Intractable computational problems are a barrier for algorithm designers. Cryptographers are modern lemonade makers. Their lemons are these intractable problems, which they squeeze into sweet lemonade: secure cryptographic protocols. Why is a lemon even required? Because it lets us assume there is something an adversary cannot do. Intractable problems can give the honest user an advantage: for example, the honest user can multiply two large primes. The honest user knows the prime factors of the resulting number; yet, it is widely believed that a classical adversary cannot (efficiently) find these factors.
Cryptographers have been squeezing this computational intractability lemon since the 1970s. Are there any other lemons on which cryptography could be based? Quantum mechanics has quite a few peculiarities. One notable example is the no-cloning theorem, which states that quantum information cannot be cloned. Uncloneable cryptography—the main focus of this review—uses the no-cloning lemon as its main ingredient. For a broader perspective, see Figure 1 on page 80.
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