RE: [RFC] crypto: xts - limit accepted key length
From: Van Leeuwen, Pascal
Date: Tue Mar 03 2020 - 08:09:41 EST
> -----Original Message-----
> From: linux-crypto-owner@xxxxxxxxxxxxxxx <linux-crypto-owner@xxxxxxxxxxxxxxx> On Behalf Of Andrei Botila
> Sent: Tuesday, March 3, 2020 1:29 PM
> To: Van Leeuwen, Pascal <pvanleeuwen@xxxxxxxxxx>; Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>; David S. Miller
> <davem@xxxxxxxxxxxxx>
> Cc: linux-crypto@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [RFC] crypto: xts - limit accepted key length
>
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> On 3/2/2020 10:33 AM, Van Leeuwen, Pascal wrote:
> >> -----Original Message-----
> >> From: linux-crypto-owner@xxxxxxxxxxxxxxx <linux-crypto-owner@xxxxxxxxxxxxxxx> On Behalf Of Andrei Botila
> >> Sent: Monday, March 2, 2020 9:16 AM
> >> To: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>
> >> Cc: linux-crypto@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> >> Subject: [RFC] crypto: xts - limit accepted key length
> >>
> >> <<< External Email >>>
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> >>
> >> From: Andrei Botila <andrei.botila@xxxxxxx>
> >>
> >> Currently in XTS generic implementation the valid key length is
> >> repesented by any length which is even. This is a deviation from
> >> the XTS-AES standard (IEEE 1619-2007) which allows keys equal
> >> to {2 x 16B, 2 x 32B} that correspond to underlying XTS-AES-{128, 256}
> >> algorithm. XTS-AES-192 is not supported as mentioned in commit
> >> b66ad0b7aa92 ("crypto: tcrypt - remove AES-XTS-192 speed tests")) or
> >> any other length beside these two specified.
> >>
> >> If this modification is accepted then other ciphers that use XTS mode
> >> will have to be modified (camellia, cast6, serpent, twofish).
> >>
> >> Signed-off-by: Andrei Botila <andrei.botila@xxxxxxx>
> >> ---
> >> include/crypto/xts.h | 13 +++++++------
> >> 1 file changed, 7 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/include/crypto/xts.h b/include/crypto/xts.h
> >> index 0f8dba69feb4..26e764a5ae46 100644
> >> --- a/include/crypto/xts.h
> >> +++ b/include/crypto/xts.h
> >> @@ -4,6 +4,7 @@
> >>
> >> #include <crypto/b128ops.h>
> >> #include <crypto/internal/skcipher.h>
> >> +#include <crypto/aes.h>
> >> #include <linux/fips.h>
> >>
> >> #define XTS_BLOCK_SIZE 16
> >> @@ -12,10 +13,10 @@ static inline int xts_check_key(struct crypto_tfm *tfm,
> >> const u8 *key, unsigned int keylen)
> >> {
> >> /*
> >> - * key consists of keys of equal size concatenated, therefore
> >> - * the length must be even.
> >> + * key consists of keys of equal size concatenated, possible
> >> + * values are 32 or 64 bytes.
> >> */
> >> - if (keylen % 2)
> >> + if (keylen != 2 * AES_MIN_KEY_SIZE && keylen != 2 * AES_MAX_KEY_SIZE)
> >> return -EINVAL;
> >>
> >> /* ensure that the AES and tweak key are not identical */
> >> @@ -29,10 +30,10 @@ static inline int xts_verify_key(struct crypto_skcipher *tfm,
> >> const u8 *key, unsigned int keylen)
> >> {
> >> /*
> >> - * key consists of keys of equal size concatenated, therefore
> >> - * the length must be even.
> >> + * key consists of keys of equal size concatenated, possible
> >> + * values are 32 or 64 bytes.
> >> */
> >> - if (keylen % 2)
> >> + if (keylen != 2 * AES_MIN_KEY_SIZE && keylen != 2 * AES_MAX_KEY_SIZE)
> >> return -EINVAL;
> >>
> >> /* ensure that the AES and tweak key are not identical */
> >> --
> >> 2.17.1
> >
> > Hmm ... in principle IEEE-1619 also defines XTS *only* for AES. So by that same
> > reasoning, you should also not allow any usage of XTS beyond AES. Yet it is
> > actually being actively used(?) with other ciphers in the Linux kernel. Which is
> > not wrong perse, as the construct works with any block cipher with a 128 bit
> > block size. And is secure as long as that blockcipher is secure.
> >
> > So why not allow 192 bit AES keys? Or some keysize that some other algorithm
> > may require, as I'm not sure all ciphers it is used with have 128 or 256 bit keys.
> >
> > The modulo 2 check was just to ensure you indeed provided 2 full cipher keys,
> > any other error checking was deferred to the cipher algorithm's setkey.
> >
> > Note that such a change would also allow all hardware drivers implementing
> > xts to follow suit and report an error, otherwise they will fail the selftests ...
> >
> > Regards,
> > Pascal van Leeuwen
> > Silicon IP Architect Multi-Protocol Engines, Rambus Security
> > Rambus ROTW Holding BV
> > +31-73 6581953
> >
> > Note: The Inside Secure/Verimatrix Silicon IP team was recently acquired by Rambus.
> > Please be so kind to update your e-mail address book with my new e-mail address.
> >
> >
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> Hi,
>
> The problem here is that implementations adhering strictly to
> the IEEE 1619-2007 standard will have problems when receiving
> key sizes different than 256/512 bit. Currently in crypto/testmgr.c
> when fuzz testing is enabled it generates random keys with sizes
> such as 192 bits. This is a problem because it will check the
> XTS SW implementation result with the one generated by the hardware
> implementations and the test will fail if the hardware is adhering
> strictly to the standard.
> This is also the case for our CAAM accelerator which is accepting
> only XTS-AES-{128, 256} and currently fails when fuzz testing is
> enabled and it receives 192 bit keys.
> Maybe we can find a solution to limit this key size check only for AES.
>
> Regards,
> Andrei
But our hardware can do XTS with AES with 192 bit keys. For whatever that
is worth. Note that this is _not_ an IEEE 1619-2007 implementation (which
would have to be AES only), it is a basic XTS mode wrapper implementation,
supporting any 128 bit blockcipher underneath. And, hence, all it's keysizes.
What is wrong with software fallback for the 192 bit keys in your driver?
Regards,
Pascal van Leeuwen
Silicon IP Architect Multi-Protocol Engines, Rambus Security
Rambus ROTW Holding BV
+31-73 6581953
Note: The Inside Secure/Verimatrix Silicon IP team was recently acquired by Rambus.
Please be so kind to update your e-mail address book with my new e-mail address.
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