Bob is happy to be sending encrypted data, and Alice is happy to be receiving data encrypted using her own public key; both are unaware of Trudy's presence. Should Bob and Alice meet later and discuss their interaction, Alice will have received exactly what Bob sent, so nothing will be detected as being amiss. This is one example of the so-called man-in-the-middle attack (more appropriately here, a "woman-in-the-middle" attack). It is also sometimes known as a bucket-brigade attack, since Trudy's passing of data between Alice and Bob resembles the passing of buckets of water along a chain of people (a so-called "bucket brigade") who are putting out a fire using a remote source of water.