To store private API keys securely in your Android app, you should follow these best practices:

• Avoid Storing Secrets in Code: Do not hardcode API keys directly into your app's code. This is not secure and exposes the keys to anyone who can reverse engineer the APK.

• Use BuildConfig Fields: Instead, store your API keys in the build.gradle file as buildConfigField entries. This way, the keys are compiled into the BuildConfig class and are not directly visible in the source code. However, be aware that this does not fully protect the keys as they can still be found in the APK if it is reverse-engineered 3.

android {
    buildTypes {
        release {
            buildConfigField 'String', 'API_KEY', "\"your-api-key\""
        }
    }
}

String apiKey = BuildConfig.API_KEY;

Then, in your code, you can access the key like this:

• Use ProGuard: Enable ProGuard to obfuscate your code and make it harder to reverse engineer. This can help protect your API keys even if they are stored in the APK 3.

• Native Libraries with NDK: Store secrets in native libraries using the NDK. This method involves obfuscating the secret with a reversible XOR operator, storing the obfuscated secret in a NDK binary as a hexadecimal array, and not persisting the obfuscating string in the binary to prevent the compiler from disclosing the secret 3.

• Restrict API Keys: Restrict your API keys to specific platforms, IP addresses, or CIDR subnets. This can help prevent unauthorized use of your keys 5.

• Server-Side Signing: For sensitive operations, consider signing requests server-side rather than client-side. This can prevent exposure of signing secrets and API keys 5.

• Separate API Keys for Each App: Use separate API keys for each app to limit the scope of each key. If one key is compromised, you can delete or regenerate it without affecting other keys 5.

• Use a Digital Signature: In addition to an API key, use digital signatures to secure your API requests. This adds an extra layer of security 5.

• Environment Variables: Store secrets in environment variables or separate files that are not included in the source control system. This keeps the secrets out of your source code and prevents accidental exposure 5.

Remember, no method is completely foolproof, and the security of your API keys depends on a combination of these practices and the overall security of your app's infrastructure.