Factory mode tool

[slave]

hello
i just want to ask
as i know there a special bitfile published (a key?) by sended trought tftp to the old cable modems sb4100 etc
will opened the factory mode on - thats right
for sb5101 and up this file is unknown and we can turn on factory mode only by internall access to the modem
question is what is this exactly bitfile ? is the isp send same type bitfile just with diffrent key to other new cable modems ?
i do not want any working solutions about that never i just need to know whatever this is - is this is realised by isp by same way remote for new modems like for old ? if yes then i think this is cipher text msg and only issuer can decrypt their value
this should be based some kind of encryption and some of answers i should find by dissasembling new cable modems firmware

or maybe bitfile is some kind of backdoor and on new devices only isp is able to send (safe open informations) trought config file
like i know that some of new modems can have opened telnet/ssh by config files
but if the config is random dynamic type the things are closed