Verifiable Random Functions (VRFs)
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dipapado@bu.edu
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jvcelak@ns1.com
public key cryptography
hashing
authenticated denial
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).
A Verifiable Random Function
(VRF) 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.
A key application of the VRF is to provide privacy against
offline enumeration (e.g. dictionary attacks) on data stored in a
hash-based data structure.
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, 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
.
The following terminology is used through this document:
The private key for the VRF.
The public key for the VRF.
The input to be hashed by the VRF.
The VRF hash output.
The VRF proof.
The Prover holds the private VRF key SK and public VRF key PK.
The Verifier holds the public VRF key PK.

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
beta = VRF_hash(SK, alpha)

The VRF_hash algorithm is deterministic, in
the sense that it always produces 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
pi = VRF_prove(SK, alpha)

The VRFs defined in this document allow anyone to deterministically
obtain the VRF hash output beta directly from the proof value pi as
beta = VRF_proof2hash(pi)

Notice that this means that
VRF_hash(SK, alpha) = VRF_proof2hash(VRF_prove(SK, alpha))

The proof pi allows a Verifier 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
VRF_verify(PK, alpha, pi)

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 means 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. Uniqueness must hold
even for an adversarial Prover that knows the VRF secret key SK.
"Full uniqueness" states that a computationally-bounded adversary cannot
choose
a VRF public key PK,
a VRF 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 or with bad randomness.
Like any cryprographic hash function, VRFs need to be
collision resistant. Collison resistance must hold
even for an adversarial Prover that knows the VRF secret key SK.
More percisely, "full collision resistance" states that
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.
Pseudorandomness ensures that when an adversarial Verifier sees
a VRF hash output beta without its corresponding VRF proof pi,
then beta is indistinguishable from a random value.
More percisely, 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 the Prover, or to any other party that knows the private
VRF key SK! Such a party can easily distinguish beta from
a random value by comparing beta 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).
Also, 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
generated with bad randomness.)
[TODO: The following property is not needed for applications
that use VRFs to prevent enumeration of hash-based data structures.
However, we noticed that some other applications of VRF rely on
this property. As we have not yet found a formal definition
of this property in the literature, we write it down here. ]
Pseudorandomness, as defined in , does not
hold if the VRF keys were generated adversarially.
There is, however, a different type of pseudorandomness that could hold
even if the VRF keys are generated adversarially, as long as
the VRF input alpha is unpredictable. Suppose
the VRF keys are generated by an adversary. Then, a VRF hash output
beta should look pseudorandom to the adversary as long as (1) its corresponding
VRF hash alpha is chosen randomly and independently of the VRF key, (2) alpha
is unknown to the adversary, (3) the corresponding proof pi is unknown
to the adversary, and (4) the VRF public key chosen by the adversary is valid.
[TODO: It should be possible to get the EC-VRF to satisfy this property,
as long as verifiers run an VRF_validate_key() key function upon receipt
of VRF public keys. However, we need to work out exactly what properties are needed from
the VRF public keys in order for this property to hold. Some
additional checks might need to be added to the ECVRF_validate_key() function.
Need to work out what are these checks.]
The RSA Full Domain Hash VRF (RSA-FDH-VRF) is a VRF that satisfies
the "trusted uniqueness", "trusted
collision resistance", and "full pseudorandomness" properties defined in .
Its security follows from the
standard RSA assumption in the random oracle model. Formal
security proofs are in .
The VRF computes the proof pi as a deterministic RSA signature on
input alpha using the RSA Full Domain Hash Algorithm
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 .
In this document, the notation from is used.
Parameters used:
(n, e) - RSA public key
K - RSA private key
k - length in octets of the RSA modulus n

Fixed options:
Hash - cryptographic hash function
hLen - output length in octets of hash function Hash

Constraints on options:
Cryptographic security of Hash is at least as high as the cryptographic security level of the RSA key

Primitives used:
I2OSP - Coversion of a nonnegative integer to an octet string as defined in
Section 4.1 of
OS2IP - Coversion of an octet string to a nonnegative integer as defined in
Section 4.2 of
RSASP1 - RSA signature primitive as defined in
Section 5.2.1 of
RSAVP1 - RSA verification primitive as defined in
Section 5.2.2 of
MGF1 - Mask Generation Function based on a hash function as defined in
Section B.2.1 of

RSAFDHVRF_prove(K, alpha)
Input:
K - RSA private key
alpha - VRF hash input, an octet string

