Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Functions

Sharon Goldberg <sharon.goldbe@gmail.com> Wed, 26 July 2017 11:42 UTC

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From: Sharon Goldberg <sharon.goldbe@gmail.com>
Date: Wed, 26 Jul 2017 14:42:11 +0300
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To: Thomas Garcia <tgarcia.3141@gmail.com>
Cc: Dan Brown <danibrown@blackberry.com>, "jan@ns1.com" <jan@ns1.com>, "cfrg@irtf.org" <cfrg@irtf.org>, Dimitrios Papadopoulos <dipapado@umd.edu>, Leonid Reyzin <reyzin@cs.bu.edu>
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Subject: Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Functions
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Hi Thomas,

1. It seems that one of the properties that you are interested that the VRF
> will have is determinism. On the other hand, the EC-VRF value is dependent
> on the choice of some random number k. How are these two facts compatible?
> Am I missing something?
>

We require determinism from the VRF hash output, which is obtained from the
VRF proof via the "proof2hash" function.

The EC-VRF proof has 3 values: gamma, c, s
Values c and s depends on the choice of random number k
But value gamma does not depend on k.

We get determinism because the VRF hash output (derived using "proof2hash")
depends only on gamma, but not on c and s.

Slide 28 from my CFRG presentations gives a pictorial explanation of this:
http://www.cs.bu.edu/~goldbe/papers/VRF_ietf99_print.pdf

2. The value c calculated in EC-VRF is only 128 bits long for Ed25519. In
> normal usage of Ed25519 the signature uses the full length of the hash
> output. Doesn't this expose the signature to collision attacks?
>
> No. We worked this out in our proofs of security.  Section B.2.1 of this
paper explains why this works.

