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Add support for the XChaCha20 stream cipher. XChaCha20 is the application of the XSalsa20 construction (https://cr.yp.to/snuffle/xsalsa-20081128.pdf) to ChaCha20 rather than to Salsa20. XChaCha20 extends ChaCha20's nonce length from 64 bits (or 96 bits, depending on convention) to 192 bits, while provably retaining ChaCha20's security. XChaCha20 uses the ChaCha20 permutation to map the key and first 128 nonce bits to a 256-bit subkey. Then, it does the ChaCha20 stream cipher with the subkey and remaining 64 bits of nonce. We need XChaCha support in order to add support for the Adiantum encryption mode. Note that to meet our performance requirements, we actually plan to primarily use the variant XChaCha12. But we believe it's wise to first add XChaCha20 as a baseline with a higher security margin, in case there are any situations where it can be used. Supporting both variants is straightforward. Since XChaCha20's subkey differs for each request, XChaCha20 can't be a template that wraps ChaCha20; that would require re-keying the underlying ChaCha20 for every request, which wouldn't be thread-safe. Instead, we make XChaCha20 its own top-level algorithm which calls the ChaCha20 streaming implementation internally. Similar to the existing ChaCha20 implementation, we define the IV to be the nonce and stream position concatenated together. This allows users to seek to any position in the stream. I considered splitting the code into separate chacha20-common, chacha20, and xchacha20 modules, so that chacha20 and xchacha20 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 of separate modules. 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> |
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| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.