Keys are knowledge of signs, must "know them all"

"I preached in the grove, on the keys of the kingdom, charity, &c. The keys are certain signs and words by which false spirits and personages may be detected from true, which cannot be revealed to the Elders till the Temple is completed. The rich can only get them in the Temple, the poor may get them on the mountain top as did Moses. The rich cannot be saved without charity, giving to feed the poor when and how God requires, as well as building. There are signs in heaven, earth and hell; the Elders must know them all, to be endowed with power, to finish their work and prevent imposition. The devil knows many signs, but does not know the sign of the Son of Man, or Jesus. No one can truly say he knows God until he has handled something and this can only be in the holiest of holies." (Joseph Smith, HC 4:35, May 1, 1842)