Posts Tagged ‘cryptographic systems

Eavesdropping Detection

The quantum cryptography protocols described above provide two parties with nearly identical secret keys along with an estimate of the discrepancy between the shared key. In quantum cryptographic systems, the discrepancy between the shared secret key can be due to either a third party eavesdropping or imperfections in the transmission line and/or equipment. For security purposes, the discrepancy is always assumed to be from third party eavesdropping, since there is no way to distinguish the true cause of the discrepancy.

Using the BB84 encoding scheme, Alice and Bob check for eavesdropping by comparing a portion of their remaining bits that they would use for a shared secret key. If a third party were eavesdropping, errors would be introduced into Bob’s photon measurements. If a number of bits beyond a threshold differ, the derived key is discarded and the process QKD (Quantum Key Distribution) process is repeated.¬†Using the B92 encoding scheme, Bob can determine if a third party was eavesdropping directly from the measurements of the photon. As stated in the previous section, if the measurement of the photon is greater than 1 then the receiver can be certain that no one was eavesdropping otherwise the photon is discarded.

Using the Ekert encoding scheme, Bob and Alice check for eavesdropping by comparing their rejected bits, called the rejected key. If their comparison satisfies Bell’s inequality, then a third party has been detected and the entire process is repeated. Otherwise the raw key is retained. Bell’s inequality is essentially a method for determining the probability that two bits do not match given the measurement basis used.

Given an error rate between the shared key, two processes can be used to reduce the erroneous bits and reduce a third party’s knowledge of the key to a negligibly small value. The two processes are privacy amplification and information reconciliation, which are used together.

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