An anonymous reader writes: We frequently hear how present computer security depends on the difficulty of prime factorization, which quantum computers perform instantly. Already we see a trickle of news stories showing progress on developing quantum computers and predictions of when the researchers will produce a useful computer. At that time, anyone will be able to decrypt intercepted or cached encrypted data such as credit card, medical, backups, and personal communications. Which ciphers depend on prime factorization? Are there ciphers which a classical computer can encrypt/decrypt but which a quantum computer cannot break? How do you envision the switchover occuring?