Verifiable Random Functions (VRFs)
draftirtfcfrgvrf07
The information below is for an old version of the document.
Document  Type 
This is an older version of an InternetDraft that was ultimately published as RFC 9381.



Authors  Sharon Goldberg , Leonid Reyzin , Dimitrios Papadopoulos , Jan Včelák  
Last updated  20200619 (Latest revision 20200618)  
Replaces  draftgoldbevrf  
RFC stream  Internet Research Task Force (IRTF)  
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IETF conflict review  conflictreviewirtfcfrgvrf, conflictreviewirtfcfrgvrf, conflictreviewirtfcfrgvrf, conflictreviewirtfcfrgvrf, conflictreviewirtfcfrgvrf, conflictreviewirtfcfrgvrf  
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draftirtfcfrgvrf07
CFRG S. Goldberg InternetDraft L. Reyzin Intended status: Standards Track Boston University Expires: December 20, 2020 D. Papadopoulos Hong Kong University of Science and Techology J. Vcelak NS1 June 18, 2020 Verifiable Random Functions (VRFs) draftirtfcfrgvrf07 Abstract A Verifiable Random Function (VRF) is the publickey 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 hashbased 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). Status of This Memo This InternetDraft is submitted in full conformance with the provisions of BCP 78 and BCP 79. InternetDrafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as InternetDrafts. The list of current Internet Drafts is at https://datatracker.ietf.org/drafts/current/. InternetDrafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use InternetDrafts as reference material or to cite them other than as "work in progress." This InternetDraft will expire on December 20, 2020. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/licenseinfo) in effect on the date of Goldberg, et al. Expires December 20, 2020 [Page 1] InternetDraft VRF June 2020 publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Requirements . . . . . . . . . . . . . . . . . . . . . . 3 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. VRF Algorithms . . . . . . . . . . . . . . . . . . . . . . . 4 3. VRF Security Properties . . . . . . . . . . . . . . . . . . . 5 3.1. Full Uniqueness or Trusted Uniqueness . . . . . . . . . . 5 3.2. Full Collison Resistance or Trusted Collision Resistance 5 3.3. Full Pseudorandomness or Selective Pseudorandomness . . . 5 3.4. A randomoraclelike unpredictability property . . . . . 6 4. RSA Full Domain Hash VRF (RSAFDHVRF) . . . . . . . . . . . 7 4.1. RSAFDHVRF Proving . . . . . . . . . . . . . . . . . . . 8 4.2. RSAFDHVRF Proof To Hash . . . . . . . . . . . . . . . . 8 4.3. RSAFDHVRF Verifying . . . . . . . . . . . . . . . . . . 9 5. Elliptic Curve VRF (ECVRF) . . . . . . . . . . . . . . . . . 10 5.1. ECVRF Proving . . . . . . . . . . . . . . . . . . . . . . 12 5.2. ECVRF Proof To Hash . . . . . . . . . . . . . . . . . . . 12 5.3. ECVRF Verifying . . . . . . . . . . . . . . . . . . . . . 13 5.4. ECVRF Auxiliary Functions . . . . . . . . . . . . . . . . 14 5.4.1. ECVRF Hash To Curve . . . . . . . . . . . . . . . . . 14 5.4.2. ECVRF Nonce Generation . . . . . . . . . . . . . . . 17 5.4.3. ECVRF Hash Points . . . . . . . . . . . . . . . . . . 18 5.4.4. ECVRF Decode Proof . . . . . . . . . . . . . . . . . 19 5.5. ECVRF Ciphersuites . . . . . . . . . . . . . . . . . . . 19 5.6. When the ECVRF Keys are Untrusted . . . . . . . . . . . . 22 5.6.1. ECVRF Validate Key . . . . . . . . . . . . . . . . . 22 6. Implementation Status . . . . . . . . . . . . . . . . . . . . 24 7. Security Considerations . . . . . . . . . . . . . . . . . . . 25 7.1. Key Generation . . . . . . . . . . . . . . . . . . . . . 25 7.1.1. Uniqueness and collision resistance with untrusted keys . . . . . . . . . . . . . . . . . . . . . . . . 25 7.1.2. Pseudorandomness with untrusted keys . . . . . . . . 26 7.2. Selective vs Full Pseudorandomness . . . . . . . . . . . 26 7.3. Proper pseudorandom nonce for ECVRF . . . . . . . . . . . 27 7.4. Sidechannel attacks . . . . . . . . . . . . . . . . . . 27 7.5. Proofs Provide No Secrecy for VRF Input . . . . . . . . . 27 7.6. Prehashing . . . . . . . . . . . . . . . . . . . . . . . 28 7.7. Hash function domain separation and futureproofing . . . 28 8. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 29 Goldberg, et al. Expires December 20, 2020 [Page 2] InternetDraft VRF June 2020 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 30 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 30 10.1. Normative References . . . . . . . . . . . . . . . . . . 30 10.2. Informative References . . . . . . . . . . . . . . . . . 32 Appendix A. Test Vectors for the ECVRFs . . . . . . . . . . . . 33 A.1. ECVRFP256SHA256TAI . . . . . . . . . . . . . . . . . . 33 A.2. ECVRFP256SHA256SSWU . . . . . . . . . . . . . . . . . 34 A.3. ECVRFEDWARDS25519SHA512TAI . . . . . . . . . . . . . . 36 A.4. ECVRFEDWARDS25519SHA512ELL2 . . . . . . . . . . . . . 37 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39 1. Introduction 1.1. Rationale A Verifiable Random Function (VRF) [MRV99] is the publickey 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 hashbased data structure. In this application, a Prover holds the VRF private key and uses the VRF hashing to construct a hashbased 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. 1.2. Requirements 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]. 1.3. Terminology The following terminology is used through this document: SK: The private key for the VRF. PK: The public key for the VRF. alpha or alpha_string: The input to be hashed by the VRF. beta or beta_string: The VRF hash output. Goldberg, et al. Expires December 20, 2020 [Page 3] InternetDraft VRF June 2020 pi or pi_string: The VRF proof. Prover: The Prover holds the private VRF key SK and public VRF key PK. Verifier: The Verifier holds the public VRF key PK. 2. VRF Algorithms A VRF comes with a key generation algorithm that generates a public VRF key PK and private VRF key SK. The prover 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 prover also uses the private key SK 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_proof_to_hash(pi) Notice that this means that VRF_hash(SK, alpha) = VRF_proof_to_hash(VRF_prove(SK, alpha)) and thus this document will specify VRF_prove and VRF_proof_to_hash rather than VRF_hash. 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, beta = VRF_proof_to_hash(pi)) if pi is valid, and INVALID otherwise. Goldberg, et al. Expires December 20, 2020 [Page 4] InternetDraft VRF June 2020 3. VRF Security Properties VRFs are designed to ensure the following security properties. 3.1. Full Uniqueness or Trusted Uniqueness 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 private key SK. More precisely, "full uniqueness" states that a computationally bounded adversary cannot choose a VRF public key PK, a VRF input alpha, and two proofs pi1 and pi2 such that VRF_verify(PK, alpha, pi1) outputs (VALID, beta1), VRF_verify(PK, alpha, pi2) outputs (VALID, beta2), and beta1 is not equal to beta2. A slightly weaker security property called "trusted uniqueness" 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 other words, uniqueness might not hold if keys were generated in an invalid manner or with bad randomness. 3.2. Full Collison Resistance or Trusted Collision Resistance Like any cryprographic hash function, VRFs need to be collision resistant. Collison resistance must hold even for an adversarial Prover that knows the VRF private key SK. More precisely, "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 private 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. 