The history of cryptography is a history of failures. Stronger ciphers replaced broken ones, to be in turn broken again. Quantum cryptography is offering a hope to end this replacement cycle, for its security premises on the laws of quantum physics and not on limitations of human ingenuity and computing. But, can our nascent quantum technology implement quantum cryptography securely? The talk introduces today's quantum cryptography techniques, then shows how to compromise their security by exploiting imperfections in engineering implementations.