Alice and Bob have a secure communications system. They play mental poker, simultaneously sign contracts, even exchange digital cash. Their protocols are secure. Their algorithms are top-notch. Unfortunately, they buy their keys from Eve's “Keys-R-Us,” whose slogan is “You can trust us: Security is the middle name of someone our ex-mother-in-law's travel agent met at the Kwilc-E-Mart.”
Eve doesn't have to break the algorithms. She doesn't have to rely on subtle flaws in the protocols. She can use their keys to read all of Alice's and Bob's message traffic without lifting a cryptanalytic finger.
In the real world, key management is the hardest part of cryptography. Designing secure cryptographic algorithms and protocols isn't easy, but you can rely on a large body of academic research. Keeping the keys secret is much harder.
Cryptanalysts often attack both symmetric and public-key cryptosystems through their key management. Why should Eve bother going through all the trouble of trying to break the cryptographic algorithm if she can recover the key because of sloppy key storage procedures? Why should she spend $10 million building a cryptanalysis machine if she can spend $1000 bribing a clerk? Spending a million dollars to buy a well-placed communications clerk in a diplomatic embassy can be a bargain. The Walkers sold U.S. Navy encryption keys to the Soviets for years. The CIA's director of counterintelligence went for less than $2 million, wife included. ...