Updating to draft 11 (#80)

* draft-11

* Fix CI complaints

* Address review comments, CHANGELOG entry, minor fixes

CHANGELOG.md

@@ -1,5 +1,13 @@
 # Changelog
 
+## 0.4.0-pre.4 (July 9, 2022)
+* Updated to be in sync with draft-irtf-cfrg-voprf-11
+* Adds the evaluate() function to the servers to calculate the output of the OPRF
+  directly
+* Renames the former evaluate() function to blind_evaluate to match the spec
+* Fixes the order of parameters for PoprfClient::blind to align it with the
+  other clients
+
 ## 0.4.0-pre.3 (July 1, 2022)
 * Updated to be in sync with draft-irtf-cfrg-voprf-10, with
   the only difference from -09 being a constant string change

Cargo.toml

@@ -9,7 +9,7 @@ name = "voprf"
 readme = "README.md"
 repository = "https://github.com/novifinancial/voprf/"
 rust-version = "1.57"
-version = "0.4.0-pre.3"
+version = "0.4.0-pre.4"
 
 [features]
 alloc = []

README.md

@@ -16,7 +16,7 @@ Installation
 Add the following line to the dependencies of your `Cargo.toml`:
 
 ```
-voprf = "0.4.0-pre.3"
+voprf = "0.4.0-pre.4"
 ```
 
 ### Minimum Supported Rust Version

src/common.rs

@@ -11,7 +11,7 @@ use core::convert::TryFrom;
 
 use derive_where::derive_where;
 use digest::core_api::BlockSizeUser;
-use digest::{Digest, OutputSizeUser};
+use digest::{Digest, Output, OutputSizeUser};
 use generic_array::sequence::Concat;
 use generic_array::typenum::{IsLess, IsLessOrEqual, Unsigned, U11, U2, U256};
 use generic_array::{ArrayLength, GenericArray};
@@ -153,7 +153,7 @@ where
     <CS::Hash as OutputSizeUser>::OutputSize:
         IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
 {
-    // https://www.ietf.org/archive/id/draft-irtf-cfrg-voprf-10.html#section-2.2.1
+    // https://www.ietf.org/archive/id/draft-irtf-cfrg-voprf-11.html#section-2.2.1
 
     let (m, z) = compute_composites::<CS, _, _>(Some(k), b, cs, ds, mode)?;
 
@@ -216,7 +216,7 @@ where
     <CS::Hash as OutputSizeUser>::OutputSize:
         IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
 {
-    // https://www.ietf.org/archive/id/draft-irtf-cfrg-voprf-10.html#section-2.2.2
+    // https://www.ietf.org/archive/id/draft-irtf-cfrg-voprf-11.html#section-2.2.2
     let (m, z) = compute_composites::<CS, _, _>(None, b, cs, ds, mode)?;
     let t2 = (a * &proof.s_scalar) + &(b * &proof.c_scalar);
     let t3 = (m * &proof.s_scalar) + &(z * &proof.c_scalar);
@@ -285,7 +285,7 @@ where
     <CS::Hash as OutputSizeUser>::OutputSize:
         IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
 {
-    // https://www.ietf.org/archive/id/draft-irtf-cfrg-voprf-10.html#section-2.2.1
+    // https://www.ietf.org/archive/id/draft-irtf-cfrg-voprf-11.html#section-2.2.1
 
     let elem_len = <CS::Group as Group>::ElemLen::U16.to_be_bytes();
 
@@ -437,12 +437,61 @@ where
     <CS::Hash as OutputSizeUser>::OutputSize:
         IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
 {
-    let dst = GenericArray::from(STR_HASH_TO_GROUP).concat(create_context_string::<CS>(mode));
+    let hashed_point = hash_to_group::<CS>(input, mode)?;
-    let hashed_point =
-        CS::Group::hash_to_curve::<CS::Hash>(&[input], &dst).map_err(|_| Error::Input)?;
     Ok(hashed_point * blind)
 }
 
+/// Hashes `input` to a point on the curve
+pub(crate) fn hash_to_group<CS: CipherSuite>(
+    input: &[u8],
+    mode: Mode,
+) -> Result<<CS::Group as Group>::Elem>
+where
+    <CS::Hash as OutputSizeUser>::OutputSize:
+        IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
+{
+    let dst = GenericArray::from(STR_HASH_TO_GROUP).concat(create_context_string::<CS>(mode));
+    CS::Group::hash_to_curve::<CS::Hash>(&[input], &dst).map_err(|_| Error::Input)
+}
+
+/// Internal function that finalizes the hash input for OPRF, VOPRF & POPRF.
+/// Returned values can only fail with [`Error::Input`].
+pub(crate) fn server_evaluate_hash_input<CS: CipherSuite>(
+    input: &[u8],
+    info: Option<&[u8]>,
+    issued_element: GenericArray<u8, <<CS as CipherSuite>::Group as Group>::ElemLen>,
+) -> Result<Output<CS::Hash>>
+where
+    <CS::Hash as OutputSizeUser>::OutputSize:
+        IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
+{
+    // OPRF & VOPRF
+    // hashInput = I2OSP(len(input), 2) || input ||
+    //             I2OSP(len(issuedElement), 2) || issuedElement ||
+    //             "Finalize"
+    // return Hash(hashInput)
+    //
+    // POPRF
+    // hashInput = I2OSP(len(input), 2) || input ||
+    //             I2OSP(len(info), 2) || info ||
+    //             I2OSP(len(issuedElement), 2) || issuedElement ||
+    //             "Finalize"
+
+    let mut hash = CS::Hash::new()
+        .chain_update(i2osp_2(input.as_ref().len()).map_err(|_| Error::Input)?)
+        .chain_update(input.as_ref());
+    if let Some(info) = info {
+        hash = hash
+            .chain_update(i2osp_2(info.as_ref().len()).map_err(|_| Error::Input)?)
+            .chain_update(info.as_ref());
+    }
+    Ok(hash
+        .chain_update(i2osp_2(issued_element.as_ref().len()).map_err(|_| Error::Input)?)
+        .chain_update(issued_element)
+        .chain_update(STR_FINALIZE)
+        .finalize())
+}
+
 /// Generates the contextString parameter as defined in
 /// <https://datatracker.ietf.org/doc/draft-irtf-cfrg-voprf/>
 pub(crate) fn create_context_string<CS: CipherSuite>(mode: Mode) -> GenericArray<u8, U11>