Output:
pi - proof, an octet string of length k

Steps:
EM = MGF1(alpha, k - 1)
m = OS2IP(EM)
s = RSASP1(K, m)
pi = I2OSP(s, k)
Output pi

RSAFDHVRF_proof2hash(pi)
Input:
pi - proof, an octet string of length k

Output:
beta - VRF hash output, an octet string of length hLen

Steps:
beta = Hash(pi)
Output beta

RSAFDHVRF_verify((n, e), alpha, pi)
Input:
(n, e) - RSA public key
alpha - VRF hash input, an octet string
pi - proof to be verified, an octet string of length n

Output:
"VALID" or "INVALID"

Steps:
s = OS2IP(pi)
m = RSAVP1((n, e), s)
EM = I2OSP(m, k - 1)
EM' = MGF1(alpha, k - 1)
If EM and EM' are equal, output "VALID";
else output "INVALID".

The Elliptic Curve Verifiable Random Function (EC-VRF) is a VRF that
satisfies the trusted uniqueness, trusted collision resistance,
and full pseudorandomness properties defined in .
The security of this VRF follows from the decisional
Diffie-Hellman (DDH) assumption in the random oracle model. Formal security proofs are
in .
Fixed options:
F - finite field
2n - length, in octets, of a field element in F
E - elliptic curve (EC) defined over F
m - length, in octets, of an EC point encoded as an octet string
G - subgroup of E of large prime order
q - prime order of group G
cofactor - number of points on E divided by q
g - generator of group G
Hash - cryptographic hash function
hLen - output length in octets of Hash

Constraints on options:
Field elements in F have bit lengths divisible by 16
hLen is equal to 2n

Parameters used:
y = g^x - VRF public key, an EC point
x - VRF private key, an integer where 0 < x < q
check this with leo

Notation and primitives used:
p^k - when p is an EC point: point multiplication, i.e. k repetitions of group operation on EC point p. when p is an integer: exponentiation
|| - octet string concatenation
I2OSP - nonnegative integer conversion to octet string as defined in
Section 4.1 of
OS2IP - Coversion of an octet string to a nonnegative integer as defined in
Section 4.2 of
EC2OSP - conversion of EC point to an m-octet string
as specified in
OS2ECP - conversion of an m-octet string to EC point
as specified in .
OS2ECP returns INVALID if the octet string does not convert to a valid EC point.
RS2ECP - conversion of a random 2n-octet string to an
EC point as specified in

Note: this function is made more efficient by taking in the public
VRF key y, as well as the private VRF key x.
ECVRF_prove(y, x, alpha)
Input:
y - public key, an EC point
x - private key, an integer
alpha - VRF input, an octet string

Output:
pi - VRF proof, octet string of length m+3n

Steps:
h = ECVRF_hash_to_curve(y, alpha)
gamma = h^x
choose a random integer nonce k from [0, q-1]
c = ECVRF_hash_points(g, h, y, gamma, g^k, h^k)
s = k - c*x mod q (where * denotes integer multiplication)
pi = EC2OSP(gamma) || I2OSP(c, n) || I2OSP(s, 2n)
Output pi

ECVRF_proof2hash(pi)
Input:
pi - VRF proof, octet string of length m+3n

Output:
"INVALID", or
beta - VRF hash output, octet string of length 2n

Steps:
D = ECVRF_decode_proof(pi)
If D is "INVALID", output "INVALID" and stop
(gamma, c, s) = D
beta = Hash(EC2OSP(gamma^cofactor))
Output beta

ECVRF_verify(y, pi, alpha)
Input:
y - public key, an EC point
pi - VRF proof, octet string of length 5n+1
alpha - VRF input, octet string

Output:
"VALID" or "INVALID"

Steps:
D = ECVRF_decode_proof(pi)
If D is "INVALID", output "INVALID" and stop
(gamma, c, s) = D
u = y^c * g^s (where * denotes EC point addition, i.e. a group operation on two EC points)
h = ECVRF_hash_to_curve(y, alpha)
v = gamma^c * h^s (where * denotes EC point addition)
c' = ECVRF_hash_points(g, h, y, gamma, u, v)
If c and c' are equal, output "VALID";
else output "INVALID"

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(y, alpha) algorithm
implements ECVRF_hash_to_curve in a simple and
generic way that works for any elliptic curve.
The running time of this algorithm depends on alpha.
For the ciphersuites specified
in , this algorithm
is expected to find a valid curve point after approximately two attempts
(i.e., when ctr=1) on average. See also .
However, because the running time of algorithm depends on alpha,
this algorithm SHOULD be avoided in
applications where it is important that
the VRF input alpha remain secret.
ECVRF_hash_to_curve1(y, alpha)
Input:
alpha - value to be hashed, an octet string
y - public key, an EC point

