YOUR PROBLEM
I'm currently using Spotify in my Android app, but I am required to use a Secret in order to refresh tokens and such. I would like to transmit the secret from my Backend to the app, so the secret does not reside in the APK and cannot be found when decompiling. I've read a lot only about securing secrets in your app, via various ways like proxies, just using your own backend, putting the code into native C++ code (NDK) in the app or using the Hash of the app to determine whether the app is calling the backend, and not some guy behind his computer trying to steal the secrets.
Congratulations in your efforts to understand the problem, it seems that you went to a great extent to understand that secrets in a mobile app can always be extracted by static binary analysis, but I don't see any mention to instrumentation frameworks like:
Frida
Inject your own scripts into black box processes. Hook any function, spy on crypto APIs or trace private application code, no source code needed. Edit, hit save, and instantly see the results. All without compilation steps or program restarts.
or
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.
but many others more exist, and all of them will hook into your code at runtime and extract any secret you store in your mobile app, no matter how securely you store it, even if you use hardware backed keystores, that run in a trusted execution environment:
Android Hardware-backed Keystore
The availability of a trusted execution environment in a system on a chip (SoC) offers an opportunity for Android devices to provide hardware-backed, strong security services to the Android OS, to platform services, and even to third-party apps.
At some point the secret retrieved from this keystore will need to be used to make the request to your third party service, and at this point all an attacker needs to do is to hook on the call to that function and extract the secret when passed to it.
So no matter what you do in the end a secret in a mobile app can always be extracted, it just depends on the skill set of the attacker and the time and effort he is willing to put in.
This being said, it leads me to the point I am always advising developers to not do, that is calling Third Party services from within their mobile app.
THIRD PARTY SERVICES ACCESS FROM A MOBILE APP
Found options:
Proxy: It means routing it through my own server, don't want that
Own backend: Same as proxy, don't want all request to got trough my own service
Yes I read you don't want to use a proxy or your backend, but that is the best chance you have to secure the access to your third party service, in this case Shopify.
I wrote this article that explains why you should not do it from your mobile app, from where I quote:
Generally, all Third Party APIs require a secret in the form of an API key, Access Token or some other mechanism for a remote client to identify itself to the backend server with which it wishes to communicate. Herein lies the crux of the problem of accessing it from within your mobile app, because you will need to ship the required secret(s) within the code (the coloured keys in the above graphic).
Now you may say that you have obfuscated the secret within your code, hidden it in the native C code, assembled it dynamically at runtime, or even encrypted it. However, in the end all an attacker needs to do in order to extract this secret is to reverse engineer the binary with static binary analysis, or hook an instrumentation framework like Frida into the function at runtime which will return the secret. Alternatively an attacker can inspect the traffic between the mobile app and the Third Party API it is connecting to by executing a MitM (man-in-the-middle).
With the secret in their possession, the attacker can cause a lot of damage to an organization. The damage can be monetary, reputational and/or regulatory. Financially, the attacker can use the extracted secret to access your cloud provider and your pay-per-call Third Party APIs in your name, thus causing you additional costs. Further, you may be financially hurt by the exfiltration of data which may be sold to your competitors or used to commit fraud. Reputationally you can be impacted when the attacker uses the extracted secret to post on your behalf on social networks, creating a public relations nightmare. Another reputational damage can occur when an attacker uses the Third Party API and violates its terms & conditions (for example where frequent usage of the API triggers rate limits) such that you get blocked from using the service, creating pain for your end users. Last but not least are regulatory troubles caused when the extracted secret is the only mechanism of protecting access to confidential information from your Third Party API. If the attacker can retrieve confidential information such as Personal Identifiable Information (PII), regulatory fines connected to violations of GDPR in Europe, or the new CCPA Data Privacy Law in the California, may be enforced against your business.
So the take home message is that any secret you ship in your code must be considered public from the moment you release your app or push the code to a public repository. By now it should be clear that the best approach is to completely avoid accessing Third Party APIs from within a mobile app; instead you should always delegate this access to a backend you can trust and control, such as a Reverse Proxy.
You now may say that the problem have just shifted from the mobile app into the reverse proxy or backend server, and that's a positive thing, because the backend or reverse proxy is under your control, but a mobile app is out of your control, because it's in the client side, therefore the attacker can do whatever he wants with it.
In the backend or reverse proxy you are not exposing the secrets to access the third party services to the public, and any abuse an attacker wants to do in your behalf against that third party service will need to pass through a place you control, therefore you can apply as many defense mechanisms as you can afford and is required by law for your use case.
SECURITY IN DEPTH
putting the code into native C++ code (NDK)
When hiding the secret in native C code it's not easy to find it with static binary analysis, at least for script kids and seasonal hackers, it needs a better skill set that the majority may not have, thus I really recommend you to use it as an extra layer of security, but to protect a secret to access your own services, not third party ones as I already mentioned before.
If you really decide to follow my advice and shift your efforts to defend the third party secret in place you have control off, like your own backend, then I recommend you to read my answer to the question How to secure an API REST for mobile app? for the sections on Securing the API Server and A Possible Better Solution.
If you read the above answer then you have realized that if you keep the access to third party services in your backend, you can lock down your API server to your mobile app with a very high degree of confidence by using the Mobile App Attestation concept.
DO YOU WANT TO GO THE EXTRA MILE?
I saw that you are well informed, thus you already know what I am about to share, but in any response I gave to a security question I always like to reference the excellent work from the OWASP foundation, thus If you allow I will do it here to :)
For Mobile Apps
OWASP Mobile Security Project - Top 10 risks
The OWASP Mobile Security Project is a centralized resource intended to give developers and security teams the resources they need to build and maintain secure mobile applications. Through the project, our goal is to classify mobile security risks and provide developmental controls to reduce their impact or likelihood of exploitation.
OWASP - Mobile Security Testing Guide:
The Mobile Security Testing Guide (MSTG) is a comprehensive manual for mobile app security development, testing and reverse engineering.
For APIS
OWASP API Security Top 10
The OWASP API Security Project seeks to provide value to software developers and security assessors by underscoring the potential risks in insecure APIs, and illustrating how these risks may be mitigated. In order to facilitate this goal, the OWASP API Security Project will create and maintain a Top 10 API Security Risks document, as well as a documentation portal for best practices when creating or assessing APIs.