src/lib.rs

@@ -8,7 +8,7 @@
 //! An implementation of a verifiable oblivious pseudorandom function (VOPRF)
 //!
 //! Note: This implementation is in sync with
-//! [draft-irtf-cfrg-voprf-10](https://www.ietf.org/archive/id/draft-irtf-cfrg-voprf-10.html),
+//! [draft-irtf-cfrg-voprf-11](https://www.ietf.org/archive/id/draft-irtf-cfrg-voprf-11.html),
 //! but this specification is subject to change, until the final version
 //! published by the IETF.
 //!
@@ -36,18 +36,19 @@
 //!   VOPRF, where a public input can be supplied to the PRF computation
 //!
 //! In all of these modes, the protocol begins with a client blinding, followed
-//! by a server evaluation, and finishes with a client finalization.
+//! by a server evaluation, and finishes with a client finalization and server
+//! evaluation.
 //!
 //! ## Base Mode
 //!
-//! In base mode, an [OprfClient] interacts with an [OprfServer]
+//! In base mode, an [OprfClient] interacts with an [OprfServer] to compute the
-//! to compute the output of the OPRF.
+//! output of the OPRF.
 //!
 //! ### Server Setup
 //!
 //! The protocol begins with a setup phase, in which the server must run
-//! [OprfServer::new()] to produce an instance of itself. This instance
+//! [OprfServer::new()] to produce an instance of itself. This instance must be
-//! must be persisted on the server and used for online client evaluations.
+//! persisted on the server and used for online client evaluations.
 //!
 //! ```
 //! # #[cfg(feature = "ristretto255")]
@@ -64,11 +65,10 @@
 //!
 //! ### Client Blinding
 //!
-//! In the first step, the client chooses an input, and runs
+//! In the first step, the client chooses an input, and runs [OprfClient::blind]
-//! [OprfClient::blind] to produce an [OprfClientBlindResult],
+//! to produce an [OprfClientBlindResult], which consists of a [BlindedElement]
-//! which consists of a [BlindedElement] to be sent to the server and an
+//! to be sent to the server and an [OprfClient] which must be persisted on the
-//! [OprfClient] which must be persisted on the client for the final
+//! client for the final step of the VOPRF protocol.
-//! step of the VOPRF protocol.
 //!
 //! ```
 //! # #[cfg(feature = "ristretto255")]
@@ -84,11 +84,11 @@
 //!     .expect("Unable to construct client");
 //! ```
 //!
-//! ### Server Evaluation
+//! ### Server Blind Evaluation
 //!
 //! In the second step, the server takes as input the message from
 //! [OprfClient::blind] (a [BlindedElement]), and runs
-//! [OprfServer::evaluate] to produce [EvaluationElement] to be sent to
+//! [OprfServer::blind_evaluate] to produce [EvaluationElement] to be sent to
 //! the client.
 //!
 //! ```
@@ -107,13 +107,13 @@
 //! # use voprf::OprfServer;
 //! # let mut server_rng = OsRng;
 //! # let server = OprfServer::<CipherSuite>::new(&mut server_rng).unwrap();
-//! let server_evaluate_result = server.evaluate(&client_blind_result.message);
+//! let server_evaluate_result = server.blind_evaluate(&client_blind_result.message);
 //! ```
 //!
 //! ### Client Finalization
 //!
-//! In the final step, the client takes as input the message from
+//! In the final step on the client side, the client takes as input the message
-//! [OprfServer::evaluate] (an [EvaluationElement]), and runs
+//! from [OprfServer::evaluate] (an [EvaluationElement]), and runs
 //! [OprfClient::finalize] to produce an output for the protocol.
 //!
 //! ```
@@ -132,7 +132,7 @@
 //! # use voprf::OprfServer;
 //! # let mut server_rng = OsRng;
 //! # let server = OprfServer::<CipherSuite>::new(&mut server_rng).unwrap();
-//! # let message = server.evaluate(&client_blind_result.message);
+//! # let message = server.blind_evaluate(&client_blind_result.message);
 //! let client_finalize_result = client_blind_result
 //!     .state
 //!     .finalize(b"input", &message)
@@ -141,10 +141,47 @@
 //! println!("VOPRF output: {:?}", client_finalize_result.to_vec());
 //! ```
 //!
+//! ### Server Evaluation
+//!
+//! Optionally, if the server has direct access to the PRF input, then it need
+//! not perform the oblivious computation and can simply run
+//! [OprfServer::evaluate] to generate an output which matches the output
+//! produced by an execution of the oblivious protocol on the same input and
+//! key.
+//!
+//! ```
+//! # #[cfg(feature = "ristretto255")]
+//! # type CipherSuite = voprf::Ristretto255;
+//! # #[cfg(not(feature = "ristretto255"))]
+//! # type CipherSuite = p256::NistP256;
+//! # use voprf::OprfClient;
+//! # use rand::{rngs::OsRng, RngCore};
+//! #
+//! # let mut client_rng = OsRng;
+//! # let client_blind_result = OprfClient::<CipherSuite>::blind(
+//! #     b"input",
+//! #     &mut client_rng,
+//! # ).expect("Unable to construct client");
+//! # use voprf::OprfServer;
+//! # let mut server_rng = OsRng;
+//! # let server = OprfServer::<CipherSuite>::new(&mut server_rng).unwrap();
+//! # let message = server.blind_evaluate(&client_blind_result.message);
+//! let client_finalize_result = client_blind_result
+//!     .state
+//!     .finalize(b"input", &message)
+//!     .expect("Unable to perform client finalization");
+//!
+//! let server_evaluate_result = server
+//!     .evaluate(b"input")
+//!     .expect("Unable to perform the server evaluation");
+//!
+//! assert_eq!(client_finalize_result, server_evaluate_result);
+//! ```
+//!
