We Keep Coding

Mobile authentication using QR in web application

I have doubts about what the correct schema should be for the next technical solution.
I need to authenticate a user in a mobile application by reading a QR code, the user being previously authenticated in a web application.
The use case consists in that the user uses a web application located in an intranet, but needs to be able to upload images from a mobile device that will be connected to the internet.
The mobile application will consume a public API exposed on the internet through a API Gateway.
The API Gateway will connect to the backend to upload the images.
As a requirement, when the user needs to use the mobile device to capture and upload images, they should not authenticate again, since they have an open session in the web application, and simply use a QR code to authenticate the device. Logically the QR will not use the user's credentials.
My idea is to make use of Oauth 2.0 with the following flow to authenticate mobile device:
The web application requests API Gateway to generate an authorization token and it responds with a UUID.
The web application will display the authorization token using a QR received from the API Gateway.
The mobile device will read the QR, and request an access token to the API Gateway with the authorization token.
The API Gateway validates the authorization token and generates the access token that is returned to the mobile device.
The mobile device makes calls to the public API (API Gateway) using the access token.
My question is whether it is the correct scheme, or there is another standard solution.
Thanks!!

Your scheme will work BUT it's not reaching its fullest security potential considering the fact that you can transfer a new generated authorization token from an already authorized device directly to another one(via QR code read by camera); this fact would make step 3 and 4 an unnecessary vulnerability(it's also a redundancy,there already is a token!, why get another one?);
The following alternative along with good cryptography can make the later authorized device connection almost impossible to intrude.the idea is that by adding a symmetric encryption layer before sending the data in step 5 and using a key which is exchanged over another medium(the already authorized device and server) the encrypted data can never be exposed;
step 3 replacement: read the authorization token;
step 4 replacement: check the secure hash derivation of the authorization token(instead of the token itself) with the server to see if its valid;
token0=read_auth_token_from_camera()
public_token=hash_function(token0) //the useless exposed token
if(check_token_with_server_for_authenticity(public_token)==true)
continue_to_step_5() //it's authorized
else
handle_the_scenario()
step 5 replacement: encrypt your request and authorization token with another hash derivation of the authorization token then make calls to the server API;
token2=another_hash_function(token0)
request="i am top secret data"
encryption_key=token0
encrypted_request=encryption_function( token2 + request , encryption_key)
send_to_server( public_token+encrypted_request)
//notice that token2 is unknown to the intruder because its encrypted,but it is known to the server; hence the authenticity of each request can be checked by the server;
how it is more secure: in this alternative way the actual authorization token is never really exchanged between the server and the new client; So if an intruder could hypothetically break the SSl/TLS layer and capture the public token, the intruder won't be able to send any requests on your behalf or modify the data in requests;

If this scheme fulfills your desired application flow then its correct. Their is no standard way to authenticate android devices if you are going for self-made authentication.
One thing that I think you need to add is a check whether the client is actually your android application and not any other application using similar authentication flow. If you can cater this than its good to go.

You can use FireBase ML Kit for a good solution and it can also performs a lot AI-based functions to your apps.
You can check it from here:
https://firebase.google.com

Rest API security for mobile apps

I'm in the process of designing a mobile application that will need to connect to a server-side process for its business logic and data transactions. I'm writing my server-side code in Java using Spring Boot and I intend to create a Rest API in order for the mobile app to connect to the server.
I'm just doing some research at the moment for the best way to secure the connection between mobile app and server. What I'd like to do is allow the user on the mobile app to log in once they open the app and for them to use the app for as long as they like and for their access to time out after a period of inactivity.
Can anyone recommend any very simple reading on this? I've looked at OAuth2 but that appears to be for logging into web services using another account (like Google, GitHub, etc).
Would it be acceptable to login over https (SSL) by passing username and password to a rest endpoint and returning some sort of token (a GUID?). Then have the client (mobile app) pass that GUID with each subsequent call so the server can verify the call? Is it better to just do everything over SSL in this scenario?
I have done a fair bit of research but I don't seem to be able to find anything that quite matches what I'm trying to do.
Hope someone can help
Thanks

