Network Working Group | S. Goldberg |
Internet-Draft | Boston University |
Intended status: Standards Track | D. Papadopoulos |
Expires: September 14, 2017 | University of Maryland |
J. Vcelak | |
NS1 | |
March 13, 2017 |
Verifiable Random Functions (VRFs)
draft-goldbe-vrf-00
A Verifiable Random Function (VRF) is the public-key version of a keyed cryptographic hash. Only the holder of the private key can compute the hash, but anyone with public key can verify the correctness of the hash. VRFs are useful for preventing enumeration of hash-based data structures. This document specifies several VRF constructions that are secure in the cryptographic random oracle model. One VRF uses RSA and the other VRF uses Eliptic Curves (EC).
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A Verifiable Random Function (VRF) [MRV99] is the public-key version of a keyed cryptographic hash. Only the holder of the private VRF key can compute the hash, but anyone with corresponding public key can verify the correctness of the hash.
The main application of the VRF is to protect the privacy of data records stored in a hash-based data structure against a querying adversary. In this application, a prover holds the VRF secret key and uses the VRF hashing to construct a hash-based data structure on the input data. Due to the nature of the VRF hashing, only the prover can answer queries about whether or not some data is stored in the data structure. Anyone who knows the public VRF key can verify that the prover has answered the queries correctly. However no offline inferences (i.e. inferences without querying the prover) can be made about the data stored in the data strucuture.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
The following terminology is used through this document:
A VRF comes with a key generation algorithm that generates a public VRF key PK and private VRF key SK.
A VRF hashes an input alpha using the private VRF key SK to obtain a VRF hash output beta:
The VRF_hash algorithm MUST be deterministic, in the sense that it will always produce the same output beta given a pair of inputs (SK, alpha). The private key SK is also used to construct a proof pi that beta is the correct hash output
The VRFs defined in this document allow anyone to deterministically obtain the VRF hash output beta directly from the proof value pi as
Notice that this means that
The proof pi allows anyone holding the public key PK to verify that beta is the correct VRF hash of input alpha under key PK. Thus, the VRF also comes with an algorithm
that outputs VALID if beta=VRF_proof2hash(pi) is correct VRF hash of alpha under key PK, and outputs INVALID otherwise.
VRFs are designed to ensure the following security properties.
Uniqueness states that, for any fixed public VRF key and for any input alpha, there is a unique VRF output beta that can be proved to be valid, even for a computationally-bounded adversary that knows the VRF secret key SK.
More precisely, full uniqueness states that a computationally bounded adversary cannot choose a VRF public key PK, a VR input alpha, two different VRF hash outputs beta1 and beta2, and two proofs pi1 and pi2 such that VRF_verify(PK, alpha, pi1) and VRF_verify(PK, alpha, pi2) both output VALID.
A slightly weaker security property called "trusted uniquness" sufficies for many applications. Trusted uniqueness is the same as full uniqueness, but it must hold only if the VRF keys PK and SK were generated in a trustworthy manner. In otherwords, uniqueness might not hold if keys were generated in an invalid manner.
Suppose the public and private VRF keys (PK, SK) were generated in a trustworthy manner.
Pseudorandomness ensures that the VRF hash output beta (without its corresponding VRF proof pi) on any adversarially-chosen "target" VRF input alpha looks indistinguishable from random for any computationally bounded adversary who does not know the private VRF key SK. This holds even if the adversary also gets to choose other VRF inputs alpha' and observe their corresponding VRF hash outputs beta' and proofs pi'.
With "full pseudorandomness", the adversary is allowed to choose the target VRF input alpha at any time, even after it observes VRF outputs beta' and proofs pi' on a variety of chosen inputs alpha'.
"Selective pseudorandomness" is a weaker security property which suffices in many applications. Here, the adversary must choose the target VRF input alpha independently of the public VRF key PK, and before it observes VRF outputs beta' and proofs pi' on inputs alpha' of its choice.
It is important to remember that the VRF output beta does not look random to a party that knows the private VRF key SK! Such a party can easily distinguish beta from a random value by comparing it to the result of VRF_hash(SK, alpha).
Also, the VRF output beta does not look random to any party that knows valid VRF proof pi corresponding to the VRF input alpha, even if this party does not know the private VRF key SK. Such a party can easily distinguish beta from a random value by checking whether VRF_verify(PK, alpha, pi) returns "VALID" and beta = VRF_proof2hash(pi).
