Possibility of Chosen Plaintext Attack (CPA) in real-world scenario? In CPA, it is said that the adversaries get ciphertext for the choice of plaintext of adversaries through an encryption oracle. Is this a realistic assumption in real-world, in which the adversaries get ciphertext for the choice of their plaintexts? If so, please provide me with some example scenarios illustrating how adversaries obtain ciphertexts for their plaintexts?
Bruce Schneier foresaw your skepticism and directly answered this question in "Applied Cryptography":


  Known-plaintext attacks and chosen-plaintext attacks are more common than you might think. It is not unheard-of for a cryptanalyst to get a plaintext message that has been encrypted or to bribe someone to encrypt a chosen message. You may not even have to bribe someone; if you give a message to an ambassador, you will probably find that it gets encrypted and sent back to his country for consideration. Many messages have standard beginnings and endings that might be known to the cryptanalyst. Encrypted source code is especially vulnerable because of the regular appearance of keywords: #define, struct, else, return. Encrypted executable code has the same kinds of problems: functions, loop structures, and so on. Known-plaintext attacks (and even chosen-plaintext attacks) were successfully used against both the Germans and the Japanese during World War II. David Kahn's books [794, 795, 796] have historical examples of these kinds of attacks.


Here are the referenced books:


D. Kahn, The Codebreakers: The Story of Secret Writing, New York: Macmillan Publishing Co., 1967.
D. Kahn, Kahn on Codes, New York: Macmillan Publishing Co., 1983.
D. Kahn, Seizing the Enigma, Boston: Houghton Mifflin Co., 1991.