 //! ## Verifiable Mode
 //!
-//! In verifiable mode, a [VoprfClient] interacts with a [VoprfServer]
+//! In verifiable mode, a [VoprfClient] interacts with a [VoprfServer] to
-//! to compute the output of the VOPRF. In order to verify the server's
+//! compute the output of the VOPRF. In order to verify the server's
 //! computation, the client checks a server-generated proof against the server's
 //! public key. If the proof fails to verify, then the client does not receive
 //! an output.
@@ -156,8 +193,8 @@
 //! ### Server Setup
 //!
 //! The protocol begins with a setup phase, in which the server must run
-//! [VoprfServer::new()] to produce an instance of itself. This instance
+//! [VoprfServer::new()] to produce an instance of itself. This instance must be
-//! must be persisted on the server and used for online client evaluations.
+//! persisted on the server and used for online client evaluations.
 //!
 //! ```
 //! # #[cfg(feature = "ristretto255")]
@@ -182,10 +219,9 @@
 //! ### Client Blinding
 //!
 //! In the first step, the client chooses an input, and runs
-//! [VoprfClient::blind] to produce a [VoprfClientBlindResult], which
+//! [VoprfClient::blind] to produce a [VoprfClientBlindResult], which consists
-//! consists of a [BlindedElement] to be sent to the server and a
+//! of a [BlindedElement] to be sent to the server and a [VoprfClient] which
-//! [VoprfClient] which must be persisted on the client for the final step
+//! must be persisted on the client for the final step of the VOPRF protocol.
-//! of the VOPRF protocol.
 //!
 //! ```
 //! # #[cfg(feature = "ristretto255")]
@@ -201,11 +237,11 @@
 //!     .expect("Unable to construct client");
 //! ```
 //!
-//! ### Server Evaluation
+//! ### Server Blind Evaluation
 //!
 //! In the second step, the server takes as input the message from
 //! [VoprfClient::blind] (a [BlindedElement]), and runs
-//! [VoprfServer::evaluate] to produce a [VoprfServerEvaluateResult],
+//! [VoprfServer::blind_evaluate] to produce a [VoprfServerEvaluateResult],
 //! which consists of an [EvaluationElement] to be sent to the client along with
 //! a proof.
 //!
@@ -226,15 +262,15 @@
 //! # let mut server_rng = OsRng;
 //! # let server = VoprfServer::<CipherSuite>::new(&mut server_rng).unwrap();
 //! let VoprfServerEvaluateResult { message, proof } =
-//!     server.evaluate(&mut server_rng, &client_blind_result.message);
+//!     server.blind_evaluate(&mut server_rng, &client_blind_result.message);
 //! ```
 //!
 //! ### Client Finalization
 //!
 //! In the final step, the client takes as input the message from
-//! [VoprfServer::evaluate] (an [EvaluationElement]), the proof, and the
+//! [VoprfServer::blind_evaluate] (an [EvaluationElement]), the proof, and the
-//! server's public key, and runs [VoprfClient::finalize] to produce an
+//! server's public key, and runs [VoprfClient::finalize] to produce an output
-//! output for the protocol.
+//! for the protocol.
 //!
 //! ```
 //! # #[cfg(feature = "ristretto255")]
@@ -252,7 +288,7 @@
 //! # use voprf::VoprfServer;
 //! # let mut server_rng = OsRng;
 //! # let server = VoprfServer::<CipherSuite>::new(&mut server_rng).unwrap();
-//! # let server_evaluate_result = server.evaluate(
+//! # let server_evaluate_result = server.blind_evaluate(
 //! #     &mut server_rng,
 //! #     &client_blind_result.message,
 //! # );
@@ -269,6 +305,51 @@
 //! println!("VOPRF output: {:?}", client_finalize_result.to_vec());
 //! ```
 //!
+//! ### Server Evaluation
+//!
+//! Optionally, if the server has direct access to the PRF input, then it need
+//! not perform the oblivious computation and can simply run
+//! [VoprfServer::evaluate] to generate an output which matches the output
+//! produced by an execution of the oblivious protocol on the same input and
+//! key.
+//!
+//! ```
+//! # #[cfg(feature = "ristretto255")]
+//! # type CipherSuite = voprf::Ristretto255;
+//! # #[cfg(not(feature = "ristretto255"))]
+//! # type CipherSuite = p256::NistP256;
+//! # use voprf::VoprfClient;
+//! # use rand::{rngs::OsRng, RngCore};
+//! #
+//! # let mut client_rng = OsRng;
+//! # let client_blind_result = VoprfClient::<CipherSuite>::blind(
+//! #     b"input",
+//! #     &mut client_rng,
+//! # ).expect("Unable to construct client");
+//! # use voprf::VoprfServer;
+//! # let mut server_rng = OsRng;
+//! # let server = VoprfServer::<CipherSuite>::new(&mut server_rng).unwrap();
+//! # let server_evaluate_result = server.blind_evaluate(
+//! #     &mut server_rng,
+//! #     &client_blind_result.message,
+//! # );
+//! let client_finalize_result = client_blind_result
+//!     .state
+//!     .finalize(
+//!         b"input",
+//!         &server_evaluate_result.message,
+//!         &server_evaluate_result.proof,
+//!         server.get_public_key(),
+//!     )
+//!     .expect("Unable to perform client finalization");
+//!
+//! let server_evaluate_result = server
+//!     .evaluate(b"input")
+//!     .expect("Unable to perform the server evaluation");
+//!
+//! assert_eq!(client_finalize_result, server_evaluate_result);
+//! ```
+//!
 //! # Advanced Usage
 //!
 //! There are two additional (and optional) extensions to the core VOPRF
@@ -279,9 +360,9 @@
 //!
 //! It is sometimes desirable to generate only a single, constant-size proof for
 //! an unbounded number of VOPRF evaluations (on arbitrary inputs).
