I just found this very nice paper on cryptography & complexity theory on the arXiv:

Jörg Rothe: "Some Facets of Complexity Theory and Cryptography: A Five-Lectures Tutorial"

from the abstract:

In this tutorial, selected topics of cryptology and of computational complexity theory are presented. We give a brief overview of the history and the foundations of classical cryptography, and then move on to modern public-key cryptography. Particular attention is paid to cryptographic protocols and the problem of constructing the key components of such protocols such as one-way functions. A function is one-way if it is easy to compute, but hard to invert. We discuss the notion of one-way functions both in a cryptographic and in a complexity-theoretic setting. We also consider interactive proof systems and present some interesting zero-knowledge protocols. In a zero-knowledge protocol one party can convince the other party of knowing some secret information without disclosing any bit of this information. Motivated by these protocols, we survey some complexity-theoretic results on interactive proof systems and related complexity classes.

It's accessible with a minimum amount of knowledge in computer science and/or mathematics. I guess it's even funny without knowing anything (but then you should skip the "mathematical" parts). A man-in-the-middle-attack on an insecure channel is illustrated by a picture of Erich Mielke :-)

If you ever wondered how PGP works, this is a good place to learn it!

some more bibliographic info:
57 pages, 17 figures, Lecture Notes for the 11th Jyvaskyla Summer School
Journal reference: ACM Computing Surveys, volume 34, issue 4, pp. 504--549, December 2002
Cite as: arXiv:cs/0111056v2 [cs.CC]