What is the Sept everyone's taking about?
Sept is the solution devised by SciresM to make CFW work on 7.x.
Basically, sept is a payload that is fed to the TSEC firmware in order to make 7.x boot. Rather than burning any TSEC flaws/glitches, the payload that is fed is a perfectly legitimate one according to the TSEC firmware.
This is because the payload in question is a
signed payload. What this means is that when the TSEC firmware checks to see if the payload that it's being fed is legitimate, it indeed is. The way this is accomplished is by using a non-public (and this one doesn't and shouldn't be made public since then it can't be fixed) hardware exploit that permitted SciresM et al. to get the neccesary keys needed to sign this payload. This relevant exploit at the time of writing is only in the hands of very few people and the only ones that publicly have said they have access to this exploit is Hexkyz and SciresM.
Of course, this presents a unique opportunity for the Atmosphere team. As a lot of the other CFW (most obviously TX, but ReiNX to a lesser extent as well) make use of various parts of Atmosphere (SX OS is Atmosphere with some features shoddily coded on top, while a lot of ReiNXs sysmodules are directly forked from a very old version of Atmosphere[1]), they opted to include a splash screen into the payload that specifically forces any CFW that uses sept to credit Atmosphere for this payload.
Basically, unless this hardware exploit is made public, nobody except the Atmosphere team can build a signed version of sept[2] and as a result, nobody can remove this splash and get a signed payload out of it. (If you tamper with the resulting payload the signature[3] breaks.)
I hope that more or less covers it.
[1]:
https://github.com/Reisyukaku/NX_Sysmodules - Note that such a connection is vehemently denied by Rei 'n co when directly confronted, but if you check repo history, you'll find this specific commit:
https://github.com/Reisyukaku/NX_Sysmodules/commit/ec72f51531165f78d8d567abd6287a49d91e2800 which "removes Atmosphere DRM" (this is in fact a check for exosphere) and also renames all the existing sysmodule paths to ReiNX.
[2]: Not an issue though. For nightly/developer builds, you can just supply a release version of sept and it'll use that instead of trying to compile it. License wise, this is where Atmospheres GLPv2 comes into play, as the loophole that permitted them to do this (called tivoization) was only fixed in the v3.
[3]: Think of a signature like it's a verification in a file that says "yes I was created right". Take for example the code 291901. If I wanted to add a signature to this code to make sure that this code is indeed correct, a really easy trick would be to simply add up all the numbers and take the last number that comes out. 2+9+1+9+0+1 = 22, so the signature would be 2. The resulting code I show then is 2919012. If someone were to change the number to 2919112, the signature would be invalid and as a result, I know that this code is not a valid code according to my rules. In reality, the actual process for this is much more advanced thanks to the power of computers and a well made signature shouldn't be so easily breakable as my example (since all it would take here is someone knowing that the way to get the signature number (with codes like these it's called a control character) is to count it all up and suddenly it's easily fakeable), but yo uget the idea.
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