Finally, the VRF output beta may not look random if VRF key generation was not done in a trustworthy fashion. (For example, if VRF keys were not generated with good randomness.)
Finally, like any cryprographic hash function, VRFs need to be collision resistant. Specifically, it should be computationally infeasible for an adversary to find two distinct VRF inputs alpha1 and alpha2 that have the same VRF hash beta, even if that adversary knows the secret VRF key SK.
For most applications, a slightly weaker security property called "trusted collision resistance" suffices. Trusted collision resistance is the same as collision resistance, but it holds only if PK and SK were generated in a trustworthy manner.
The RSA Full Domain Hash VRF (RSA-FDH-VRF) is VRF that satisfies the trusted uniqueness, full pseudorandomness, and trusted collision resistance properties defined in Section 3. Its security follows from the standard RSA assumption in the random oracle model. Formal security proofs are in [nsec5ecc].
The VRF computes the proof pi as a deterministic RSA signature on input alpha using the RSA Full Domain Hash Algorithm [RFC8017] parametrized with the selected hash algorithm. RSA signature verification is used to verify the correctness of the proof. The VRF hash output beta is simply obtained by hashing the proof pi with the selected hash algorithm.
The key pair for RSA-FDH-VRF MUST be generated in a way that it satisfies the conditions specified in Section 3 of [RFC8017].
In this document, the notation from [RFC8017] is used.
Used parameters:
Fixed options:
Options constraints:
Used primitives:
RSAFDHVRF_prove(K, alpha)
Input:
Output:
Steps:
RSAFDHVRF_proof2hash(pi)
Input:
Output:
Steps:
RSAFDHVRF_verify((n, e), alpha, pi)
Input:
Output:
Steps:
The Elliptic Curve Verifiable Random Function (EC-VRF) is VRF that satisfies the trusted uniqueness, full pseudorandomness, and trusted collision resistance properties defined in Section 3. The security of this VRF follows from the decisional Diffie-Hellman (DDH) assumption in the cyclic group in the random oracle model. Formal security proofs are in [nsec5ecc].
The key pair generation primitive is specified in Section 3.2.1 of [SECG1].
Fixed options:
Options constraints:
Used parameters:
Used primitives:
ECVRF_prove(g^x, x, alpha)
Input:
Output:
Steps:
ECVRF_proof2hash(pi)
Input:
Output:
Steps:
ECVRF_verify(g^x, pi, alpha)
Input:
Output:
Steps:
The ECVRF_hash_to_curve algorithm takes in an octet string alpha and converts it to h, an EC point in G.
The following ECVRF_hash_to_curve1(alpha, g^x) algorithm implements ECVRF_hash_to_curve in a simple and generic way that works for any elliptic curve that supports point compression.
However, this algorithm MUST NOT be used in applications where the VRF input alpha must be kept secret. This is because the running time of the hashing algorithm depends on alpha, and so it is susceptible to timing attacks. That said, the amount of information obtained from such a timing attack is likely to be small, since the algorithm is expected to find a valid curve point after only two attempts (i.e., when ctr=1) on average (see [Icart09]).
ECVRF_hash_to_curve1(alpha, g^x)
Input:
Output:
Steps:
The initial octet 0x02 in the octet string created in step B represents that the point in compressed form has positive y-coefficient [SECG1]. Similarly, the 0x03 octet in step D represents negative y-coefficient.
For applications where VRF input alpha must be kept secret, the following ECVRF_hash_to_curve algorithm MAY be used to used as generic way to hash an octet string onto any elliptic curve.
[TODO: If there interest, we could look into specifying the generic deterministic time hash_to_curve algorithm from [Icart09]. ]
ECVRF_hash_points(p_1, p_2, ..., p_j)
Input:
Output:
Steps:
ECVRF_decode_proof(pi)
Input:
Output:
Steps:
[Seeking feedback on this section!]
This document defines EC-VRF-P256-SHA256 as follows:
This document defines EC-VRF-ED25519-SHA256 as follows:
[TODO: Should we add an EC-VRF-ED25519-SHA256-Elligator ciphersuite where the Elligator hash function is used for ECVRF_hash-to-curve?]
[TODO: Add an Ed448 ciphersuite?]
