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From: Eric Biggers <ebiggers@kernel.org>
To: "Theodore Y. Ts'o" <tytso@mit.edu>
Cc: Satya Tangirala <satyat@google.com>,
	linux-api@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net,
	linux-fscrypt@vger.kernel.org, keyrings@vger.kernel.org,
	linux-mtd@lists.infradead.org, linux-crypto@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
	Paul Crowley <paulcrowley@google.com>
Subject: Re: [PATCH v7 09/16] fscrypt: add an HKDF-SHA512 implementation
Date: Mon, 29 Jul 2019 13:29:52 -0700
Message-ID: <20190729202951.GG169027@gmail.com> (raw)
In-Reply-To: <20190728193949.GI6088@mit.edu>

On Sun, Jul 28, 2019 at 03:39:49PM -0400, Theodore Y. Ts'o wrote:
> On Fri, Jul 26, 2019 at 03:41:34PM -0700, Eric Biggers wrote:
> > From: Eric Biggers <ebiggers@google.com>
> > 
> > Add an implementation of HKDF (RFC 5869) to fscrypt, for the purpose of
> > deriving additional key material from the fscrypt master keys for v2
> > encryption policies.  HKDF is a key derivation function built on top of
> > HMAC.  We choose SHA-512 for the underlying unkeyed hash, and use an
> > "hmac(sha512)" transform allocated from the crypto API.
> > 
> > We'll be using this to replace the AES-ECB based KDF currently used to
> > derive the per-file encryption keys.  While the AES-ECB based KDF is
> > believed to meet the original security requirements, it is nonstandard
> > and has problems that don't exist in modern KDFs such as HKDF:
> > 
> > 1. It's reversible.  Given a derived key and nonce, an attacker can
> >    easily compute the master key.  This is okay if the master key and
> >    derived keys are equally hard to compromise, but now we'd like to be
> >    more robust against threats such as a derived key being compromised
> >    through a timing attack, or a derived key for an in-use file being
> >    compromised after the master key has already been removed.
> > 
> > 2. It doesn't evenly distribute the entropy from the master key; each 16
> >    input bytes only affects the corresponding 16 output bytes.
> > 
> > 3. It isn't easily extensible to deriving other values or keys, such as
> >    a public hash for securely identifying the key, or per-mode keys.
> >    Per-mode keys will be immediately useful for Adiantum encryption, for
> >    which fscrypt currently uses the master key directly, introducing
> >    unnecessary usage constraints.  Per-mode keys will also be useful for
> >    hardware inline encryption, which is currently being worked on.
> > 
> > HKDF solves all the above problems.
> > 
> > Signed-off-by: Eric Biggers <ebiggers@google.com>
> 
> Unless I missed something there's nothing here which is fscrypt
> specific.  Granted that it's somewhat unlikely that someone would want
> to implement (the very bloated) IKE from IPSEC in the kernel, I wonder
> if there might be other users of HKDF, and whether this would be
> better placed in lib/ or crypto/ instead of fs/crypto?
> 

This is standard HKDF-SHA512; only the choice of parameters is fscrypt-specific.
So it could indeed use a common implementation of HKDF if one were available.

However, I don't think there are any other HKDF users in the kernel currently.
Also, while there was a patch to support HKDF via the crypto_rng API, there was
no consensus about whether this was actually the best way to add KDF support:
https://lore.kernel.org/linux-crypto/2423373.Zd5ThvQH5g@positron.chronox.de

So for now, to avoid unnecessarily blocking this patchset I think we should just
go with this implementation in fs/crypto/.  It can always be changed later, once
we decide on the best way to add KDFs to the crypto API.

[To be clear: this patch already uses "hmac(sha512)" from the crypto API.  It's
only the actual HKDF part that we're talking about here.

Also, its correctness is tested by the ciphertext verification xfstests.]

- Eric

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Thread overview: 51+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-26 22:41 [PATCH v7 00/16] fscrypt: key management improvements Eric Biggers
2019-07-26 22:41 ` [PATCH v7 01/16] fs, fscrypt: move uapi definitions to new header <linux/fscrypt.h> Eric Biggers
2019-07-28 15:08   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 02/16] fscrypt: use FSCRYPT_ prefix for uapi constants Eric Biggers
2019-07-26 22:41 ` [PATCH v7 03/16] fscrypt: use FSCRYPT_* definitions, not FS_* Eric Biggers
2019-07-26 22:41 ` [PATCH v7 04/16] fscrypt: add ->ci_inode to fscrypt_info Eric Biggers
2019-07-28 15:09   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 05/16] fscrypt: refactor v1 policy key setup into keysetup_legacy.c Eric Biggers
2019-07-28 15:40   ` Theodore Y. Ts'o
2019-07-29 19:37     ` Eric Biggers
2019-07-26 22:41 ` [PATCH v7 06/16] fscrypt: add FS_IOC_ADD_ENCRYPTION_KEY ioctl Eric Biggers
2019-07-28 18:50   ` Theodore Y. Ts'o
2019-07-29 19:46     ` Eric Biggers
2019-07-29 20:14       ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 07/16] fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY ioctl Eric Biggers
2019-07-28 19:24   ` Theodore Y. Ts'o
2019-07-29 19:58     ` Eric Biggers
2019-07-31 18:38       ` Eric Biggers
2019-07-31 23:38         ` Theodore Y. Ts'o
2019-08-01  1:11           ` [f2fs-dev] " Eric Biggers
2019-08-01  5:31             ` Theodore Y. Ts'o
2019-08-01 18:35               ` Eric Biggers
2019-08-01 18:46                 ` Eric Biggers
2019-08-01 22:04               ` Eric Biggers
2019-08-02  4:38                 ` Eric Biggers
2019-08-12 14:16                   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 08/16] fscrypt: add FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl Eric Biggers
2019-07-28 19:30   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 09/16] fscrypt: add an HKDF-SHA512 implementation Eric Biggers
2019-07-28 19:39   ` Theodore Y. Ts'o
2019-07-29 20:29     ` Eric Biggers [this message]
2019-07-29 21:42       ` James Bottomley
2019-07-26 22:41 ` [PATCH v7 10/16] fscrypt: v2 encryption policy support Eric Biggers
2019-07-28 21:17   ` Theodore Y. Ts'o
2019-07-29 20:46     ` Eric Biggers
2019-07-26 22:41 ` [PATCH v7 11/16] fscrypt: allow unprivileged users to add/remove keys for v2 policies Eric Biggers
2019-07-28 21:22   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 12/16] fscrypt: require that key be added when setting a v2 encryption policy Eric Biggers
2019-07-28 21:24   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 13/16] ext4: wire up new fscrypt ioctls Eric Biggers
2019-07-28 21:24   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 14/16] f2fs: " Eric Biggers
2019-07-30  0:36   ` Jaegeuk Kim
2019-08-02  8:10   ` Chao Yu
2019-08-02 17:31     ` Eric Biggers
2019-08-04  9:42       ` [f2fs-dev] " Chao Yu
2019-07-26 22:41 ` [PATCH v7 15/16] ubifs: " Eric Biggers
2019-07-30  0:39   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 16/16] fscrypt: document the new ioctls and policy version Eric Biggers
2019-07-29  2:00   ` Theodore Y. Ts'o
2019-07-29 21:36     ` Eric Biggers

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