# Attack on BB84 algorithm for quantum key exchange?

I have a question which is related to the BB84 cryptosystem.

We are not able to send single photons so instead we send $K$ photons at a time all with the same polarization. An enemy can separate one of these from the remaining $K-1$.

If the enemy can detect the polarisation by only looking at one of the photons, the others may remain undisturbed and so the enemy will not be detected - is this correct?

Why can the enemy find our common key if $K=3$?

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In 2001, Scarani, Acín, Ribordy and Gisin proposed a protocol (SARG04 pdf, wiki) which is robust against PNS attacks while using the same physical modulation than BB84. This protocol works by exchanging classical information which is more ambiguous than the basis. However, as soon as $K\ge3$ and the losses are over 50%, this protocol is subject to an unambiguous discrimination attack.