One way of semi-resolving the issue of the paper falling into another's hands is writing the password and adding characters to it on paper that you know not to

One way of semi-resolving the issue of the paper falling into another's hands is writing the password and adding characters to it on paper that you know not to add when using it as a password. For example, when we visit my mother-in-law, she likes to hand over her ATM card for us to do local errands. My wife, knowing that she'll forget the PIN, writes it as a telephone number, where she knows the offset/method to extract the PIN.