OAUTH2 IS NOT ONLY FOR WEB
Can anyone recommend any very simple reading on this?
I've looked at OAuth2 but that appears to be for logging into web services using another account (like Google, GitHub, etc).
No OAUTH2 is not only for web apps is also for mobile apps and you read this article for a more in depth explanation, but I will leave you with the article introduction:
Like single-page apps, mobile apps also cannot maintain the confidentiality of a client secret. Because of this, mobile apps must also use an OAuth flow that does not require a client secret. The current best practice is to use the Authorization Flow along with launching an external browser, in order to ensure the native app cannot modify the browser window or inspect the contents. If the service supports PKCE, then that adds a layer of security to the mobile and native app flow.
The linked article is very brief, you will need to follow the next chapters to get the full picture.
DO NOT ROLL YOUR OWN AUTHENTICATION / AUTHORIZATION SOLUTION
Would it be acceptable to login over https (SSL) by passing username and password to a rest endpoint and returning some sort of token (a GUID?). Then have the client (mobile app) pass that GUID with each subsequent call so the server can verify the call?
While you can do it I strongly advise you to use an already established OAUTH2 or OPENID connect solution, because they are developed and maintained by experts in the field and battle tested by millions of web and mobile apps using them. This enables to identify ans fix security issues much more quickly that anyone could do in their own in-house solution.
OAuth2
OAuth 2.0 is the industry-standard protocol for authorization. OAuth 2.0 supersedes the work done on the original OAuth protocol created in 2006. OAuth 2.0 focuses on client developer simplicity while providing specific authorization flows for web applications, desktop applications, mobile phones, and living room devices. This specification and its extensions are being developed within the IETF OAuth Working Group.
OpenID Connect
OpenID Connect 1.0 is a simple identity layer on top of the OAuth 2.0 protocol. It allows Clients to verify the identity of the End-User based on the authentication performed by an Authorization Server, as well as to obtain basic profile information about the End-User in an interoperable and REST-like manner.
OpenID Connect performs many of the same tasks as OpenID 2.0, but does so in a way that is API-friendly, and usable by native and mobile applications. OpenID Connect defines optional mechanisms for robust signing and encryption. Whereas integration of OAuth 1.0a and OpenID 2.0 required an extension, in OpenID Connect, OAuth 2.0 capabilities are integrated with the protocol itself.
So for your authentication/authorization needs I would recommend you to go with an OpenID connect solution, that leverages OAuth2 under the hood.
SSL IS ALWAYS NECESSARY?
Is it better to just do everything over SSL in this scenario?
SSL must be always used for everything, http MUST not be used at all in any situation, because once you allow an http request you are vulnerable to a man in the middle attack and I strongly recommend you to read this article from a well know security researcher, Troy Hunt, to see how even a static website must use https and he goes to a great extent to explain why and names very important attack vectors that can harm an application not using https, like WiFi hot-spots hijacking, DNS Hijeacking, Router Exploits, China great cannon, and others.
IMPROVE SSL WITH CERTIFICATE PINNING
Communicating using https is the way to go for any kind of application but developers must be aware that an attacker in control of device where the application is installed can spy https traffic by doing a man in the middle attack with a custom certificate installed in the device the mobiel app is installed, enabling this way for him to understand how the mobile app communicates with the API server in order to mount automated attacks to abuse from same API.
Certificate Pinning
Pinning is the process of associating a host with their expected X509 certificate or public key. Once a certificate or public key is known or seen for a host, the certificate or public key is associated or 'pinned' to the host. If more than one certificate or public key is acceptable, then the program holds a pinset (taking from Jon Larimer and Kenny Root Google I/O talk). In this case, the advertised identity must match one of the elements in the pinset.
You can read this article, with code sample to see how easy is to implement certificate pinning, how it can be difficult to maintain in the operational side, and with a video to see how an attacker can bypass pass certificate pinning in the client side by using Xposed framework.
xPosed
