The paper presents a theoretical quantum key distribution (QKD) scheme that utilizes EPR (Entangled Pair of Photons) pairs. This scheme is designed to be efficient and high-capacity, as it uses all EPR pairs except those reserved for eavesdropper detection. Each EPR pair carries 2 bits of key code, significantly increasing the key capacity compared to other schemes like BB84, which uses only 1 bit per EPR pair. The scheme involves Alice producing an ordered sequence of EPR pairs, selecting one particle from each pair to form an EPR partner sequence, and sending it to Bob. Bob measures a subset of these particles and informs Alice, who then measures the corresponding particles. They compare results to check for eavesdropping. If no eavesdropping is detected, Alice sends the remaining particles to Bob, who performs Bell-basis measurements. A second eavesdropping check is conducted using a subset of these measurements. The protocol is secure against direct measurement, intercept-resend, and opaque attacks. It is more efficient than the BB84 protocol, achieving 100% information-theoretic efficiency, and has a higher capacity due to the use of 2-bit key codes. The scheme can also be generalized to distribute secret keys to multiple legitimate users. While the practical implementation of the scheme is challenging due to the complexity of Bell state measurements, the operations are feasible in principle. The authors acknowledge financial support from various institutions and thank the referee for helpful comments.The paper presents a theoretical quantum key distribution (QKD) scheme that utilizes EPR (Entangled Pair of Photons) pairs. This scheme is designed to be efficient and high-capacity, as it uses all EPR pairs except those reserved for eavesdropper detection. Each EPR pair carries 2 bits of key code, significantly increasing the key capacity compared to other schemes like BB84, which uses only 1 bit per EPR pair. The scheme involves Alice producing an ordered sequence of EPR pairs, selecting one particle from each pair to form an EPR partner sequence, and sending it to Bob. Bob measures a subset of these particles and informs Alice, who then measures the corresponding particles. They compare results to check for eavesdropping. If no eavesdropping is detected, Alice sends the remaining particles to Bob, who performs Bell-basis measurements. A second eavesdropping check is conducted using a subset of these measurements. The protocol is secure against direct measurement, intercept-resend, and opaque attacks. It is more efficient than the BB84 protocol, achieving 100% information-theoretic efficiency, and has a higher capacity due to the use of 2-bit key codes. The scheme can also be generalized to distribute secret keys to multiple legitimate users. While the practical implementation of the scheme is challenging due to the complexity of Bell state measurements, the operations are feasible in principle. The authors acknowledge financial support from various institutions and thank the referee for helpful comments.