je ne sais rien
I don't know anything for sure.
Je ne sais rien avec certitude.
I swear that I don't know anything.
Je jure que je ne sais rien.
However, officially I don't know anything.
Cependant, officiellement, je ne suis au courant de rien.
— Investigation of a Citizen Above Suspicion (1970)
You act as is I don't know anything.
Tu fais comme si je ne savais rien.
— Hum Dil De Chuke Sanam (1999)
I don't know anything as to his past.
Je ne sais rien de son passé.
All I know is that I don't know anything.
Tout ce que je sais, c’est que je ne sais rien.