Algebraic attack
A method of cryptanalytic attack used against block ciphers that exhibit a significant amount of mathematical structure.

Algorithmic attack (Formulaic attack)
Algorithmic attacks are in some ways much more difficult to perform because they generally require an extremely high degree of knowledge in mathematics. Rather than going after the entire keyspace, the codebreaker will try and find flaws in the algorithm that causes it to be reduced to a problem of decreased complexity.

Birthday attack
A brute-force attack used to find collisions. It gets its name from the surprising result that the probability of two or more people in a group of 23 sharing the same birthday is greater than 1/2.

Brute Force Attack
Brute Force Attack is a form of attack in which each possibility is tried until success is obtained. Typically, a ciphertext is deciphered under different keys until plaintext is recognized.

Chosen ciphertext attack
An attack where the cryptanalyst may choose the ciphertext to be decrypted.

Chosen plaintext attack
A form of cryptanalysis where the cryptanalyst may choose the plaintext to be encrypted

Ciphertext-only attack
A form of cryptanalysis where the cryptanalyst has some ciphertext but nothing else. Modern cryptosystems are not weak against ciphertext-only attacks, however, in practice it is often possible to guess the plaintext, as many types of messages have fixed format headers. For example, many classical attacks use frequency analysis of the ciphertext, however, this does not work well against modern ciphers.

Dictionary attack
A brute force attack that tries passwords and or keys from a precompiled list of values. This is often done as a precomputation attack.

Differential cryptanalysis
A chosen plaintext attack relying on the analysis of the evolution of the differences between two plaintexts.

Known plaintext attack
A form of cryptanalysis where the cryptanalyst knows both the plaintext and the associated ciphertext.

Meet-in-the-middle attack
A known plaintext attack against double encryption with two separated keys where the attacker encrypts a plaintext with a key and "decrypts" the original ciphertext with another key and hopes to get the same value.

Middleperson attack
The idea of Middleperson attack is that when two parties, A and B, are exchanging keys for secure communication, an adversary intercepts the signals that A and B send to each other, and performs a key exchange with A and B separately. A and B will end up using a different key, each of which is known to the adversary. The adversary can then decrypt any communication from A with the key he shares with A, and then resends the communication to B by encrypting it again with the key he shares with B. Both A and B will think that they are communicating securely, but in fact the adversary is hearing everything. The best way to prevent the middleperson attack is to use a public-key cryptosystem capable of providing digital signatures. The parties must know each other's public keys in advance. After the shared secret has been generated, the parties send digital signatures of it to each other.

Precomputation attack
An attack where the adversary precomputes a look-up table of values used to crack encryption or passwords. See also dictionary attack.