-//! [VoprfClient] and [VoprfServer] support a batch API for handling
+//! [VoprfClient] and [VoprfServer] support a batch API for handling this case.
-//! this case. In the following example, we show how to use the batch API to
+//! In the following example, we show how to use the batch API to produce a
-//! produce a single proof for 10 parallel VOPRF evaluations.
+//! single proof for 10 parallel VOPRF evaluations.
 //!
 //! First, the client produces 10 blindings, storing their resulting states and
 //! messages:
@@ -305,8 +386,8 @@
 //! }
 //! ```
 //!
-//! Next, the server calls the [VoprfServer::batch_evaluate_prepare] and
+//! Next, the server calls the [VoprfServer::batch_blind_evaluate_prepare] and
-//! [VoprfServer::batch_evaluate_finish] function on a set of client
+//! [VoprfServer::batch_blind_evaluate_finish] function on a set of client
 //! messages, to produce a corresponding set of messages to be returned to the
 //! client (returned in the same order), along with a single proof:
 //!
@@ -332,15 +413,15 @@
 //! # use voprf::VoprfServer;
 //! let mut server_rng = OsRng;
 //! # let server = VoprfServer::<CipherSuite>::new(&mut server_rng).unwrap();
-//! let prepared_evaluation_elements = server.batch_evaluate_prepare(client_messages.iter());
+//! let prepared_evaluation_elements = server.batch_blind_evaluate_prepare(client_messages.iter());
 //! let prepared_elements: Vec<_> = prepared_evaluation_elements.collect();
 //! let VoprfServerBatchEvaluateFinishResult { messages, proof } = server
-//!     .batch_evaluate_finish(&mut server_rng, client_messages.iter(), &prepared_elements)
+//!     .batch_blind_evaluate_finish(&mut server_rng, client_messages.iter(), &prepared_elements)
 //!     .expect("Unable to perform server batch evaluate");
 //! let messages: Vec<_> = messages.collect();
 //! ```
 //!
-//! If `alloc` is available, `VoprfServer::batch_evaluate` can be called
+//! If `alloc` is available, `VoprfServer::batch_blind_evaluate` can be called
 //! to avoid having to collect output manually:
 //!
 //! ```
@@ -367,15 +448,15 @@
 //! let mut server_rng = OsRng;
 //! # let server = VoprfServer::<CipherSuite>::new(&mut server_rng).unwrap();
 //! let VoprfServerBatchEvaluateResult { messages, proof } = server
-//!     .batch_evaluate(&mut server_rng, &client_messages)
+//!     .batch_blind_evaluate(&mut server_rng, &client_messages)
 //!     .expect("Unable to perform server batch evaluate");
 //! # }
 //! ```
 //!
-//! Then, the client calls [VoprfClient::batch_finalize] on the client
+//! Then, the client calls [VoprfClient::batch_finalize] on the client states
-//! states saved from the first step, along with the messages returned by the
+//! saved from the first step, along with the messages returned by the server,
-//! server, along with the server's proof, in order to produce a vector of
+//! along with the server's proof, in order to produce a vector of outputs if
-//! outputs if the proof verifies correctly.
+//! the proof verifies correctly.
 //!
 //! ```
 //! # #[cfg(feature = "alloc")] {
@@ -401,7 +482,7 @@
 //! # let mut server_rng = OsRng;
 //! # let server = VoprfServer::<CipherSuite>::new(&mut server_rng).unwrap();
 //! # let VoprfServerBatchEvaluateResult { messages, proof } = server
-//! #     .batch_evaluate(&mut server_rng, &client_messages)
+//! #     .batch_blind_evaluate(&mut server_rng, &client_messages)
 //! #     .expect("Unable to perform server batch evaluate");
 //! let client_batch_finalize_result = VoprfClient::batch_finalize(
 //!     &[b"input"; 10],
@@ -420,17 +501,17 @@
 //! ## Metadata
 //!
 //! The optional metadata parameter included in the POPRF mode allows clients
-//! and servers to cryptographically bind additional data to the
+//! and servers to cryptographically bind additional data to the VOPRF output.
-//! VOPRF output. This metadata is known to both parties at the start of the
+//! This metadata is known to both parties at the start of the protocol, and is
-//! protocol, and is inserted under the server's evaluate step and the client's
+//! inserted under the server's blind evaluate step and the client's finalize
-//! finalize step. This metadata can be constructed with some type of
+//! step. This metadata can be constructed with some type of higher-level domain
-//! higher-level domain separation to avoid cross-protocol attacks or related
+//! separation to avoid cross-protocol attacks or related issues.
-//! issues.
 //!
 //! The API for POPRF mode is similar to VOPRF mode, except that a [PoprfServer]
 //! and [PoprfClient] are used, and that each of the functions accept an
 //! additional (and optional) info parameter which represents the public input.
-//! See <https://www.ietf.org/archive/id/draft-irtf-cfrg-voprf-10.html#name-poprf-public-input>
+//! See
+//! <https://www.ietf.org/archive/id/draft-irtf-cfrg-voprf-11.html#name-poprf-public-input>
 //! for more detailed information on how this public input should be used.
 //!
 //! # Features
@@ -464,7 +545,8 @@
 //!   automatically enable the `ristretto255-u64` feature and requires Rust
 //!   nightly.
 //!
-//! [curve25519-dalek]: (https://doc.dalek.rs/curve25519_dalek/index.html#backends-and-features)
+//! [curve25519-dalek]:
+//!     (https://doc.dalek.rs/curve25519_dalek/index.html#backends-and-features)
 