3.3. Full Pseudorandomness or Selective Pseudorandomness 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 precisely, suppose the public and private VRF keys (PK, SK) were generated in a trustworthy manner. Pseudorandomness ensures that the Goldberg, et al. Expires December 20, 2020 [Page 5] InternetDraft VRF June 2020 VRF hash output beta (without its corresponding VRF proof pi) on any adversariallychosen "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, beta). 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.) 3.4. A randomoraclelike unpredictability property Pseudorandomness, as defined in Section 3.3, does not hold if the VRF keys were generated adversarially. For instance, if an adversary outputs VRF keys that are deterministically generated (or hardcoded and publicly known), then the outputs are easily derived by anyone. There is, however, a different type of unpredictability that is desirable in certain VRF applications (such as [GHMVZ17] and [DGKR18]). This property is similar to the unpredictability achieved by an (ordinary, unkeyed) cryptographic hash function: if the input has enough entropy (i.e., cannot be predicted), then the correct output is indistinguishable from uniform. A formal definition of this property appears in Section 3.2 of [DGKR18]. The VRF schemes presented in this specification are Goldberg, et al. Expires December 20, 2020 [Page 6] InternetDraft VRF June 2020 believed to satisfy this property if the public key was generated in a trustworthy manner. Additionally, the ECVRF is believed to also satisify this property even if the public key was not generated in a trustworthy manner, as long as the public key satisfies the key validation procedure in Section 5.6. 4. RSA Full Domain Hash VRF (RSAFDHVRF) The RSA Full Domain Hash VRF (RSAFDHVRF) is a VRF that satisfies the "trusted uniqueness", "trusted collision resistance", and "full pseudorandomness" properties defined in Section 3. Its security follows from the standard RSA assumption in the random oracle model. Formal security proofs are in [PWHVNRG17]. 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 RSAFDHVRF 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. Parameters used: (n, e)  RSA public key K  RSA private key k  length in octets of the RSA modulus n (k must be less than 2^32) Fixed options: Hash  cryptographic hash function hLen  output length in octets of hash function Hash Primitives used: I2OSP  Conversion of a nonnegative integer to an octet string as defined in Section 4.1 of [RFC8017] OS2IP  Conversion of an octet string to a nonnegative integer as defined in Section 4.2 of [RFC8017] Goldberg, et al. Expires December 20, 2020 [Page 7] InternetDraft VRF June 2020 RSASP1  RSA signature primitive as defined in Section 5.2.1 of [RFC8017] RSAVP1  RSA verification primitive as defined in Section 5.2.2 of [RFC8017] MGF1  Mask Generation Function based on the hash function Hash as defined in Section B.2.1 of [RFC8017]   octet string concatenation 4.1. RSAFDHVRF Proving RSAFDHVRF_prove(K, alpha_string) Input: K  RSA private key alpha_string  VRF hash input, an octet string Output: pi_string  proof, an octet string of length k Steps: 1. one_string = 0x01 = I2OSP(1, 1), a single octet with value 1 2. EM = MGF1(one_string  I2OSP(k, 4)  I2OSP(n, k)  alpha_string, k  1) 3. m = OS2IP(EM) 4. s = RSASP1(K, m) 5. pi_string = I2OSP(s, k) 6. Output pi_string 4.2. RSAFDHVRF Proof To Hash RSAFDHVRF_proof_to_hash(pi_string) Input: pi_string  proof, an octet string of length k Goldberg, et al. Expires December 20, 2020 [Page 8] InternetDraft VRF June 2020 Output: beta_string  VRF hash output, an octet string of length hLen Important note: RSAFDHVRF_proof_to_hash should be run only on pi_string that is known to have been produced by RSAFDHVRF_prove, or from within RSAFDHVRF_verify as specified in Section 4.3. Steps: 1. two_string = 0x02 = I2OSP(2, 1), a single octet with value 2 2. beta_string = Hash(two_string  pi_string) 3. Output beta_string 4.3. RSAFDHVRF Verifying RSAFDHVRF_verify((n, e), alpha_string, pi_string) Input: (n, e)  RSA public key alpha_string  VRF hash input, an octet string pi_string  proof to be verified, an octet string of length n Output: ("VALID", beta_string), where beta_string is the VRF hash output, an octet string of length hLen; or "INVALID" Steps: 1. s = OS2IP(pi_string) 2. m = RSAVP1((n, e), s) 3. EM = I2OSP(m, k  1) 4. one_string = 0x01 = I2OSP(1, 1), a single octet with value 1 5. EM' = MGF1(one_string  I2OSP(k, 4)  I2OSP(n, k)  alpha_string, k  1) Goldberg, et al. Expires December 20, 2020 [Page 9] InternetDraft VRF June 2020 6. If EM and EM' are equal, output ("VALID", RSAFDHVRF_proof_to_hash(pi_string)); else output "INVALID". 5. Elliptic Curve VRF (ECVRF) The Elliptic Curve Verifiable Random Function (ECVRF) is a VRF that satisfies the trusted uniqueness, trusted collision resistance, and full pseudorandomness properties defined in Section 3. The security of this VRF follows from the decisional DiffieHellman (DDH) assumption in the random oracle model. Formal security proofs are in [PWHVNRG17]. To additionally satisfy "full uniqueness" and "full collision resistance", the Verifier MUST additionally perform the validation procedure specified in Section 5.6 upon receipt of the public VRF key. Notation used: Elliptic curve operations are written in additive notation, with P+Q denoting point addition and x*P denoting scalar multiplication of a point P by a scalar x x^y  x raised to the power y x*y  x multiplied by y s  t  concatenation of octet strings s and t Fixed options (specified in Section 5.5): F  finite field 2n  length, in octets, of a field element in F, rounded up to the nearest even integer E  elliptic curve (EC) defined over F ptLen  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 qLen  length of q in octets, i.e., smallest integer such that 2^(8qLen)>q (note that in the typical case, qLen equals 2n or is close to 2n) Goldberg, et al. Expires December 20, 2020 [Page 10] InternetDraft VRF June 2020 cofactor  number of points on E divided by q B  generator of group G Hash  cryptographic hash function hLen  output length in octets of Hash; must be at least 2n ECVRF_hash_to_curve  a function that hashes strings to an EC point. ECVRF_nonce_generation  a function that derives a pseudorandom nonce from SK and the input as part of ECVRF proving. suite_string  a single nonzero octet specifying the ECVRF ciphersuite, which determines the above options as well as type conversions and parameter generation Type conversions (specified in Section 5.5): int_to_string(a, len)  conversion of nonnegative integer a to to octet string of length len string_to_int(a_string)  conversion of an octet string a_string to a nonnegative integer point_to_string  conversion of EC point to an ptLenoctet string string_to_point  conversion of an ptLenoctet string to EC point. string_to_point returns INVALID if the octet string does not convert to a valid EC point. Note that with certain software libraries (for big integer and elliptic curve arithmetic), the int_to_string and point_to_string conversions are not needed. For example, in some implementations, EC point operations will take octet strings as inputs and produce octet strings as outputs, without introducing a separate elliptic curve point type. Parameters used (the generation of these parameters is specified in Section 5.5): SK  VRF private key x  VRF secret scalar, an integer Note: depending on the ciphersuite used, the VRF secret scalar may be equal to SK; else, it is derived from SK Goldberg, et al. Expires December 20, 2020 [Page 11] InternetDraft VRF