AES-256-XTS related question on a multithreaded program

Dim Xr dimxrss at
Fri Jul 21 15:40:41 UTC 2023

To answer my own question, after checking the documentation,
I think that the answer is NO, you can't give the same buffers
to multiple threads.

I would appreciate if anyone can confirm this, because I've faced
some pretty weird bugs (which are not deterministic, I mean in the
most cases my program seems to work correctly, but in some cases
it doesn't) and I can't find why!

"In particular, being configured for threads support does not
imply that all OpenSSL objects are thread-safe.
To emphasize: *most objects are not safe for simultaneous use*."


On Thu, Jul 20, 2023 at 12:21 AM Dim Xr <dimxrss at> wrote:

> Hi all!
> At the moment I'm experimenting with OpenSSL-3.1.0, and I'm
> trying to encrypt in parallel a block of data.
> Let's say that I have a block of 4096 bytes stored in a buffer.
> Can I pass to worker threads a pointer to this buffer (let' call
> it input_buf), a pointer to an output buffer (let's call it output_buf)
>  and a corresponding tweak value and let them work in parallel?
> So each thread (in short) will compute something like (error handling
> is abbreviated):
> cipher = EVP_CIPHER_fetch(NULL, "AES-256-XTS", NULL);
> EVP_EncryptInit_ex2(ctx, cipher, key, NULL, NULL);
> EVP_EncryptInit_ex2(ctx, NULL, NULL, tweak, NULL);
> EVP_EncryptUpdate(ctx, outbuf + relative_bytes, &encrypt_size,
> input_buf + relative_bytes, inputsize);
> To put it another way, can multiple threads work **on the same
> input and output buffers** (but in different regions of course) in
> parallel?
> Thanks,
> Dimitris
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