A software diversity system including an executable provider to provide an executable program including component blocks such that different combinations of blocks are operative to perform a functionally encryption keys functionally equivalent data transformation, a cipher to encrypt the component blocks with cryptographic keys, a key selector to select a first selection of keys for a first device, such that the first selection is operative to decrypt a first combination of the blocks operative when executed to perform the same functionally equivalent data transformation, and select a second selection of keys for a second device, such that the second selection is operative to decrypt a second combination of the blocks operative when executed to perform the same functionally equivalent data transformation, and a transfer module to prepare for transfer the first and second selection of cryptographic keys for transfer to the first and second device, respectively. Related apparatus and methods are also included.