From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6C33EC7618F for ; Sun, 28 Jul 2019 19:40:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4EF5320578 for ; Sun, 28 Jul 2019 19:40:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726173AbfG1TkK (ORCPT ); Sun, 28 Jul 2019 15:40:10 -0400 Received: from outgoing-auth-1.mit.edu ([18.9.28.11]:46025 "EHLO outgoing.mit.edu" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726108AbfG1TkJ (ORCPT ); Sun, 28 Jul 2019 15:40:09 -0400 Received: from callcc.thunk.org (96-72-102-169-static.hfc.comcastbusiness.net [96.72.102.169] (may be forged)) (authenticated bits=0) (User authenticated as tytso@ATHENA.MIT.EDU) by outgoing.mit.edu (8.14.7/8.12.4) with ESMTP id x6SJdo7V012074 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Sun, 28 Jul 2019 15:39:52 -0400 Received: by callcc.thunk.org (Postfix, from userid 15806) id C00FF4202F5; Sun, 28 Jul 2019 15:39:49 -0400 (EDT) Date: Sun, 28 Jul 2019 15:39:49 -0400 From: "Theodore Y. Ts'o" To: Eric Biggers Cc: linux-fscrypt@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-mtd@lists.infradead.org, linux-api@vger.kernel.org, linux-crypto@vger.kernel.org, keyrings@vger.kernel.org, Paul Crowley , Satya Tangirala Subject: Re: [PATCH v7 09/16] fscrypt: add an HKDF-SHA512 implementation 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-Disposition: inline In-Reply-To: <20190726224141.14044-10-ebiggers@kernel.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-crypto-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-crypto@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