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t m= m1m2. . .mn be a given message of length n, which Alice wishes to encrypt. For each plaintext letter mi, where 1 a‰¤ ia‰¤ n, Alice adds the plaintext numbers to the key numbers. The result is taken modulo 30. For example, the last letter of the plaintext from Fig.3.2, “D,” is encoded by “m12=03.” The corresponding key is “m12= 28,” so we have c12= 3 + 28 = 31. Since 31 a‰¡ 1 mod 30, our plaintext letter “D” is encrypted as “B.”

Decryption works similarly by subtracting, character by character, the key letters from the corresponding cipher text letters. So the encryption and decryption can be written as respectively ci= (mi+ ki) mod 30 and mi= (ciaˆ’ ki) mod 30, 1 a‰¤ i a‰¤ n.

mONE-TIMEPADM141304281906120426150003
k061302011406071805132826
C202606290313192201111301
cUG.DNTWBLNB

Fig.3.2 Encryption and decryption example for the one time pad

3.3. PROTOCOLS OF QKD
BB84 (and DARPA Project) - uses polarization of photons to encode the bits of information - relies on “uncertainty” to keep Eve from learning the secret key.
Ekert - uses entangled photon states to encode the bits - relies on the fact that the information defining the key only 'comes into being' after measurements performed by Alice and Bob.
3.4. LIMITATIONS
Cryptographic technology in use today relies on the hardness of certain mathematical problems. Classical cryptography faces the following two problems which are as follows.
The security of many classical cryptosystems is based on the hardness of problems such as integer factoring or the discrete logarithm problem. But since these problems typically are not probablyhard, the corresponding cryptosystems are potentially insecure.
The theory of quantum computation has yielded new methods to tackle these mathematical problems in a much more efficient way. Although there are still numerous challenges to overcome before a working quantum computer of sufficient power can be built, in theory many classical ciphers might be broken by such a powerful machine.
However, while quantum computation seems to be a severe challenge to classical cryptography in a possibly not so distant future, at the same time it offers new possibilities to build encryption methods that are safe even against attacks performed by means of a quantum computer. Quantum cryptography extends the power of classical cryptography by protecting the secrecy of messages using the physical laws of quantum mechanics.

4、量子密码学——4. QUANTUM CRYPTOGRAPHY
Quantum Cryptography, or Quantum Key Distribution (QKD), uses quantum mechanics to guarantee secure communication. It enables two parties to produce a shared random bit string known only to them, which can be used as a key to encrypt and decrypt messages. An important and unique property of quantum cryptography is the ability of the two communicating users to detect the presence of any third party trying to gain knowledge of the key. This result from a fundamental part of quantum mechanics: the process of measuring a quantum system in general disturbs the system. A third party trying to eavesdrop on the key must in some way measure it, thus introducing detectable anomalies. By using quantum superposition's or quantum entanglement and transmitting information in quantum states, a communication system can be implemented which detects eavesdropping. If