Is TLS Enough? A Retrospective on Application-Layer Encryption

Years ago, I was part of a heated debate that every engineering team eventually faces: Is standard TLS enough, or do we need custom application-layer encryption? We were implementing a payment solution. The provider required a backend-to-backend integration, meaning we had to take user credit card data, send it to our server, and then forward it to the provider. My argument was that the TLS layer would be enough for it. The rest of the team disagreed. They didn't have a technical counter-argument, it was just a "lack of trust". We ended up building a complex Dual-Keypair System: The app keypair: Used to sign requests so the server could verify the data actually came from our app (Authenticity). The server keypair: The app used the server's public key to encrypt the payload, ensuring only our backend could read it (Confidentiality). It worked, but years later I realized the hidden parts we didn't consider. The scale: We were small then. But if you scale to multiple server instances, you