Everyone likely to read this knows that root can unlock a lot of doors for Android modding, and an unlocked bootloader opens even more. But for HTC phones, S-OFF is the ultimate in control, allowing users essentially unlimited access to every piece of code on the device. The Revolutionary dev team has released a new S-OFF method for the HTC One. You may remember these guys from exploits on most of the major HTC phones from 2011 and earlier.
S-OFF refers to a signature check and write access security in locked HTC phones. When it's on, the phone checks for signatures on certain portions of the software - the radio, recovery, certain parts of the system drive, and a few others - and keeps the user from writing to restricted areas. S-OFF grants users access to essentially every part of the phone, without the signature and security check, and opens up parts of NAND memory as well. Now, here's where things get esoteric: modders already had access to pretty much all of this before, thanks to a change in the way HTC is handling signature checks on the One.
If you've already got root and a custom bootloader and you're wondering why you need this, well, you don't - it's just one more vector for modding the phone that grants a few more tools. Specifically, users with S-Off can flash a custom HBOOT with extended commands, perform a SIM unlock, downgrade to older versions of official firmware, and remove the "relocked" text from the HBOOT screen, among other smaller tweaks.
This method is being presented in a preview edition. It is not for the inexperienced. There are no less than eight steps which require knowledge of ADB, bootloaders, and command lines... and may need several tries to achieve success. In other words, if your only experience with deep-level mods is root and maybe a custom recovery or two, you probably shouldn't try it. This is the kind of thing that can all too easily brick your phone. The Revolutionary team will almost certainly create a safer, easier method once they've gotten feedback from preview users.
Still not dissuaded? Then hit the link below to head to XDA, where the full set of tools and instructions await you. It would help these guys out a lot if you left feedback on the XDA thread with your success or failure.