QubitCore is a CubeSat-compatible quantum key distribution (QKD) payload designed for space-based quantum communication missions. The payload integrates a miniaturized entangled photon source, two optical receiving modules, and an onboard time-tagging unit in a compact architecture. Each receiving module includes a dedicated optical design and four single-photon detectors for coincidence and correlation measurements. Entanglement-based BBM92 QKD protocol is fully implemented and optimized for space operation. QubitCore is designed as a self-contained payload suitable for CubeSat platforms, enabling secure key distribution experiments and demonstrations in orbit.