rfactordec (and rfactorgmdec too obviously) supports also the encrypting of the file:
rfactordec -e -s 0x2eb8f5cc9b14ea3b input.gmt output.gmt
the number after -s is the signature number of the game for which you want to decrypt the file.
for example the one I used there is that of ARCA Sim Racing, the others are:
Code:
0x38af5637e81bc9a0 = rFactor
0x2eb8f5cc9b14ea3b = ARCA Sim Racing
0x6a9d37283a9f3d9f = Simulador Turismo Carretera
0xde4139f961fa2817 = Top Race Simulador 2009
0x38af3150902cc55b = Superleague Formula