When a cryptosystem is susceptible to chosen-ciphertext attack , implementers must be careful to avoid situations in which an attackers might be able to decrypt chosen ciphertexts ( i.e. , avoid providing a decryption scheme ) . When a cryptosystem is vulnerable to chosen-ciphertext attack , implementers must be careful to avoid situations in which an adversary might be able to decrypt chosen-ciphertexts ( i.e. , avoid providing a decryption oracle ) .