Last week, Japanese blog Mac Otakara reported that Apple’s suppliers are currently only able to produce enough 18W USB-C power adapters for inclusion with 2018 iPhones, suggesting the fast-chargers won’t initially be sold separately.
Today, the same source has claimed that third-party fast chargers may require USB-C Authentication or C-AUTH to charge the iPhones at full speeds. Without it, compatible iPhones may limit charging speeds to a max of 2.5W (via MacRumors).
The main purpose of USB-C Authentication is to protect against non-compliant USB chargers and to mitigate risks from maliciously embedded hardware or software in USB devices.
[Translated from source] The next iPhone might be applicable to “C-AUTH”. Those who are working on it say that if an unauthorized item is used, USB PD 3.0 standard policy that does not supply the power exceeding “2.5W (5V/0.5A)” is applied.
If USB-C-Lightning cable and USB-C power supply are used, current items of Apple can use power supply by 15 w(5V/3A) as USB-C standard. But from the next iPhone and thereafter, as USB PD 3.0 standard is added, security is increased whereas restriction of the standard might be added.
USB-compliant fast chargers are available from a wide variety of brands. It wouldn’t be a bad idea to check the list to see if the manufacturer is actually a member of the USB Implementers Forum or not.