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## Sunday, March 10, 2013

### What is meant by Quantum Cryptography?

The quantum mechanical effects when used for carrying out cryptography tasks are called the quantum cryptography. This technology is also used for breaking the cryptographic systems. And the quantum mechanical effects used include:
1. The quantum computation and
2. The quantum communication

Some very popular examples of uses of quantum cryptography are as follows:
1. For the secure exchange of the quantum key distribution or key
2. Use of quantum computers that used for breaking in to the systems using signature schemes such as ElGamal and RSA and public – key encryption.

The major advantage of quantum cryptography is that by using it, a number of cryptographic tasks are completed that are almost impossible to be completed through the classical communication i.e., the non – quantum effects.

## Applications of Quantum Cryptography

1. Quantum key distribution:
- This is the most widely used application.
- It can be described as the use of quantum communication for establishing a key that is shared by two parties (usually referred to as ‘Alice’ and ‘bob’) without involvement of a third party (called Eve) knowing anything regarding the key, even if it eavesdrop on the communication between the two parties.
This happens as follows:
- The bits of the key are encoded by Alice as quantum data and are sent to Bob.
- Now if Eve eavesdrops, the message will be disturbed, making Alice and Bob know about it.
- Thus, we can say that this is a form of encrypted communication.
- Further, the QKD’s security is proven mathematically without restricting the eavesdropper’s abilities.
- In CKD (classical key distribution) this is not possible.
- This is commonly known as the ‘unconditional security’.
- However, the laws of quantum mechanics also apply and there is a need for Alice and Bob to authenticate each other. - It should not be possible for Eve to impersonate as Alice or Bob.
- This can lead to man – in – the – middle – attack.

2. Quantum commitment:
- This was another task that the researchers tried to achieve with the unconditional security offered by QKD.
- Quantum commitment is actually a scheme in which apart Alice can fix a certain value i.e., to commit where it cannot be changed by Alice anymore and it is ensured that Bob won’t learn anything about it, until and unless it is decided by Alice to be revealed to Bob.
- The most common use of these schemes is in the cryptographic protocols.
- Oblivious transfers can be performed by constructing an unconditionally secure protocol from a quantum channel and commitment.
- With such transfers any distributed computations can be implemented securely.

3. Bounded – and noisy – quantum – storage model (BQSM):
- This model provides a possible way for constructing quantum commitment and OTs that unconditionally secure.
- This model assumes that a known constant Q limits the amount of quantum data stored by an adversary.
- However, no limit is imposed up on the classical data.
Idea behind this model is:
- The number of quantum bits exchanged by the involving parties is more than Q.
- This amount of information cannot be stored even by a dishonest party since the memory limit of adversary if Q quantum bits.
- This will lead it to 2 options: either discard the data or measure it.
- So now the OTs can be implemented.