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=-2.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FSL_HELO_FAKE,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 58C01C433FF for ; Mon, 29 Jul 2019 20:29:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1CB1320693 for ; Mon, 29 Jul 2019 20:29:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1564432196; bh=7LK4u/xXVtAq5xQMkemCjB4sBIVdQGteWvKYNQ+TBCY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=MIfUuzegFVCCUgqosw95Zjxvp5G+JgoL9nxndaEXosHiXz3DEzMg6WWQeu0c4UitM ZvoVPPTgEv+8rlx9/CZGYPlqURTO+zZ4w83npQgH5rdTD8Q/PWr4Byy9G8ln3tFIA4 8rZDSKrjBl7PlCzLUuJ9cWQaXV+U1maXcYM9EApg= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728817AbfG2U3z (ORCPT ); Mon, 29 Jul 2019 16:29:55 -0400 Received: from mail.kernel.org ([198.145.29.99]:42898 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726713AbfG2U3z (ORCPT ); Mon, 29 Jul 2019 16:29:55 -0400 Received: from gmail.com (unknown [104.132.1.77]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id B2FB620679; Mon, 29 Jul 2019 20:29:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1564432194; bh=7LK4u/xXVtAq5xQMkemCjB4sBIVdQGteWvKYNQ+TBCY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=DPew4TPf3PkKUXWtCLWWIQsryuZDmRsZW9HWBPCFmul6GOPKrMhWsOP2j0s6E1587 pPpWGjXg0oLEblY9YgIrbRyuoJDVlQC0LdMHo0X3YE5xtPBbo3Oz6TZzJnytmQbINO 1di8FZ/AmOwFvTEDEuqxMDphn/LIvGf9Grr0aIA8= Date: Mon, 29 Jul 2019 13:29:52 -0700 From: Eric Biggers To: "Theodore Y. Ts'o" 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: <20190729202951.GG169027@gmail.com> Mail-Followup-To: "Theodore Y. Ts'o" , 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 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-Disposition: inline In-Reply-To: <20190728193949.GI6088@mit.edu> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@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