Oblivious Pseudorandom Functions (OPRFs) using Prime-Order Groups
draft-irtf-cfrg-voprf-02
The information below is for an old version of the document.
Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9497.
|
|
---|---|---|---|
Authors | Alex Davidson , Nick Sullivan , Christopher A. Wood | ||
Last updated | 2019-11-04 (Latest revision 2019-07-24) | ||
Replaces | draft-sullivan-cfrg-voprf | ||
RFC stream | Internet Research Task Force (IRTF) | ||
Formats | |||
IETF conflict review | conflict-review-irtf-cfrg-voprf, conflict-review-irtf-cfrg-voprf, conflict-review-irtf-cfrg-voprf, conflict-review-irtf-cfrg-voprf, conflict-review-irtf-cfrg-voprf, conflict-review-irtf-cfrg-voprf | ||
Additional resources | Mailing list discussion | ||
Stream | IRTF state | Active RG Document | |
Consensus boilerplate | Unknown | ||
Document shepherd | (None) | ||
IESG | IESG state | Became RFC 9497 (Informational) | |
Telechat date | (None) | ||
Responsible AD | (None) | ||
Send notices to | (None) |
draft-irtf-cfrg-voprf-02
Davidson, et al. Expires May 7, 2020 [Page 20] Internet-Draft OPRFs November 2019 4.8.2. Evaluation phase The evaluation phase of the OPRF results in a client receiving pseudorandom function evaluations from the server. It is important that the client is able to link the computation that it performs in the first step, with the output that it receives from the server. In other words, the client must store the data (r,M) output by OPRF_Blind(x). When it receives Z from the server, it must then use (r,M) as inputs to OPRF_Blind. In the batched setting, the client stores multiple values (ri,Mi) and sends each Mi to the server. Both client and server should preserve this ordering throughout the evaluation phase so that the client can successfully finalize the output in the final step. 4.8.3. Additional requirements The client input to the OPRF evaluation phase is a set of bytes x. These bytes are RECOMMENDED to be uniformly distributed. If the bytes are sampled from a predictable distribution instead, then it is likely that the server will also be able to predict the client's input to the OPRF. Therefore client privacy is reduced. Protocols that embed an OPRF evaluation MUST specify exactly how group elements are encoded in messages. The server need not not preserve any information during the evaluation exchange. For efficiency and client-privacy reasons, we recommend that all data received from the client in the evaluation phase is destroyed after the server has responded. In the VOPRF setting, when the server sends the response, it needs to indicate which version of key that it has used. This enables the client to retrieve the correct commitment from the public registry. The server MUST include a key identifier as part of its response, to ensure that the client can verify the contents of D correctly. 5. NIZK Discrete Logarithm Equality Proof For the VOPRF protocol we require that V is able to verify that P has used its private key k to evaluate the PRF. We can do this by showing that the original commitment (G,Y) output by VOPRF_Setup(l) satisfies log_G(Y) == log_M(Z) where Z is the output of VOPRF_Eval(k,G,Y,M). This may be used, for example, to ensure that P uses the same private key for computing the VOPRF output and does not attempt to "tag" Davidson, et al. Expires May 7, 2020 [Page 21] Internet-Draft OPRFs November 2019 individual verifiers with select keys. This proof must not reveal the P's long-term private key to V. Consequently, this allows extending the OPRF protocol with a (non- interactive) discrete logarithm equality (DLEQ) algorithm built on a Chaum-Pedersen [ChaumPedersen] proof. This proof is divided into two procedures: DLEQ_Generate and DLEQ_Verify. These are specified below. 5.1. DLEQ_Generate Input: k: Evaluator secret key. G: Public fixed generator of GG. Y: Evaluator public key (= kG). M: An element in GG. Z: An element in GG. H_3: A hash function from GG to {0,1}^L, modeled as a random oracle. Output: D: DLEQ proof (c, s). Steps: 1. r <-$ GF(p) 2. A := rG and B := rM 3. c <- H_3(G,Y,M,Z,A,B) (mod p) 4. s := (r - ck) (mod p) 5. Output D := (c, s) We note here that it is essential that a different r value is used for every invocation. If this is not done, then this may leak the key k in a similar fashion as is possible in Schnorr or (EC)DSA scenarios where fresh randomness is not used. 5.2. DLEQ_Verify Davidson, et al. Expires May 7, 2020 [Page 22] Internet-Draft OPRFs November 2019 Input: G: Public