GAGA SSO Mobile SDK — Developer Guide

One Unified GAGA ID per player. The same account & tokens work across Game Client, Website and Web Shop (true SSO). Desktop SDK is out of SOW, but auth is plain REST so a PC client can reuse the C++ bridge + web login later — platform accepts android | ios | pc | web.

Solid blue = request, dashed grey = response. The verifier stays on the device; only the challenge is sent at /login, and the verifier is revealed only at /auth/token.

Two values the SDK generates fresh for every login — they prove the app that started the flow is the same one finishing it. This is what protects a public client (a mobile app can't safely hold a client secret).

code_verifier = a high-entropy random string that never leaves the device. code_challenge = its SHA-256 hash, safe to send out because a hash can't be reversed.

code_verifier  = base64url( random(32 bytes) )        // 43 chars, no padding
code_challenge = base64url( SHA256(code_verifier) )   // code_challenge_method = "S256"

Where each value goes in the flow

Step 2  GET /login?...&code_challenge=<challenge>&code_challenge_method=S256   ← challenge only
Step 6  POST /auth/token { code, code_verifier }                              ← verifier
        backend recomputes SHA256(verifier), compares to the stored challenge

Why it matters on mobile: the callback returns on a deep link (gagasdk://auth/callback?code=…) that a malicious app could intercept. Without PKCE, a stolen code could be exchanged for tokens. With PKCE it can't — the attacker has no code_verifier and the hash can't be reversed, so the stolen code is worthless.

val verifier = Base64.encodeToString(ByteArray(32).also { SecureRandom().nextBytes(it) },
    Base64.URL_SAFE or Base64.NO_PADDING or Base64.NO_WRAP)
val challenge = Base64.encodeToString(
    MessageDigest.getInstance("SHA-256").digest(verifier.toByteArray()),
    Base64.URL_SAFE or Base64.NO_PADDING or Base64.NO_WRAP)

var b = [UInt8](repeating: 0, count: 32); _ = SecRandomCopyBytes(kSecRandomDefault, 32, &b)
let verifier = Data(b).base64URLEncodedString()
let challenge = Data(SHA256.hash(data: Data(verifier.utf8))).base64URLEncodedString()

Do: use S256 (never plain) · a new pair per login · keep the verifier in memory only · also send a random state for CSRF. Don't: log, persist, or reuse the verifier.