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fscrypt: Add HCTR2 support for filename encryption

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HCTR2 is a tweakable, length-preserving encryption mode that is intended
for use on CPUs with dedicated crypto instructions.  HCTR2 has the
property that a bitflip in the plaintext changes the entire ciphertext.
This property fixes a known weakness with filename encryption: when two
filenames in the same directory share a prefix of >= 16 bytes, with
AES-CTS-CBC their encrypted filenames share a common substring, leaking
information.  HCTR2 does not have this problem.

More information on HCTR2 can be found here: "Length-preserving
encryption with HCTR2": https://eprint.iacr.org/2021/1441.pdf

Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
This commit is contained in:
Nathan Huckleberry 2022-05-20 18:15:01 +00:00 • committed by Herbert Xu
parent 9d2c0b485c
commit 6b2a51ff03
5 changed files with 38 additions and 10 deletions
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3
include/uapi/linux/fscrypt.h
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@ -27,7 +27,8 @@
#define FSCRYPT_MODE_AES_128_CBC 5
#define FSCRYPT_MODE_AES_128_CTS 6
#define FSCRYPT_MODE_ADIANTUM 9
/* If adding a mode number > 9, update FSCRYPT_MODE_MAX in fscrypt_private.h */
#define FSCRYPT_MODE_AES_256_HCTR2 10
/* If adding a mode number > 10, update FSCRYPT_MODE_MAX in fscrypt_private.h */
/*
* Legacy policy version; ad-hoc KDF and no key verification.