Hi all,
I am working on SAMA5D3 board with atmel-aes driver, I have a question
about sg.
This board does not support XTS mode, however we want to add this
functionality.
As a recall XTS mode :
For a 512 bytes block:
1) We encrypt the given IV with the second part of the key [ECB].
2) We compute the 32 tweaks value (with GF multiplication)
3) We Xor the plaintext with the tweaks (called XOR_1)
4) We encrypt the result with ECB and the first half of the key
5) Again we xor the result with tweaks (called XOR_2)
So if I want to add my own XTS mode I need to make all those
operations.
I have seen in the code there is a alignement issue to handle, if source
data is aligned
we can encrypt it directly. If source data is not aligned we call a
function to copy into a buffer :
625 if (!src_aligned) {
626 sg_copy_to_buffer(src, sg_nents(src), dd->buf, len);
...
Moreover we are working with dm-crypt.
My question is : is it possible to make the XOR operation directly on
scatterlist if data is aligned ?
Or I am forced to use the sg_copy_to/from_buffer.
We have tested and here are the results :
1) The easy solution is to copy the src data into the buffer and to xor
it with tweaks for XOR_1 and XOR_2
2) If we xor only the ciphertext [dst] (XOR_2) directly with the
scatterlist and compute the src XOR_1 with the buffer, it is working.
3) If we xor direcly with the scatterlist for XOR_1 and XOR_2 we have an
error at the mount step using dm-crypt :
[269132.780000] EXT4-fs (dm-0): ext4_check_descriptors: Block bitmap for
group 0 not in group
(block 16843203)!
[269132.790000] EXT4-fs (dm-0): group descriptors corrupted!
If you can help me on this point.
Thanks.
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