path: root/crypto/tcrypt.c
diff options
authorEric Biggers <>2018-11-16 17:26:31 -0800
committerHerbert Xu <>2018-11-20 14:26:56 +0800
commit059c2a4d8e164dccc3078e49e7f286023b019a98 (patch)
tree1848d90f72c8c531ed3fe9793cabcb8a04c490b3 /crypto/tcrypt.c
parent16aae3595a9d41c97d983889b341c455779c2ecf (diff)
crypto: adiantum - add Adiantum support
Add support for the Adiantum encryption mode. Adiantum was designed by Paul Crowley and is specified by our paper: Adiantum: length-preserving encryption for entry-level processors ( See our paper for full details; this patch only provides an overview. Adiantum is a tweakable, length-preserving encryption mode designed for fast and secure disk encryption, especially on CPUs without dedicated crypto instructions. Adiantum encrypts each sector using the XChaCha12 stream cipher, two passes of an ε-almost-∆-universal (εA∆U) hash function, and an invocation of the AES-256 block cipher on a single 16-byte block. On CPUs without AES instructions, Adiantum is much faster than AES-XTS; for example, on ARM Cortex-A7, on 4096-byte sectors Adiantum encryption is about 4 times faster than AES-256-XTS encryption, and decryption about 5 times faster. Adiantum is a specialization of the more general HBSH construction. Our earlier proposal, HPolyC, was also a HBSH specialization, but it used a different εA∆U hash function, one based on Poly1305 only. Adiantum's εA∆U hash function, which is based primarily on the "NH" hash function like that used in UMAC (RFC4418), is about twice as fast as HPolyC's; consequently, Adiantum is about 20% faster than HPolyC. This speed comes with no loss of security: Adiantum is provably just as secure as HPolyC, in fact slightly *more* secure. Like HPolyC, Adiantum's security is reducible to that of XChaCha12 and AES-256, subject to a security bound. XChaCha12 itself has a security reduction to ChaCha12. Therefore, one need not "trust" Adiantum; one need only trust ChaCha12 and AES-256. Note that the εA∆U hash function is only used for its proven combinatorical properties so cannot be "broken". Adiantum is also a true wide-block encryption mode, so flipping any plaintext bit in the sector scrambles the entire ciphertext, and vice versa. No other such mode is available in the kernel currently; doing the same with XTS scrambles only 16 bytes. Adiantum also supports arbitrary-length tweaks and naturally supports any length input >= 16 bytes without needing "ciphertext stealing". For the stream cipher, Adiantum uses XChaCha12 rather than XChaCha20 in order to make encryption feasible on the widest range of devices. Although the 20-round variant is quite popular, the best known attacks on ChaCha are on only 7 rounds, so ChaCha12 still has a substantial security margin; in fact, larger than AES-256's. 12-round Salsa20 is also the eSTREAM recommendation. For the block cipher, Adiantum uses AES-256, despite it having a lower security margin than XChaCha12 and needing table lookups, due to AES's extensive adoption and analysis making it the obvious first choice. Nevertheless, for flexibility this patch also permits the "adiantum" template to be instantiated with XChaCha20 and/or with an alternate block cipher. We need Adiantum support in the kernel for use in dm-crypt and fscrypt, where currently the only other suitable options are block cipher modes such as AES-XTS. A big problem with this is that many low-end mobile devices (e.g. Android Go phones sold primarily in developing countries, as well as some smartwatches) still have CPUs that lack AES instructions, e.g. ARM Cortex-A7. Sadly, AES-XTS encryption is much too slow to be viable on these devices. We did find that some "lightweight" block ciphers are fast enough, but these suffer from problems such as not having much cryptanalysis or being too controversial. The ChaCha stream cipher has excellent performance but is insecure to use directly for disk encryption, since each sector's IV is reused each time it is overwritten. Even restricting the threat model to offline attacks only isn't enough, since modern flash storage devices don't guarantee that "overwrites" are really overwrites, due to wear-leveling. Adiantum avoids this problem by constructing a "tweakable super-pseudorandom permutation"; this is the strongest possible security model for length-preserving encryption. Of course, storing random nonces along with the ciphertext would be the ideal solution. But doing that with existing hardware and filesystems runs into major practical problems; in most cases it would require data journaling (like dm-integrity) which severely degrades performance. Thus, for now length-preserving encryption is still needed. Signed-off-by: Eric Biggers <> Reviewed-by: Ard Biesheuvel <> Signed-off-by: Herbert Xu <>
Diffstat (limited to 'crypto/tcrypt.c')
1 files changed, 12 insertions, 0 deletions
diff --git a/crypto/tcrypt.c b/crypto/tcrypt.c
index 5fb120474902..0590a9204562 100644
--- a/crypto/tcrypt.c
+++ b/crypto/tcrypt.c
@@ -2320,6 +2320,18 @@ static int do_test(const char *alg, u32 type, u32 mask, int m, u32 num_mb)
test_cipher_speed("ctr(sm4)", DECRYPT, sec, NULL, 0,
+ case 219:
+ test_cipher_speed("adiantum(xchacha12,aes)", ENCRYPT, sec, NULL,
+ 0, speed_template_32);
+ test_cipher_speed("adiantum(xchacha12,aes)", DECRYPT, sec, NULL,
+ 0, speed_template_32);
+ test_cipher_speed("adiantum(xchacha20,aes)", ENCRYPT, sec, NULL,
+ 0, speed_template_32);
+ test_cipher_speed("adiantum(xchacha20,aes)", DECRYPT, sec, NULL,
+ 0, speed_template_32);
+ break;
case 300:
if (alg) {
test_hash_speed(alg, sec, generic_hash_speed_template);