fixed generator of GG. Y: Evaluator public key. M: An element in GG. Z: An element in GG. D: DLEQ proof (c, s). Output: True if log_G(Y) == log_M(Z), False otherwise. Steps: 1. A' := (sG + cY) 2. B' := (sM + cZ) 3. c' <- H_3(G,Y,M,Z,A',B') (mod p) 4. Output c == c' (mod p) 5.3. Batched VOPRF evaluation Common applications (e.g. [PrivacyPass]) require V to obtain multiple PRF evaluations from P. In the VOPRF case, this would also require generation and verification of a DLEQ proof for each Zi received by V. This is costly, both in terms of computation and communication. To get around this, applications use a 'batching' procedure for generating and verifying DLEQ proofs for a finite number of PRF evaluation pairs (Mi,Zi). For n PRF evaluations: o Proof generation is slightly more expensive from 2n modular exponentiations to 2n+2. o Proof verification is much more efficient, from 4n modular exponentiations to 2n+4. o Communications falls from 2n to 2 group elements. Therefore, since P is usually a powerful server, we can tolerate a slight increase in proof generation complexity for much more efficient communication and proof verification. In this section, we describe algorithms for batching the DLEQ generation and verification procedure. For these algorithms we require an additional random oracle H_5: {0,1}^a x ZZ^3 -> {0,1}^b that takes an inputs of a binary string of length a and three integer values, and outputs an element in {0,1}^b. Davidson, et al. Expires May 7, 2020 [Page 23] Internet-Draft OPRFs November 2019 We can instantiate the random oracle function H_4 using the same hash function that is used for H_1,H_2,H_3. For H_5, we can also use a similar instantiation, or we can use a variable-length output generator. For example, for groups with an order of 256-bit, valid instantiations include functions such as SHAKE-256 [SHAKE] or HKDF- Expand-SHA256 [RFC5869]. 5.3.1. Batched_DLEQ_Generate Input: k: Evaluator secret key. G: Public fixed generator of group GG. Y: Evaluator public key (= kG). n: Number of PRF evaluations. [ Mi ]: An array of points in GG of length n. [ Zi ]: An array of points in GG of length n. H_4: A hash function from GG^(2n+2) to {0,1}^a, modeled as a random oracle. H_5: A hash function from {0,1}^a x ZZ^2 to {0,1}^b, modeled as a random oracle. label: An integer label value for the splitting the domain of H_5 Output: D: DLEQ proof (c, s). Steps: 1. seed <- H_4(G,Y,[Mi,Zi])) 2. for i in [n]: di <- H_5(seed,i,label) 3. c1,...,cn := (int)d1,...,(int)dn 4. M := c1M1 + ... + cnMn 5. Z := c1Z1 + ... + cnZn 6. Output D <- DLEQ_Generate(k,G,Y,M,Z) 5.3.2. DLEQ_Batched_Verify Davidson, et al. Expires May 7, 2020 [Page 24] Internet-Draft OPRFs November 2019 Input: G: Public fixed generator of group GG. Y: Evaluator public key. [ Mi ]: An array of points in GG of length n. [ Zi ]: An array of points in GG of length n. D: DLEQ proof (c, s). Output: True if log_G(Y) == log_(Mi)(Zi) for each i in 1...n, False otherwise. Steps: 1. seed <- H_4(G,Y,[Mi,Zi])) 2. i' := i 3. for i in [n]: 1. di <- H_5(seed,i',info) 2. if di > p: 1. i' = i'+1 2. i = i-1 // decrement and try again 3. continue 4. c1,...,cn := (int)d1,...,(int)dn 5. M := c1M1 + ... + cnMn 6. Z := c1Z1 + ... + cnZn 7. Output DLEQ_Verify(G,Y,M,Z,D) 5.3.3. Modified protocol execution The VOPRF protocol from Section Section 4 changes to allow specifying multiple blinded PRF inputs [ Mi ] for i in 1...n. P computes the array [ Zi ] and replaces DLEQ_Generate with DLEQ_Batched_Generate over these arrays. The same applies to the algorithm VOPRF_Eval. The same applies for replacing DLEQ_Verify with DLEQ_Batched_Verify when V verifies the response from P and during the algorithm VOPRF_Unblind. 5.3.4. Random oracle instantiations for proofs We can instantiate the random oracle function H_4 using the same hash function that is used for H_1,H_2,H_3. For H_5, we can also use a similar instantiation, or we can use a variable-length output generator. For example, for groups with an order of 256-bit, valid instantiations include functions such as SHAKE-256 [SHAKE] or HKDF- Expand-SHA256 [RFC5869]. Davidson, et al. Expires May 7, 2020 [Page 25] Internet-Draft OPRFs November 2019 Input: [ ri ]: Random scalars in [1, p - 1]. G: Public fixed generator of group GG. Y: Evaluator public key. [ Mi ]: Blinded elements of GG. [ Zi ]: Server-generated elements in GG. D: A batched DLEQ proof object. Output: N: element in GG, or "error". Steps: 1. N := (r^(-1))Z 2. If 1 = DLEQ_Batched_Verify(G,Y,[ Mi ],[ Zi ],D), output N 3. Output "error" 6. Supported ciphersuites This section specifies supported VOPRF group and hash function instantiations. We only provide ciphersuites in the EC setting as these provide the most efficient way of instantiating the OPRF. Our instantiation includes considerations for providing the DLEQ proofs that make the instantiation a VOPRF. Supporting OPRF operations alone can be allowed by simply dropping the relevant components. For reasons that are detailed in Section 8.1, we only consider ciphersuites that provide strictly greater than 128 bits of security [NIST]. 