crypto: chacha20-generic - add XChaCha20 support

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>

1 files changed, 9 insertions, 5 deletions

diff --git a/crypto/Kconfig b/crypto/Kconfig
index 62dbd1a99fa3..75ebd1a2746c 100644
--- a/crypto/Kconfig
+++ b/crypto/Kconfig
@@ -1403,18 +1403,22 @@ config CRYPTO_SALSA20
 	  Bernstein <djb@cr.yp.to>. See <http://cr.yp.to/snuffle.html>
 
 config CRYPTO_CHACHA20
-	tristate "ChaCha20 cipher algorithm"
+	tristate "ChaCha20 stream cipher algorithms"
 	select CRYPTO_BLKCIPHER
 	help
-	  ChaCha20 cipher algorithm, RFC7539.
+	  The ChaCha20 and XChaCha20 stream cipher algorithms.
 
 	  ChaCha20 is a 256-bit high-speed stream cipher designed by Daniel J.
 	  Bernstein and further specified in RFC7539 for use in IETF protocols.
-	  This is the portable C implementation of ChaCha20.
-
-	  See also:
+	  This is the portable C implementation of ChaCha20.  See also:
 	  <http://cr.yp.to/chacha/chacha-20080128.pdf>
 
+	  XChaCha20 is the application of the XSalsa20 construction to ChaCha20
+	  rather than to Salsa20.  XChaCha20 extends ChaCha20's nonce length
+	  from 64 bits (or 96 bits using the RFC7539 convention) to 192 bits,
+	  while provably retaining ChaCha20's security.  See also:
+	  <https://cr.yp.to/snuffle/xsalsa-20081128.pdf>
+
 config CRYPTO_CHACHA20_X86_64
 	tristate "ChaCha20 cipher algorithm (x86_64/SSSE3/AVX2)"
 	depends on X86 && 64BIT