June 2020 Y = x*B  VRF public key, an EC point 5.1. ECVRF Proving ECVRF_prove(SK, alpha_string) Input: SK  VRF private key alpha_string = input alpha, an octet string Output: pi_string  VRF proof, octet string of length ptLen+n+qLen Steps: 1. Use SK to derive the VRF secret scalar x and the VRF public key Y = x*B (this derivation depends on the ciphersuite, as per Section 5.5; these values can be cached, for example, after key generation, and need not be rederived each time) 2. H = ECVRF_hash_to_curve(Y, alpha_string) 3. h_string = point_to_string(H) 4. Gamma = x*H 5. k = ECVRF_nonce_generation(SK, h_string) 6. c = ECVRF_hash_points(H, Gamma, k*B, k*H) (see Section 5.4.3) 7. s = (k + c*x) mod q 8. pi_string = point_to_string(Gamma)  int_to_string(c, n)  int_to_string(s, qLen) 9. Output pi_string 5.2. ECVRF Proof To Hash ECVRF_proof_to_hash(pi_string) Input: pi_string  VRF proof, octet string of length ptLen+n+qLen Goldberg, et al. Expires December 20, 2020 [Page 12] InternetDraft VRF June 2020 Output: "INVALID", or beta_string  VRF hash output, octet string of length hLen Important note: ECVRF_proof_to_hash should be run only on pi_string that is known to have been produced by ECVRF_prove, or from within ECVRF_verify as specified in Section 5.3. Steps: 1. D = ECVRF_decode_proof(pi_string) (see Section 5.4.4) 2. If D is "INVALID", output "INVALID" and stop 3. (Gamma, c, s) = D 4. three_string = 0x03 = int_to_string(3, 1), a single octet with value 3 5. zero_string = 0x00 = int_to_string(0, 1), a single octet with value 0 6. beta_string = Hash(suite_string  three_string  point_to_string(cofactor * Gamma)  zero_string) 7. Output beta_string 5.3. ECVRF Verifying ECVRF_verify(Y, pi_string, alpha_string) Input: Y  public key, an EC point pi_string  VRF proof, octet string of length ptLen+n+qLen alpha_string  VRF input, octet string Output: ("VALID", beta_string), where beta_string is the VRF hash output, octet string of length hLen; or "INVALID" Goldberg, et al. Expires December 20, 2020 [Page 13] InternetDraft VRF June 2020 Steps: 1. D = ECVRF_decode_proof(pi_string) (see Section 5.4.4) 2. If D is "INVALID", output "INVALID" and stop 3. (Gamma, c, s) = D 4. H = ECVRF_hash_to_curve(Y, alpha_string) 5. U = s*B  c*Y 6. V = s*H  c*Gamma 7. c' = ECVRF_hash_points(H, Gamma, U, V) (see Section 5.4.3) 8. If c and c' are equal, output ("VALID", ECVRF_proof_to_hash(pi_string)); else output "INVALID" 5.4. ECVRF Auxiliary Functions 5.4.1. ECVRF Hash To Curve The ECVRF_hash_to_curve algorithm takes in the VRF input alpha and converts it to H, an EC point in G. This algorithm is the only place the VRF input alpha is used for proving and verfying. See Section 7.6 for further discussion. This section specifies a number of such algorithms, which are not compatible with each other. The choice of a particular algorithm from the options specified in this section is made in Section 5.5. 5.4.1.1. ECVRF_hash_to_curve_try_and_increment The following ECVRF_hash_to_curve_try_and_increment(Y, alpha_string) 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_string. For the ciphersuites specified in Section 5.5, this algorithm is expected to find a valid curve point after approximately two attempts (i.e., when ctr=1) on average. However, because the running time of algorithm depends on alpha_string, this algorithm SHOULD be avoided in applications where it is important that the VRF input alpha remain secret. ECVRF_hash_to_try_and_increment(Y, alpha_string) Goldberg, et al. Expires December 20, 2020 [Page 14] InternetDraft VRF June 2020 Input: Y  public key, an EC point alpha_string  value to be hashed, an octet string Output: H  hashed value, a finite EC point in G Fixed option (specified in Section 5.5): arbitrary_string_to_point  conversion of an arbitrary octet string to an EC point. Steps: 1. ctr = 0 2. PK_string = point_to_string(Y) 3. one_string = 0x01 = int_to_string(1, 1), a single octet with value 1 4. zero_string = 0x00 = int_to_string(0, 1), a single octet with value 0 5. H = "INVALID" 6. While H is "INVALID" or H is EC point at infinity: A. ctr_string = int_to_string(ctr, 1) B. hash_string = Hash(suite_string  one_string  PK_string  alpha_string  ctr_string  zero_string) C. H = arbitrary_string_to_point(hash_string) D. If H is not "INVALID" and cofactor > 1, set H = cofactor * H E. ctr = ctr + 1 7. Output H Goldberg, et al. Expires December 20, 2020 [Page 15] InternetDraft VRF June 2020 5.4.1.2. ECVRF_hash_to_curve_h2c_suite The ECVRF_hash_to_curve_h2c_suite(Y, alpha_string) algorithm implements ECVRF_hash_to_curve using one of the several hashtocurve options defined in [ID.irtfcfrghashtocurve]. The specific choice of the hashtocurve option (called Suite ID in [ID.irtfcfrghashtocurve]) is given by the h2c_suite_ID_string parameter. ECVRF_hash_to_curve_h2c_suite(Y, alpha_string) Input: alpha_string  value to be hashed, an octet string Y  public key, an EC point Output: H  hashed value, a finite EC point in G Fixed option (specified in Section 5.5): h2c_suite_ID_string  a hashtocurve suite ID, encoded in ASCII (see discussion below) Steps 1. PK_string = point_to_string(Y) 2. string_to_hash = PK_string  alpha_string 3. H = encode(string_to_hash) (the encode function is discussed below) 4. Output H The encode function is provided by the hashtocurve suite whose ID is h2c_suite_ID_string, as specified in [ID.irtfcfrghashtocurve], Section 8. The domain separation tag DST, a parameter to the hashtocurve suite, SHALL be set to "ECVRF_"  h2c_suite_ID_string  suite_string where "ECVRF_" is represented as a 6byte ASCII encoding (in hexadecimal, octets 45 43 56 52 46 5F). Goldberg, et al. Expires December 20, 2020 [Page 16] InternetDraft VRF June 2020 5.4.2. ECVRF Nonce Generation The following algorithms generate the nonce value k in a deterministic pseudorandom fashion. This section specifies a number of such algorithms, which are not compatible with each other. The choice of a particular algorithm from the options specified in this section is made in Section 5.5. 5.4.2.1. ECVRF Nonce Generation From RFC 6979 ECVRF_nonce_generation_RFC6979(SK, h_string) Input: SK  an ECVRF secret key h_string  an octet string Output: k  an integer between 1 and q1 The ECVRF_nonce_generation function is as specified in [RFC6979] Section 3.2 where Input m is set equal to h_string The "suitable for DSA or ECDSA" check in step h.3 is omitted The hash function H is Hash and its output length hlen is set as hLen*8 The secret key x is set equal to the VRF secret scalar x The prime q is the same as in this specification qlen is the binary length of q, i.e., the smallest integer such that 2^qlen > q All the other values and primitives as defined in [RFC6979] 5.4.2.2. ECVRF Nonce Generation From RFC 8032 The following is from Steps 23 of Section 5.1.6 in [RFC8032]. ECVRF_nonce_generation_RFC8032(SK, h_string) Input: Goldberg, et al. Expires December 20, 2020 [Page 17] InternetDraft VRF June 2020 SK  an ECVRF secret key h_string  an octet string Output: k  an integer between 0 and q1 Steps: 1. hashed_sk_string = Hash(SK) 2. truncated_hashed_sk_string = hashed_sk_string[32]...hashed_sk_string[63] 3. k_string = Hash(truncated_hashed_sk_string  h_string) 4. k = string_to_int(k_string) mod q 5.4.3. ECVRF Hash Points ECVRF_hash_points(P1, P2, ..., PM) Input: P1...PM  EC points in G Output: c  hash value, integer between 0 and 2^(8n)1 Steps: 1. two_string = 0x02 = int_to_string(2, 1), a single octet with value 2 2. Initialize str = suite_string  two_string 3. for PJ in [P1, P2, ... PM]: str = str  point_to_string(PJ) 4. zero_string = 0x00 = int_to_string(0, 1), a