There is a compromise between the two extremes of, on the one hand, not knowing any of the plaintext at all, and, on the other, knowing all of it. In the Cryptonomicon that falls under the heading of cribs . A crib is an educated guess as to what words or phrases might be present in the message. For example if you were decrypting German messages from World War II, you might guess that the plaintext included the phrase " HEIL HITLER " or " SIEG HEIL ." You might pick out a sequence of ten characters at random and say, "Let’s assume that this represented HEIL HITLER . If that is the case, then what would it imply about the remainder of the message?"