Question

I’d like to encrypt a file with aes256 using OpenSSL with C.
I did find a pretty nice example here.

Should I first read the whole file into a memory buffer and than do the aes256, or should I do it partial with a ~16K buffer?

Any snippets or hints?

Answer 1

Loading the whole file in a buffer can get inefficient to impossible on larger files – do this only if all your files are below some size limit.

OpenSSL’s EVP API (which is also used by the example you linked) has an EVP_EncryptUpdate function, which can be called multiple times, each time providing some more bytes to encrypt. Use this in a loop together with reading in the plaintext from a file into a buffer, and writing out the ciphertext to another file (or the same one). (Analogously for decryption.)

Of course, instead of inventing a new file format (which you are effectively doing here), think about implementing the OpenPGP Message format (RFC 4880). There are less chances to make mistakes which might destroy your security – and as an added bonus, if your program somehow ceases to work, your users can always use the standard tools (PGP or GnuPG) to decrypt the file.