Under the constraints we specified, Eve will be limited to simply
measuring the photon or letting it go. If she measures it, she may
produce a pulse on her own that she sends in place of the measured
photon. This is a so called intercept-resend attack. In [2],
it is argued that the best such attack is to measure the photon in the
Breidbart basis (i.e. a basis oriented at 22.5 degrees) and resend a
pulse in a state equal to the one measured. This gives a guess at the
bit sent by Alice which is correct with probability
, and leads to an error rate for Bob of at least 0.25. We
will call this a *Single Intercept-Resend* attack (SIR).

Louis Salvail 2001-06-15