6.1. VOPRF-curve448-HKDF-SHA512-ELL2: o GG: curve448 [RFC7748] o H_1: curve448-SHA512-ELL2-RO [I-D.irtf-cfrg-hash-to-curve] * label: voprf_h2c o H_2: HMAC_SHA512 [RFC2104] o H_3: SHA512 o H_4: SHA512 o H_5: HKDF-Expand-SHA512 Davidson, et al. Expires May 7, 2020 [Page 26] Internet-Draft OPRFs November 2019 6.2. VOPRF-p384-HKDF-SHA512-ICART: o GG: secp384r1 [SEC2] o H_1: P384-SHA512-ICART-RO [I-D.irtf-cfrg-hash-to-curve] * label: voprf_h2c o H_2: HMAC_SHA512 [RFC2104] o H_3: SHA512 o H_4: SHA512 o H_5: HKDF-Expand-SHA512 6.3. VOPRF-p521-HKDF-SHA512-SSWU: o GG: secp521r1 [SEC2] o H_1: P521-SHA512-SSWU-RO [I-D.irtf-cfrg-hash-to-curve] * label: voprf_h2c o H_2: HMAC_SHA512 [RFC2104] o H_3: SHA512 o H_4: SHA512 o H_5: HKDF-Expand-SHA512 We remark that the 'label' field is necessary for domain separation of the hash-to-curve functionality. 7. Recommended protocol integration We describe some recommendations and suggestions on the topic of integrating the (V)OPRF protocol from Section 4 into wider protocols. It should be noted that since [JKK14] provides a security proof of the VPRF construction in the UC security model, then any UC-secure protocol that uses the OPRF construction as an atomic instantiation will remain UC-secure. As a result we recommend that any protocol that wishes to include an OPRF stage does so by implementing all OPRF evaluation functionality as a contiguous block of operations during the protocol. This does not include the OPRF setup phase, which should be run before the Davidson, et al. Expires May 7, 2020 [Page 27] Internet-Draft OPRFs November 2019 entire protocol interaction. For example, such an instantiation for a wider protocol W would look like the following. ================================================================ OPRF setup phase ================================================================ > ... > BEGIN(protocol W) > ... > PAUSE(protocol W) ================================================================ OPRF evaluation phase ================================================================ > RESTART(protocol W) > ... > END(protocol W) In other words, no messages from protocol W should take place during the OPRF protocol instantiation. This DOES NOT preclude the participants in protocol W from using the outputs of the OPRF evaluation, once the OPRF protocol is complete. Note that the OPRF protocol can involve batched evaluations, as well as single evaluations. 7.1. Setup phase In the VOPRF setting, the server must send to the client (p,Y) where p is the prime used in instantiating the group used for the VOPRF operations, and Y is a commitment to the server key k. From this information, the client and server must agree on a generator G for the group description. It is important that the generator G of GG is not chosen by the server, and that it is agreed upon before the protocol starts. In the elliptic curve setting, we recommend that G is chosen as the standard generator for the curve. As we mentioned above, if an implementer wants to embed OPRF evaluation as part of a wider protocol, then we recommend that this setup phase should occur before all communication takes place; including all communication required for the wider protocol. We recommend that any server implementation only implements one group instantiation at any one time. This means that the client does not have to pick a specific instantiation when it sends the first evaluation message. Davidson, et al. Expires May 7, 2020 [Page 28] Internet-Draft OPRFs November 2019 7.2. Evaluation phase The evaluation phase of the OPRF results in a client receiving pseudorandom function evaluations from the server. It is important that the client is able to link the computation that it performs in the first step, with the output that it receives from the server. In other words, the client must store the data (r,M) output by OPRF_Blind(x). When