Charles works by sitting between a server and a phone. It decrypts traffic from the server, displays this to the user to 'inspect', and then re-encrypts the tr

Charles works by sitting between a server and a phone. It decrypts traffic from the server, displays this to the user to 'inspect', and then re-encrypts the traffic to be sent to the phone. It re-encrypts the traffic using its own SSL certificate (technically a CA, but no need to get bogged down in details). In many modern apps, code has been added such that an app will only accept traffic which has been encrypted by the original server certificate (ie: the Uber iOS app will only accept traffic which has been encrypted by a certificate from www.uber.com). When Charles attempts to sit between this traffic, the app will not allow network connections, and thus no traffic can be inspected by Charles. This procedure of an app only permitting traffic encrypted by a predefined certificate is known as "certificate pinning" or "pinning" for short. It is becoming more and more common for native apps.