FairMormon does an excellent job of explaining where Brigham Young and the Twelve got their Priesthood Authority and Keys:

http://en.fairmormon.org/Criticism_of_Mormonism/Books/Passing_the_Heavenly_Gift/Source_of_authority_of_Brigham_Young_and_the_apostles_after_Joseph%27s_death

You can download a copy of this at this link:

The Source of Authority of Brigham Young

I found this information extremely valuable, because I never had access to it before.

Enoch

Other Articles about the Priesthood:

Men Will Never Be Given the Priestesshood