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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 f...
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