Re: XTS does not get the expected result for non-byte-aligned NIST vectors

On Wednesday, May 19, 2021 at 11:53:54 PM UTC-4 [email protected] wrote:

> I intend to use Crypto++  XTS-AES for encrypting data.  I tested XTS-AES 
> with the following NIST vector and got an unexpected 
> CT= 54a8629d76db46d0c516fca52c9c903baa3a635ddd56f09760f63252c8b46140 .   
>
> I call the following function:
> XTS_Mode< AES >::Encryption e;
> e.SetKeyWithIV(in_key, KeySize, in_iv, IVSize);
>     StringSource(in_plain_txt,svSize(plain_txt, 1),true, 
> new StreamTransformationFilter(e,
> new StringSink(cipher),
>                 StreamTransformationFilter::NO_PADDING
> ) // StreamTransformationFilter
> ); // StringSource
>
> My questions:
>
>    - Does  Crypto++  XTS-AES  not support non-byte aligned data 
>    encryption and decryption
>    
>
XTS mode should support non-aligned data. For SSE, it uses _mm_loadu_si128 
and _mm_storeu_si128, which are unaligned loads and stores. The class also 
uses 'GetWord<word64>(false, LITTLE_ENDIAN_ORDER, ...)' and 
'PutWord<word64>(false, LITTLE_ENDIAN_ORDER, ...)'. The 'false' says the 
data is not aligned, so a memcpy is used. Also see 
https://github.com/weidai11/cryptopp/blob/master/xts.cpp.

We've seen some trouble with bad code generation when two pointers happen 
to be the same in strcipher.cpp. But I don't believe XTS uses it. Also see 
https://github.com/weidai11/cryptopp/issues/1010.

Do you have a reproducer?

Jeff

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