Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Functions
Philip L <philippl.lee@gmail.com> Sun, 30 July 2017 17:02 UTC
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From: Philip L <philippl.lee@gmail.com>
Date: Sun, 30 Jul 2017 13:02:20 -0400
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To: Thomas Garcia <tgarcia.3141@gmail.com>
Cc: Sharon Goldberg <sharon.goldbe@gmail.com>, "cfrg@irtf.org" <cfrg@irtf.org>, "jan@ns1.com" <jan@ns1.com>, Leonid Reyzin <reyzin@cs.bu.edu>, Dimitrios Papadopoulos <dipapado@umd.edu>
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Subject: Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Functions
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Please excuse the spam. I thought for just a moment somehow that the function was verifiably random... like one of those hard problems (for decidability?). Just a random thought... On Thu, Jul 27, 2017 at 4:51 AM, Thomas Garcia <tgarcia.3141@gmail.com> wrote: > > Thanks for helping me clear those things up! > > Thomas G. > > On Wed, Jul 26, 2017 at 12:42 PM, Sharon Goldberg <sharon.goldbe@gmail.com > > wrote: > >> 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/2017/099.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>; 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 >> > > > _______________________________________________ > Cfrg mailing list > Cfrg@irtf.org > https://www.irtf.org/mailman/listinfo/cfrg > >
- [Cfrg] draft-goldbe-vrf: Verifiable Random Functi… Sharon Goldberg
- Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Fu… Dan Brown
- Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Fu… Tony Arcieri
- Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Fu… Paterson, Kenny
- Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Fu… Dan Brown
- Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Fu… Watson Ladd
- Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Fu… Tony Arcieri
- Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Fu… Blumenthal, Uri - 0553 - MITLL
- Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Fu… Sharon Goldberg
- Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Fu… Thomas Garcia
- Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Fu… Sharon Goldberg
- Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Fu… Thomas Garcia
- Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Fu… Philip L
- Re: [Cfrg] draft-goldbe-vrf: Verifiable Random Fu… Sharon Goldberg