single octet with value 0 5. str = str  zero_string 6. c_string = Hash(str) Goldberg, et al. Expires December 20, 2020 [Page 18] InternetDraft VRF June 2020 7. truncated_c_string = c_string[0]...c_string[n1] 8. c = string_to_int(truncated_c_string) 9. Output c 5.4.4. ECVRF Decode Proof ECVRF_decode_proof(pi_string) Input: pi_string  VRF proof, octet string (ptLen+n+qLen octets) Output: "INVALID", or Gamma  EC point c  integer between 0 and 2^(8n)1 s  integer between 0 and 2^(8qLen)1 Steps: 1. let gamma_string = pi_string[0]...p_string[ptLen1] 2. let c_string = pi_string[ptLen]...pi_string[ptLen+n1] 3. let s_string =pi_string[ptLen+n]...pi_string[ptLen+n+qLen1] 4. Gamma = string_to_point(gamma_string) 5. if Gamma = "INVALID" output "INVALID" and stop. 6. c = string_to_int(c_string) 7. s = string_to_int(s_string) 8. Output Gamma, c, and s 5.5. ECVRF Ciphersuites This document defines ECVRFP256SHA256TAI as follows: o suite_string = 0x01 = int_to_string(1, 1), a single octet with value 1. Goldberg, et al. Expires December 20, 2020 [Page 19] InternetDraft VRF June 2020 o The EC group G is the NIST P256 elliptic curve, with curve parameters as specified in [FIPS1864] (Section D.1.2.3) and [RFC5114] (Section 2.6). For this group, 2n = qLen = 32 and cofactor = 1. o The key pair generation primitive is specified in Section 3.2.1 of [SECG1] (q, B, SK, and PK in this document correspond to in n, G, d, and Q in Section 3.2.1 of [SECG1]). In this ciphersuite, the secret scalar x is equal to the private key SK. o The ECVRF_nonce_generation function is as specified in Section 5.4.2.1. o The int_to_string function is the I2OSP function specified in Section 4.1 of [RFC8017]. (This is big endian representation.) o The string_to_int function is the OS2IP function specified in Section 4.2 of [RFC8017]. (This is big endian representation.) o The point_to_string function converts an EC point to an octet string according to the encoding specified in Section 2.3.3 of [SECG1] with point compression on. This implies ptLen = 2n + 1 = 33. (Note that certain software implementations do not introduce a separate elliptic curve point type and instead directly treat the EC point as an octet string per above encoding. When using such an implementation, the point_to_string function can be treated as the identity function.) o The string_to_point function converts an octet string to an EC point according to the encoding specified in Section 2.3.4 of [SECG1]. This function MUST output INVALID if the octet string does not decode to an EC point. o The hash function Hash is SHA256 as specified in [RFC6234], with hLen = 32. o The ECVRF_hash_to_curve function is as specified in Section 5.4.1.1, with arbitrary_string_to_point(s) = string_to_point(0x02  s) (where 0x02 is a single octet with value 2, 0x02=int_to_string(2, 1)). The input s to arbitrary_string_to_point is a 32octet string and the output is either an EC point or "INVALID". This document defines ECVRFP256SHA256SSWU as identical to ECVRF P256SHA256TAI, except that: o suite_string = 0x02 = int_to_string(2, 1), a single octet with value 2. Goldberg, et al. Expires December 20, 2020 [Page 20] InternetDraft VRF June 2020 o the ECVRF_hash_to_curve function is as specified in Section 5.4.1.2 with h2c_suite_ID_string = P256_XMD:SHA 256_SSWU_NU_ (the suite is defined in [ID.irtfcfrghashtocurve] Section 8.1) This document defines ECVRFEDWARDS25519SHA512TAI as follows: o suite_string = 0x03 = int_to_string(3, 1), a single octet with value 3. o The EC group G is the edwards25519 elliptic curve with parameters defined in Table 1 of [RFC8032]. For this group, 2n = qLen = 32 and cofactor = 8. o The private key and generation of the secret scalar and the public key are specified in Section 5.1.5 of [RFC8032] o The ECVRF_nonce_generation function is as specified in Section 5.4.2.2. o The int_to_string function as specified in the first paragraph of Section 5.1.2 of [RFC8032]. (This is little endian representation.) o The string_to_int function interprets the string as an integer in littleendian representation. o The point_to_string function converts an EC point to an octect string according to the encoding specified in Section 5.1.2 of [RFC8032]. This implies ptLen = 2n = 32. (Note that certain software implementations do not introduce a separate elliptic curve point type and instead directly treat the EC point as an octet string per above encoding. When using such and implementation, the point_to_string function can be treated as the identity function.) o The string_to_point function converts an octet string to an EC point according to the encoding specified in Section 5.1.3 of [RFC8032]. This function MUST output INVALID if the octet string does not decode to an EC point. o The hash function Hash is SHA512 as specified in [RFC6234], with hLen = 64. o The ECVRF_hash_to_curve function is as specified in Section 5.4.1.1, with arbitrary_string_to_point(s) = string_to_point(s[0]...s[31]). Goldberg, et al. Expires December 20, 2020 [Page 21] InternetDraft VRF June 2020 This document defines ECVRFEDWARDS25519SHA512ELL2 as identical to ECVRFEDWARDS25519SHA512TAI, except: o suite_string = 0x04 = int_to_string(4, 1), a single octet with value 4. o the ECVRF_hash_to_curve function is as specified in Section 5.4.1.2 with h2c_suite_ID_string = edwards25519_XMD:SHA 512_ELL2_NU_ (the suite is defined in [ID.irtfcfrghashtocurve] Section 8.4.) 5.6. When the ECVRF Keys are Untrusted The ECVRF as specified above is a VRF that satisfies the "trusted uniqueness", "trusted collision resistance", and "full pseudorandomness" properties defined in Section 3. In order to obtain "full uniqueness" and "full collision resistance" (which provide protection against a malicious VRF public key), the Verifier MUST perform the following additional validation procedure upon receipt of the public VRF key. The public VRF key MUST NOT be used if this procedure returns "INVALID". Note that this procedure is not sufficient if the elliptic curve E or the point B, the generator of group G, is untrusted. If the prover is untrusted, the Verifier MUST obtain E and B from a trusted source, such as a ciphersuite specification, rather than from the prover. 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. 5.6.1. ECVRF Validate Key ECVRF_validate_key(PK_string) Input: PK_string  public key, an octet string Output: "INVALID", or Y  public key, an EC point Steps: Goldberg, et al. Expires December 20, 2020 [Page 22] InternetDraft VRF June 2020 1. Y = string_to_point(PK_string) 2. If Y is "INVALID", output "INVALID" and stop 3. If cofactor*Y is the EC point at infinty, output "INVALID" and stop 4. Output Y Note that if the cofactor = 1, then Step 3 need not multiply Y by the cofactor; instead, it suffices to output "INVALID" if Y is the point at infinity. Moreover, when cofactor>1, it is not necessary to verify that Y is in the subgroup G; Step 3 suffices. Therefore, if the cofactor is small, the total number of points that could cause Step 3 to output "INVALID" may be small, and it may be more efficient to simply check Y against a fixed list of such points. For example, the following algorithm can be used for the edwards25519 curve: 1. Y = string_to_point(PK_string) 2. If Y is "INVALID", output "INVALID" and stop 3. y_string = PK_string 4. oneTwentySeven_string = 0x7F = int_to_string(127, 1) (a single octet with value 127) 5. y_string[31] = y_string[31] & oneTwentySeven_string (this step clears the highorder bit of octet 31) 6. bad_pk[0] = int_to_string(0, 32) 7. bad_pk[1] = int_to_string(1, 32) 8. bad_y2 = 2707385501144840649318225287225658788936804267575313519 463743609750303402022 9. bad_pk[2] = int_to_string(bad_y2, 32) 10. bad_pk[3] = int_to_string(pbad_y2, 32) 11. bad_pk[4] = int_to_string(p1, 32) 12. bad_pk[5] = int_to_string(p, 32) 13. bad_pk[6] = int_to_string(p+1, 32) Goldberg, et al. Expires December 20, 2020 [Page 23] InternetDraft VRF June 2020 14. If y_string is in bad_pk[0]...bad_pk[6], output "INVALID" and stop 15. Output Y (bad_pk[0], bad_pk[2], bad_pk[3] each match two bad public keys, depending on the sign of the xcoordinate, which was cleared in step 5, in order to make sure that it does not affect the comparison. bad_pk[1] and bad_pk[4] each match one bad public key, because xcoordinate is 0 for these two public keys. bad_pk[5] and bad_pk[6] are simply bad_pk[0] and bad_pk[1] shifted by p, in case the ycoordinate had not been modular reduced by p. There is no need to shift the other bad_pk values by p, because they will exceed 2^255. These bad keys, which represent all points of order 1, 2, 4, and 8, have been obtained by converting the points specified in [X25519] to Edwards coordinates.) 