This paper introduces a method to make quantum key distribution (QKD) secure using decoy states, which allows for the first time most long-distance QKD experiments to be unconditionally secure. The method uses decoy states to detect eavesdropping attacks, enabling both unconditional security and improved performance. The security of QKD is based on fundamental physics rather than computational assumptions, and the method allows for the use of existing hardware, avoiding the need for new experimental developments. The key idea is that Alice prepares additional decoy states along with standard BB84 states. These decoy states help detect eavesdropping by allowing Alice and Bob to determine bounds on the fraction of single-photon signals and the quantum bit error rate (QBER). By measuring the yields and QBER of decoy states, Alice and Bob can obtain reliable bounds on these parameters, leading to a more secure and efficient QKD protocol. The paper shows that the use of decoy states significantly improves the secure key generation rate, allowing for longer distances and higher performance. The method is combined with the GLLP analysis, leading to a more robust security proof. The results demonstrate that with decoy states, secure QKD can be achieved over distances of over 140 km using current technology, surpassing previous experimental results. The paper also discusses the implications of the results, showing that the new method allows for secure QKD at much longer distances than previously thought possible. The method is applicable to various QKD setups and can be used in multiparty quantum cryptographic protocols. The results are supported by experimental demonstrations and theoretical analysis, showing the effectiveness of the decoy state method in enhancing the security and performance of QKD.This paper introduces a method to make quantum key distribution (QKD) secure using decoy states, which allows for the first time most long-distance QKD experiments to be unconditionally secure. The method uses decoy states to detect eavesdropping attacks, enabling both unconditional security and improved performance. The security of QKD is based on fundamental physics rather than computational assumptions, and the method allows for the use of existing hardware, avoiding the need for new experimental developments. The key idea is that Alice prepares additional decoy states along with standard BB84 states. These decoy states help detect eavesdropping by allowing Alice and Bob to determine bounds on the fraction of single-photon signals and the quantum bit error rate (QBER). By measuring the yields and QBER of decoy states, Alice and Bob can obtain reliable bounds on these parameters, leading to a more secure and efficient QKD protocol. The paper shows that the use of decoy states significantly improves the secure key generation rate, allowing for longer distances and higher performance. The method is combined with the GLLP analysis, leading to a more robust security proof. The results demonstrate that with decoy states, secure QKD can be achieved over distances of over 140 km using current technology, surpassing previous experimental results. The paper also discusses the implications of the results, showing that the new method allows for secure QKD at much longer distances than previously thought possible. The method is applicable to various QKD setups and can be used in multiparty quantum cryptographic protocols. The results are supported by experimental demonstrations and theoretical analysis, showing the effectiveness of the decoy state method in enhancing the security and performance of QKD.