Quantum cryptography with entangled photon pairs can be more powerful than protocols based on single photons or weak coherent pulses: they can tolerate higher losses and thus allow the distribution of quantum secret keys (QKD) over longer distances [1], they also provide a way towards device-independent quantum cryptography [2]. However, in order to enable a wide use of entangled photon pairs in future quantum telecommunication systems, further developments are needed to demonstrate performant sources that can be easily fabricated and integrated into Telecom fiber networks. Here we present a source consisting of an aluminium gallium arsenide waveguide generating photon pairs in the Telecom band by type II spontaneous parametric down-conversion [3]. Such a device has already been proven to work under electrical pumping [4]. Thanks to the very small birefringence of the guided modes, the pairs are directly generated in a polarization-entangled Bell state, without the need for any post-compensation. Moreover, as the photons are emitted over a large bandwidth (about 100 nm) with a joint spectrum that exhibits frequency anticorrelation, the same source can be used to simultaneously distribute keys among multiple pairs of users by using standard Telecom wavelength demultiplexers [5]. Here, we experimentally show the distribution of quantum secret keys with the BBM92 QKD protocol [6] between four different pairs of users with a commercial 100 GHz demultiplexer (0.8 nm channel width and spacing). Under CW pumping conditions, using free-running InGaAs single-photon detectors, we achieve a secret key rate of 0.21 bits/s and a qubit error rate (QBER) of 6.9% over 50 km of standard optical fiber. Our results, obtained with a robust and simple experimental set-up, open the way towards the implementation of practical device-independent quantum communication protocols.