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From: Paul Crowley <paulcrowley@google.com>
To: Eric Biggers <ebiggers@kernel.org>
Cc: Satya Tangirala <satyat@google.com>,
	Theodore Ts'o <tytso@mit.edu>,
	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, Jaegeuk Kim <jaegeuk@kernel.org>,
	linux-ext4@vger.kernel.org
Subject: Re: [PATCH v8 12/20] fscrypt: add an HKDF-SHA512 implementation
Date: Tue, 6 Aug 2019 13:43:27 -0700
Message-ID: <CA+_SqcBkR_8Z9EUTpK-dEW4PN+9P5OgJnqYDHtOhG+P1LjotPA@mail.gmail.com> (raw)
In-Reply-To: <20190805162521.90882-13-ebiggers@kernel.org>

On Mon, 5 Aug 2019 at 09:28, Eric Biggers <ebiggers@kernel.org> 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.
>
> Reviewed-by: Theodore Ts'o <tytso@mit.edu>
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Looks good, feel free to add:

Reviewed-by: Paul Crowley <paulcrowley@google.com>

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Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-05 16:25 [PATCH v8 00/20] fscrypt: key management improvements Eric Biggers
2019-08-05 16:25 ` [PATCH v8 01/20] fs, fscrypt: move uapi definitions to new header <linux/fscrypt.h> Eric Biggers
2019-08-05 16:25 ` [PATCH v8 02/20] fscrypt: use FSCRYPT_ prefix for uapi constants Eric Biggers
2019-08-05 16:25 ` [PATCH v8 03/20] fscrypt: use FSCRYPT_* definitions, not FS_* Eric Biggers
2019-08-05 16:25 ` [PATCH v8 04/20] fscrypt: add ->ci_inode to fscrypt_info Eric Biggers
2019-08-05 16:25 ` [PATCH v8 05/20] fscrypt: rename fscrypt_master_key to fscrypt_direct_key Eric Biggers
2019-08-12 22:20   ` Theodore Y. Ts'o
2019-08-05 16:25 ` [PATCH v8 06/20] fscrypt: refactor key setup code in preparation for v2 policies Eric Biggers
2019-08-12 22:38   ` Theodore Y. Ts'o
2019-08-05 16:25 ` [PATCH v8 07/20] fscrypt: move v1 policy key setup to keysetup_v1.c Eric Biggers
2019-08-12 22:53   ` Theodore Y. Ts'o
2019-08-05 16:25 ` [PATCH v8 08/20] fscrypt: rename keyinfo.c to keysetup.c Eric Biggers
2019-08-12 22:53   ` Theodore Y. Ts'o
2019-08-05 16:25 ` [PATCH v8 09/20] fscrypt: add FS_IOC_ADD_ENCRYPTION_KEY ioctl Eric Biggers
2019-08-05 16:25 ` [PATCH v8 10/20] fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY ioctl Eric Biggers
2019-08-13  0:06   ` Theodore Y. Ts'o
2019-08-14 22:35     ` Eric Biggers
2019-08-05 16:25 ` [PATCH v8 11/20] fscrypt: add FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl Eric Biggers
2019-08-05 16:25 ` [PATCH v8 12/20] fscrypt: add an HKDF-SHA512 implementation Eric Biggers
2019-08-06 20:43   ` Paul Crowley [this message]
2019-08-05 16:25 ` [PATCH v8 13/20] fscrypt: v2 encryption policy support Eric Biggers
2019-08-06 20:44   ` Paul Crowley
2019-08-13  0:39   ` Theodore Y. Ts'o
2019-08-05 16:25 ` [PATCH v8 14/20] fscrypt: allow unprivileged users to add/remove keys for v2 policies Eric Biggers
2019-08-13  0:14   ` Theodore Y. Ts'o
2019-08-05 16:25 ` [PATCH v8 15/20] fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS ioctl Eric Biggers
2019-08-13  0:15   ` Theodore Y. Ts'o
2019-08-05 16:25 ` [PATCH v8 16/20] fscrypt: require that key be added when setting a v2 encryption policy Eric Biggers
2019-08-05 16:25 ` [PATCH v8 17/20] ext4: wire up new fscrypt ioctls Eric Biggers
2019-08-05 16:25 ` [PATCH v8 18/20] f2fs: " Eric Biggers
2019-08-05 16:25 ` [PATCH v8 19/20] ubifs: " Eric Biggers
2019-08-05 16:25 ` [PATCH v8 20/20] fscrypt: document the new ioctls and policy version Eric Biggers
2019-08-13  0:49   ` Theodore Y. Ts'o
2019-08-14 22:37 ` [PATCH v8 00/20] fscrypt: key management improvements Eric Biggers

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