From: James Bottomley <James.Bottomley@HansenPartnership.com> To: Eric Biggers <ebiggers@kernel.org>, "Theodore Y. Ts'o" <tytso@mit.edu> Cc: Satya Tangirala <satyat@google.com>, 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 <paulcrowley@google.com> Subject: Re: [PATCH v7 09/16] fscrypt: add an HKDF-SHA512 implementation Date: Mon, 29 Jul 2019 14:42:34 -0700 Message-ID: <1564436554.12726.38.camel@HansenPartnership.com> (raw) In-Reply-To: <20190729202951.GG169027@gmail.com> On Mon, 2019-07-29 at 13:29 -0700, Eric Biggers wrote: > 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 <ebiggers@google.com> [...] > > > HKDF solves all the above problems. > > > > > > Signed-off-by: Eric Biggers <ebiggers@google.com> > > > > 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. Well, I'm still trying to add the TPM ones, but they're based on SP800- 108 for arbitrary keys and SP800-56A for elliptic curve ones. These are similar to the RFC5869 except that they do extract/expand in a single operation. Plus, of course, the TPM mandates we use the name algorithm (usually sha256, which is what I hardcoded) as the hash. Note: since you don't use the extract step either in your implementation, effectively you're equivalent to SP800-108 as well. This is effectively the same reason as the TPM: we need deterministic keys, so we've nowhere to get the salt from that would persist. > 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.chro > nox.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. Right, once you have the hmac + hash available, the rest is easy, so this is what we have for the TPM KDFa: static void KDFa(u8 *key, int keylen, const char *label, u8 *u, u8 *v, int bytes, u8 *out) { u32 counter; const __be32 bits = cpu_to_be32(bytes * 8); for (counter = 1; bytes > 0; bytes -= SHA256_DIGEST_SIZE, counter++, out += SHA256_DIGEST_SIZE) { SHASH_DESC_ON_STACK(desc, sha256_hash); __be32 c = cpu_to_be32(counter); hmac_init(desc, key, keylen); crypto_shash_update(desc, (u8 *)&c, sizeof(c)); crypto_shash_update(desc, label, strlen(label)+1); crypto_shash_update(desc, u, SHA256_DIGEST_SIZE); crypto_shash_update(desc, v, SHA256_DIGEST_SIZE); crypto_shash_update(desc, (u8 *)&bits, sizeof(bits)); hmac_final(desc, key, keylen, out); } } I honestly think these things are so simplistic with the correct hmac that it would make it more confusing to try to produce a general KDF than it would simply to hard code them where we need them. James ______________________________________________________ Linux MTD discussion mailing list http://lists.infradead.org/mailman/listinfo/linux-mtd/
next prev parent reply index Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-07-26 22:41 [PATCH v7 00/16] fscrypt: key management improvements Eric Biggers 2019-07-26 22:41 ` [PATCH v7 01/16] fs, fscrypt: move uapi definitions to new header <linux/fscrypt.h> Eric Biggers 2019-07-28 15:08 ` Theodore Y. Ts'o 2019-07-26 22:41 ` [PATCH v7 02/16] fscrypt: use FSCRYPT_ prefix for uapi constants Eric Biggers 2019-07-26 22:41 ` [PATCH v7 03/16] fscrypt: use FSCRYPT_* definitions, not FS_* Eric Biggers 2019-07-26 22:41 ` [PATCH v7 04/16] fscrypt: add ->ci_inode to fscrypt_info Eric Biggers 2019-07-28 15:09 ` Theodore Y. Ts'o 2019-07-26 22:41 ` [PATCH v7 05/16] fscrypt: refactor v1 policy key setup into keysetup_legacy.c Eric