Humans as Smart Cards
Valery pointed to a great quote in the “Network Security – Private Communication in a Public World” by Kaufman, Perlman and Speciener, Prentice Hall 1995 ISBN 0-13-061466-1.
Humans are incapable of securely storing high-quality cryptographic keys, and they have unacceptable speed and accuracy when performing cryptographic operations. (They are also large, expensive to maintain, difficult to manage, and they pollute environment. It is astonishing that these devices continue to be manufactured and deployed. But they are sufficiently pervasive that we must design our protocols around their limitations.)The way I talk about it is that there is an impedence mismatch between the human brain and digital security requirements.