From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Biggers 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> References: <20190726224141.14044-1-ebiggers@kernel.org> <20190726224141.14044-10-ebiggers@kernel.org> <20190728193949.GI6088@mit.edu> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20190728193949.GI6088@mit.edu> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-mtd" Errors-To: linux-mtd-bounces+gldm-linux-mtd-36=gmane.org@lists.infradead.org To: "Theodore Y. Ts'o" Cc: Satya Tangirala , 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 List-Id: linux-api@vger.kernel.org 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 > > > > 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 > > 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 ______________________________________________________ Linux MTD discussion mailing list http://lists.infradead.org/mailman/listinfo/linux-mtd/