From Kaufman, Perlman, and Speciner Network Security—Private
Communication in a Public World book
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.)