• Eric Biggers's avatar
    crypto: chacha - add XChaCha12 support · aa762409
    Eric Biggers authored
    Now that the generic implementation of ChaCha20 has been refactored to
    allow varying the number of rounds, add support for XChaCha12, which is
    the XSalsa construction applied to ChaCha12.  ChaCha12 is one of the
    three ciphers specified by the original ChaCha paper
    (https://cr.yp.to/chacha/chacha-20080128.pdf: "ChaCha, a variant of
    Salsa20"), alongside ChaCha8 and ChaCha20.  ChaCha12 is faster than
    ChaCha20 but has a lower, but still large, security margin.
    
    We need XChaCha12 support so that it can be used in the Adiantum
    encryption mode, which enables disk/file encryption on low-end mobile
    devices where AES-XTS is too slow as the CPUs lack AES instructions.
    
    We'd prefer XChaCha20 (the more popular variant), but it's too slow on
    some of our target devices, so at least in some cases we do need the
    XChaCha12-based version.  In more detail, the problem is that Adiantum
    is still much slower than we're happy with, and encryption still has a
    quite noticeable effect on the feel of low-end devices.  Users and
    vendors push back hard against encryption that degrades the user
    experience, which always risks encryption being disabled entirely.  So
    we need to choose the fastest option that gives us a solid margin of
    security, and here that's XChaCha12.  The best known attack on ChaCha
    breaks only 7 rounds and has 2^235 time complexity, so ChaCha12's
    security margin is still better than AES-256's.  Much has been learned
    about cryptanalysis of ARX ciphers since Salsa20 was originally designed
    in 2005, and it now seems we can be comfortable with a smaller number of
    rounds.  The eSTREAM project also suggests the 12-round version of
    Salsa20 as providing the best balance among the different variants:
    combining very good performance with a "comfortable margin of security".
    
    Note that it would be trivial to add vanilla ChaCha12 in addition to
    XChaCha12.  However, it's unneeded for now and therefore is omitted.
    
    As discussed in the patch that introduced XChaCha20 support, I
    considered splitting the code into separate chacha-common, chacha20,
    xchacha20, and xchacha12 modules, so that these algorithms could be
    enabled/disabled independently.  However, since nearly all the code is
    shared anyway, I ultimately decided there would have been little benefit
    to the added complexity.
    Reviewed-by: 's avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
    Acked-by: 's avatarMartin Willi <martin@strongswan.org>
    Signed-off-by: 's avatarEric Biggers <ebiggers@google.com>
    Signed-off-by: 's avatarHerbert Xu <herbert@gondor.apana.org.au>
    aa762409
chacha_generic.c 5.99 KB