The encryption key was to the archive that Assange shared with the Guardian journalist working on the story, David Leigh. The encrypted archive was apparently

The encryption key was to the archive that Assange shared with the Guardian journalist working on the story, David Leigh. The encrypted archive was apparently mirrored on several sites, meaning that when Leigh published the key, anyone could access the full archive of unredacted cables. I guess Assange could be faulted for working with traditional journalists who aren't so competent with technology. That would be an ironic reversal of the usual complaints about WikiLeaks. But at the very least, Leigh should have known to at least ask Assange before publishing the encryption key. The Guardian turned around and blamed WikiLeaks, throwing it under the bus for the Guardian's own screw-up.