https://eprint.iacr.org/2017/099.pdf

Thanks,
Sharon



> Thank you for your comments. Indeed, VRFs have been around since 1999.
>> They are really "verifiable PRFs" but the name is by now standard in the
>> crypto literature, and changing it will cause more confusion than keeping
>> it.
>>
>> In terms of the VRF's security properties, there are three:  uniqueness,
>> collision resistance, and pseudorandomness. These are defined in our draft (
>> https://tools.ietf.org/html/draft-goldbe-vrf-01#section-3).
>>
>> How VRFs prevent dictionary attacks: a public hash is subject to a
>> dictionary attack because, given the output, an adversary can evaluate the
>> hash on different inputs and see what hits the outputs. In a VRF, the
>> adversary can't evaluate the hash on different inputs.
>>
>> You say it's not surprising that "random oracle model proof can prove the
>> output of a hash to be random". But actually, the output of a hash is NOT
>> random if the input and the hash function are known. This is because the
>> output of a hash is deterministic (every input maps to a unique output).
>> That's exactly what enables dictionary attacks. This is in contrast to a
>> VRF, where the output is (pseudo)random even given knowledge of the input.
>> Only once the VRF proof is given, does the VRF output stop looking random.
>> Note that random oracles are not essential for VRFs, and non-random-oracles
>> constructions exist (but are less efficient than what we propose to
>> standardize).
>>
>> As far as use cases, here are a few:
>>
>> In the NSEC5 use case for DNSSEC, you have sensitive data (domain names)
>> and you sign consecutive pairs of hashes of domain names in order to be
>> able to prove absence of a name. If you just use standard hashing, whenever
>> signed hash values are disclosed, your sensitive data is subject to
>> dictionary attacks. VRFs solve that problem, and sensitive names don't have
>> to be disclosed at all. More details are in https://eprint.iacr.org/201
>> 7/099.pdf
>>
>> In CONIKS (also Google Key Transparency, Signal secure messaging, Yahoo!
>> Coname) you also have some sensitive data (user names) that are put into a
>> Merkle-like authenticated data structure. If you just use standard hashing,
>> whenever hash values are disclosed, sensitive data is subject to dictionary
>> attacks. If you use VRFs, sensitive data again can be disclosed only on an
>> as-needed basis. More details are in  https://eprint.iacr.org/2014/
>> 1004.pdf.
>>
>> In a cryptocurrency use case, you wish perform a coin flip that is
>> deterministic and provably correct, but cannot be done by just anyone.
>> More details are in https://people.csail.mit.ed
>> u/nickolai/papers/gilad-algorand-eprint.pdf and possibly other
>> cryptocurrency papers.
>>
>> Bryan Ford mentioned an additional usecase at the mic on Tuesday:
>> distributed password protection protocols
>>
>> Many of these use case are already putting VRFs into production use (esp.
>> the CONIKS one).  You can see a list of the various implementations we have
>> found in the "implementation status" section of our draft.  One of the
>> reasons we think this spec is so important is that we found flaws in
>> several of the implementations that can be used to trivially break
>> uniqueness.  (See eg: https://github.com/google/
>> keytransparency/issues/567)
>>
>> Thanks,
>> Sharon
>>
>> On Fri, Jul 21, 2017 at 7:32 PM, Dan Brown <danibrown@blackberry.com>
>> wrote:
>>
>>> Answering myself below: VRFs have been around since 1999, so are not so
>>> new.  ‎Still don't like the name, and still have trouble seeing the value.
>>>
>>> *From: *Dan Brown
>>> *Sent: *Tuesday, July 18, 2017 2:29 PM
>>> *To: *Sharon Goldberg; cfrg@irtf.org
>>> *Cc: *jan@ns1.com; Leonid Reyzin; Dimitrios Papadopoulos
>>> *Subject: *Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Functions
>>>
>>> Hi Sharon and CFRG,
>>>
>>>
>>>
>>> On VRFs, my uncertain comments to consider at your leisure:
>>>
>>>
>>>
>>> Is it fair to say VRFs are relatively new?  If so, then maybe a little
>>> more caution is needed about their use.  It seems a tad hasty that it is
>>> being used already.
>>>
>>>
>>>
>>> To me, it seems that VRFs are basically signatures, with an extra
>>> feature.  My concern is that this extra feature might get overused, before
>>> it is thoroughly reviewed.
>>>
>>>
>>>
>>> It is unsurprising to me that random oracle model proof can prove the
>>> output of a hash to be random.  My intuitive concern is that at least
>>> informally, this is kind of circular.  Hashes often have some
>>> non-random-ish properties that might affect the extra security (over
>>> signatures) that VRFs are aiming for.  I guess I would much prefer a proof
>>> saying if the hash has (well-studied) properties XYZ, then your
>>> construction are VRFs.  (Maybe you have this already?  If so, then tell me
>>> so.)
>>>
>>>
>>>
>>> Since, VRFs require sending the “proofs” on the wire, I find it hard to
>>> see how it could be used to prevent dictionary attacks.  I assume that you
>>> are saying the proofs must be encrypted when one needs to avoid dictionary
>>> models?  I suppose all the details are there in I-D and papers, but for
>>> now, I am confused about the threat model (which parties have keys, etc.,
>>> if they require a secure channel and mutual trust, why just use plain old
>>> hash,…). To resist dictionary attacks, were already have PAKEs and
>>> PBHashing.  Now this?
>>>
>>>
>>>
>>> Finally, on a bikeshed-coloring note, I object to the name “verifiable
>>> random function”, on several grounds.
>>>
>>>
>>>
>>>    1. It is not a function.  It is at least four functions, keygen,
>>>    sign, verify, and hashify.
>>>    2. If you make it into a keyed function F_sk(m), as in
>>>    prooftohash(sign_sk(m)), it is not verifiable.
>>>    3. Verification requires the intermediate proof, which is certainly
>>>    not even pseudorandom (it is easy to distinguish valid signatures from
>>>    random).
>>>    4. It is pseudorandom, not random.  (The keys are random, but many
>>>    crypto has keys, without having “random” in its name: encryption, MAC,
>>>    signatures, key exchange, …, they also don’t verifiable or random in their
>>>    names either.)
>>>    5. The similar phrase “verifiably random”, albeit as a misnomer, has
>>>    past precedents, see NIST P-256 and Brainpool, etc.  When I see VRF, I
>>>    think a function, that aims to VR in that sense, and great, now we can
>>>    improve on Brainpool, etc.
>>>    6. “Random function” should be reserved for the ideal random mapping
>>>    concept, for example, as studied by Flajolet-Odlyzko (ok they only studied
>>>    the case of equal size domain and range).  The random oracle model, is the
>>>    idea of approximating this ideal, etc.  An actual approximation should not
>>>    be name as the ideal (sorry, I’m kind of repeating my point 4).
>>>
>>>
>>>
>>> Please forgive the fact that my comments above are not very constructive
>>> (or if the tone is wrong).  This is a new topic for me, so I am reluctant
>>> too many suggestions.  Nonetheless, I suggest (0) waiting a little, (1) a
>>> non-random-oracle security proof (if you don’t have it yet), (2) re-naming
>>> the scheme to something like re-hashable (or digestible) signatures (and
>>> re-name the various parts, i.e. proof -> signature, etc.).
>>>
>>>
>>>
>>> Best regards,
>>>
>>>
>>>
>>> Dan
>>>
>>>
>>>
>>>
>>>
>>> *From:* Cfrg [mailto:cfrg-bounces@irtf.org] *On Behalf Of *Sharon
>>> Goldberg
>>> *Sent:* Wednesday, July 12, 2017 5:42 AM
>>> *To:* cfrg@irtf.org
>>> *Cc:* jan@ns1.com; Dimitrios Papadopoulos <dipapado@umd.edu>du>; Leonid
>>> Reyzin <reyzin@cs.bu.edu>
>>> *Subject:* [Cfrg] draft-goldbe-vrf: Verifiable Random Functions
>>>
>>>
>>>
>>> Dear CFRG,
>>>
>>> I'm presenting at next week's meeting on Verifiable Random Functions. A
>>> VRF is the public-key version of keyed cryptographic hash. Only the holder
>>> of the VRF secret key can compute the hash, but anyone with the public key
>>> can verify it.  VRFs can be used to prevent dictionary attacks on
>>> hash-based data structures, and have applications to key transparency
>>> (CONIKS), DNSSEC (NSEC5), and cryptocurrencies (Algorand).
>>>
>>> In advance of the meeting, please see:
>>>
>>> 1) Our substantially updated -01 draft:
>>> https://datatracker.ietf.org/doc/draft-goldbe-vrf/
>>>
>>> 2) Our project page, with links to various VRF implementations:
>>> https://www.cs.bu.edu/~goldbe/projects/vrf
>>>
>>> Comments welcome.  Thanks,
>>>
>>> Sharon
>>>
>>> --
>>> Sharon Goldberg
>>> Computer Science, Boston University
>>> http://www.cs.bu.edu/~goldbe
>>>
>>
>>
>>
>> --
>> ---
>> Sharon Goldberg
>> Computer Science, Boston University
>> http://www.cs.bu.edu/~goldbe
>>
>> _______________________________________________
>> Cfrg mailing list
>> Cfrg@irtf.org
>> https://www.irtf.org/mailman/listinfo/cfrg
>>
>>
>


-- 
---
Sharon Goldberg
Computer Science, Boston University
http://www.cs.bu.edu/~goldbe