The XTS configuration option MBEDTLS_CIPHER_MODE_XTS currently only enables
XTS for AES. So, don't say it enables XTS for "symmetric ciphers", just
AES. This helps to avoid being misleading.
THe function `mbedtls_gf128mul_x_ble()` doesn't multiply by x, x^4, and
x^8. Update the function description to properly describe what the function
does.
mbedtls_aes_crypt_xts() currently takes a `bits_length` parameter, unlike
the other block modes. Change the parameter to accept a bytes length
instead, as the `bits_length` parameter is not actually ever used in the
current implementation.
Add a new context structure for XTS. Adjust the API for XTS to use the new
context structure, including tests suites and the benchmark program. Update
Doxgen documentation accordingly.
AES-XEX is a building block for other cryptographic standards and not yet a
standard in and of itself. We'll just provide the standardized AES-XTS
algorithm, and not AES-XEX. The AES-XTS algorithm and interface provided
can be used to perform the AES-XEX algorithm when the length of the input
is a multiple of the AES block size.
If we're unlucky with memory placement, gf128mul_table_bbe may spread over
two cache lines and this would leak b >> 63 to a cache timing attack.
Instead, take an approach that is less likely to make different memory
loads depending on the value of b >> 63 and is also unlikely to be compiled
to a condition.
XTS mode is fully known as "xor-encrypt-xor with ciphertext-stealing".
This is the generalization of the XEX mode.
This implementation is limited to an 8-bits (1 byte) boundary, which
doesn't seem to be what was thought considering some test vectors [1].
This commit comes with tests, extracted from [1], and benchmarks.
Although, benchmarks aren't really nice here, as they work with a buffer
of a multiple of 16 bytes, which isn't a challenge for XTS compared to
XEX.
[1] http://csrc.nist.gov/groups/STM/cavp/documents/aes/XTSTestVectors.zip
As seen from the first benchmark run, AES-XEX was running pourly (even
slower than AES-CBC). This commit doubles the performances of the
current implementation.
The test cases come from the XTS test vectors given by the CAVP initiative
from NIST (see [1]).
As mentioned in a previous commit, XEX is a simpler case of XTS.
Therefore, to construct the test_suite_aes.xex.data file, extraction of
the XEX-possible cases has been done on the given test vectors.
All of the extracted test vectors pass the tests on a Linux x86_64 machine.
[1] http://csrc.nist.gov/groups/STM/cavp/documents/aes/XTSTestVectors.zip
XEX mode, known as "xor-encrypt-xor", is the simple case of the XTS
mode, known as "XEX with ciphertext stealing". When the buffers to be
encrypted/decrypted have a length divisible by the length of a standard
AES block (16), XTS is exactly like XEX.
