When I worked in a secure area I had six x eight digit safe combinations to remember, and I had to change them every three months.

Two of the safes had to be accessed with senior security staff looking on, so there was no question of an aide memoire.

I used a system whereby each eight figure combination was itself multiplied/divided by another figure familiar only to me and one other person, who lived on a different continent!

The resultant figures, all six of them, I did openly write down.

This used to cause apoplexy among the security people, but I used to put my figures on a piece of paper, leave them all day and challenge them to work out the combination.

Since these people were trained counter-intelligence staff they could go right back to the date of my great grandfather's marriage, but they never managed to crack my code!