2 Commits

Author	SHA1	Message	Date
Eric Biggers	aa7624093c	crypto: chacha - add XChaCha12 support ... 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: Ard Biesheuvel <ard.biesheuvel@linaro.org> Acked-by: Martin Willi <martin@strongswan.org> Signed-off-by: Eric Biggers <ebiggers@google.com> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>	2018-11-20 14:26:55 +08:00
Eric Biggers	1ca1b91794	crypto: chacha20-generic - refactor to allow varying number of rounds ... In preparation for adding XChaCha12 support, rename/refactor chacha20-generic to support different numbers of rounds. The justification for needing XChaCha12 support is explained in more detail in the patch "crypto: chacha - add XChaCha12 support". The only difference between ChaCha{8,12,20} are the number of rounds itself; all other parts of the algorithm are the same. Therefore, remove the "20" from all definitions, structures, functions, files, etc. that will be shared by all ChaCha versions. Also make ->setkey() store the round count in the chacha_ctx (previously chacha20_ctx). The generic code then passes the round count through to chacha_block(). There will be a ->setkey() function for each explicitly allowed round count; the encrypt/decrypt functions will be the same. I decided not to do it the opposite way (same ->setkey() function for all round counts, with different encrypt/decrypt functions) because that would have required more boilerplate code in architecture-specific implementations of ChaCha and XChaCha. Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Acked-by: Martin Willi <martin@strongswan.org> Signed-off-by: Eric Biggers <ebiggers@google.com> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>	2018-11-20 14:26:55 +08:00