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 the 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.)

Kaufman, Perlman and Speciner