it receives Z from the server, it must then use (r,M) as inputs to OPRF_Blind. In the batched setting, the client stores multiple values (ri,Mi) and sends each Mi to the server. Both client and server should preserve this ordering throughout the evaluation phase so that the client can successfully finalize the output in the final step. 7.3. Client-specific considerations 7.3.1. Inputs The client input to the OPRF evaluation phase is a set of bytes x. These bytes do not have to be uniformly distributed. However, we should note that if the bytes are sampled from a predictable distribution, then it is likely that the server will also be able to predict the client's input to the OPRF. Therefore the utility of client privacy is reduced somewhat. 7.3.2. Output The client receives y = H_2(DST, x .. N) at the end of the protocol. We suggest that clients store the pair (x, y) as bytes. This allows the client to use the the output of the protocol in conjunction with the input used to create it later. 7.3.3. Messages The client message contains a group element and should be encoded as bytes. In the elliptic curve setting this corresponds to an encoded curve point. Both compressed and uncompressed point encodings should be supported by the server. The length of the point encoding should be enough to determine the encoding of the point. 7.4. Server-specific considerations 7.4.1. Setup As mentioned previously, the server should pick a single group instantiation and advertise this as the only way of evaluating the OPRF. Davidson, et al. Expires May 7, 2020 [Page 29] Internet-Draft OPRFs November 2019 7.4.2. Inputs The server input to the evaluation phase is a key k. This key can be stored simply as bytes. The key must be protected at all times. If the server ever suspects that the key has been compromised then it must be rotated immediately. In addition, the key should be rotated somewhat frequently for security reasons to reduce the impact of an unknown compromise. For more information on appropriate key schedules, see Section 8.5. Every time the server key is rotated, a new setup phase will have to be run. The server should publish public key commitments (Y) to a public, trusted registry to avoid notifying all client's individually. The registry should be considered tamper-proof from the client perspective and should retain a history of all edits. We recommend that all commitments come with an expiry date to enforce rotation policies, and optionally a signature using a long-term signing key (with public verification key made available via another public beacon). The signature is only necessary to prevent active attackers that may be able to route the client to an untrusted registry. Below, we recommend the following proposed JSON structure for holding public commitment data. { "Y": <bytes_of_commitment>, "expiry": <date-of-expiry>, "sig": <commitment_signature> } This data should be retrieved and validated by the client when verifying VOPRF messages from the server. For efficiency reasons, the client may want to cache the value of "Y" and "expiry". Any commitment that has expired should not be used by the client. Each commitment should be versioned according to some obvious convention. After a key rotation the server should append a new commitment object with a new version tag. 7.4.3. Outputs The server need not not preserve any information during the evaluation exchange. For efficiency and client-privacy reasons, we recommend that all data received from the client in the evaluation phase is destroyed after the server has responded. Davidson, et al. Expires May 7, 2020 [Page 30] Internet-Draft OPRFs November 2019 7.4.4. Messages In the VOPRF setting, when the server sends the response, it needs to indicate which version of key that it has used. This enables the client to retrieve the correct commitment from the public registry. We recommend that the server sends it's response as a JSON object that specifies separate members for the values Z and D, along with the key version that is used. 8. Security Considerations This section discusses the cryptographic security of our protocol, along with some suggestions and trade-offs that arise from the implementation of the implementation of an OPRF. 8.1. Cryptographic security We discuss the cryptographic security of the OPRF protocol from Section 4, relative to the necessary cryptographic assumptions that need to be made. 8.1.1. Computational hardness assumptions Each assumption states that