6. Implementation Status A reference C++ implementation of ECVRFP256SHA256TAI, ECVRF P256SHA256SSWU, ECVRFEDWARDS25519SHA512TAI, and ECVRF EDWARDS25519SHA512ELL2 is available at <https://github.com/reyzin/ ecvrf>. This implementation is neither secure nor especially effecient, but can be used to generate test vectors. A Python implementation of a previous version of ECVRF EDWARDS25519SHA512ELL2 is available at <https://github.com/integritychain/draftirtfcfrgvrf05>. A C implementation of a previous version of ECVRF EDWARDS25519SHA512ELL2 is available at <https://github.com/algorand/libsodium/tree/draftirtfcfrgvrf 03/src/libsodium/crypto_vrf/ietfdraft03>. A Rust implemention of a previous version of ECVRFP256SHA256TAI, as well as variants for the sect163k1 and secp256k1 curves, is available at <https://crates.io/crates/vrf>. A C implemention of a variant of this VRF for the secp256k1 curve is available at <https://github.com/aergoio/secp256k1vrf>. An implementation of an earlier, slightly different, version of RSA FDHVRF (SHA256) and ECVRFP256SHA256TAI was first developed as a part of the NSEC5 project [ID.vcelaknsec5] and is available at <http://github.com/fcelda/nsec5crypto>. The Key Transparency project at Google uses a VRF implemention that is similar to the ECVRFP256SHA256TAI, with a few minor changes Goldberg, et al. Expires December 20, 2020 [Page 24] InternetDraft VRF June 2020 including the use of SHA512 instead of SHA256. Its implementation is available at <https://github.com/google/keytransparency/blob/master/core/vrf/ vrf.go> An implementation by Yahoo! similar to the ECVRF is available at <https://github.com/r2ishiguro/vrf>. An implementation similar to ECVRF is available as part of the CONIKS implementation in Golang at <https://github.com/conikssys/coniks go/tree/master/crypto/vrf>. Open Whisper Systems also uses a VRF very similar to ECVRF EDWARDS25519SHA512ELL2, called VXEdDSA, and specified here <https://whispersystems.org/docs/specifications/xeddsa/> and here <https://moderncrypto.org/mailarchive/curves/2017/000925.html>. Implementations in C and Java are available at <https://github.com/signalapp/curve25519java> and <https://github.com/wavesplatform/curve25519java>. 7. Security Considerations 7.1. Key Generation Applications that use the VRFs defined in this document MUST ensure that that the VRF key is generated correctly, using good randomness. 7.1.1. Uniqueness and collision resistance with untrusted keys The ECVRF as specified in Section 5.1Section 5.5 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 ECVRF uses keys that could be generated adversarially, then the the Verfier MUST first perform the validation procedure ECVRF_validate_key(PK) (specified in Section 5.6) 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 ECVRF with public key Y satisfies the "full uniqueness" and "full collision resistance" properties. The RSAFDHVRF 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 Goldberg, et al. Expires December 20, 2020 [Page 25] InternetDraft VRF June 2020 permutation). Meanwhile, the "full uniqueness" and "full collision resistance" are properties that hold even if VRF keys are generated by an adversary. The RSAFDHVRF defined in this document does not have these properties. However, if adversarial key generation is a concern, the RSAFDHVRF 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. 7.1.2. Pseudorandomness with untrusted keys 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. 7.2. Selective vs Full Pseudorandomness [PWHVNRG17] presents cryptographic reductions to an underlying hard problem (e.g. Decisional Diffie Hellman for the ECVRF, or the standard RSA assumption for RSAFDHVRF) 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. o 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. o If full pseudorandomness is required for the application, the application may increase security parameters to make up for the loose security reduction. For RSAFDHVRF, this means increasing the RSA key length. For ECVRF, this means increasing the cryptographic strength of the EC group G. For both RSAFDHVRF and ECVRF the cryptographic strength of the hash function Hash may also potentially need to be increased. Goldberg, et al. Expires December 20, 2020 [Page 26] InternetDraft VRF June 2020 7.3. Proper pseudorandom nonce for ECVRF The security of the ECVRF defined in this document relies on the fact that nonce k used in the ECVRF_prove algorithm is chosen uniformly and pseudorandomly modulo q, and is unknown to the advesrary. 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 nonce generation methods specified in the ECVRF ciphersuites of Section 5.5 are designed with this requirement in mind. 7.4. Sidechannel attacks Side channel attacks on cryptographic primatives are an important issue. Here we discuss only one such side channel: timing attacks that can be used to leak information about the VRF input alpha. Implementers should take care to avoid sidechannel attacks that leak information about the VRF private key SK (and the nonce k used in the ECVRF). The ECVRF_hash_to_curve_try_and_increment 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 onthefly. This is because the 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. ECVRFP256SHA256SSWU and ECVRFEDWARDS25519SHA512ELL2 can be made to run in time independent of alpha, following recommendations in [ID.irtfcfrghashtocurve]. 7.5. Proofs Provide No Secrecy for VRF Input The VRF proof pi is not designed to provide secrecy and, in general, may reveal the VRF input alpha. Anyone who knows PK and pi is able to perform an offline dictionary attack to search for alpha, by verifying guesses for alpha using VRF_verify. This is in contrast to the VRF hash output beta which, without the proof, is pseudorandom and thus is designed to reveal no information about alpha. Goldberg, et al. Expires December 20, 2020 [Page 27] InternetDraft VRF June 2020 7.6. Prehashing The VRFs specified in this document allow for readonce access to the input alpha for both signing and verifying. Thus, additional prehashing of alpha (as specified, for example, in [RFC8032] for EdDSA signatures) is not needed, even for applications that need to handle long alpha or to support the InitializedUpdateFinalize (IUF) interface (in such an interface, alpha is not supplied all at once, but rather in pieces by a sequence of calls to Update). The ECVRF, in particular, uses alpha only in ECVRF_hash_to_curve. The curve point H becomes the representative of alpha thereafter. Note that the suite_string octet and the public key are hashed together with alpha in ECVRF_hash_to_curve, which ensures that the curve (including the generator B) and the public key are included indirectly into subsequent hashes. 