 #![cfg_attr(not(test), deny(unsafe_code))]
 #![no_std]

src/oprf.rs

@@ -17,8 +17,8 @@ use generic_array::GenericArray;
 use rand_core::{CryptoRng, RngCore};
 
 use crate::common::{
-    derive_key_internal, deterministic_blind_unchecked, i2osp_2, BlindedElement, EvaluationElement,
+    derive_key_internal, deterministic_blind_unchecked, hash_to_group, i2osp_2,
-    Mode, STR_FINALIZE,
+    server_evaluate_hash_input, BlindedElement, EvaluationElement, Mode, STR_FINALIZE,
 };
 #[cfg(feature = "serde")]
 use crate::serialization::serde::Scalar;
@@ -202,9 +202,25 @@ where
     /// Computes the second step for the multiplicative blinding version of
     /// DH-OPRF. This message is sent from the server (who holds the OPRF key)
     /// to the client.
-    pub fn evaluate(&self, blinded_element: &BlindedElement<CS>) -> EvaluationElement<CS> {
+    pub fn blind_evaluate(&self, blinded_element: &BlindedElement<CS>) -> EvaluationElement<CS> {
         EvaluationElement(blinded_element.0 * &self.sk)
     }
+
+    /// Computes the output of the OPRF on the server side
+    ///
+    /// # Errors
+    /// [`Error::Input`]  if the `input` is longer then [`u16::MAX`].
+    pub fn evaluate(&self, input: &[u8]) -> Result<Output<<CS as CipherSuite>::Hash>> {
+        let input_element = hash_to_group::<CS>(input, Mode::Oprf)?;
+        if CS::Group::is_identity_elem(input_element).into() {
+            return Err(Error::Input);
+        };
+        let evaluated_element = input_element * &self.sk;
+
+        let issued_element = CS::Group::serialize_elem(evaluated_element);
+
+        server_evaluate_hash_input::<CS>(input, None, issued_element)
+    }
 }
 
 /////////////////////////
@@ -312,7 +328,7 @@ mod tests {
         let mut rng = OsRng;
         let client_blind_result = OprfClient::<CS>::blind(input, &mut rng).unwrap();
         let server = OprfServer::<CS>::new(&mut rng).unwrap();
-        let message = server.evaluate(&client_blind_result.message);
+        let message = server.blind_evaluate(&client_blind_result.message);
         let client_finalize_result = client_blind_result.state.finalize(input, &message).unwrap();
         let res2 = prf::<CS>(input, server.get_private_key(), &[], Mode::Oprf);
         assert_eq!(client_finalize_result, res2);
@@ -343,6 +359,34 @@ mod tests {
         assert_eq!(client_finalize_result, res2);
     }
 
+    fn server_evaluate<CS: CipherSuite>()
+    where
+        <CS::Hash as OutputSizeUser>::OutputSize:
+            IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
+    {
+        let input = b"input";
+        let mut rng = OsRng;
+        let client_blind_result = OprfClient::<CS>::blind(input, &mut rng).unwrap();
+        let server = OprfServer::<CS>::new(&mut rng).unwrap();
+        let server_result = server.blind_evaluate(&client_blind_result.message);
+
+        let client_finalize = client_blind_result
+            .state
+            .finalize(input, &server_result)
+            .unwrap();
+
+        // We expect the outputs from client and server to be equal given an identical
+        // input
+        let server_evaluate = server.evaluate(input).unwrap();
+        assert_eq!(client_finalize, server_evaluate);
+
+        // We expect the outputs from client and server to be different given different
+        // inputs
+        let wrong_input = b"wrong input";
+        let server_evaluate = server.evaluate(wrong_input).unwrap();
+        assert!(client_finalize != server_evaluate);
+    }
+
     fn zeroize_oprf_client<CS: CipherSuite>()
     where
         <CS::Hash as OutputSizeUser>::OutputSize:
@@ -370,7 +414,7 @@ mod tests {
         let mut rng = OsRng;
         let client_blind_result = OprfClient::<CS>::blind(input, &mut rng).unwrap();
         let server = OprfServer::<CS>::new(&mut rng).unwrap();
-        let mut message = server.evaluate(&client_blind_result.message);
+        let mut message = server.blind_evaluate(&client_blind_result.message);
 
         let mut state = server;
         unsafe { ptr::drop_in_place(&mut state) };
@@ -390,6 +434,7 @@ mod tests {
 
             base_retrieval::<Ristretto255>();
             base_inversion_unsalted::<Ristretto255>();
+            server_evaluate::<Ristretto255>();
 
             zeroize_oprf_client::<Ristretto255>();
             zeroize_oprf_server::<Ristretto255>();
@@ -397,6 +442,7 @@ mod tests {
 
         base_retrieval::<NistP256>();
         base_inversion_unsalted::<NistP256>();
+        server_evaluate::<NistP256>();
 
         zeroize_oprf_client::<NistP256>();
         zeroize_oprf_server::<NistP256>();

src/poprf.rs

@@ -20,9 +20,10 @@ use generic_array::GenericArray;
 use rand_core::{CryptoRng, RngCore};
 
