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>

crypto/Kconfig

@@ -1403,10 +1403,10 @@ config CRYPTO_SALSA20
 	  Bernstein <djb@cr.yp.to>. See <http://cr.yp.to/snuffle.html>
 
 config CRYPTO_CHACHA20
-	tristate "ChaCha20 stream cipher algorithms"
+	tristate "ChaCha stream cipher algorithms"
 	select CRYPTO_BLKCIPHER
 	help
-	  The ChaCha20 and XChaCha20 stream cipher algorithms.
+	  The ChaCha20, XChaCha20, and XChaCha12 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.
@@ -1419,6 +1419,10 @@ config CRYPTO_CHACHA20
 	  while provably retaining ChaCha20's security.  See also:
 	  <https://cr.yp.to/snuffle/xsalsa-20081128.pdf>
 
+	  XChaCha12 is XChaCha20 reduced to 12 rounds, with correspondingly
+	  reduced security margin but increased performance.  It can be needed
+	  in some performance-sensitive scenarios.
+
 config CRYPTO_CHACHA20_X86_64
 	tristate "ChaCha20 cipher algorithm (x86_64/SSSE3/AVX2)"
 	depends on X86 && 64BIT

crypto/chacha_generic.c

 /*
- * ChaCha20 (RFC7539) and XChaCha20 stream cipher algorithms
+ * ChaCha and XChaCha stream ciphers, including ChaCha20 (RFC7539)
  *
  * Copyright (C) 2015 Martin Willi
  * Copyright (C) 2018 Google LLC
@@ -106,6 +106,13 @@ int crypto_chacha20_setkey(struct crypto_skcipher *tfm, const u8 *key,
 }
 EXPORT_SYMBOL_GPL(crypto_chacha20_setkey);
 
+int crypto_chacha12_setkey(struct crypto_skcipher *tfm, const u8 *key,
+			   unsigned int keysize)
+{
+	return chacha_setkey(tfm, key, keysize, 12);
+}
+EXPORT_SYMBOL_GPL(crypto_chacha12_setkey);
+
 int crypto_chacha_crypt(struct skcipher_request *req)
 {
 	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
@@ -168,6 +175,21 @@ static struct skcipher_alg algs[] = {
 		.setkey			= crypto_chacha20_setkey,
 		.encrypt		= crypto_xchacha_crypt,
 		.decrypt		= crypto_xchacha_crypt,
+	}, {
+		.base.cra_name		= "xchacha12",
+		.base.cra_driver_name	= "xchacha12-generic",
+		.base.cra_priority	= 100,
+		.base.cra_blocksize	= 1,
+		.base.cra_ctxsize	= sizeof(struct chacha_ctx),
+		.base.cra_module	= THIS_MODULE,
+
+		.min_keysize		= CHACHA_KEY_SIZE,
+		.max_keysize		= CHACHA_KEY_SIZE,
+		.ivsize			= XCHACHA_IV_SIZE,
+		.chunksize		= CHACHA_BLOCK_SIZE,
+		.setkey			= crypto_chacha12_setkey,
+		.encrypt		= crypto_xchacha_crypt,
+		.decrypt		= crypto_xchacha_crypt,
 	}
 };
 
@@ -191,3 +213,5 @@ MODULE_ALIAS_CRYPTO("chacha20");
 MODULE_ALIAS_CRYPTO("chacha20-generic");
 MODULE_ALIAS_CRYPTO("xchacha20");
 MODULE_ALIAS_CRYPTO("xchacha20-generic");
+MODULE_ALIAS_CRYPTO("xchacha12");
+MODULE_ALIAS_CRYPTO("xchacha12-generic");

crypto/testmgr.c

@@ -3576,6 +3576,12 @@ static const struct alg_test_desc alg_test_descs[] = {
 		.suite = {
 			.hash = __VECS(aes_xcbc128_tv_template)
 		}
+	}, {
+		.alg = "xchacha12",
+		.test = alg_test_skcipher,
+		.suite = {
+			.cipher = __VECS(xchacha12_tv_template)
+		},
 	}, {
 		.alg = "xchacha20",
 		.test = alg_test_skcipher,

include/crypto/chacha.h

@@ -5,6 +5,11 @@
  * XChaCha extends ChaCha's nonce to 192 bits, while provably retaining ChaCha's
  * security.  Here they share the same key size, tfm context, and setkey
  * function; only their IV size and encrypt/decrypt function differ.
+ *
+ * The ChaCha paper specifies 20, 12, and 8-round variants.  In general, it is
+ * recommended to use the 20-round variant ChaCha20.  However, the other
+ * variants can be needed in some performance-sensitive scenarios.  The generic
+ * ChaCha code currently allows only the 20 and 12-round variants.
  */
 
 #ifndef _CRYPTO_CHACHA_H
@@ -40,6 +45,8 @@ void crypto_chacha_init(u32 *state, struct chacha_ctx *ctx, u8 *iv);
 
 int crypto_chacha20_setkey(struct crypto_skcipher *tfm, const u8 *key,
 			   unsigned int keysize);
+int crypto_chacha12_setkey(struct crypto_skcipher *tfm, const u8 *key,
+			   unsigned int keysize);
 
 int crypto_chacha_crypt(struct skcipher_request *req);
 int crypto_xchacha_crypt(struct skcipher_request *req);

lib/chacha.c

@@ -21,7 +21,7 @@ static void chacha_permute(u32 *x, int nrounds)
 	int i;
 
 	/* whitelist the allowed round counts */
-	WARN_ON_ONCE(nrounds != 20);
+	WARN_ON_ONCE(nrounds != 20 && nrounds != 12);
 
 	for (i = 0; i < nrounds; i += 2) {
 		x[0]  += x[4];    x[12] = rol32(x[12] ^ x[0],  16);
@@ -70,7 +70,7 @@ static void chacha_permute(u32 *x, int nrounds)
  * chacha_block - generate one keystream block and increment block counter
  * @state: input state matrix (16 32-bit words)
  * @stream: output keystream block (64 bytes)
- * @nrounds: number of rounds (currently must be 20)
+ * @nrounds: number of rounds (20 or 12; 20 is recommended)
  *
  * This is the ChaCha core, a function from 64-byte strings to 64-byte strings.
  * The caller has already converted the endianness of the input.  This function
@@ -96,7 +96,7 @@ EXPORT_SYMBOL(chacha_block);
  * hchacha_block - abbreviated ChaCha core, for XChaCha
  * @in: input state matrix (16 32-bit words)
  * @out: output (8 32-bit words)
- * @nrounds: number of rounds (currently must be 20)
+ * @nrounds: number of rounds (20 or 12; 20 is recommended)
  *
  * HChaCha is the ChaCha equivalent of HSalsa and is an intermediate step
  * towards XChaCha (see https://cr.yp.to/snuffle/xsalsa-20081128.pdf).  HChaCha