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.
For example, as far as we know, DES is reasonably strong even under an
adaptive chosen plaintext attack (the attack Biham and Shamir
used). Of course, we do not have access to the secrets of US
government cryptanalytic services. Still, it is the working assumption
that DES is reasonably strong under known plaintext and
triple-DES
is very strong under all attacks.
To summarize, the basic types of cryptanalytic attacks in order of difficulty for the attacker, hardest first, are:
Under the following attacks, the attacker has the far less likely or plausible ability to `trick' the sender into encrypting or decrypting arbitrary plaintexts or ciphertexts. Codes that resist these attacks are considered to have the utmost security.