Various methods and systems for selectively protecting against chosen plaintext attacks when encrypting data for storage on an untrusted storage system are disclosed. One method involves generating an encryption key for use in encrypting data and generating an identifier for the data. Generation of the encryption key is based upon a hash of the data to be encrypted, and generation of the identifier is based upon the data to be encrypted and/or the encryption key. The method also involves detecting whether an encrypted copy of the data is already stored by a storage system, based upon the identifier. The method also detects whether a higher level of security has been specified for the data and, if so, modifies the data to be encrypted or the encryption key, based upon a client-specific value, prior to generating the identifier.