  We derive a proof of security for the Differential Phase Shift Quantum Key
Distribution (DPSQKD) protocol under the assumption that Eve is restricted to
individual attacks. The security proof is derived by bounding the average
collision probability, which leads directly to a bound on Eve's mutual
information on the final key. The security proof applies to realistic sources
based on pulsed coherent light. We then compare individual attacks to
sequential attacks and show that individual attacks are more powerful.