7.7. Hash function domain separation and futureproofing Hashing is used for different purposes in the two VRFs (namely, in the RSAFDHVRF, in MGF1 and in proof_to_hash; in the ECVRF, in hash_to_curve, nonce_generation, hash_points, and proof_to_hash). The theoretical analysis assumes each of these functions is a separate random oracle. This analysis still holds even if the same hash function is used, as long as the four queries made to the hash function for a given SK and alpha are overwhelmingly unlikely to equal each other or to any queries made to the hash function for the same SK and different alpha. This is indeed the case for the RSA FDHVRF defined in this document, because the first octets of the input to the hash function used in MGF1 and in proof_to_hash are different. This is also the case for the ECVRF ciphersuites defined in this document, because: o inputs to the hash function used during nonce_generation are unlikely to equal inputs used in hash_to_curve, proof_to_hash, and hash_points. This follows since nonce_generation inputs a secret to the hash function that is not used by honest parties as input to any other hash function, and is not available to the adversary o the second octets of the inputs to the hash function used in proof_to_hash, hash_points, and ECVRF_hash_to_curve_try_and_increment are all different o the last octet of the input to the hash function used in proof_to_hash, hash_points, and ECVRF_hash_to_curve_try_and_increment is always zero, and therefore different from the last octet of the input to the hash Goldberg, et al. Expires December 20, 2020 [Page 28] InternetDraft VRF June 2020 function used in ECVRF_hash_to_curve_h2c_suite, which is set equal to the nonzero length of the domain separation tag by [ID.irtfcfrghashtocurve]. For the RSA VRF, if future designs need to specify variants of the design in this document, such variants should use different first octets in inputs to MGF1 and to the hash funciton used in proof_to_hash, in order to avoid the possibility that an adversary can obtain a VRF output under one variant, and then claim it was obtained under another variant For the elliptic curve VRF, if future designs need to specify variants (e.g., additional ciphersuites) of the design in this document, then, to avoid the possibility that an adversary can obtain a VRF output under one variant, and then claim it was obtained under another variant, they should specify a different suite_string constant. This way, the inputs to the hash_to_curve hash function used in producing H are guaranteed to be different; since all the other hashing done by the prover depends on H, inputs all the hash functions used by the prover will also be different as long as hash_to_curve is collision resistant. 8. Change Log 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 draftgoldbevrf01. 01  Minor updates, mostly highlighting TODO items. 02  Added specification of elligator2 for Curve25519, along with ciphersuites for ECVRFED25519SHA512Elligator. Changed ECVRF ED25519SHA256 suite_string to ECVRFED25519SHA512. (This change made because Ed25519 in [RFC8032] signatures use SHA512 and not SHA256.) Made ECVRF nonce generation a separate component, so that nonces are determinsitic. In ECVRF proving, changed + to  (and made corresponding verification changes) in order to be consistent with EdDSA and ECDSA. Highlighted that ECVRF_hash_to_curve acts like a prehash. Added "suites" variable to ECVRF for futureproofing. Ensured domain separation for hash functions by modifying hash_points and added discussion about domain separation. Updated todos in the "additional pseudorandomness property" section. Added an discussion of secrecy into security considerations. Removed B and PK=Y from ECVRF_hash_points because they are already present via H, which is computed via hash_to_curve using the suite_string (which identifies B) and Y. Goldberg, et al. Expires December 20, 2020 [Page 29] InternetDraft VRF June 2020 03  Changed Ed25519 conversions to littleendian, to match RFC 8032; added simple key validation for Ed25519; added Simple SWU cipher suite; clarified Elligator and removed the extra x0 bit, to make Montgomery and Edwards Elligator the same; added domain separation for RSA VRF; improved notation throughout; added nonce generation as a section; changed counter in tryandincrement from four bytes to one, to avoid endian issues; renamed tryand increment ciphersuites to TAI; added qLen as a separate paremeter; changed output length to hLen for ECVRF, to match RSAVRF; made Verify return beta so unverified proofs don't end up in proof_to_hash; added test vectors. 04  Clarified handling of optional arguments x and PK in ECVRF_prove. Edited implementation status to bring it up to date. 05  Renamed ed25519 into the more commonly used edwards25519. Corrected ECVRF_nonce_generation_RFC6979 (thanks to Gorka Irazoqui Apecechea and Mario Cao Cueto for finding the problem) and corresponding test vectors for the P256 suites. Added a reference to the Rust implementation. 06  Made some variable names more descriptive. Added a few implementation references. 07  Incorporated hashtocurve draft by reference to replace our own Elligator2 and Simple SWU. Clarified discussion of EC parameters and functions. Added a 0 octet to all hashing to enforce domain separation from hashing done inside hashtocurve. 9. Contributors 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, David C. Lawerence, Trevor Perrin, Annie Yousar, Stanislav Smyshlyaev, Liliya Akhmetzyanova, Tony Arcieri, Sergey Gorbunov, Sam Scott, Nick Sullivan, Christopher Wood, Marek Jankowski, Derek TingHaye Leung, Adam Suhl, Gary Belvinm, Piotr Nojszewski, Gorka Irazoqui Apecechea, and Mario Cao Cueto provided valuable input to this draft. Riad Wahby was very helpful with the integration of the hashtocurve draft. 10. References 10.1. Normative References Goldberg, et al. Expires December 20, 2020 [Page 30] InternetDraft VRF June 2020 [FIPS1864] National Institute for Standards and Technology, "Digital Signature Standard (DSS)", FIPS PUB 1864, July 2013, <https://csrc.nist.gov/publications/detail/fips/186/4/ final>. [ID.irtfcfrghashtocurve] FazHernandez, A., Scott, S., Sullivan, N., Wahby, R., and C. Wood, "Hashing to Elliptic Curves", draftirtfcfrg hashtocurve07 (work in progress), April 2020. