Sounds like a fun project. If you’re aiming for real security, start with a strong asymmetric key pair for signing the spells, then use a symmetric cipher—AES‑256 is a good default—to encrypt the content. Store the symmetric key in a hardware security module or a secure enclave, and only allow the magical girl’s device to decrypt it. For the “bulletproof” part, add a key‑derived salt per spell, use PBKDF2 or Argon2, and always verify a hash or MAC to catch tampering. Keep the key management simple, but don’t forget that the most common weakness is people storing private keys in plain text somewhere. Make the UI friendly, but don’t compromise the crypto. Let me know if you need a sample key‑exchange flow or a quick audit of your prototype.

Use ECDH over a short‑lived session key. The wizard’s “spell scroll” animation can carry the public key in a QR‑style glyph. The client (your app) does an ECDH with the server’s static public key, derives a shared secret, then hashes it with a salt to get the AES key. Store that AES key in the OS secure store—Keychain on iOS, Keystore on Android. No HSM needed, just the built‑in hardware encryption. For the UI, keep the graphics lightweight. Pre‑render the pixelated scrolls to a texture once, then just blit them. Run any cryptographic ops off the main thread; use async callbacks. Double‑buffer the canvas so you never see a partially drawn frame. And if you want the scroll to look like it’s floating, animate a simple alpha tween rather than re‑drawing the whole sprite each frame. That’ll keep the interface smooth while you still show those cute spell scrolls.