(cryptography) An attack on a cryptosystem that relies only on the attacker's knowledge of some ciphertexts encrypted with that system and key. Less powerful than a known-plaintext attack.

(cryptography) An attack on a cryptosystem that relies on the attacker's ability to choose any ciphertext of their liking and decrypt it into the corresponding plaintext with that system and key. More powerful than a chosen-plaintext attack.

(cryptography) An attack on a cryptographic hash function that is able to find a second preimage for a hash and its preimage; that is, given a hash and an input that has that specific hash, it is able to find (faster than by brute force) another input with the same hash.

In a battle or dispute, attacking the opposition first is better than waiting for them to attack.

(immunology) A formation of transmembrane channels on the surface of a pathogenic bacterial cell that disrupts the phospholipid bilayer of the target cell resulting in its destruction.

Alternative spelling of attack is the best form of defence

(Canada, informal, education, derogatory) Standardized tests conducted by the Education Quality and Accountability Office in the province of Ontario.

plural of complement membrane attack complex