I am not understanding the following from "Lattice Cryptography for the Internet" by C. Peikert (pages 9):
We remark that a work of Ding et al. DXL14 proposes a different reconciliation method for lower bandwidth "approximate agreement," in the context of a key exchange against a passive adversary. However, we observe that the agreed-upon bit produced by their protocol is necessarily biased, not uniform, so it should not be used directly as a secret key (and the protocol as described does not satisfy the standard definition of passive security for key exchange).
I don't understand the biased part and how it is related to passive security for key exchange. Maybe I just don't understand the definition of passive security for key exchange. I don't know. Any help would be appreciated.