the problems specified below are computationally difficult to solve in relation to sp (the security parameter). In other words, the probability that an adversary has in solving the problem is bounded by a function negl(sp), where negl(sp) < 1/f(sp) for all polynomial functions f(). Let GG = GG(sp) be a group with prime-order p, and let FFp be the finite field of order p. 8.1.1.1. Discrete-log (DL) problem Given G, a generator of GG, and H = hG for some h in FFp; output h. 8.1.1.2. Decisional Diffie-Hellman (DDH) problem Sample a uniformly random bit d in {0,1}. Given (G, aG, bG, C), where: o G is a generator of GG; o a,b are elements of FFp; o if d == 0: C = abG; else: C is sampled uniformly GG(sp). Output d' == d. Davidson, et al. Expires May 7, 2020 [Page 31] Internet-Draft OPRFs November 2019 8.1.2. Protocol security As aforementioned, our OPRF and VOPRF constructions are based heavily on the 2HashDH-NIZK construction given in [JKK14], except for considerations on how we instantiate the NIZK DLEQ proof system. This means that the cryptographic security of our construction is also based on the assumption that the One-More Gap DH is computationally difficult to solve. The (N,Q)-One-More Gap DH (OMDH) problem asks the following. Given: - G, kG, G_1, ... , G_N where G, G1, ... GN are elements of the group GG; - oracle access to an OPRF functionality using the key k; - oracle access to DDH solvers. Find Q+1 pairs of the form below: (G_{j_s}, kG_{j_s}) where the following conditions hold: - s is a number between 1 and Q+1; - j_s is a number between 1 and N for each s; - Q is the number of allowed queries. The original paper [JKK14] gives a security proof that the 2HashDH- NIZK construction satisfies the security guarantees of a VOPRF protocol Section 3.1 under the OMDH assumption in the universal composability (UC) security model. Without the NIZK proof system, the protocol instantiates an OPRF protocol only. See the paper for further details. 8.1.3. Q-strong-DH oracle A side-effect of our OPRF design is that it allows instantiation of a oracle for constructing Q-strong-DH (Q-sDH) samples. The Q-Strong-DH problem asks the following. Given G1, G2, h*G2, (h^2)*G2, ..., (h^Q)*G2; for G1 and G2 generators of GG. Output ( (1/(k+c))*G1, c ) where c is an element of FFp The assumption that this problem is hard was first introduced in [BB04]. Since then, there have been a number of cryptanalytic studies that have reduced the security of the assumption below that implied by the group instantiation (for example, [BG04] and [Cheon06]). In summary, the attacks reduce the security of the group instantiation by log_2(Q) bits. Davidson, et al. Expires May 7, 2020 [Page 32] Internet-Draft OPRFs November 2019 As an example, suppose that a group instantiation is used that provides 128 bits of security. Then an adversary with access to a Q-sDH oracle and makes Q=2^20 queries can reduce the security of the instantiation by log_2(2^20) = 20 bits. Notice that it is easy to instantiate a Q-sDH oracle using the OPRF functionality that we provide. A client can just submit sequential queries of the form (G, kG, (k^2)G, ..., (k^(Q-1))G), where each query is the output of the previous interaction. This means that any client that submit Q queries to the OPRF can use the aforementioned attacks to reduce security of the group instantiation by log_2(Q) bits. Recall that from a malicious client's perspective, the adversary wins if they can distinguish the OPRF interaction from a protocol that computes the ideal functionality provided by the PRF. 8.1.4. Implications for ciphersuite choices The OPRF instantiations that we recommend in this document are informed by the cryptanalytic discussion above. In particular, choosing elliptic curves configurations that describe 128-bit group instantiations would appear to in fact instantiate an OPRF with 128-log_2(Q) bits of security. While it would require an informed and persistent attacker to launch a highly expensive attack to reduce security to anything much below 100 bits of security, we see this possibility as something that may result in problems in the future. Therefore, all of our ciphersuites in Section 6 come with a minimum group instantiation corresponding to 196 bits of security. This would require an adversary to launch a minimum of Q = 2^(68) queries to reduce security to 128 bits using the Q-sDH attacks. As a result, it appears prohibitively expensive to launch credible attacks on these parameters with our current understanding of the attack surface. 