file encryption and decryption utility
an encrypted file :
file_header one block and several other blocks ... the last block tends to smaller than 10M
|num of blocks| 8bytes| |block header(encrypted) | 128 bytes | |block header(encrypted) | 128 bytes | |content data(original ) | <1282 B | || | ||| ||| |_____|| | content data (original) |10M - 1282B | | content data (original) |all left data| or ||| ... ... ||| |block tail (encrypted) | 128 B | |block tail (encrypted) | 128 B | ||| |||