 use crate::common::{
-    create_context_string, derive_keypair, deterministic_blind_unchecked, generate_proof, i2osp_2,
+    create_context_string, derive_keypair, deterministic_blind_unchecked, generate_proof,
-    verify_proof, BlindedElement, EvaluationElement, Mode, PreparedEvaluationElement, Proof,
+    hash_to_group, i2osp_2, server_evaluate_hash_input, verify_proof, BlindedElement,
-    STR_FINALIZE, STR_HASH_TO_SCALAR, STR_INFO,
+    EvaluationElement, Mode, PreparedEvaluationElement, Proof, STR_FINALIZE, STR_HASH_TO_SCALAR,
+    STR_INFO,
 };
 #[cfg(feature = "serde")]
 use crate::serialization::serde::{Element, Scalar};
@@ -89,8 +90,8 @@ where
     /// # Errors
     /// [`Error::Input`] if the `input` is empty or longer than [`u16::MAX`].
     pub fn blind<R: RngCore + CryptoRng>(
-        blinding_factor_rng: &mut R,
         input: &[u8],
+        blinding_factor_rng: &mut R,
     ) -> Result<PoprfClientBlindResult<CS>> {
         let blind = CS::Group::random_scalar(blinding_factor_rng);
         Self::deterministic_blind_unchecked_inner(input, blind)
@@ -248,7 +249,7 @@ where
     /// # Errors
     /// - [`Error::Info`] if the `info` is longer than `u16::MAX`.
     /// - [`Error::Protocol`] if the protocol fails and can't be completed.
-    pub fn evaluate<R: RngCore + CryptoRng>(
+    pub fn blind_evaluate<R: RngCore + CryptoRng>(
         &self,
         rng: &mut R,
         blinded_element: &BlindedElement<CS>,
@@ -257,7 +258,7 @@ where
         let PoprfServerBatchEvaluatePrepareResult {
             mut prepared_evaluation_elements,
             prepared_tweak,
-        } = self.batch_evaluate_prepare(iter::once(blinded_element), info)?;
+        } = self.batch_blind_evaluate_prepare(iter::once(blinded_element), info)?;
 
         let prepared_evaluation_element = prepared_evaluation_elements.next().unwrap();
         let prepared_evaluation_elements = core::array::from_ref(&prepared_evaluation_element);
@@ -265,7 +266,7 @@ where
         let PoprfServerBatchEvaluateFinishResult {
             mut messages,
             proof,
-        } = Self::batch_evaluate_finish(
+        } = Self::batch_blind_evaluate_finish(
             rng,
             iter::once(blinded_element),
             prepared_evaluation_elements,
@@ -286,7 +287,7 @@ where
     /// - [`Error::Info`] if the `info` is longer than `u16::MAX`.
     /// - [`Error::Protocol`] if the protocol fails and can't be completed.
     #[cfg(feature = "alloc")]
-    pub fn batch_evaluate<'a, R: RngCore + CryptoRng, IE>(
+    pub fn batch_blind_evaluate<'a, R: RngCore + CryptoRng, IE>(
         &self,
         rng: &mut R,
         blinded_elements: &'a IE,
@@ -300,13 +301,13 @@ where
         let PoprfServerBatchEvaluatePrepareResult {
             prepared_evaluation_elements,
             prepared_tweak,
-        } = self.batch_evaluate_prepare(blinded_elements.into_iter(), info)?;
+        } = self.batch_blind_evaluate_prepare(blinded_elements.into_iter(), info)?;
 
         let prepared_evaluation_elements: Vec<_> = prepared_evaluation_elements.collect();
 
         // This can't fail because we know the size of the inputs.
         let PoprfServerBatchEvaluateFinishResult { messages, proof } =
-            Self::batch_evaluate_finish::<_, _, Vec<_>>(
+            Self::batch_blind_evaluate_finish::<_, _, Vec<_>>(
                 rng,
                 blinded_elements.into_iter(),
                 &prepared_evaluation_elements,
@@ -319,15 +320,15 @@ where
         Ok(PoprfServerBatchEvaluateResult { messages, proof })
     }
 
-    /// Alternative version of `batch_evaluate` without
+    /// Alternative version of `batch_blind_evaluate` without
     /// memory allocation. Returned [`PreparedEvaluationElement`] have to
     /// be [`collect`](Iterator::collect)ed and passed into
-    /// [`batch_evaluate_finish`](Self::batch_evaluate_finish).
+    /// [`batch_blind_evaluate_finish`](Self::batch_blind_evaluate_finish).
     ///
     /// # Errors
     /// - [`Error::Info`] if the `info` is longer than `u16::MAX`.
     /// - [`Error::Protocol`] if the protocol fails and can't be completed.
-    pub fn batch_evaluate_prepare<'a, I: Iterator<Item = &'a BlindedElement<CS>>>(
+    pub fn batch_blind_evaluate_prepare<'a, I: Iterator<Item = &'a BlindedElement<CS>>>(
         &self,
         blinded_elements: I,
         info: Option<&[u8]>,
@@ -349,14 +350,14 @@ where
         })
     }
 
-    /// See [`batch_evaluate_prepare`](Self::batch_evaluate_prepare) for more
+    /// See [`batch_blind_evaluate_prepare`](Self::batch_blind_evaluate_prepare)
-    /// details.
+    /// for more details.
     ///
     /// # Errors
     /// [`Error::Batch`] if the number of `blinded_elements` and
     /// `prepared_evaluation_elements` don't match or is longer then
     /// [`u16::MAX`]
-    pub fn batch_evaluate_finish<
+    pub fn batch_blind_evaluate_finish<
         'a,
         'b,
         R: RngCore + CryptoRng,
@@ -398,6 +399,29 @@ where
         Ok(PoprfServerBatchEvaluateFinishResult { messages, proof })
     }
 