8.2. Hashing to curve A critical aspect of implementing this protocol using elliptic curve group instantiations is a method of instantiating the function H1, that maps inputs to group elements. In the elliptic curve setting, this must be a deterministic function that maps arbitrary inputs x (as bytes) to uniformly chosen points in the curve. In the security proof of the construction H1 is modeled as a random oracle. This implies that any instantiation of H1 must be pre-image and collision resistant. In Section 6 we give instantiations of this functionality based on the functions described in Davidson, et al. Expires May 7, 2020 [Page 33] Internet-Draft OPRFs November 2019 [I-D.irtf-cfrg-hash-to-curve]. Consequently, any OPRF implementation must adhere to the implementation and security considerations discussed in [I-D.irtf-cfrg-hash-to-curve] when instantiating the function H1. 8.3. Timing Leaks To ensure no information is leaked during protocol execution, all operations that use secret data MUST be constant time. Operations that SHOULD be constant time include: H_1() (hashing arbitrary strings to curves) and DLEQ_Generate(). As mentioned previously, [I-D.irtf-cfrg-hash-to-curve] describes various algorithms for constant-time implementations of H_1. 8.4. User segregation The aim of the OPRF functionality is to allow clients receive pseudorandom function evaluations on their own inputs, without compromising their own privacy with respect to the server. In many applications (for example, [PrivacyPass]) the client may choose to reveal their original input, after an invocation of the OPRF protocol, along with their OPRF output. This can prove to the server that it has received a valid OPRF output in the past. Since the server does not reveal learn anything about the OPRF output, it should not be able to link the client to any previous protocol instantiation. Consider a malicious server that manages to segregate the user base into different sets. Then this reduces the effective privacy of all of the clients involved, since the client above belongs to a smaller set of users than previously hoped. In general, if the user-base of the OPRF functionality is quite small, then the obliviousness of clients is limited. That is, smaller user-bases mean that the server is able to identify client's with higher certainty. In summary, an OPRF instantiation effectively comes with an additional privacy parameter pp. If all clients of the OPRF make one query and then subsequently reveal their OPRF input afterwards, then the server should be link the revealed input to a protocol instantiation with probability 1/pp. Below, we provide a few techniques that could be used to abuse client-privacy in the OPRF construction by segregating the user-base, along with some mitigations. Davidson, et al. Expires May 7, 2020 [Page 34] Internet-Draft OPRFs November 2019 8.4.1. Linkage patterns If the server is able to ascertain patterns of usage for some clients - such as timings associated with usage - then the effective privacy of the clients is reduced to the number of users that fit each usage pattern. Along with early registration patterns, where early adopters initially have less privacy due to a low number of registered users, such problems are inherent to any anonymity- preserving system. 8.4.2. Evaluation on multiple keys Such an attack consists of the server evaluating the OPRF on multiple different keys related to the number of clients that use the functionality. As an extreme, the server could evaluate the OPRF with a different key for each client. If the client then revealed their hidden information at a later date then the server would immediately know which initial request they launched. The VOPRF variant helps mitigate this attack since each server evaluation can be bound to a known public key. However, there are still ways that the VOPRF construction can be abused. In particular: o If the server successfully provisions a large number of keys that are trusted by clients, then the server can divide the user-base by the number of keys that are currently in use. As such, clients should only trust a small number (2 or 3 ideally) of server keys at any one time. Additionally, a tamper-proof audit log system akin to existing work on Key Transparency [keytrans] could be used to ensure that a server is abiding by the key policy. This would force the server to be held accountable for their key updates, and thus