Provided are a computer program product, system, and method to allocate blocks of memory in a memory device having a plurality of blocks. At least one unencrypted memory allocation function coded in an application is executed to request allocation of unencrypted blocks in the memory device. An encrypted memory allocation function coded in the application is executed to request allocation of encrypted blocks in the memory device. At least one unencrypted Input/Output (I/O) request function coded in the application indicating an I/O operation to perform against the unencrypted blocks in the memory device is executed. At least one encrypted I/O request function coded in the application indicating an I/O operation to perform against the encrypted blocks in the memory device is executed. 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.