Xposed is a framework for modules that can change the behavior of the system and apps without touching any APKs. That's great because it means that modules can work for different versions and even ROMs without any changes (as long as the original code was not changed too much). It's also easy to undo.
EDIT:
Nowadays, you can use the Mobile Certificate Pinning Generator to help you with implementing certificate pinning in your mobile app:
That will give you a ready to use pinning configuration for Android and iOS:
RESEARCHING FOR A SOLUTION
Before I point you out to a possible solution I would like to make clear the distinction between 2 concepts that developers frequently are not aware off or take as being the same thing...
The Difference Between WHO and WHAT is Accessing the API Server
The WHO is the user of the mobile app that you can authenticate,authorize and identify in several ways, like using OpenID Connect or OAUTH2 flows.
Now you need a way to identify WHAT is calling your API server and here things become more tricky than most developers may think. The WHAT is the thing making the request to the API server, is it really your genuine mobile app or is a bot, an automated script or an attacker manually poking around your API server with a tool like Postman?
Well to identify the WHAT developers tend to resort to an API key that usually they hard-code in the code of their mobile app and some go the extra mile and compute it at run-time in the mobile app, thus becomes a dynamic secret in opposition to the former approach that is a static secret embedded in the code.
Some Mobile API Security Techniques
I have done a fair bit of research but I don't seem to be able to find anything that quite matches what I'm trying to do.
You can start by read this series of articles about Mobile API Security techniques to understand how Https, Certificate Pinning, APi Keys, HMAC, OAuth2 and other techniques can be used to protect the communication channel between your mobile app and the API serve, and how they can be bypassed.
To solve the problem of WHAT is accessing your mobile app you need to use one or all the solutions mentioned in the series of articles about Mobile API Security Techniques that I mentioned above and accepted that they can only make unauthorized access to your API server harder to bypass but not impossible.
A better solution can be employed by using a Mobile App Attestation solution that will enable the API server to know is receiving only requests from a genuine mobile app.
A POSSIBLE BETTER SOLUTION
The use of a Mobile App Attestation solution will enable the API server to know WHAT is sending the requests, thus allowing to respond only to requests from a genuine mobile app while rejecting all other requests from unsafe sources.
The role of a Mobile App Attestation service is to guarantee at run-time that your mobile app was not tampered or is not running in a rooted device by running a SDK in the background that will communicate with a service running in the cloud to attest the integrity of the mobile app and device is running on.
On successful attestation of the mobile app integrity a short time lived JWT token is issued and signed with a secret that only the API server and the Mobile App Attestation service in the cloud are aware. In the case of failure on the mobile app attestation the JWT token is signed with a secret that the API server does not know.
Now the App must sent with every API call the JWT token in the headers of the request. This will allow the API server to only serve requests when it can verify the signature and expiration time in the JWT token and refuse them when it fails the verification.
Once the secret used by the Mobile App Attestation service is not known by the mobile app, is not possible to reverse engineer it at run-time even when the App is tampered, running in a rooted device or communicating over a connection that is being the target of a Man in the Middle Attack.
This is a positive model where false positives do not occur, thus the API server is able to deny requests with the confidence of not blocking legit users of your mobile app.
The Mobile App Attestation service already exists as a SAAS solution at Approov(I work here) that provides SDKs for several platforms, including iOS, Android, React Native and others. The integration will also need a small check in the API server code to verify the JWT token issued by the cloud service. This check is necessary for the API server to be able to decide what requests to serve and what ones to deny.
CONCLUSION
Properly securing a mobile app and the API server is a task composed of several layers of defense that you must put together in order to protect it.
How many layers to use will depend on the data your are protecting, the value it have for the business, the damage it can cause when leaked in a data breach and how much you may be penalized by law enforcement, like GDPR in Europe.

