Known-plaintext attack: The attacker knows or can guess the plaintext for some parts of the ciphertext. The task is to decrypt the rest of the ciphertext blocks using this information. This may be done by determining the key used to encrypt the data, or via some shortcut.
Chosen-plaintext attack: The attacker is able to have any text he likes encrypted with the unknown key. The task is to determine the key used for encryption. Some encryption methods, particularly RSA, are extremely vulnerable to chosen-plaintext attacks. When such algorithms are used, extreme care must be taken to design the entire system so that an attacker can never have chosen plaintext encrypted.
Man-in-the-middle attack: This attack is relevant for cryptographic communication and key exchange protocols. The idea is that when two parties are exchanging keys for secure communications (e.g., using Diffie-Hellman), an adversary puts himhelf between the parties on the communication line. The adversary then performs a separate key exchange with each party. The parties will end up using a different key, each of which is known to the adversary. The adversary will then decrypt any communications with the proper key, and encrypt them with the other key for sending to the other party. The parties will think that they are communicating securely, but in fact the adversary is hearing everything.
One way to prevent man-in-the-middle attacks is that both sides compute a cryptographic hash function of the key exchange (or at least the encryption keys), sign it using a digital signature algorithm, and send the signature to the other side. The recipient then verifies that the signature came from the desired other party, and that the hash in the signature matches that computed locally. This method is used e.g. in Photuris.
Timing Attack: This very recent attack is based on repeatedly measuring the exact execution times of modular exponentiation operations. It is relevant to at least RSA, Diffie-Hellman, and Elliptic Curve methods. More information is available in the original paper and various followup articles.
Discover the Algorithm Attack: Some crypt systems keep the algorithm secret as well as the key (Clipper). These algorithms can be attacked in part by discovering the algorithm via disassembly or such.
Dumb Attacks: Guess key by disassembling netscapes code. Read data sent of SSL chanel via disk cache. be root and just search thru ram for ither the key or the data itself. Find someone logged in yet absent.
Brute Force Attacks: Break system by just trying every
possible key, one by one. Takes 2**key_length time. Yech!!
Clipper is the chip that the U.S. government tried to get everybody to use. It had a secret algorithm and key escrow, which ment the governbment had a password that would crack every message, and would give these to police for the price of a warent.
DES is another government standard, widely used by banks and such. Government limited key length to 56 bits so they could crack it themselves. Often now used with longer keys in a thing called tripple-DES.
RC4 is the algorithm used by the secure socket layer. The algorithm used to be a trade secret until someone posted the code on the net. Has been broken! He used 120+ machines for 8 days.