Output:
h - hashed value, a finite EC point in G

Steps:
ctr = 0
pk = EC2OSP(y)
h = "INVALID"
While h is "INVALID" or h is EC point at infinity:
CTR = I2OSP(ctr, 4)
ctr = ctr + 1
attempted_hash = Hash(pk || alpha || CTR)
h = RS2ECP(attempted_hash)
If h is not "INVALID" and cofactor > 1, set h = h^cofactor

Output h

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
. Note also for the Ed25519 curve
(but not the P256 curve), the Elligator algorithm
could be used here.]
ECVRF_hash_points(p_1, p_2, ..., p_j)
Input:
p_i - EC point in G

Output:
h - hash value, integer between 0 and 2^(8n)-1

Steps:
P = empty octet string
for p_i in [p_1, p_2, ... p_j]:
P = P || EC2OSP(p_i)
h1 = Hash(P)
h2 = first n octets of h1
h = OS2IP(h2)
Output h

ECVRF_decode_proof(pi)
Input:
pi - VRF proof, octet string (m+3n octets)

Output:
"INVALID", or
gamma - EC point
c - integer between 0 and 2^(8n)-1
s - integer between 0 and 2^(16n)-1

Steps:
let gamma', c', s' be pi split after m-th and m+n-th octet
gamma = OS2ECP(gamma')
if gamma = "INVALID" output "INVALID" and stop.
c = OS2IP(c')
s = OS2IP(s')
Output gamma, c, and s

This document defines EC-VRF-P256-SHA256 as follows:
The EC group G is the NIST-P256 elliptic curve, with curve parameters
as specified in (Section D.1.2.3)
and (Section 2.6). For this group,
2n = 32 and cofactor = 1.
The key pair generation primitive is specified in
Section 3.2.1 of .
EC2OSP is specified in Section 2.3.3 of with point compression on.
This implies m = 2n + 1 = 33.
OS2ECP is specified in Section 2.3.4 of .
RS2ECP(h) = OS2ECP(0x02 || h). The input h is a 32-octet string
and the output is either an EC point or "INVALID".
The hash function Hash is SHA-256 as specified in .
The ECVRF_hash_to_curve function is as specified in .

This document defines EC-VRF-ED25519-SHA256 as follows:
The EC group G is the Ed25519
elliptic curve with parameters defined in Table 1 of
.
For this group, 2n = 32 and cofactor = 8.
The key pair generation primitive is specified in Section 5.1.5
of
EC2OSP is specified in Section 5.1.2 of . This implies m = 2n = 32.
OS2ECP is specified in Section 5.1.3 of .
RS2ECP is equivalent to OS2ECP.
The hash function Hash is SHA-256 as specified in .
The ECVRF_hash_to_curve function is as specified in .

[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?]
[NOTE: In the unlikely case that future versions of this spec
use a elliptic curve group G that does not also come with a specification
of the group generator g, then we can still have full uniqueness and full
collision resistance by adding an check to ECVRF_validate_key(PK) that
ensures that g is a point on the elliptic curve and g^cofactor is not
the EC point at infinity.]
The EC-VRF as specified above is a VRF that satisfies the
"trusted uniqueness", "trusted collision resistance", and
"full pseudorandomness" properties defined in .
If the elliptic curve parameters (including the generator g) are trusted,
but the VRF public key PK is not trusted, this VRF can be modified
to additionally satisfy "full uniqueness",
and "full collision resistance". This is done by
additionally requiring the Verifier to perform
the following validation procedure upon receipt of the public
VRF key.
The Verifier MUST perform this validation procedure when the
entity that generated the public VRF key is untrusted.
The public key MUST NOT be used if this procedure returns "INVALID".
Note well that this procedure is not sufficient if the elliptic curve E
or if g, the generator of group G, is untrusted.
This procedure supposes that the public key provided to the Verifier is an octet
string. The procedure returns "INVALID" if the public key in invalid.
Otherwise, it returns y, the public key as an EC point.
ECVRF_validate_key(PK)
Input:
PK - public key, an octet string

Output:
"INVALID", or
y - public key, an EC point

Steps:
y = OS2ECP(PK)
If y is "INVALID", output "INVALID" and stop
If y^cofactor is the EC point at infinty, output "INVALID" and stop
Output y