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfceditor.org/info/rfc2119>. [RFC5114] Lepinski, M. and S. Kent, "Additional DiffieHellman Groups for Use with IETF Standards", RFC 5114, DOI 10.17487/RFC5114, January 2008, <https://www.rfceditor.org/info/rfc5114>. [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHAbased HMAC and HKDF)", RFC 6234, DOI 10.17487/RFC6234, May 2011, <https://www.rfceditor.org/info/rfc6234>. [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 2013, <https://www.rfceditor.org/info/rfc6979>. [RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch, "PKCS #1: RSA Cryptography Specifications Version 2.2", RFC 8017, DOI 10.17487/RFC8017, November 2016, <https://www.rfceditor.org/info/rfc8017>. [RFC8032] Josefsson, S. and I. Liusvaara, "EdwardsCurve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, January 2017, <https://www.rfceditor.org/info/rfc8032>. [SECG1] Standards for Efficient Cryptography Group (SECG), "SEC 1: Elliptic Curve Cryptography", Version 2.0, May 2009, <http://www.secg.org/sec1v2.pdf>. Goldberg, et al. Expires December 20, 2020 [Page 31] InternetDraft VRF June 2020 10.2. Informative References [ANSI.X9622005] "Public Key Cryptography for the Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)", ANSI X9.62, 2005. [DGKR18] David, B., Gazi, P., Kiayias, A., and A. Russell, "Ouroboros Praos: An adaptivelysecure, semisynchronous proofofstake protocol", in Advances in Cryptology  EUROCRYPT, 2018, <https://eprint.iacr.org/2017/573>. [GHMVZ17] Gilad, Y., Hemo, R., Micali, Y., Vlachos, Y., and Y. Zeldovich, "Algorand: Scaling Byzantine Agreements for Cryptocurrencies", in Proceedings of the 26th Symposium on Operating Systems Principles (SOSP), 2017, <https://eprint.iacr.org/2017/454>. [ID.vcelaknsec5] Vcelak, J., Goldberg, S., Papadopoulos, D., Huque, S., and D. Lawrence, "NSEC5, DNSSEC Authenticated Denial of Existence", draftvcelaknsec508 (work in progress), December 2018. [MRV99] Michali, S., Rabin, M., and S. Vadhan, "Verifiable Random Functions", in FOCS, 1999, <https://dash.harvard.edu/handle/1/5028196>. [PWHVNRG17] Papadopoulos, D., Wessels, D., Huque, S., Vcelak, J., Naor, M., Reyzin, L., and S. Goldberg, "Making NSEC5 Practical for DNSSEC", in ePrint Cryptology Archive 2017/099, February 2017, <https://eprint.iacr.org/2017/099.pdf>. [X25519] Bernstein, D., "How do I validate Curve25519 public keys?", 2006, <https://cr.yp.to/ecdh.html#validate>. Goldberg, et al. Expires December 20, 2020 [Page 32] InternetDraft VRF June 2020 Appendix A. Test Vectors for the ECVRFs The test vectors in this section were genereated using the reference implementation at <https://github.com/reyzin/ecvrf>. A.1. ECVRFP256SHA256TAI These two example secret keys and messages are taken from Appendix A.2.5 of [RFC6979]. SK = x = c9afa9d845ba75166b5c215767b1d6934e50c3db36e89b127b8a622b120f6721 PK = 0360fed4ba255a9d31c961eb74c6356d68c049b8923b61fa6ce669622e60f29fb6 alpha = 73616d706c65 (ASCII "sample") try_and_increment succeded on ctr = 1 H = 0272a877532e9ac193aff4401234266f59900a4a9e3fc3cfc6a4b7e467a15d06d4 k = 0d90591273453d2dc67312d39914e3a93e194ab47a58cd598886897076986f77 U = k*B = 02bb6a034f67643c6183c10f8b41dc4babf88bff154b674e377d90bde009c21672 V = k*H = 02893ebee7af9a0faa6da810da8a91f9d50e1dc071240c9706726820ff919e8394 pi = 035b5c726e8c0e2c488a107c600578ee75cb702343c153cb1eb8dec77f4b5071 b498e7c291a16dafb9ccff8c2ae1f039fa92a328d5f7e0d483ee18353067a13f69994 4a78892ff24939bcd044827eef884 beta = a3ad7b0ef73d8fc6655053ea22f9bede8c743f08bbed3d38821f0e16474b505e SK = x = c9afa9d845ba75166b5c215767b1d6934e50c3db36e89b127b8a622b120f6721 PK = 0360fed4ba255a9d31c961eb74c6356d68c049b8923b61fa6ce669622e60f29fb6 alpha = 74657374 (ASCII "test") try_and_increment succeded on ctr = 3 H = 02173119b4fff5e6f8afed4868a29fe8920f1b54c2cf89cc7b301d0d473de6b974 k = 5852353a868bdce26938cde1826723e58bf8cb06dd2fed475213ea6f3b12e961 U = k*B = 022779a2cafcb65414c4a04a4b4d2adf4c50395f57995e89e6de823250d91bc48e V = k*H = 033b4a14731672e82339f03b45ff6b5b13dee7ada38c9bf1d6f8f61e2ce5921119 pi = 034dac60aba508ba0c01aa9be80377ebd7562c4a52d74722e0abae7dc3080ddb 56c874cc95b7d29a6a65cb518fe6f4418256385f12b1eccbad023c901bb983ff707b1 09b3a3b526ca3a1e8661f7b8481a2 beta = a284f94ceec2ff4b3794629da7cbafa49121972671b466cab4ce170aa365f26d Goldberg, et al. Expires December 20, 2020 [Page 33] InternetDraft VRF June 2020 This example secret key is taken from Appendix L.4.2 of [ANSI.X9622005]. SK = x = 2ca1411a41b17b24cc8c3b089cfd033f1920202a6c0de8abb97df1498d50d2c8 PK = 03596375e6ce57e0f20294fc46bdfcfd19a39f8161b58695b3ec5b3d16427c274d alpha = 4578616d706c65207573696e67204543445341206b65792066726f6d20417 070656e646978204c2e342e32206f6620414e53492e58392d36322d32303035 (ASCII "Example using ECDSA key from Appendix L.4.2 of ANSI.X9622005") try_and_increment succeded on ctr = 1 H = 0258055c26c4b01d01c00fb57567955f7d39cd6f6e85fd37c58f696cc6b7aa761d k = 5689e2e08e1110b4dda293ac21667eac6db5de4a46a519c73d533f69be2f4da3 U = k*B = 020f465cd0ec74d2e23af0abde4c07e866ae4e5138bded5dd1196b8843f380db84 V = k*H = 036cb6f811428fc4904370b86c488f60c280fa5b496d2f34ff8772f60ed24b2d1d pi = 03d03398bf53aa23831d7d1b2937e005fb0062cbefa06796579f2a1fc7e7b8c6 679d92353c8a4fdfddb2a8540094b686cb5fb50f730d833a098a0399ccad32f3fec4d a2299891fc75ebda42baeb65e8c11 beta = 90871e06da5caa39a3c61578ebb844de8635e27ac0b13e829997d0d95dd98c19 A.2. ECVRFP256SHA256SSWU These two example secret keys and messages are taken from Appendix A.2.5 of [RFC6979]. SK = x = c9afa9d845ba75166b5c215767b1d6934e50c3db36e89b127b8a622b120f6721 PK = 0360fed4ba255a9d31c961eb74c6356d68c049b8923b61fa6ce669622e60f29fb6 alpha = 73616d706c65 (ASCII "sample") In SSWU: uniform_bytes = 5024e98d6067dec313af09ff0cbe78218324a645c2a4 b0aae2453f6fe91aa3bd9471f7b4a5fbf128e4b53f0c59603f7e In SSWU: u = df565615a2372e8b31b8771f7503bafc144e48b05688b97958cc27ce29a8d810 In SSWU: x1 = e7e39eb8a4c982426fcff629e55a3e13516cfeb62c02c369b1e750316f5e94eb In SSWU: gx1 is a nonsquare H = 02b31973e872d4a097e2cfae9f37af9f9d73428fde74ac537dda93b5f18dbc5842 k = e92820035a0a8afe132826c6312662b6ea733fc1a0d33737945016de54d02dd8 U = k*B = 031490f49d0355ffcdf66e40df788bee93861917ee713acff79be40d20cc91a30a Goldberg, et al. Expires December 20, 2020 [Page 34] InternetDraft VRF June 2020 V = k*H = 03701df0228138fa3d16612c0d720389326b3265151bc7ac696ea4d0591cd053e3 pi = 0331d984ca8fece9cbb9a144c0d53df3c4c7a33080c1e02ddb1a96a365394c78 88a39dfe7432f119228473f37db3f87ca470c63b0237432a791f18f823c1215e276b7 ac0962725ba8daec2bf90c0ccc91a beta = 21e66dc9747430f17ed9efeda054cf4a264b097b9e8956a1787526ed00dc664b SK = x = c9afa9d845ba75166b5c215767b1d6934e50c3db36e89b127b8a622b120f6721 PK = 0360fed4ba255a9d31c961eb74c6356d68c049b8923b61fa6ce669622e60f29fb6 alpha = 74657374 (ASCII "test") In SSWU: uniform_bytes = 910cc66d84a57985a1d15843dad83fd9138a109afb24 3b7fa5d64d766ec9ca3894fdcf46ebeb21a3972eb452a4232fd3 In SSWU: u = d8b0107f7e7aa36390240d834852f8703a6dc407019d6196bda5861b8fc00181 In SSWU: x1 = ccc747fa7318b9486ce4044adbbecaa084c27be6eda88eb7b7f3d688fd0968c7 In SSWU: gx1 is a square H = 03ccc747fa7318b9486ce4044adbbecaa084c27be6eda88eb7b7f3d688fd0968c7 k = febc3451ea7639fde2cf41ffd03f463124ecb3b5a79913db1ed069147c8a7dea U = k*B = 031200f9900e96f811d1247d353573f47e0d9da601fc992566234fc1a5b37749ae V = k*H = 02d3715dcfee136c7ae50e95ffca76f4ca6c29ddfb92a39c31a0d48e75c6605cd1 pi = 03f814c0455d32dbc75ad3aea08c7e2db31748e12802db23640203aebf1fa8db 2721e0499b7cecd68027a82f6095da076625a5f2f62908f1c283d5ee9b9e852d85bed f64f2452a4e5094729e101824443e beta = 8e7185d2b420e4f4681f44ce313a26d05613323837da09a69f00491a83ad25dd This example secret key is taken from Appendix L.4.2 of [ANSI.X9622005]. SK = x = 2ca1411a41b17b24cc8c3b089cfd033f1920202a6c0de8abb97df1498d50d2c8 PK = 03596375e6ce57e0f20294fc46bdfcfd19a39f8161b58695b3ec5b3d16427c274d alpha = 4578616d706c65207573696e67204543445341206b65792066726f6d20417 070656e646978204c2e342e32206f6620414e53492e58392d36322d32303035 (ASCII "Example using ECDSA key from Appendix L.4.2 of ANSI.X9622005") In SSWU: uniform_bytes = 9b81d55a242d3e8438d3bcfb1bee985a87fd144802c9 268cf9adeee160e6e9ff765569797a0f701cb4316018de2e7dd4 In SSWU: u = e43c98c2ae06d13839fedb0303e5ee815896beda39be83fb11325b97976efdce Goldberg, et al. Expires December 20, 2020 [Page 35] InternetDraft VRF June 2020 In SSWU: x1 = be9e195a50f175d3563aed8dc2d9f513a5536c1e9aee1757d86c08d32d582a86 In SSWU: gx1 is a nonsquare H = 022dd5150e5a2a24c66feab2f68532be1486e28e07f1b9a055cf38ccc16f6595ff k = 8e29221f33564f3f66f858ba2b0c14766e1057adbd422c3e7d0d99d5e142b613 U = k*B = 03a8823ff9fd16bf879261c740b9c7792b77fee0830f21314117e441784667958d V = k*H = 02d48fbb45921c755b73b25be2f23379e3ce69294f6cee9279815f57f4b422659d