  The security of quantum key distribution (QKD) has been proven for different
protocols, in particular for the BB84 protocol. It has been shown that this
scheme is robust against eventual imperfections in the state preparation, and
sending only three different states delivers the same secret key rate
achievable with four states. In this work, we prove, in a finite-key scenario,
that the security of this protocol can be maintained even with less measurement
operators on the receiver. This allows us to implement a time-bin encoding
scheme with a minimum amount of resources.