What I usually do is crafting a JSON web token (https://jwt.io/), and handle the sessions on my own.
JWT is really nice, since you only need to define a secret key on the server side. As long as your clients are able to pass the string you crafted (inside the headers for example), and as long as nobody gets to retrieve your secret key, you are sure that every data you push when creating the token was generated by you. (Don't hesitate to use the strongest encryption algorithm)

For a secure connection use HTTPS at level TLS 1.2 level. Then pin the server certificate in the app, that will prevent MITM attacks.
It is safe to pass the user name and password. You can return a time-limited token for further authentication is needed/desired.
With HTTPS everything but the address portion of the URL is encrypted. But be careful with the query string, it may end up in the server logs.

Thanks again for these replies. I've been implementing my service to run under https by using server.ssl.key-store parameters and it looks like it's working okay. I have used keytool.exe to create a trust store and I run my SpringBoot app (with Tomcat embedded) using that trust store. I can open a browser to my REST endpoint (using https this time, not http), the browser asks for authentication and when I enter my user details, it matches them against my db user and allows me to see the response from the server.
One question though, what's the point of having a trust store on the server side (Java) if I can just access the REST endpoint using any old browser and just enter my user name and password? Eventually, this REST endpoint won't be accessed via a browser, it'll be accessed using a mobile app programmatically, so I assume I'll be logging on using that with username and password over https. I thought I'd need to have a certificate of some sort on the client side in order to communicate or does it not work like this?
Thanks again

How to do Android Sync Provider server side implimentation?

I have a running J2EE based Web Application for Point of Sales hosted in cloud and database is using postgresql. Now I am building an android app for Point of Sales. For data sync I have read a lot how to create SyncAdapter etc but very few about server side developments. My first question:
How the authentication token will be created? Who will create this AuthToken? My server side RESTful webservice or Device SyncAdapter itself? Who should initiate expiry of AuthToken - the device or server webservice?
Currently my web application got many user level permission. When a data will come from device to sync I need to check the user permission before sync. Do I need to custom write these permission checking inside my server side webservices?

In short the server is responsible for creating an AuthToken. For a better understanding you should read the OAuth2 specification. Although you could write your own simpler solution, I recommend to you that you use OAuth2 to avoid some pitfalls. In your case authentication grant type "Resource Owner Password Credentials" seems most appropriate. Apache Oltu is a framework for implementing OAuth2 in Java, but you still have to handle some things like persistence of tokens on your own.
Yes, you have to the check the permissions on server side. Don't trust the client! Besides that you may restrict the apps more strictly by using different token types in Android. For example: When you're accessing the web service from within your server application and from Android apps, you could differentiate permissions based on the token type (Android vs. internal). Or maybe you provide your app's users an option to restrict the app's access only to read operations.

How would I approach secure communication from Android application to Server

I need encrypted information regarding the application users to be sent over to my server. And also for the server to verify the information sent and send back an encrypted response so the client would know what to do depending on the response. I believe I should be approaching this with JSON and POST PHP but don't know what to do for the security aspects of it.
An example would be. Client buys coins through in-app billing. Server receives notice from in-app billing and updates how many coins Client has. Client then proceeds to use coins to buy an item in the game and sends its verification and id to server which the server verifies and then sends a response code either stating that the Client is banned for doing fraudulent activity or insufficient coins or lets the Client buy the item in question.
Some questions I have.
I understand that any application or game can always be hacked on the clients end but gets progressively harder if its verifying things coinciding a server. How can I make sure that it is very hard for the client to mess with the response code and also with the sending of verification? Would SSL be enough? If so, could you direct me to an article on how to implement SSL for Android php posting?
How does Google billing verify it's users? By e-mail or device ID?

