rc-security

name	rc-security
description	Use this skill when hardening a RevenueCat integration on Android. Covers Trusted Entitlements response verification (INFORMATIONAL vs ENFORCED), why the server is always the authority, API key hygiene (public SDK key vs secret REST key), anonymous user identity, and purchase token protections RevenueCat provides automatically.
license	Apache-2.0; see LICENSE
metadata	{"author":"RevenueCat","source":"revenuecat-handbook chapter 15","keywords":["android","revenuecat","security","trusted-entitlements","entitlement-verification-mode","api-key","anonymous-user"]}

Phase 0: Intent

Tell the user: "I will review your RevenueCat security posture on Android: verification mode, API keys, user identity, and server side access decisions."

Phase 1: Discovery

Confirm what RevenueCat already covers so you can focus on the real gaps.

Concern	Who handles it	How
Receipt validation against Google Play	RevenueCat backend	Runs before awaitPurchase() returns
Purchase token reuse	RevenueCat backend	Tokens are deduplicated server side
Fabricated purchase tokens	RevenueCat backend	Fails Google Play verification, no entitlement granted
HTTPS transport to RevenueCat	SDK	Always on
Retry of token post on network failure	SDK	Retries on next app launch

Ask the project these questions:

• Is EntitlementVerificationMode set? If not, CustomerInfo responses are trusted without signature checking.
• Is the SDK configured with the public Android key (starts with goog_) or has someone accidentally pasted the secret key into the client?
• Are real users identified with Purchases.logIn(yourUserId), or is the app still relying on anonymous $RCAnonymousID:...?
• Do server endpoints that serve premium content verify entitlement server side, or do they trust a client sent flag?

Phase 2: Plan

Decide each of the following before changing code.

Decision A: Entitlement verification mode

Mode	Failed verification does what	Pick when
DISABLED (default)	No signing happens	Only for quick prototypes
INFORMATIONAL	Logged, access still granted	You want signal without risking false denials to real users
ENFORCED	EntitlementInfo.isActive returns false	You accept some false negatives from proxies or VPNs in exchange for a strict client guarantee

Decision B: Where is the authority?

Even with ENFORCED, the client is not the authority for paid content. Decide whether each premium endpoint:

1. Calls the RevenueCat REST API per request, or
2. Reads a local has_entitlement flag driven by RevenueCat webhooks.

Option 2 is cheaper at request time, option 1 has no cache staleness. Pick one, document it, do not mix per endpoint without a reason.

Decision C: User identity

If the app is in production, plan to call Purchases.logIn("your_user_id") with your own authenticated user id. Anonymous ids are device scoped identifiers, not credentials, and are shared across users of the same device.

Phase 3: Execute

3.1 Turn on response verification

Add the mode to PurchasesConfiguration:

PurchasesConfiguration.Builder(context, apiKey)
    .entitlementVerificationMode(EntitlementVerificationMode.INFORMATIONAL)
    .build()

Read the verification result when you inspect entitlements:

when (customerInfo.entitlements.verification) {
    VerificationResult.VERIFIED -> { /* response is authentic */ }
    VerificationResult.FAILED -> { /* possible tampering, log and alert */ }
    VerificationResult.NOT_REQUESTED -> { /* verification disabled */ }
    VerificationResult.VERIFIED_ON_DEVICE -> { /* verified locally */ }
}

Upgrade to ENFORCED once you have telemetry confirming FAILED is rare on real traffic:

.entitlementVerificationMode(EntitlementVerificationMode.ENFORCED)

In ENFORCED, a failed signature flips isActive to false on the affected entitlement.

3.2 Keep API keys in the right place

Key	Where it lives	What it can do
Public Android SDK key (goog_...)	Embedded in the app binary	Read and purchase for the calling user only
Secret REST API key	Your server, secret manager or env var	Full REST API, admin operations, grant entitlements

In Android code, only the public key appears:

Purchases.configure(
    PurchasesConfiguration.Builder(context, "goog_PUBLIC_android_sdk_key").build()
)

Never commit the secret key to the app repo. Grep the Android source tree for the secret key prefix and confirm zero hits before release.

3.3 Identify real users

Call logIn as soon as you have an authenticated user id:

val result = Purchases.sharedInstance.awaitLogIn("your_user_id")
val customerInfo = result.customerInfo

Do not rely on the anonymous id as a credential. It is a device scoped identifier and does not protect purchase history on shared devices.

3.4 Enforce on the server, not on the client

Even with ENFORCED mode, every server endpoint that serves paid content checks entitlement server side:

def get_premium_content(user_id):
    info = revenuecat.get_subscriber(user_id)
    if not info.entitlements["pro"].is_active:
        raise Forbidden()
    return content

Or read a local has_pro flag driven by RevenueCat webhooks and check that flag per request.

Phase 4: Verify

• Android source contains only the public SDK key. Secret key grep is clean.
• EntitlementVerificationMode is set (not DISABLED) and telemetry for VerificationResult.FAILED is watched.
• Real users are identified through Purchases.logIn. Anonymous ids are only used for pre login flows.
• Every paid content endpoint on your server verifies entitlement through the REST API or a webhook driven flag. None trust a client header.

Common mistakes

Mistake	Why it hurts	Fix
Shipping the secret key in the app	An attacker who decompiles the APK can call admin REST endpoints	Public key in app, secret key only on server
Leaving mode at DISABLED in production	A man in the middle can forge CustomerInfo responses	Set INFORMATIONAL or ENFORCED
Treating the anonymous id as a credential	It is not secret and is shared across users of the same device	Call logIn with your authenticated user id
Trusting customerInfo from the client on the server	A tampered client can claim any entitlement	Verify on the server via REST API or webhook driven DB

References

• Full chapter