Biggers 2019-07-28 15:40 ` Theodore Y. Ts'o 2019-07-29 19:37 ` Eric Biggers 2019-07-26 22:41 ` [PATCH v7 06/16] fscrypt: add FS_IOC_ADD_ENCRYPTION_KEY ioctl Eric Biggers 2019-07-28 18:50 ` Theodore Y. Ts'o 2019-07-29 19:46 ` Eric Biggers 2019-07-29 20:14 ` Theodore Y. Ts'o 2019-07-26 22:41 ` [PATCH v7 07/16] fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY ioctl Eric Biggers 2019-07-28 19:24 ` Theodore Y. Ts'o 2019-07-29 19:58 ` Eric Biggers 2019-07-31 18:38 ` Eric Biggers 2019-07-31 23:38 ` Theodore Y. Ts'o 2019-08-01 1:11 ` [f2fs-dev] " Eric Biggers 2019-08-01 5:31 ` Theodore Y. Ts'o 2019-08-01 18:35 ` Eric Biggers 2019-08-01 18:46 ` Eric Biggers 2019-08-01 22:04 ` Eric Biggers 2019-08-02 4:38 ` Eric Biggers 2019-08-12 14:16 ` Theodore Y. Ts'o 2019-07-26 22:41 ` [PATCH v7 08/16] fscrypt: add FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl Eric Biggers 2019-07-28 19:30 ` Theodore Y. Ts'o 2019-07-26 22:41 ` [PATCH v7 09/16] fscrypt: add an HKDF-SHA512 implementation Eric Biggers 2019-07-28 19:39 ` Theodore Y. Ts'o 2019-07-29 20:29 ` Eric Biggers 2019-07-29 21:42 ` James Bottomley [this message] 2019-07-26 22:41 ` [PATCH v7 10/16] fscrypt: v2 encryption policy support Eric Biggers 2019-07-28 21:17 ` Theodore Y. Ts'o 2019-07-29 20:46 ` Eric Biggers 2019-07-26 22:41 ` [PATCH v7 11/16] fscrypt: allow unprivileged users to add/remove keys for v2 policies Eric Biggers 2019-07-28 21:22 ` Theodore Y. Ts'o 2019-07-26 22:41 ` [PATCH v7 12/16] fscrypt: require that key be added when setting a v2 encryption policy Eric Biggers 2019-07-28 21:24 ` Theodore Y. Ts'o 2019-07-26 22:41 ` [PATCH v7 13/16] ext4: wire up new fscrypt ioctls Eric Biggers 2019-07-28 21:24 ` Theodore Y. Ts'o 2019-07-26 22:41 ` [PATCH v7 14/16] f2fs: " Eric Biggers 2019-07-30 0:36 ` Jaegeuk Kim 2019-08-02 8:10 ` Chao Yu 2019-08-02 17:31 ` Eric Biggers 2019-08-04 9:42 ` [f2fs-dev] " Chao Yu 2019-07-26 22:41 ` [PATCH v7 15/16] ubifs: " Eric Biggers 2019-07-30 0:39 ` Theodore Y. Ts'o 2019-07-26 22:41 ` [PATCH v7 16/16] fscrypt: document the new ioctls and policy version Eric Biggers 2019-07-29 2:00 ` Theodore Y. Ts'o 2019-07-29 21:36 ` Eric Biggers
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=1564436554.12726.38.camel@HansenPartnership.com \ --to=james.bottomley@hansenpartnership.com \ --cc=ebiggers@kernel.org \ --cc=keyrings@vger.kernel.org \ --cc=linux-api@vger.kernel.org \ --cc=linux-crypto@vger.kernel.org \ --cc=linux-ext4@vger.kernel.org \ --cc=linux-f2fs-devel@lists.sourceforge.net \ --cc=linux-fscrypt@vger.kernel.org \ --cc=linux-fsdevel@vger.kernel.org \ --cc=linux-mtd@lists.infradead.org \ --cc=paulcrowley@google.com \ --cc=satyat@google.com \ --cc=tytso@mit.edu \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: link
Linux-mtd Archive on lore.kernel.org Archives are clonable: git clone --mirror https://lore.kernel.org/linux-mtd/0 linux-mtd/git/0.git # If you have public-inbox 1.1+ installed, you may # initialize and index your mirror using the following commands: public-inbox-init -V2 linux-mtd linux-mtd/ https://lore.kernel.org/linux-mtd \ linux-mtd@lists.infradead.org public-inbox-index linux-mtd Example config snippet for mirrors Newsgroup available over NNTP: nntp://nntp.lore.kernel.org/org.infradead.lists.linux-mtd AGPL code for this site: git clone https://public-inbox.org/public-inbox.git