If Eve measures one of the two photons, she may then from her measurement result produce a pulse that she forwards to Bob, possibly containing the photon she did not measure. Thus, this is actually a single photon intercept-resend attack, where however the pulse sent to Bob contains at most 1 photon in the correct state. Thus, from the above, Eve knows the bit sent with probability at most .

Eve may choose to let only the unmeasured photon go to Bob. Bob's
error probability will then only be the normal one (, since
Eve cannot influence Bob's apparatus), but Eve runs the risk that the
photon is lost in Bob's apparatus. We call this the *Split-off*
attack (SO). As we have seen, the probability that Alice emits a
two-photon pulse is less than . Even if Eve transmits the
unmeasured photon to Bob's apparatus through a perfect fiber, it will
only contribute to the key, if it makes it through Bob's apparatus and
is detected. So Eve is left with a fraction
of all pulses for which the SO attack could work. To increase the
effect of this attack, Eve may choose to block some of the other
pulses, typically one-photon pulses that she did not measure. This
will increase the fraction of attacked pulses among those that Bob
detects. However, there is limit to the number of pulses Eve can
block: If Eve is not present, Bob expects to detect a certain number
of pulses, more precisely, the fraction of sent pulses that are
normally detected by Bob is
. Eve must maintain
approximately the number of pulses detected, since otherwise the raw
key will be shorter than expected. Consequently, the fraction
of bits in the raw key coming from pulses subjected to SO can
be at most

Eve may also inject one or more photons of her own into the pulse. If
Bob happens to measure one of these instead of the correct one, the
error probability will be at least 0.25 (again by the above
considerations on intercept-resend). So, because Eve has no control
over which of the photons Bob will measure, Bob's overall error
probability in this case will be at least
. On the other hand, there is
now a larger chance that Bob will detect the pulse - for simplicity we
will be generous to Eve and assume that Bob will always detect pulses
attacked this way. We call this the *Split-Off-Resend attack*
(SOR).