Provided are a computer program product, system, and method to allocate blocks of memory in a memory device having a plurality of blocks. An unencrypted memory allocation function requests allocation of unencrypted blocks in the memory device. An encrypted memory allocation function requests allocation of encrypted blocks in the memory device. An unencrypted Input/Output (I/O) request performs an I/O operation against the unencrypted blocks in the memory device. An encrypted I/O request function performs an I/O operation against the encrypted blocks in the memory device. An operating system uses an encryption key associated with the encrypted blocks to encrypt or decrypt data in the encrypted blocks to perform the encrypted I/O operation in response to processing the encrypted I/O request functions, wherein the unencrypted and encrypted memory allocation functions and unencrypted and encrypted I/O request functions comprise different functions in a library of functions available to the application.