An implementation of the RSA-FDH-VRF (SHA-256) and EC-VRF-P256-SHA256 was
first developed
as a part of the NSEC5 project and is available
at .
The EC-VRF implementation may be out of date as this spec has evolved.
The Key Transparency project at Google
uses a VRF implemention that is similar to
the EC-VRF-P256-SHA256, with a few minor changes
including the use of SHA-512 instead of SHA-256. Its implementation
is available
An implementation by Yahoo! similar to the EC-VRF is available at
.
An implementation similar to EC-VRF is available as part of the
CONIKS implementation in Golang at
.
Open Whisper Systems also uses a VRF very similar to
EC-VRF-ED25519-SHA512-Elligator, called VXEdDSA, and specified here:
Applications that use the VRFs defined in this
document MUST ensure that that the VRF key is generated correctly,
using good randomness.
The EC-VRF as specified in -
statisfies the "trusted uniqueness" and "trusted collision resistance" properties
as long as the VRF keys are generated correctly, with good randomness.
If the Verifier trusts the VRF keys are generated correctly, it MAY use
the public key y as is.
However, if the EC-VRF uses keys that could be generated adversarially, then the
the Verfier MUST first perform the validation procedure ECVRF_validate_key(PK)
(specified in ) upon receipt of the
public key PK as an octet string. If the validation procedure
outputs "INVALID", then the public key MUST not be used.
Otherwise, the procedure will output a valid public key y,
and the EC-VRF with public key y satisfies the "full uniqueness" and
"full collision resistance" properties.
The RSA-FDH-VRF statisfies the "trusted uniqueness" and "trusted collision resistance" properties
as long as the VRF keys are generated correctly, with good randomness.
These properties may not hold if the keys are generated adversarially
(e.g., if RSA is not permutation). Meanwhile,
the "full uniqueness" and "full collision resistance" are
properties that hold even if VRF keys are generated by an adversary.
The RSA-FDH-VRF defined in this document does not have these properties.
However, if adversarial key generation is a concern, the
RSA-FDH-VRF may be modifed to have these
properties by adding additional cryptographic checks
that its public key has the right form. These modifications are left for future specification.
Without good randomness, the "pseudorandomness"
properties of the VRF may not hold. Note that 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.
presents cryptographic reductions to an
underlying hard problem (e.g. Decisional Diffie Hellman for the EC-VRF, or the
standard RSA assumption for RSA-FDH-VRF) 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 therefore have two options.
They may choose to ensure that selective pseudorandomness is sufficient for
the application. That is, that
pseudorandomness of outputs matters only for inputs that are chosen
independently of the VRF key.
If full pseudorandomness is required for the application, the application
may increase
security parameters to make up for the loose security reduction.
For RSA-FDH-VRF, this means increasing the RSA key length. For
EC-VRF, this means increasing the cryptographic strength of the EC group
G. For both RSA-FDH-VRF and EC-VRF the cryptographic strength of the
hash function Hash may also potentially need to be increased.

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
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.
That said, for most inputs the amount of information obtained from
such a timing attack is likely to be small (1 bit, on average), since the algorithm
is expected to find a valid curve point after only two attempts.
However, there might be inputs which cause the algorithm to make many attempts
before it finds a valid curve point; for such inputs, the information leaked
in a timing attack will be more than 1 bit.
Note to RFC Editor: if this document does not obsolete an existing RFC,
please remove this appendix before publication as an RFC.
00 - Forked this document from draft-vcelak-nsec5-04.
Cleaned up the definitions of VRF algorithms.
Added security definitions for VRF and security considerations.
Parameterized EC-VRF so it could support curves other than
P-256 and Ed25519.
01 - Fixed ECVRF to work when cofactor > 1.
Changed ECVRF_proof2hash(pi) so that it outputs a value raised
to the cofactor and then processed by the cryptographic hash function Hash.
Included the VRF public key y as input to the hash function
ECVRF_hash_to_curve1.
Cleaned up ciphersuites and ECVRF description so that it works with
EC point encodings for both P256 and Ed25519 curves.
Added ECVRF_validate_key so that EC-VRF can satisfy "full
uniqueness" and "full collision" resistance.
Updated implementation status.
Added "an additional pseudorandomness property" to security
definitions.

Leonid Reyzin (Boston University) is a major contributor 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.
Digital Signature Standard (DSS)
National Institute for Standards and Technology
SEC 1: Elliptic Curve Cryptography
Standards for Efficient Cryptography Group (SECG)
Making NSEC5 Practical for DNSSEC Deployments
Verifiable Random Functions
How to Hash into Elliptic Curves
Note to RFC Editor: please remove this appendix before publication as an RFC.
Open issue.