Imagine that two users, Alice and Bob, have a conversation about a sensitive subject over email. Alice is vigilant about protecting the secrecy of the discussion and encrypts all copies of her sent and received emails. Bob, on the other hand, is well-meaning but negligent, and does not bother to encrypt any of his on-disk files. Worse, he unwittingly stores his email files in the portion of his hard drive that is shared with the rest of the Internet via a peer-to-peer client. One night, unbeknownst to either Alice or Bob, someone on the peer-to-peer network peruses Bob's shared space, finds his email files, and reads their private conversation. Despite Alice's best efforts and Bob's best intentions, the privacy of their discussion has leaked.
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