  To prove the security of quantum key distribution (QKD) protocols, several
assumptions have to be imposed on users' devices. From an experimental point of
view, it is preferable that such theoretical requirements are feasible and the
number of them is small. In this paper, we provide a security proof of a QKD
protocol where the usage of any light source is allowed as long as it emits two
independent and identically distributed (i.i.d.) states. Our QKD protocol is
composed of two parts: the first part is characterization of the photon-number
statistics of the emitted signals up to three-photons based on the method [Opt.
Exp. 27, 5297 (2019)], followed by running our differential-phase-shift (DPS)
protocol [npj Quantum Inf. 5, 87 (2019)]. It is remarkable that as long as the
light source emits two i.i.d. states, even if we have no prior knowledge of the
light source, we can securely employ it in the QKD protocol. As this result
substantially simplifies the requirements on light sources, it constitutes a
significant contribution on realizing truly secure quantum communication.