It's all written here. In the coins example, you'll likely need an unmanaged in-app billing. Unmanaged means that Google Play won't store the transaction and you are responsible of tracking users (by email, device ID, both or an username).
Regarding security, it really depend on your requirements. Google Play's in-app billing doesn't require you to deploy your own server. You don't even need to talk to the network: it's all managed by the Play app, which acts like a Proxy. Assuming you want to use your own server, SSL only grants a transport security, ie integrity and confidentiality of the message, not typically guaranteed by HTTP, are now guaranteed by SSL. This prevents man-in-the-middle attacks and the like, but it really doesn't protect your app from application-level flaws.
If your backend has a standard RESTful interface, you need to authenticate users against at least one private piece of information, like a password. Otherwise, anyone could make requests impersonating 1234567890 or john#example.com, which are publid data. In other words, you simply need to create accounts for users and communicating over HTTPS.

How to secure Android App that pulls data from OAuth protected resource

My company is building a RESTful API that will return moderately sensitive information (i.e. financial information, but not account numbers). I have control over the RESTful API code/server and also am building the Android app. I've setup the API to use OAuth 2 with authorization code grant flow (with client ID and secret), and I auto-approve users without them having to approve the client since we own both client and provider. We use CAS for SSO and I am using this for the Authorization server as part of the OAuth 2 process when the user logs in to retrieve the token.
I am contemplating various ways to secure the data on the Android app. I've concluded that storing the client id and secret on the device is definitely not going to happen, but am thinking that storing the auth token might work, since it is only risk to the individual user (and really only if they happen to have a rooted phone).
Here are two options I have thought of. They both require me to have a sort of proxy server that is CAS protected, does the dance with the API server, and returns the auth token. This gets rid of the need for storing the client id and secret in the app code.
Here are what I've come up with:
1) Require the user to enter their password to access data each time they startup the App. This is definitely the most foolproof method. If this were done, I'd probably want to save the userID for convenience, but in that case couldn't use the CAS login (since it's web-based). I might be able to use a headless browser on the backend to log the user into CAS and retrieve the token based on what they enter in the Android form, but this seems hacky. Saving the userID is similar to what the Chase app does (if you happen to use this one) - it saves the userID but not your password between sessions.
2) Store the auth token on the Android device. This is a little less secure, but almost foolproof. When the user starts the app for the first time, open the webpage to the CAS login of the proxy server that returns the token (similar to https://developers.google.com/accounts/docs/MobileApps). After the user logs in and the token is returned to the app, encrypt it and store it private to the application. Also, use ProGuard to obfuscate the code, making the encryption algorithm more difficult to reverse engineer. I could also work in a token refresh, but I think this would be more of a false sense of security.
3) Don't use CAS but come up with another way to get an auth token for the service.
Any advice of how others have implemented similar scenarios (if it's been done)?
Thanks.

Well the reason why standards like OAuth are developed is that not everyone has to rethink the same attack vectors again and again. So most often it is your best choice to stick to something already available instead of baking your own thing.
The first problem with clients that are not capable of secretly storing data is that the user's data could be accessed by some attacker. As it is technically not possible to prevent this (code obfuscation won't help you against an expert attacker), the access token in OAuth 2 typically expires after short time and doesn't give an attacker full access (bounded by scope). Certainly you shouldn't store any refresh token on such a device.
The second problem is client impersonation. An attacker could steal your client secret and access your API in his own (maybe malicious) app. The user would still have to login there himself. The OAuth draft there requires the server to do everything it can to prevent this, but it is really hard.
The authorization server MUST authenticate the client whenever possible. If the authorization server cannot authenticate the client due to the client's nature, the authorization server MUST require the registration of any redirection URI used for receiving authorization responses, and SHOULD utilize other means to protect resource owners from such potentially malicious clients. For example, the authorization server can engage the resource owner to assist in identifying the client and its origin.
I think Google are the first to try another approach to authenticate a client on such devices, by checking the signature of the application, but they are not yet ready for prime time. If you want more insight into that approach, see my answer here.
For now, your best bet is to stay on the OAuth way, i.e. having the access token, client ID and client secrect (when using the authorization code grant flow) on the device, and configure your server to do additional checks. If you feel more secure obfuscating these, just do it, but always think of it as if these values were publicly available.