From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Theodore Y. Ts'o" Subject: Re: [PATCH v7 09/16] fscrypt: add an HKDF-SHA512 implementation Date: Sun, 28 Jul 2019 15:39:49 -0400 Message-ID: <20190728193949.GI6088@mit.edu> References: <20190726224141.14044-1-ebiggers@kernel.org> <20190726224141.14044-10-ebiggers@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20190726224141.14044-10-ebiggers@kernel.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-f2fs-devel-bounces@lists.sourceforge.net To: Eric Biggers 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 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? Other than that, looks good. Feel free to add: Reviewed-by: Theodore Ts'o