If necessary I would tattoo them on my buttocks! XD

Seriously now, I think the best is on paper, in a safe place where only I can access and no one else. If possible, under lock and key.

A pendrive can also be but it could be ruined as it happened to me with another one where I had some photos that became corrupted (every now and then you have to use the device so that it does not degrade with the passage of time).