higher key update frequencies can be better managed on the client-side. o If the server rotates their key frequently, then this may result in client's holding out-of-date information from a past interaction. Such information can also be used to segregate the user-base based on the last time that they accessed the OPRF protocol. Similarly to the above, server key rotations must be kept to relatively infrequent intervals (such as once per month). This will prevent too many clients from being segregated into different groups related to the time that they accessed the functionality. There are viable reasons for rotating the server key (for protecting against malicious clients) that we address more closely in Section 8.5. Since key provisioning requires careful handling, all public keys should be accessible from a client-trusted registry with a way of Davidson, et al. Expires May 7, 2020 [Page 35] Internet-Draft OPRFs November 2019 auditing the history of key updates. We also recommend that public keys have a corresponding expiry date that clients can use to prevent the server from using keys that have been provisioned for a long period of time. 8.5. Key rotation Since the server's key is critical to security, the longer it is exposed by performing (V)OPRF operations on client inputs, the longer it is possible that the key can be compromised. For instance, if the key is kept in production for a long period of time, then this may grant the client the ability to hoard large numbers of tokens. This has negative impacts for some of the applications that we consider in Section 9. As another example, if the key is kept in circulation for a long period of time, then it also allows the clients to make enough queries to launch more powerful variants of the Q-sDH attacks from Section 8.1.3. To combat attacks of this nature, regular key rotation should be employed on the server-side. A suitable key-cycle for a key used to compute (V)OPRF evaluations would be between one week and six months. As we discussed in Section 8.4.2, key rotation cycles that are too frequent (in the order of days) can lead to large segregation of the wider user base. As such, the length of the key cycles represent a trade-off between greater server key security (for shorter cycles), and better client privacy (for longer cycles). In situations where client privacy is paramount, longer key cycles should be employed. Otherwise, shorter key cycles can be managed if the server uses a Key Transparency-type system [keytrans]; this allows clients to publicly audit their rotations. 9. Applications This section describes various applications of the (V)OPRF protocol. 9.1. Privacy Pass This VOPRF protocol is used by the Privacy Pass system [PrivacyPass] to help Tor users bypass CAPTCHA challenges. Their system works as follows. Client C connects - through Tor - to an edge server E serving content. Upon receipt, E serves a CAPTCHA to C, who then solves the CAPTCHA and supplies, in response, n blinded points. E verifies the CAPTCHA response and, if valid, signs (at most) n blinded points, which are then returned to C along with a batched DLEQ proof. C stores the tokens if the batched proof verifies correctly. When C attempts to connect to E again and is prompted with a CAPTCHA, C uses one of the unblinded and signed points, or Davidson, et al. Expires May 7, 2020 [Page 36] Internet-Draft OPRFs November 2019 tokens, to derive a shared symmetric key sk used to MAC the CAPTCHA challenge. C sends the CAPTCHA, MAC, and token input x to E, who can use x to derive sk and verify the CAPTCHA MAC. Thus, each token is used at most once by the system. The Privacy Pass implementation uses the P-256 instantiation of the VOPRF protocol. For more details, see [DGSTV18]. 9.2. Private Password Checker In this application, let D be a collection of plaintext passwords obtained by prover P. For each password p in D, P computes VOPRF_Eval on H_1(p), where H_1 is as described above, and stores the result in a separate collection D'. P then publishes D' with Y, its public key. If a client C wishes to query D' for a password p', it runs the VOPRF protocol using p as input x to obtain output y. By construction, y will be the OPRF evaluation of p hashed onto the curve. C can then search D' for y to determine if there is a match. Concrete examples of important applications in the password domain include: o password-protected storage [JKK14], [JKKX16]; o perfectly-hiding password management [SJKS17]; o password-protected secret-sharing [JKKX17]. 