密码学：秘密通信的数学

Cryptography is the math of communicating in secret, and it’s as high stakes as math gets. The submarine story and dozens more like it highlight a catch-22 that plagued cryptography for millennia: to speak in code, you must first agree on a code. If I want to send you a letter but distrust my mail carrier, I can encrypt my message with a cipher. Snoops won’t be able to read it, but neither will you. If I send a follow-up note explaining the cipher, the mail carrier can intercept that, too—we’re right back where we started. Called the key-distribution problem, this cryptography pitfall seems to imply that to establish a private communication channel, we effectively need privacy to begin with. …