  This paper consists of musings that originate mainly from conversations with
other physicists, as together we've tried to learn some cryptography, but also
from conversations with a couple of classical cryptographers. The main thrust
of the paper is an attempt to explore the ramifications for cryptographic
security of incorporating physics into our thinking at every level. I begin by
discussing two fundamental cryptographic principles, namely that security must
not rely on secrecy of the protocol and that our local environment must be
secure, from a physical perspective. I go on to explain why by definition a
particular cryptographic task, oblivious transfer, is inconsistent with a
belief in the validity of quantum mechanics. More precisely, oblivious transfer
defines states and operations that do not exist in any (complex) Hilbert space.
I go on to argue the fallaciousness of a "black box" approach to quantum
cryptography, in which classical cryptographers just trust physicists to
provide them with secure quantum cryptographic sub-protocols, which they then
attempt to incorporate into larger cryptographic systems. Lest quantum
cryptographers begin to feel too smug, I discuss the fact that current
implementations of quantum key distribution are only technologically secure,
and not "unconditionally" secure as is sometimes claimed. I next examine the
concept of a secure lab from a physical perspective, and end by making some
observations about the cryptographic significance of the (often overlooked)
necessity for parties who wish to communicate having established physical
reference frames.
