Can we encrypt a single file using AES cryptography?

pie

Hi, I need to test and debug sending stuff to other people and I make use of lualfs to copy a single file (that contains useful info) from |D| to a specific path. That file has to be sent back to me through email.

I was wondering if I could encrypt this file while copying it to keep its content hidden, instead of parsing and encrypting every single string inside it.


This is the working code, thanks to hgy29:

require "lfs"     local key = "GiderosGideros11" local iv = "GiderosRules2023" local paddingType = 1     local function cryptCopy(src, dst) local srcf = io.open(src, "rb") local dstf = io.open(dst, "wb") local size = 2^13 -- good buffer size (8K) while true do local block = srcf:read(size) if not block then break end block = Cryptography.aesEncrypt(block,key,iv,paddingType) dstf:write(block) end srcf:close() dstf:close() end   local function decryptCopy(src, dst) local srcf = io.open(src, "rb") local dstf = io.open(dst, "wb") local size = 2^13 + 16 -- good buffer size (8K + 16 for padding) while true do local block = srcf:read(size) if not block then break end block = Cryptography.aesDecrypt(block,key,iv,paddingType) dstf:write(block) end srcf:close() dstf:close() end   cryptCopy ("|D|myfile.sqlite", "C:/tmp/myfile.crypt") decryptCopy ("C:/tmp/myfile.crypt", "C:/tmp/myfile.sqlite")

[edit: updated after success]

hgy29

You'll definitely want to use padding if your input blocks aren't a multiple of 16 bytes, since AES operates only with 16 bytes blocks.
You use a 8k buffer, which is 16 byte aligned so your code should work, but you use "r" for reading your files, even the crypted one, while you should really use "rb", to avoid line endings to be converted by the operating system.

pie

Thank you @hgy29, that is indeed a good point

Unfortunately I get the same exact error ( Invalid PKCS#7 padding value) using "rb" instead of "r" for reading files if I set paddingType as 0 or 1.

hgy29

Yes, it is obvious on second thought if you give a 16 byte aligned buffer and ask for padding, then gideros needs to add 16 more bytes of padding, because the padding byte indicates the padding size, so 0 isn't allowed. so you write blocks of 8k+16, but you only read 8k when decoding. Try to read buffers of 8k+16 in the decrypting function

pie

It's obvious for you.. it works!
Thank you a lot, we're lucky for having you here!

updating the code in the first post to use for anyone later

MoKaLux

thank you guys it goes straight to the wiki