Some think think Syfo Dias is just an alias for Palpatine, Because if you remember Palpatine/ Sidious wants the clone war (when Dooku tells him ones started he sounds quite pleased) so ordering a clone army is a good start! Sifo Dyas was a jedi who died after the battle of Naboo , and Palpatine just used his name to get the clones.
Who agrees?
Here is another theory I found
Master Sifo-Dyas isn't darth sidious nor darth tyrannus (dooku) Because remember in episode 2 when Dooku catched Obi-Wan and told him: Qui-Gon alreaddy knew about the separatists but he wouldn't agree the truth... So to protect the republic... Master Jinn ask to creat the clone army under the name of Sifo-Dyas. Remember also, Qui-Gon found Anakin, so logicly he has to be the author of the clone army, that will support the empire of Darth Vador. But all these things have been created with good intentions from Qui-Gon... Remember, Qui-Gon is the only one who hasn't disappear when he died... Qui-Gon has been a powerful Jedi but he was ignoring that he was dark... Qui-Gon is "dark" because of his master Dooku, and because is the one who made Anakin a Jedi... So Qui-Gon must be the one that asked the army to complete this. what do you think?
OR
Sifo-Dyas did not actually die, but faked his death and became Senator Palpatine, and eventually the Emporer. It's a far stretch, but remember the Jedi council can't detect the Emporer as a member of the dark side, so its possible that he initially was Sifo-Dyas. If you take the 'fo' out of Sifo-Dyas, you get Si-Dyas (Sidious). Who else could Sifo-Dyas be? It must be the emporer.