Re: [PATCH 4/6] crypto: hkdf - RFC5869 Key Derivation Function

Eric Biggers
2019-01-14T17:53:20Z

On Mon, Jan 14, 2019 at 10:30:39AM +0100, Stephan Müller wrote:
> Am Samstag, 12. Januar 2019, 06:12:54 CET schrieb Eric Biggers:
>
> Hi Eric,
>
> [...]
>
> > > The extract and expand phases use different instances of the underlying
> > > keyed message digest cipher to ensure that while the extraction phase
> > > generates a new key for the expansion phase, the cipher for the
> > > expansion phase can still be used. This approach is intended to aid
> > > multi-threaded uses cases.
> >
> > I think you partially misunderstood what I was asking for. One HMAC tfm is
> > sufficient as long as HKDF-Expand is separated from HKDF-Extract, which
> > you've done. So just use one HMAC tfm, and in crypto_hkdf_seed() key it
> > with the 'salt', and then afterwards with the 'prk'.
>
> Ok, thanks for the clarification. I will remove the 2nd HMAC TFM then.
> >
> > Also everywhere in this patchset, please avoid using the word "cipher" to
> > refer to algorithms that are not encryption/decryption. I know a lot of
> > the crypto API docs do this, but I think it is a mistake and confusing.
> > Hash algorithms and KDFs are not "ciphers".
>
> As you wish, I will refer to specific name of the cryptographic operation.
>
> [...]
>
> > > + * NOTE: In-place cipher operations are not supported.
> > > + */
> >
> > What does an "in-place cipher operation" mean in this context? That the
> > 'info' buffer must not overlap the 'dst' buffer?
>
> Correct, no overlapping.
>
> > Maybe
> > crypto_rng_generate() should check that for all crypto_rngs? Or is it
> > different for different crypto_rngs?
>
> This is the case in general for all KDFs (and even RNGs). It is no technical
> or cryptographic error to have overlapping buffers. The only issue is that the
> result will not match the expected value.
>
> The issue is that the input buffer to the generate function is an input to
> every round of the KDF. If the input and output buffer overlap, starting with
> the 2nd iteration of the KDF, the input is the output of the 1st round. Again,
> I do not think it is a cryptographic error though.
>
> (To support my conclusion: A colleague of mine has proposed an update to the
> HKDF specification where the input data changes for each KDF round. This
> proposal was considered appropriate by one of the authors of HKDF.)
>
> If the requested output is smaller or equal to the output block size of the
> KDF, overlapping buffers are even harmless since the implementation will
> calculate the correct output.
>
> Due to that, I removed the statement. But I am not sure we should add a
> technical block to deny overlapping input/output buffers.
>
> [...]
> > >
> > > + desc->flags = crypto_shash_get_flags(expand_kmd) &
> > > + CRYPTO_TFM_REQ_MAY_SLEEP;
> >
> > This line setting desc->flags doesn't make sense. How is the user meant to
> > control whether crypto_rng_generate() can sleep or not? Or can it always
> > sleep? Either way this part is wrong since the user can't get access to the
> > HMAC tfm to set this flag being checked for.
>
> Could you please help me why a user should set this flag? Isn't the
> implementation specifying that flag to allow identifying whether the
> implementation could or could not sleep? Thus, we simply copy the sleeping
> flag from the lower level keyed message digest implementation.
>
> At least that is also the implementation found in crypto/hmac.c.
>
> [...]

Whether the crypto_shash* stuff can sleep is controlled on a per-request basis,
not a per-implementation basis. So I don't understand what you are talking
about here.

>
> > > + if (dlen < h) {
> > > + u8 tmpbuffer[CRYPTO_HKDF_MAX_DIGESTSIZE];
> > > +
> > > + err = crypto_shash_finup(desc, &ctr, 1, tmpbuffer);
> > > + if (err)
> > > + goto out;
> > > + memcpy(dst, tmpbuffer, dlen);
> > > + memzero_explicit(tmpbuffer, h);
> > > + goto out;
> > > + } else {
> >
> > No need for the 'else'.
>
> Could you please help me why that else branch is not needed? If the buffer to
> be generated is equal or larger than the output block length of the keyed
> message digest, I would like to directly operate on the output buffer to avoid
> a memcpy.

I'm simply saying you don't need the 'else' keyword as the previous block ends
with a goto.

> >
> > > + err = crypto_shash_finup(desc, &ctr, 1, dst);
> > > + if (err)
> > > + goto out;
> > > +
> > > + prev = dst;
> > > + dst += h;
> > > + dlen -= h;
> > > + ctr++;
> > > + }
> > > + }
>
> [...]
> >
> > > + struct crypto_shash *extract_kmd = ctx->extract_kmd;
> > > + struct crypto_shash *expand_kmd = ctx->expand_kmd;
> > > + struct rtattr *rta = (struct rtattr *)seed;
> > > + SHASH_DESC_ON_STACK(desc, extract_kmd);
> > > + u32 saltlen;
> > > + unsigned int h = crypto_shash_digestsize(extract_kmd);
> > > + int err;
> > > + const uint8_t null_salt[CRYPTO_HKDF_MAX_DIGESTSIZE] = { 0 };
> >
> > static const
> >
>
> Why would I want to turn that buffer into a static variable? All we need it
> for is in case there is no salt provided.
>
> [...]
>
> > > +
> > > + if (!RTA_OK(rta, slen))
> > > + return -EINVAL;
> > > + if (rta->rta_type != 1)
> > > + return -EINVAL;
> > > + if (RTA_PAYLOAD(rta) < sizeof(saltlen))
> > > + return -EINVAL;
> > > + saltlen = *((u32 *)RTA_DATA(rta));
> >
> > I'm guessing you copied the weird "length as a rtattr payload" approach from
> > the authenc template. I think it's not necessary. And it's overly
> > error-prone, as shown by the authenc template getting the parsing wrong for
> > years and you making the exact same mistake again here...
> > (See https://patchwork.kernel.org/patch/10732803/) How about just using a
> > u32 at the beginning without the 'rtattr' preceding it?
>
> I was not sure whether this approach would be acceptable. I very much would
> love to have a u32 pre-pended only without the RTA business.
>
> I updated the implementation accordingly.
> >
> [...]
>
> >
> > > + alg = &salg->base;
> >
> > Check here that the underlying algorithm really is "hmac(" something?
>
> I added a check for the presence of salg->setkey.
> >
> > Alternatively it may be a good idea to simplify usage by making the template
> > just take the unkeyed hash directly, like "hkdf(sha512)". And if any users
> > really need to specify a specific HMAC implementation then another template
> > usable as "hkdf_base(hmac(sha512))" could be added later.
> >
>
> I would not suggest this, because that rounds contrary to the concept of the
> kernel crypto API IMHO. The caller has to provide the wrapping cipher. It is
> perfectly viable to allow a caller to invoke a specific keyed message digest.
>

Sure, but it would not conform to the HKDF specification. Are you sure it is
okay to specify an arbitrary keyed hash?

> [...]
>
> Thank you very much for your code review.
>
> Ciao
> Stephan
>
>

- Eric