Método
criptoanalítico en el cual el atacante
puede obtener el texto en claro correspondiente a cualquier texto cifrado por
él elegido. [Ribagorda:1997]
(I) A cryptanalysis
technique in which the analyst tries to determine the key from knowledge of
plain text that corresponds to cipher text selected (i.e., dictated) by the
analyst. [RFC4949:2007]
A chosen-ciphertext attack
(CCA) is an attack model for cryptanalysis in which the cryptanalyst chooses a
ciphertext and causes it to be decrypted with an unknown key. Specific forms of
this attack are sometimes termed "lunchtime" or "midnight"
attacks, referring to a scenario in which an attacker gains access to an
unattended decryption machine. A device which provides decryptions of chosen
ciphertexts (either by accident or by design) is generically referred to as a
"decryption oracle".
http://en.wikipedia.org/wiki/Chosen_ciphertext_attack