A chosen plaintext attack is any form of cryptanalysis which presumes that the attacker has the capability to choose arbitrary plaintexts to be encrypted and obtain the corresponding ciphertexts. The attack would gain some further information, typically the secret key.
This appears, at first glance, to be an unrealistic model; it would certainly be unlikely that an attacker could persuade a human cryptographer to encrypt large amounts of plaintexts of the attacker's choosing. Modern cryptography, on the other hand, is implemented in software or hardware and is used for a diverse range of applications; for many cases, a chosen-plaintext attack is often very feasible. In addition, any cipher that can prevent chosen-plaintext attacks is then also guaranteed to be secure against known-plaintext and ciphertext-only attacks; this is a conservative approach to security.
Two forms of chosen-plaintext attack can be distinguished:
Batch chosen-plaintext attack, where the cryptanalyst chooses all plaintexts before any of them is encrypted. This is often the meaning of an unqualified use of "chosen-plaintext attack".
Adaptive chosen-plaintext attack, where the cryptanalyst makes a series of interactive queries, choosing subsequent plaintexts based on the information from the previous encryptions.
The bits in the plaintext are divided into two categories, ``fixed'' and ``changing''; a selection of the bits of the ciphertext are chosen as ``target'' bits.
A number of chosenplaintexts are encrypted, all with the same fixed bits and with changing bits chosen at random; the attack is a success if a collision in the target bits of the ciphertext is generated.
This attack may be applied to  by choosing the first two blocks of the plaintext as the ``changing'' bits, and all of the output except the second two blocks as the ``target'' bits.
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