An implementation of the RSA-FDH-VRF (SHA-256) and EC-VRF-P256-SHA256 was developed as a part of the NSEC5 project [I-D.vcelak-nsec5] and is available at <http://github.com/fcelda/nsec5-crypto>.
The Key Transparency project at Google uses a VRF implemention that is almost identical to the EC-VRF-P256-SHA256 specified here, with a few minor changes including the use of SHA-512 instead of SHA-256. Its implementation is available <https://github.com/google/keytransparency/blob/master/core/vrf/vrf.go>
Open Whisper Systems also uses a VRF very similar to EC-VRF-ED25519-SHA512-Elligator, called VXEdDSA, and specified here: <https://whispersystems.org/docs/specifications/xeddsa/>
Applications that use the VRFs defined in this document MUST ensure that that the VRF key is generated correctly, using good randomness. Without good randomness, pseudorandomness properties of the VRF may not hold. Also, trusted uniqueness and trusted collision-resistance may also not hold if the keys are generated adversarially (e.g., the RSA modulus is not a product of two primes for the RSA-FDH-VRF or the public key g^x is not valid point in the prime-order group G for the EC).
Full uniqueness and full collision-resistance (as opposed to trusted uniqueness and trusted collision-resistance) are properties that hold even if VRF keys are generated by an adversary. The VRFs defined in this document do not have these properties. However, they may be modifed to have these properties if adversarial key generation is a concern. The modification consists of additional cryptographic proofs that keys have of the correct form. These modifications are left for future specification.
Note that for the RSA-FDH-VRF, it might be possible to construct such a proof using the [GQ88] identification protocol made non-interactive using the Fiat-Shamir heuristic in the random oracle model.
However, it is not possible to guarantee pseudorandomness in the face of adversarially generated VRF keys. This is because an adversary can always use bad randomness to generate the VRF keys, and thus, the VRF output may not be pseudorandom.
Applications that use the EC-VRF defined in this document MUST ensure that the random nonce k used in the ECVRF_prove algorithm is chosen with proper randomness. Otherwise, an adversary may be able to recover the private VRF key x (and thus break pseudorandomness of the VRF) after observing several valid VRF proofs pi.
The EC-VRF_hash_to_curve algorithm defined in Section 5.4.1.1 should not be used in applications where the VRF input alpha is secret and is hashed by the VRF on-the-fly. This is because the EC-VRF_hash_to_curve algorithm's running time depends on the VRF input alpha, and thus creates a timing channel that can be used to learn information about alpha.
[nsec5ecc] presents cryptographic reductions to an underlying hard problem (e.g. Decisional Diffie Hellman, or the standard RSA assumption) that prove the VRFs specificied in this document possess full pseudorandomness as well as selective pseudorandomness. However, the cryptographic reductions are tighter for selective pseudorandomness than for full pseudorandomness. This means the the VRFs have quantitavely stronger security guarentees for selective pseudorandomness.
Applications that are concerned about tightness of cryptographic reductions therefor have two options.
Note to RFC Editor: if this document does not obsolete an existing RFC, please remove this appendix before publication as an RFC.
Leonid Reyzin (Boston University) made major contributions to this document. This document also would not be possible without the work of Moni Naor (Weizmann Institute), Sachin Vasant (Cisco Systems), and Asaf Ziv (Facebook). Shumon Huque (Salesforce) and David C. Lawerence (Akamai) provided valuable input to this draft.
[nsec5ecc] | Papadopoulos, D., Wessels, D., Huque, S., Vcelak, J., Naor, M., Reyzin, L. and S. Goldberg, "NSEC5 from Elliptic Curves", in ePrint Cryptology Archive 2017/099, February 2017. |
[MRV99] | Michali, S., Rabin, M. and S. Vadhan, "Verifiable Random Functions", in FOCS, 1999. |
[CP92] | Chaum, D. and C. Pederson, "Wallet databases with observers", in FOCS, 1992. |
[Icart09] | Icart, T., "How to Hash into Elliptic Curves", in CRYPTO, 2009. |
[GQ88] | Guillou, L. and JJ. Quisquater, "A Practical Zero-Knowledge Protocol Fitted to Security Microprocessor Minimizing Both Transmission and Memory", in Advances in Cryptology - EUROCRYPT '88, 1988. |
Note to RFC Editor: please remove this appendix before publication as an RFC.