Conventional (symmetric cipher[?] or secret key) encryption systems, in which the same key is used to encrypt and decrypt a text, are sometimes vulnerable to this attack. If you know that f(foo) = bar, and you know both foo and bar, (because you chose foo and arranged to have the encryption system (f() in this case) convert it to bar), it is often possible to recover the value of f. One of the advantages of hybrid crypto systems[?] such as Pretty Good Privacy is that they are less often subject to this form of attack. The 'message' encrypted with using an asymmetric key algorithm (aka public key / private key algorithm[?]) is a session key which should have been randomly chosen, or the hash of plaintext message, not the plaintext itself.
But note that the RSA asymmetric key algorithm is inherently susceptible to a chosen ciphertext attack.
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