Let me try to break down your question to multiple subquestions/assumption:

Assumptions:

a) Keychain is safe place

Actually, it's not that safe. If your application is installed on jailbroked device, a hacker will be able to get your keys from the keychain

Questions:

a) Is there a way to put some key into an app (binary which is delivered form AppStore) and be completely secure?

Short answer is NO. As soon as there is something in your binary, it could be reverse engineered.

b) Will obfuscation help?

Yes. It will increase time for a hacker to figure it out. If the keys which you have in app will "cost" less than a time spend on reverse engineering - generally speaking, you are good.

However, in most cases, security through obscurity is bad practice, It gives you a feeling that you are secure, but you aren't.

So, this could be one of security measures, but you need to have other security measures in place too.

c) What should I do in such case?*

It's hard to give you a good solution without knowing background what you are trying to do.

As example, why everybody should have access to the same Amazon S3? Do they need to read-only or write (as pointed out by Kendall Helmstetter Gein).

I believe one of the most secure scenarios would be something like that:

• Your application should be passcode protected
• First time you enter your application it requests a user to authenticate (enter his username, password) to the server
• This authenticates against your server or other authentication provider (e.g. Google)
• The server sends some authentication token to a device (quite often it's some type of cookie).
• You encrypt this token based on hash of your application passcode and save it in keychain in this form
• And now you can do one of two things:
  • hand over specific keys from the server to the client (so each client will have their own keys) and encrypt them with the hash of your application passcode
  • handle all operation with S3 on the server (and require client to send)

This way your protect from multiple possible attacks.

c) Whoooa.... I don't plan to implement all of this stuff which you just wrote, because it will take me months. Is there anything simpler?

I think it would be useful, if you have one set of keys per client.

If even this is too much then download encrypted keys from the server and save them in encrypted form on the device and have decryption key hardcoded into your app. I would say it's minimally invasive and at least your binary doesn't have keys in it.

P.S. Both Kendall and Rob are right.

Update 1 (based on new info)

First of all, have you seen in app purchase programming guide.

There is very good drawing under Server Product Model. This model protects against somebody who didn't buy new levels. There will be no amazon keys embedded in your application and your server side will hand over levels when it will receive receipt of purchase.

There is no perfect solution to protect against somebody who purchased the content (and decided to rip it off from your application), because at the end of days your application will have the content downloaded to a device and will need it in plain (unencrypted form) at some point of time.

If you are really concerned about this case, I would recommend to encrypt all your assets and hand over it in encrypted form from the server together with encryption key. Encryption key should be generated per client and asset should be encrypted using it.

This won't stop any advanced hacker, but at least it will protect from somebody using iExplorer and just copying files (since they will be encrypted).

Update 2

One more thing regarding update 1. You should store files unencrypted and store encryption key somewhere (e.g. in keychain).

In case your game requires internet connection, the best idea is to not store encryption key on the device at all. You can get it from the server each time when your app is started.

DO NOT store an S3 key used for write in your app! In short order someone sniffing traffic will see the write call to S3, in shorter order they will find that key and do whatever they like.

The ONLY way an application can write content to S3 with any degree of security is by going through a server you control.

If it's a key used for read-only use, meaning your S3 cannot be read publicly but the key can be used for read-only access with no ability to write, then you could embed it in the application but anyone wanting to can pull it out.

To lightly obscure pre-loaded sensitive data you could encrypt it in a file and the app can read it in to memory and decrypt before storing in the keychain. Again, someone will be able to get to these keys so it better not matter much if they can.

Edit:

Based on new information you are probably better off just embedding the secrets in code. Using a tool like iExplorer a causal user can easily get to a core data database or anything else in your application bundle, but object files are somewhat encrypted. If they have a jailbroken device they can easily get the un-encrypted versions but it still can be hard to find meaningful strings, perhaps store them in two parts and re-assemble in code.

Again it will not stop a determined hacker but it's enough to keep most people out.

You might want to also add some code that would attempt to ask your server if there's any override secrets it can download. That way if the secrets are leaked you could quickly react to it by changing the secrets used for your app, while shutting out anyone using a copied secret. To start with there would be no override to download. You don't want to have to wait for an application update to be able to use new keys.

There is no good way to hide a secret in a piece of code you send your attacker. As with most things of this type, you need to focus more on how to mitigate the problem when the key does leak rather than spend unbounded time trying to protect it. For instance, generating different keys for each user allows you to disable a key if it is being used abusively. Or working through a intermediary server allows you to control the protocol (i.e. the server has the key and is only willing to do certain things with it).

It is not a waste of time to do a little obfuscating. That's fine. But don't spend a lot of time on it. If it's in the program and it's highly valuable, then it will be hacked out. Focus on how to detect when that happens, and how to recover when it does. And as much as possible, move that kind of sensitive data into some other server that you control.