pi = 039f8d9cdc162c89be2871cbcb1435144739431db7fab437ab7bc4e2651a9e99 d5288aac70a5e4bd07df303c1d460eb6336bb5fa95436a07c2f6b7aec6fef7cc4846e a901ee1e238dee12bf752029b0b2e beta = 4fbadf33b42a5f42f23a6f89952d2e634a6e3810f15878b46ef1bb85a04fe95a A.3. ECVRFEDWARDS25519SHA512TAI These three example secret keys and messages are taken from Section 7.1 of [RFC8032]. SK = 9d61b19deffd5a60ba844af492ec2cc44449c5697b326919703bac031cae7f60 PK = d75a980182b10ab7d54bfed3c964073a0ee172f3daa62325af021a68f707511a alpha = (the empty string) x = 307c83864f2833cb427a2ef1c00a013cfdff2768d980c0a3a520f006904de94f try_and_increment succeded on ctr = 0 H = 91bbed02a99461df1ad4c6564a5f5d829d0b90cfc7903e7a5797bd658abf3318 k = 7100f3d9eadb6dc4743b029736ff283f5be494128df128df2817106f345b8594b 6d6da2d6fb0b4c0257eb337675d96eab49cf39e66cc2c9547c2bf8b2a6afae4 U = k*B = aef27c725be964c6a9bf4c45ca8e35df258c1878b838f37d9975523f09034071 V = k*H = 5016572f71466c646c119443455d6cb9b952f07d060ec8286d678615d55f954f pi = 8657106690b5526245a92b003bb079ccd1a92130477671f6fc01ad16f26f723f 5e8bd1839b414219e8626d393787a192241fc442e6569e96c462f62b8079b9ed83ff2 ee21c90c7c398802fdeebea4001 beta = 90cf1df3b703cce59e2a35b925d411164068269d7b2d29f3301c03dd757876 ff66b71dda49d2de59d03450451af026798e8f81cd2e333de5cdf4f3e140fdd8ae SK = 4ccd089b28ff96da9db6c346ec114e0f5b8a319f35aba624da8cf6ed4fb8a6fb PK = 3d4017c3e843895a92b70aa74d1b7ebc9c982ccf2ec4968cc0cd55f12af4660c alpha = 72 (1 byte) x = 68bd9ed75882d52815a97585caf4790a7f6c6b3b7f821c5e259a24b02e502e51 try_and_increment succeded on ctr = 1 H = 5b659fc3d4e9263fd9a4ed1d022d75eaacc20df5e09f9ea937502396598dc551 k = 42589bbf0c485c3c91c1621bb4bfe04aed7be76ee48f9b00793b2342acb9c167c ab856f9f9d4febc311330c20b0a8afd3743d05433e8be8d32522ecdc16cc5ce Goldberg, et al. Expires December 20, 2020 [Page 36] InternetDraft VRF June 2020 U = k*B = 1dcb0a4821a2c48bf53548228b7f170962988f6d12f5439f31987ef41f034ab3 V = k*H = fd03c0bf498c752161bae4719105a074630a2aa5f200ff7b3995f7bfb1513423 pi = f3141cd382dc42909d19ec5110469e4feae18300e94f304590abdced48aed593 f7eaf3eb2f1a968cba3f6e23b386aeeaab7b1ea44a256e811892e13eeae7c9f6ea899 2557453eac11c4d5476b1f35a08 beta = eb4440665d3891d668e7e0fcaf587f1b4bd7fbfe99d0eb2211ccec90496310 eb5e33821bc613efb94db5e5b54c70a848a0bef4553a41befc57663b56373a5031 SK = c5aa8df43f9f837bedb7442f31dcb7b166d38535076f094b85ce3a2e0b4458f7 PK = fc51cd8e6218a1a38da47ed00230f0580816ed13ba3303ac5deb911548908025 alpha = af82 (2 bytes) x = 909a8b755ed902849023a55b15c23d11ba4d7f4ec5c2f51b1325a181991ea95c try_and_increment succeded on ctr = 0 H = bf4339376f5542811de615e3313d2b36f6f53c0acfebb482159711201192576a k = 38b868c335ccda94a088428cbf3ec8bc7955bfaffe1f3bd2aa2c59fc31a0febc5 9d0e1af3715773ce11b3bbdd7aba8e3505d4b9de6f7e4a96e67e0d6bb6d6c3a U = k*B = 2bae73e15a64042fcebf062abe7e432b2eca6744f3e8265bc38e009cd577ecd5 V = k*H = 88cba1cb0d4f9b649d9a86026b69de076724a93a65c349c988954f0961c5d506 pi = 9bc0f79119cc5604bf02d23b4caede71393cedfbb191434dd016d30177ccbf80 e29dc513c01c3a980e0e545bcd848222d08a6c3e3665ff5a4cab13a643bef812e284c 6b2ee063a2cb4f456794723ad0a beta = 645427e5d00c62a23fb703732fa5d892940935942101e456ecca7bb217c61c 452118fec1219202a0edcf038bb6373241578be7217ba85a2687f7a0310b2df19f A.4. ECVRFEDWARDS25519SHA512ELL2 These three example secret keys and messages are taken from Section 7.1 of [RFC8032]. SK = 9d61b19deffd5a60ba844af492ec2cc44449c5697b326919703bac031cae7f60 PK = d75a980182b10ab7d54bfed3c964073a0ee172f3daa62325af021a68f707511a alpha = (the empty string) x = 307c83864f2833cb427a2ef1c00a013cfdff2768d980c0a3a520f006904de94f In Elligator2: uniform_bytes = d620782a206d9de584b74e23ae5ee1db5ca529 8b3fc527c4867f049dee6dd419b3674967bd614890f621c128d72269ae In Elligator2: u = 30f037b9745a57a9a2b8a68da81f397c39d46dee9d047f86c427c53f8b29a55c In Elligator2: gx1 = 8cb66318fb2cea01672d6c27a5ab662ae33220961607f69276080a56477b4a08 In Elligator2: gx1 is a square H = b8066ebbb706c72b64390324e4a3276f129569eab100c26b9f05011200c1bad9 k = b5682049fee54fe2d519c9afff73bbfad724e69a82d5051496a42458f817bed7a 386f96b1a78e5736756192aeb1818a20efb336a205ffede351cfe88dab8d41c Goldberg, et al. Expires December 20, 2020 [Page 37] InternetDraft VRF June 2020 U = k*B = 762f5c178b68f0cddcc1157918edf45ec334ac8e8286601a3256c3bbf858edd9 V = k*H = 4652eba1c4612e6fce762977a59420b451e12964adbe4fbecd58a7aeff5860af pi = 7d9c633ffeee27349264cf5c667579fc583b4bda63ab71d001f89c10003ab46f 25898f6bd7d4ed4c75f0282b0f7bb9d0e61b387b76db60b3cbf34bf09109ccb33fab7 42a8bddc0c8ba3caf5c0b75bb04 beta = 9d574bf9b8302ec0fc1e21c3ec5368269527b87b462ce36dab2d14ccf80c53 cccf6758f058c5b1c856b116388152bbe509ee3b9ecfe63d93c3b4346c1fbc6c54 SK = 4ccd089b28ff96da9db6c346ec114e0f5b8a319f35aba624da8cf6ed4fb8a6fb PK = 3d4017c3e843895a92b70aa74d1b7ebc9c982ccf2ec4968cc0cd55f12af4660c alpha = 72 (1 byte) x = 68bd9ed75882d52815a97585caf4790a7f6c6b3b7f821c5e259a24b02e502e51 In Elligator2: uniform_bytes = 04ae20a9ad2a2330fb33318e376a2448bd77bb 99e81d126f47952b156590444a9225b84128b66a2f15b41294fa2f2f6d In Elligator2: u = 3092f033b16d4d5f74a3f7dc7091fe434b449065152b95476f121de899bb773d In Elligator2: gx1 = 25d7fe7f82456e7078e99fdb24ef2582b4608357cdba9c39a8d535a3fd98464d In Elligator2: gx1 is a nonsquare H = 76ac3ccb86158a9104dff819b1ca293426d305fd76b39b13c9356d9b58c08e57 k = 88bf479281fd29a6cbdffd67e2c5ec0024d92f14eaed58f43f22f37c4c37f1d41 e65c036fbf01f9fba11d554c07494d0c02e7e5c9d64be88ef78cab7544e444d U = k*B = 8ec26e77b8cb3114dd2265fe1564a4efb40d109aa3312536d93dfe3d8d80a061 V = k*H = fe799eb5770b4e3a5a27d22518bb631db183c8316bb552155f442c62a47d1c8b pi = 47b327393ff2dd81336f8a2ef10339112401253b3c714eeda879f12c509072ef 9bf1a234f833f72d8fff36075fd9b836da28b5569e74caa418bae7ef521f2ddd35f57 27d271ecc70b4a83c1fc8ebc40c beta = 38561d6b77b71d30eb97a062168ae12b667ce5c28caccdf76bc88e093e4635 987cd96814ce55b4689b3dd2947f80e59aac7b7675f8083865b46c89b2ce9cc735 SK = c5aa8df43f9f837bedb7442f31dcb7b166d38535076f094b85ce3a2e0b4458f7 PK = fc51cd8e6218a1a38da47ed00230f0580816ed13ba3303ac5deb911548908025 alpha = af82 (2 bytes) x = 909a8b755ed902849023a55b15c23d11ba4d7f4ec5c2f51b1325a181991ea95c In Elligator2: uniform_bytes = be0aed556e36cdfddf8f1eeddbb7356a24fad6 4cf95a922a098038f215588b216beabbfe6acf20256188e883292b7a3a In Elligator2: u = f6675dc6d17fc790d4b3f1c6acf689a13d8b5815f23880092a925af94cd6fa24 In Elligator2: gx1 = a63d48e3247c903e22fdfb88fd9295e396712a5fe576af335dbe16f99f0af26c In Elligator2: gx1 is a square H = 13d2a8b5ca32db7e98094a61f656a08c6c964344e058879a386a947a4e189ed1 k = a7ddd74a3a7d165d511b02fa268710ddbb3b939282d276fa2efcfa5aaf79cf576 087299ca9234aacd7cd674d912deba00f4e291733ef189a51e36c861b3d683b Goldberg, et al. Expires December 20, 2020 [Page 38] InternetDraft VRF June 2020 U = k*B = a012f35433df219a88ab0f9481f4e0065d00422c3285f3d34a8b0202f20bac60 V = k*H = fb613986d171b3e98319c7ca4dc44c5dd8314a6e5616c1a4f16ce72bd7a0c25a pi = 926e895d308f5e328e7aa159c06eddbe56d06846abf5d98c2512235eaa57fdce 6187befa109606682503b3a1424f0f729ca0418099fbd86a48093e6a8de26307b8d93 e02da927e6dd5b73c8f119aee0f beta = 121b7f9b9aaaa29099fc04a94ba52784d44eac976dd1a3cca458733be5cd09 0a7b5fbd148444f17f8daf1fb55cb04b1ae85a626e30a54b4b0f8abf4a43314a58 Authors' Addresses Sharon Goldberg Boston University 111 Cummington Mall Boston, MA 02215 USA EMail: goldbe@cs.bu.edu Leonid Reyzin Boston University 111 Cummington Mall Boston, MA 02215 USA EMail: reyzin@bu.edu Dimitrios Papadopoulos Hong Kong University of Science and Techology Clearwater Bay Hong Kong EMail: dipapado@cse.ust.hk Jan Vcelak NS1 16 Beaver St New York, NY 10004 USA EMail: jvcelak@ns1.com Goldberg, et al. Expires December 20, 2020 [Page 39]