path: root/crypto/chacha_generic.c
diff options
authorEric Biggers <ebiggers@google.com>2018-11-16 17:26:22 -0800
committerHerbert Xu <herbert@gondor.apana.org.au>2018-11-20 14:26:55 +0800
commitaa7624093cb7fbf4fea95e612580d8d29a819f67 (patch)
tree5c71a13973dce1c04d2e3ebe442239301320dfa1 /crypto/chacha_generic.c
parent1ca1b917940c24ca3d1f490118c5474168622953 (diff)
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>
Diffstat (limited to 'crypto/chacha_generic.c')
1 files changed, 25 insertions, 1 deletions
diff --git a/crypto/chacha_generic.c b/crypto/chacha_generic.c
index 438f15a14054..35b583101f4f 100644
--- a/crypto/chacha_generic.c
+++ b/crypto/chacha_generic.c
@@ -1,5 +1,5 @@
- * 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,
+int crypto_chacha12_setkey(struct crypto_skcipher *tfm, const u8 *key,
+ unsigned int keysize)
+ return chacha_setkey(tfm, key, keysize, 12);
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");