9.2.1. Parameter Commitments For some applications, it may be desirable for P to bind tokens to certain parameters, e.g., protocol versions, ciphersuites, etc. To accomplish this, P should use a distinct scalar for each parameter combination. Upon redemption of a token T from V, P can later verify that T was generated using the scalar associated with the corresponding parameters. 10. Acknowledgements This document resulted from the work of the Privacy Pass team [PrivacyPass]. The authors would also like to acknowledge the helpful conversations with Hugo Krawczyk. Eli-Shaoul Khedouri provided additional review and comments on key consistency. Davidson, et al. Expires May 7, 2020 [Page 37] Internet-Draft OPRFs November 2019 11. References 11.1. Normative References [BB04] "Short Signatures Without Random Oracles", n.d., <http://ai.stanford.edu/~xb/eurocrypt04a/bbsigs.pdf>. [BG04] "The Static Diffie-Hellman Problem", n.d., <https://eprint.iacr.org/2004/306>. [ChaumBlindSignature] "Blind Signatures for Untraceable Payments", n.d., <http://sceweb.sce.uhcl.edu/yang/teaching/ csci5234WebSecurityFall2011/Chaum-blind-signatures.PDF>. [ChaumPedersen] "Wallet Databases with Observers", n.d., <https://chaum.com/publications/Wallet_Databases.pdf>. [Cheon06] "Security Analysis of the Strong Diffie-Hellman Problem", n.d., <https://www.iacr.org/archive/ eurocrypt2006/40040001/40040001.pdf>. [DECAF] "Decaf, Eliminating cofactors through point compression", n.d., <https://www.shiftleft.org/papers/decaf/decaf.pdf>. [DGSTV18] "Privacy Pass, Bypassing Internet Challenges Anonymously", n.d., <https://www.degruyter.com/view/j/ popets.2018.2018.issue-3/popets-2018-0026/ popets-2018-0026.xml>. [I-D.irtf-cfrg-hash-to-curve] Faz-Hernandez, A., Scott, S., Sullivan, N., Wahby, R., and C. Wood, "Hashing to Elliptic Curves", draft-irtf-cfrg- hash-to-curve-05 (work in progress), November 2019. [JKK14] "Round-Optimal Password-Protected Secret Sharing and T-PAKE in the Password-Only model", n.d., <https://eprint.iacr.org/2014/650>. [JKKX16] "Highly-Efficient and Composable Password-Protected Secret Sharing (Or, How to Protect Your Bitcoin Wallet Online)", n.d., <https://eprint.iacr.org/2016/144>. [JKKX17] "TOPPSS: Cost-minimal Password-Protected Secret Sharing based on Threshold OPRF", n.d., <https://eprint.iacr.org/2017/363>. Davidson, et al. Expires May 7, 2020 [Page 38] Internet-Draft OPRFs November 2019 [keytrans] "Security Through Transparency", n.d., <https://security.googleblog.com/2017/01/ security-through-transparency.html>. [NIST] "Keylength - NIST Report on Cryptographic Key Length and Cryptoperiod (2016)", n.d., <https://www.keylength.com/en/4/>. [OPAQUE] "The OPAQUE Asymmetric PAKE Protocol", n.d., <https://tools.ietf.org/html/ draft-krawczyk-cfrg-opaque-02>. [PrivacyPass] "Privacy Pass", n.d., <https://github.com/privacypass/challenge-bypass-server>. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997, <https://www.rfc-editor.org/info/rfc2104>. [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.rfc-editor.org/info/rfc2119>. [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010, <https://www.rfc-editor.org/info/rfc5869>. [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January 2016, <https://www.rfc-editor.org/info/rfc7748>. [RISTRETTO] "The ristretto255 Group", n.d., <https://tools.ietf.org/html/ draft-hdevalence-cfrg-ristretto-01>. [SEC2] Standards for Efficient Cryptography Group (SECG), ., "SEC 2: Recommended Elliptic Curve Domain Parameters", n.d., <http://www.secg.org/sec2-v2.pdf>. Davidson, et al. Expires May 7, 2020 [Page 39] Internet-Draft OPRFs November 2019 [SHAKE] "SHA-3 Standard, Permutation-Based Hash and Extendable- Output Functions", n.d., <https://www.nist.gov/publications/sha-3-standard- permutation-based-hash-and-extendable-output- functions?pub_id=919061>. [SJKS17] "SPHINX, A Password Store that Perfectly Hides from Itself", n.d., <https://eprint.iacr.org/2018/695>. 11.2. URIs [1] https://tools.ietf.org/html/draft-irtf-cfrg-voprf-00 Appendix A. Test Vectors [[TODO: add when done]] Authors' Addresses Alex Davidson Cloudflare County Hall London, SE1 7GP United Kingdom Email: adavidson@cloudflare.com Nick Sullivan Cloudflare 101 Townsend St San Francisco United States of America Email: nick@cloudflare.com Christopher A. Wood Apple Inc. One Apple Park Way Cupertino, California 95014 United States of America Email: cawood@apple.com Davidson, et al. Expires May 7, 2020 [Page 40]