+    /// Computes the output of the VOPRF on the server side
+    ///
+    /// # Errors
+    /// [`Error::Input`]  if the `input` is longer then [`u16::MAX`].
+    pub fn evaluate(
+        &self,
+        input: &[u8],
+        info: Option<&[u8]>,
+    ) -> Result<Output<<CS as CipherSuite>::Hash>> {
+        let input_element = hash_to_group::<CS>(input, Mode::Poprf)?;
+        if CS::Group::is_identity_elem(input_element).into() {
+            return Err(Error::Input);
+        };
+
+        let tweak = compute_tweak::<CS>(self.sk, info)?;
+
+        let evaluated_element = input_element * &CS::Group::invert_scalar(tweak);
+
+        let issued_element = CS::Group::serialize_elem(evaluated_element);
+
+        server_evaluate_hash_input::<CS>(input, info, issued_element)
+    }
+
     /// Retrieves the server's public key
     pub fn get_public_key(&self) -> <CS::Group as Group>::Elem {
         self.pk
@@ -808,9 +832,9 @@ mod tests {
         let info = b"info";
         let mut rng = OsRng;
         let server = PoprfServer::<CS>::new(&mut rng).unwrap();
-        let client_blind_result = PoprfClient::<CS>::blind(&mut rng, input).unwrap();
+        let client_blind_result = PoprfClient::<CS>::blind(input, &mut rng).unwrap();
         let server_result = server
-            .evaluate(&mut rng, &client_blind_result.message, Some(info))
+            .blind_evaluate(&mut rng, &client_blind_result.message, Some(info))
             .unwrap();
         let client_finalize_result = client_blind_result
             .state
@@ -835,9 +859,9 @@ mod tests {
         let info = b"info";
         let mut rng = OsRng;
         let server = PoprfServer::<CS>::new(&mut rng).unwrap();
-        let client_blind_result = PoprfClient::<CS>::blind(&mut rng, input).unwrap();
+        let client_blind_result = PoprfClient::<CS>::blind(input, &mut rng).unwrap();
         let server_result = server
-            .evaluate(&mut rng, &client_blind_result.message, Some(info))
+            .blind_evaluate(&mut rng, &client_blind_result.message, Some(info))
             .unwrap();
         let wrong_pk = {
             let dst = GenericArray::from(STR_HASH_TO_GROUP)
@@ -855,6 +879,43 @@ mod tests {
         assert!(client_finalize_result.is_err());
     }
 
+    fn verifiable_server_evaluate<CS: CipherSuite>()
+    where
+        <CS::Hash as OutputSizeUser>::OutputSize:
+            IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
+    {
+        let input = b"input";
+        let info = Some(b"info".as_slice());
+        let mut rng = OsRng;
+        let client_blind_result = PoprfClient::<CS>::blind(input, &mut rng).unwrap();
+        let server = PoprfServer::<CS>::new(&mut rng).unwrap();
+        let server_result = server
+            .blind_evaluate(&mut rng, &client_blind_result.message, info)
+            .unwrap();
+
+        let client_finalize = client_blind_result
+            .state
+            .finalize(
+                input,
+                &server_result.message,
+                &server_result.proof,
+                server.get_public_key(),
+                info,
+            )
+            .unwrap();
+
+        // We expect the outputs from client and server to be equal given an identical
+        // input
+        let server_evaluate = server.evaluate(input, info).unwrap();
+        assert_eq!(client_finalize, server_evaluate);
+
+        // We expect the outputs from client and server to be different given different
+        // inputs
+        let wrong_input = b"wrong input";
+        let server_evaluate = server.evaluate(wrong_input, info).unwrap();
+        assert!(client_finalize != server_evaluate);
+    }
+
     fn zeroize_verifiable_client<CS: CipherSuite>()
     where
         <CS::Hash as OutputSizeUser>::OutputSize:
@@ -864,7 +925,7 @@ mod tests {
     {
         let input = b"input";
         let mut rng = OsRng;
-        let client_blind_result = PoprfClient::<CS>::blind(&mut rng, input).unwrap();
+        let client_blind_result = PoprfClient::<CS>::blind(input, &mut rng).unwrap();
 
         let mut state = client_blind_result.state;
         unsafe { ptr::drop_in_place(&mut state) };
@@ -888,9 +949,9 @@ mod tests {
         let info = b"info";
         let mut rng = OsRng;
         let server = PoprfServer::<CS>::new(&mut rng).unwrap();
-        let client_blind_result = PoprfClient::<CS>::blind(&mut rng, input).unwrap();
+        let client_blind_result = PoprfClient::<CS>::blind(input, &mut rng).unwrap();
         let server_result = server
-            .evaluate(&mut rng, &client_blind_result.message, Some(info))
+            .blind_evaluate(&mut rng, &client_blind_result.message, Some(info))
             .unwrap();
 
         let mut state = server;
@@ -916,6 +977,7 @@ mod tests {
 
             verifiable_retrieval::<Ristretto255>();
             verifiable_bad_public_key::<Ristretto255>();
+            verifiable_server_evaluate::<Ristretto255>();
 
             zeroize_verifiable_client::<Ristretto255>();
             zeroize_verifiable_server::<Ristretto255>();
@@ -923,6 +985,7 @@ mod tests {
 
         verifiable_retrieval::<NistP256>();
         verifiable_bad_public_key::<NistP256>();
+        verifiable_server_evaluate::<NistP256>();
 
         zeroize_verifiable_client::<NistP256>();
         zeroize_verifiable_server::<NistP256>();

src/tests/test_cfrg_vectors.rs

@@ -101,8 +101,9 @@ fn test_vectors() -> Result<()> {
         assert_ne!(ristretto_oprf_tvs.len(), 0);
         test_oprf_seed_to_key::<Ristretto255>(&ristretto_oprf_tvs)?;
         test_oprf_blind::<Ristretto255>(&ristretto_oprf_tvs)?;
-        test_oprf_evaluate::<Ristretto255>(&ristretto_oprf_tvs)?;
+        test_oprf_blind_evaluate::<Ristretto255>(&ristretto_oprf_tvs)?;
         test_oprf_finalize::<Ristretto255>(&ristretto_oprf_tvs)?;
+        test_oprf_evaluate::<Ristretto255>(&ristretto_oprf_tvs)?;
 
