To ensure absolute security of key distribution, each key distribution scheme must be proved. While the security of most classical cipher schemes cannot be proved, the security of quantum cipher schemes is guaranteed by quantum mechanics and can be strictly proved. Shor-Preskill's method with which the unconditional security of BB84 quantum key distribution scheme is proved is introduced first. Its techniques and features are summarized and the subtleties of using symmetrization and quantum entanglement purification (QEP) to reduce a QEP based BB84 to a CSS based BB84, and of using the property of CSS code to reduce the CSS based BB84 to standard BB84 protocol are revealed. Then it is proved that Shor-preskill's method cannot be applied to B92 protocol directly. A possible solution to prove the unconditional security of B92 using Shor-Preskill's method indirectly is proposed.