> Kerckhoffs's principles in «La Cryptographie Militaire» (1883)
> 3. It must be possible to communicate and remember the key without using written notes, and correspondents must be able to change or modify it at will;

So basically key rotation, meaning that some implementers have built known-broken systems before their grandparents were born.
Not having to write down the key is maybe a bit historic these days, but it's a reminder that your private keys should be encrypted and with good protection against dumps (like via a smartcard/HSM).

> 6. Lastly, given the circumstances in which it is to be used, the system must be easy to use and should not be stressful to use or require its users to know and comply with a long list of rules.

Yup, good security is user-centric, otherwise it gets broken by the human element.