# Quantum cryptography

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Quantum cryptography describes the use of quantum mechanical effects (in particular quantum communication and quantum computation) to perform cryptographic tasks or to break cryptographic systems. The use of classical (i.e., non-quantum) cryptography to protect against quantum attackers is also often considered as quantum cryptography (in this case, one also speaks of post-quantum cryptography).

Well-known examples of quantum cryptography are the use of quantum communication to securely exchange a key (quantum key distribution) and the (hypothetical) use of quantum computers that would allow the breaking of various popular public-key encryption and signature schemes (e.g., RSA and ElGamal).

The advantage of quantum cryptography lies in the fact that it allows the completion of various cryptographic tasks that are proven or conjectured to be impossible using only classical (i.e., non-quantum) communication (see below for examples). In particular, quantum mechanics guarantees that measuring quantum data disturbs that data; this can be used to detect an adversary's interference with a message.

However, researchers at the University of Toronto[1][2] and NTNU[3] have shown that undetected quantum hacking might be possible in a variety of implementations of quantum key distribution systems.

## Contents

### Quantum key distribution

Arguably the best-known application of quantum cryptography is quantum key distribution (QKD). (For the history of the field see the history section in quantum key distribution). QKD describes the process of using quantum communication to establish a shared key between two parties (usually called Alice and Bob) without a third party (Eve) learning anything about that key, even if Eve can eavesdrop on all communication between Alice and Bob. This is achieved (roughly speaking) by letting Alice encode the bits of the key as quantum data before sending them to Bob; if Eve tries to learn these bits, the messages will be disturbed and Alice and Bob will notice.

QKD is possible without imposing any computational assumptions (that is, assumptions stating that certain mathematical problems such as factoring large numbers take very long time to solve on a computer). One also speaks of "unconditional security". The only assumptions are that the laws of quantum mechanics hold (which is to a certain extent disputable due to the difficulties of unifying relativity theory and quantum mechanics), and that Alice and Bob have an authenticated channel, i.e., Eve should not be able to impersonate Alice or Bob as otherwise a man-in-the-middle attack would be possible.