crypto: lib/chacha20poly1305 - Add missing function declaration

This patch adds a declaration for chacha20poly1305_selftest to
silence a sparse warning.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto: lib/chacha20poly1305 - reimplement crypt_from_sg() routine

Reimplement the library routines to perform chacha20poly1305 en/decryption
on scatterlists, without [ab]using the [deprecated] blkcipher interface,
which is rather heavyweight and does things we don't really need.

Instead, we use the sg_miter API in a novel and clever way, to iterate
over the scatterlist in-place (i.e., source == destination, which is the
only way this library is expected to be used). That way, we don't have to
iterate over two scatterlists in parallel.

Another optimization is that, instead of relying on the blkcipher walker
to present the input in suitable chunks, we recognize that ChaCha is a
streamcipher, and so we can simply deal with partial blocks by keeping a
block of cipherstream on the stack and use crypto_xor() to mix it with
the in/output.

Finally, we omit the scatterwalk_and_copy() call if the last element of
the scatterlist covers the MAC as well (which is the common case),
avoiding the need to walk the scatterlist and kmap() the page twice.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto: chacha20poly1305 - import construction and selftest from Zinc

This incorporates the chacha20poly1305 from the Zinc library, retaining
the library interface, but replacing the implementation with calls into
the code that already existed in the kernel's crypto API.

Note that this library API does not implement RFC7539 fully, given that
it is limited to 64-bit nonces. (The 96-bit nonce version that was part
of the selftest only has been removed, along with the 96-bit nonce test
vectors that only tested the selftest but not the actual library itself)

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>