A standard cryptanalytic attack is to know some plaintext matching a
given piece of ciphertext and try to determine the key which maps one
to the other. This plaintext can be known because it is standard (a
standard greeting, a known header or trailer, …) or because it is
guessed. If text is guessed to be in a message, its position is probably
not known, but a message is usually short enough that the cryptanalyst
can assume the known plaintext is in each possible position and do
attacks for each case in parallel. In this case, the known plaintext can
be something so common that it is almost guaranteed to be in a message.
A strong encryption algorithm will be unbreakable not only under known
plaintext (assuming the enemy knows all the plaintext for a given
ciphertext) but also under “adaptive chosen plaintext” — an attack
making life much easier for the cryptanalyst. In this attack, the enemy
gets to choose what plaintext to use and gets to do this over and over,
choosing the plaintext for round N+1 only after analyzing the result of
round N.
