[Verizon Galaxy S 5 Q&A, Help & Troubleshooting] dreamwave:
I know that the Verizon bootloader is almost impenetrable as is, but would it be plausible to completely go over the head of the firmware and directly write an image with JTAG that would allow for custom software? If so, would it be possible to use the firmware from another carrier like USC or would it have to be a custom image?

EDIT: summary of the method and everything I have thusfar discovered

So, this method after a bit of evolution, got to the point it basically entailed the following: Using the SD Card debrick method (popularized by the galaxy s3 LTE variants) a modified firmware image would be written to an SD Card, and the phone would boot from that image. The main problem I ran into: it would not let me flash anything that could brick the phone, nor was I able to pull the usb cord at the right moment and try and manually brick it. I was able to flash firmware and stock tars from other variants of the phone (such as the one that runs on T-mobile), but what I found out through that is a couple things:

1. The stock tars seem mostly carrier independent, and I was without any modification able to flash a T-mobile bootloader, system image, and pit file, but within recovery and download mode it would show that because of integrated CSC, it would still change back to the original variant. This could have implications for a very simple method of removing bloat from the phone, but I'm not so sure

2. It must have a very low level method of injecting information and file verification that is not located anywhere on eMMC

The latter led me to research a TON, eventually finding that the most likely culprit is the use of Qualcomm Qfuses, non-volatile pre-set memory located directly on the SoC, to check how the bootloader is signed. They consist of a couple blocks of registers, and definitely aren't readily writable. The trusted base of the entire secure system, the same system that KNOX invokes on other systems, is within a series of Qfuses. From what I have deduced, however, they must be at some software level writable, as although the Knox counter is an e-fuse, the others (such as the warrantee bit) have been both changed upon their void and reverted when brought back to a service center. This must mean that the entire block is possible to modify

in both directions

, unlike a fuse or breaker; It seems to act more like flash memory than a "fuse." This is very good, mainly because if the service center can change it it means that jtag has not been disabled by those flags, and is enabled in at least some form. What this also means is that without another MAJOR exploit within unfortunately simple, clean code or a leak of several RSA keys from verizon, either current workarounds such as safestrap are the answer for the foreseeable future, or a method of manually changing a simgle Qfuse (the one that controls the "Qualcomm Secureboot" flag) could be used.

What I'm hopefully going to start at some point here is research into finding a way of accessing and changing that Qfuse via JTAG. I have no money for a JTAG box at the moment, so it'll have to wait, but if anyone who already has one wants to use it, hopefully this info helps :)

P.S. I figured out exactly what T-flash does in odin: it flashes the files that you input into odin to the currently inserted SD Card (or so it seems, I could be wrong but that's what it did for me)

P.P.S. Verizon, I respectfully request that...oh never mind, profanity is definitely frowned upon here :p

Also, I'm in ongoing discussion with the FCC as to block C violations by Verizon of aspects of the regulations that upon research have not really been argued to any substantial extent, so if that comes to fruition hopefully there'll be simple ODIN flashable patches for this stuff :fingers-crossed:

UPON REFLECTION: if the phone could be bricked, either by very subtly corrupted file or by interrupting a flash at the right moment, then could the debrick image from a tmobile galaxy s5 with an unlocked bootloader be used as not a method of flashing the on-board bootloader but as a kind of external boot, so a permenantly installed SD Card that would be permissive of modified kernels and such but still accepted as a boot device by the phone?