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
(https://eprint.iacr.org/2018/720.pdf)
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 <ebiggers@google.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
(cherry picked from commit 059c2a4d8e164dccc3078e49e7f286023b019a98
https://git.kernel.org/pub/scm/linux/kernel/git/herbert/cryptodev-2.6.git master)
Conflicts:
crypto/tcrypt.c
crypto/testmgr.c
(adjusted test vector formatting for old testmgr)
Bug: 112008522
Test: Among other things, I ran the relevant crypto self-tests:
1.) Build kernel with CONFIG_CRYPTO_MANAGER_DISABLE_TESTS *unset*, and
all relevant crypto algorithms built-in, including:
CONFIG_CRYPTO_ADIANTUM=y
CONFIG_CRYPTO_CHACHA20=y
CONFIG_CRYPTO_CHACHA20_NEON=y
CONFIG_CRYPTO_NHPOLY1305=y
CONFIG_CRYPTO_NHPOLY1305_NEON=y
CONFIG_CRYPTO_POLY1305=y
CONFIG_CRYPTO_AES=y
CONFIG_CRYPTO_AES_ARM=y
2.) Boot and check dmesg for test failures.
3.) Instantiate "adiantum(xchacha12,aes)" and
"adiantum(xchacha20,aes)" to trigger them to be tested. There are
many ways to do this, but one way is to create a dm-crypt target
that uses them, e.g.
key=$(hexdump -n 32 -e '16/4 "%08X" 1 "\n"' /dev/urandom)
dmsetup create crypt --table "0 $((1<<17)) crypt xchacha12,aes-adiantum-plain64 $key 0 /dev/vdc 0"
dmsetup remove crypt
dmsetup create crypt --table "0 $((1<<17)) crypt xchacha20,aes-adiantum-plain64 $key 0 /dev/vdc 0"
dmsetup remove crypt
4.) Check dmesg for test failures again.
5.) Do 1-4 on both x86_64 (for basic testing) and on arm32 (for
testing the ARM32-specific implementations). I did the arm32 kernel
testing on Raspberry Pi 2, which is a BCM2836-based device that can
run the upstream and Android common kernels.
The same ARM32 assembly files for ChaCha, NHPoly1305, and AES are
also included in the userspace Adiantum benchmark suite at
https://github.com/google/adiantum, where they have undergone
additional correctness testing.
Change-Id: Ic61c13b53facfd2173065be715a7ee5f3af8760b
Signed-off-by: Eric Biggers <ebiggers@google.com>
Add a generic implementation of NHPoly1305, an ε-almost-∆-universal hash
function used in the Adiantum encryption mode.
CONFIG_NHPOLY1305 is not selectable by itself since there won't be any
real reason to enable it without also enabling Adiantum support.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
(cherry picked from commit 26609a21a9460145e37d90947ad957b358a05288
https://git.kernel.org/pub/scm/linux/kernel/git/herbert/cryptodev-2.6.git master)
Conflicts:
crypto/testmgr.c
crypto/testmgr.h
Bug: 112008522
Test: As series, see Ic61c13b53facfd2173065be715a7ee5f3af8760b
Change-Id: If6f00c01fab530fc2458c44ca111f84604cb85c1
Signed-off-by: Eric Biggers <ebiggers@google.com>
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>
(cherry picked from commit aa7624093cb7fbf4fea95e612580d8d29a819f67
https://git.kernel.org/pub/scm/linux/kernel/git/herbert/cryptodev-2.6.git master)
Conflicts:
crypto/chacha_generic.c
(backported from skcipher to blkcipher API)
(adjusted test vector formatting for old testmgr)
Bug: 112008522
Test: As series, see Ic61c13b53facfd2173065be715a7ee5f3af8760b
Change-Id: I876a5be92e9f583effcd35a4b66a36608ac581f0
Signed-off-by: Eric Biggers <ebiggers@google.com>
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>
(cherry picked from commit de61d7ae5d3789dcba3749a418f76613fbee8414
https://git.kernel.org/pub/scm/linux/kernel/git/herbert/cryptodev-2.6.git master)
Conflicts:
crypto/chacha20_generic.c
include/crypto/chacha20.h
(backported from skcipher to blkcipher API)
(adjusted test vector formatting for old testmgr)
Bug: 112008522
Test: As series, see Ic61c13b53facfd2173065be715a7ee5f3af8760b
Change-Id: I5c878e1d6577abda11d7b737cbb650baf16b6886
Signed-off-by: Eric Biggers <ebiggers@google.com>
