EVP_aes_256_xts() problems with multiple calls to EVP_CipherUpdate

[Thulasi Goriparthi]

As 512 byte blocks are independently encrypted, they should be decrypted
similarly. This is how XTS mode is defined.
i.e Try to decrypt 512 byte blocks separately with two CipherUpdates.

Thanks,
Thulasi.

On Tue, 1 Oct 2019 at 06:43, Norm Green <norm.green at gemtalksystems.com>
wrote:

> Hi all,
>
> I'm using OpenSSL 1.1.1d on Linux with the cipher EVP_aes_256_xts() in
> order to write database/disk encryption software.
>
> When encrypting, I have problems if I call EVP_CipherUpdate() and
> encrypt the data in chunks. Encrypting only works when I encrypt the
> entire payload with one and only one call to EVP_CipherUpdate.
>
> If I try to break the data into chunks (and make more than one call to
> EVP_CipherUpdate), then decrypting the data produces garbage after the
> first chunk that was encrypted
> When decrypting, I always decrypt all data in one call to EVP_CipherUpdate
> .
>
> For example, when encrypting 1024 bytes, this pseudo-code sequence works:
>
> char payload[1024];
> char encrypted[1024];
> int destSize = sizeof(encrypted);
> EVP_CipherInit_ex();
> EVP_CipherUpdate(ctx, encrypted, &destSize, payload, sizeof(payload));
> EVP_CipherFinal(); (produces no additional data)
>
> However if I break the 1024 payload into 2 x 512 byte chunks, decrypting
> the entire 1024 bytes of cipher text produces garbage every time:
>
> char payload[1024];
> char encrypted[1024];
> int destSize = sizeof(encrypted);
> EVP_CipherInit_ex();
> EVP_CipherUpdate(ctx, encrypted, &destSize, payload, 512); // first chunk
> destSize -= 512;
> EVP_CipherUpdate(ctx, &encrypted[512], &destSize, &payload[512], 512);
> // second chunk
> EVP_CipherFinal(); (produces no additional data)
>
> I have a short C program that demonstrates the problem that I can post
> if necessary.
>
> Can anyone explain what's going on?
>
> Norm Green
> CTO, GemTalk Systems Inc.
>
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