. Replay attack: In this, the attacker does not tamper with the request. The attacker taps the request and sends it to the server multiple times in a short duration, though it is a valid request, the server processes the request multiple times, which is not needed Please read about Nonce.

3. In the first step, the user sends his credentials i.e username and password to the login service and if you have a web based application that also uses the same password it might be dangerous. If in case password in compromised, API and web everything is exposed, please use a different PIN for API access. Also, ensure decrypted token as specified by you, expires after a certain time.

4. Ensure the service (application server) tomcat. jboss never returns a server page in case of internal error, this gives the attacker extra information of the server where the app is deployed.

-- MODIFIED BASED ON SECOND POST --
Yes your correct if your using mutual SSL, but in case its a one way access you don't have the client certificates. It would be good if you just double ensured everything in the request, just like signed (signature) SOAP, one of the strong data transfer mechanism. But replay attack is a possibility with HTTPS, just handle that. Rest use tokens encryption is good. And why not ask the client to decrypt the token with the password and return the output of the decryption by this you can validate the output, if it is present in your database ? This approach the user does not send the password over the wire even if it is HTTPS ?