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 , 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).