         let ristretto_voprf_tvs = json_to_test_vectors!(
             rfc,
@@ -112,8 +113,9 @@ fn test_vectors() -> Result<()> {
         assert_ne!(ristretto_voprf_tvs.len(), 0);
         test_voprf_seed_to_key::<Ristretto255>(&ristretto_voprf_tvs)?;
         test_voprf_blind::<Ristretto255>(&ristretto_voprf_tvs)?;
-        test_voprf_evaluate::<Ristretto255>(&ristretto_voprf_tvs)?;
+        test_voprf_blind_evaluate::<Ristretto255>(&ristretto_voprf_tvs)?;
         test_voprf_finalize::<Ristretto255>(&ristretto_voprf_tvs)?;
+        test_voprf_evaluate::<Ristretto255>(&ristretto_voprf_tvs)?;
 
         let ristretto_poprf_tvs = json_to_test_vectors!(
             rfc,
@@ -123,8 +125,9 @@ fn test_vectors() -> Result<()> {
         assert_ne!(ristretto_poprf_tvs.len(), 0);
         test_poprf_seed_to_key::<Ristretto255>(&ristretto_poprf_tvs)?;
         test_poprf_blind::<Ristretto255>(&ristretto_poprf_tvs)?;
-        test_poprf_evaluate::<Ristretto255>(&ristretto_poprf_tvs)?;
+        test_poprf_blind_evaluate::<Ristretto255>(&ristretto_poprf_tvs)?;
         test_poprf_finalize::<Ristretto255>(&ristretto_poprf_tvs)?;
+        test_poprf_evaluate::<Ristretto255>(&ristretto_poprf_tvs)?;
     }
 
     let p256_oprf_tvs =
@@ -132,24 +135,27 @@ fn test_vectors() -> Result<()> {
     assert_ne!(p256_oprf_tvs.len(), 0);
     test_oprf_seed_to_key::<NistP256>(&p256_oprf_tvs)?;
     test_oprf_blind::<NistP256>(&p256_oprf_tvs)?;
-    test_oprf_evaluate::<NistP256>(&p256_oprf_tvs)?;
+    test_oprf_blind_evaluate::<NistP256>(&p256_oprf_tvs)?;
     test_oprf_finalize::<NistP256>(&p256_oprf_tvs)?;
+    test_oprf_evaluate::<NistP256>(&p256_oprf_tvs)?;
 
     let p256_voprf_tvs =
         json_to_test_vectors!(rfc, String::from("P-256, SHA-256"), String::from("VOPRF"));
     assert_ne!(p256_voprf_tvs.len(), 0);
     test_voprf_seed_to_key::<NistP256>(&p256_voprf_tvs)?;
     test_voprf_blind::<NistP256>(&p256_voprf_tvs)?;
-    test_voprf_evaluate::<NistP256>(&p256_voprf_tvs)?;
+    test_voprf_blind_evaluate::<NistP256>(&p256_voprf_tvs)?;
     test_voprf_finalize::<NistP256>(&p256_voprf_tvs)?;
+    test_voprf_evaluate::<NistP256>(&p256_voprf_tvs)?;
 
     let p256_poprf_tvs =
         json_to_test_vectors!(rfc, String::from("P-256, SHA-256"), String::from("POPRF"));
     assert_ne!(p256_poprf_tvs.len(), 0);
     test_poprf_seed_to_key::<NistP256>(&p256_poprf_tvs)?;
     test_poprf_blind::<NistP256>(&p256_poprf_tvs)?;
-    test_poprf_evaluate::<NistP256>(&p256_poprf_tvs)?;
+    test_poprf_blind_evaluate::<NistP256>(&p256_poprf_tvs)?;
     test_poprf_finalize::<NistP256>(&p256_poprf_tvs)?;
+    test_poprf_evaluate::<NistP256>(&p256_poprf_tvs)?;
 
     Ok(())
 }
@@ -286,7 +292,7 @@ where
 }
 
 // Tests sksm, blinded_element -> evaluation_element
-fn test_oprf_evaluate<CS: CipherSuite>(tvs: &[VOPRFTestVectorParameters]) -> Result<()>
+fn test_oprf_blind_evaluate<CS: CipherSuite>(tvs: &[VOPRFTestVectorParameters]) -> Result<()>
 where
     <CS::Hash as OutputSizeUser>::OutputSize:
         IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
@@ -294,7 +300,7 @@ where
     for parameters in tvs {
         for i in 0..parameters.input.len() {
             let server = OprfServer::<CS>::new_with_key(&parameters.sksm)?;
-            let message = server.evaluate(&BlindedElement::deserialize(
+            let message = server.blind_evaluate(&BlindedElement::deserialize(
                 &parameters.blinded_element[i],
             )?);
 
@@ -307,7 +313,7 @@ where
     Ok(())
 }
 
-fn test_voprf_evaluate<CS: CipherSuite>(tvs: &[VOPRFTestVectorParameters]) -> Result<()>
+fn test_voprf_blind_evaluate<CS: CipherSuite>(tvs: &[VOPRFTestVectorParameters]) -> Result<()>
 where
     <CS::Hash as OutputSizeUser>::OutputSize:
         IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
@@ -323,10 +329,11 @@ where
             blinded_elements.push(BlindedElement::deserialize(blinded_element_bytes)?);
         }
 
-        let prepared_evaluation_elements = server.batch_evaluate_prepare(blinded_elements.iter());
+        let prepared_evaluation_elements =
+            server.batch_blind_evaluate_prepare(blinded_elements.iter());
         let prepared_elements: Vec<_> = prepared_evaluation_elements.collect();
-        let VoprfServerBatchEvaluateFinishResult { messages, proof } =
+        let VoprfServerBatchEvaluateFinishResult { messages, proof } = server
-            server.batch_evaluate_finish(&mut rng, blinded_elements.iter(), &prepared_elements)?;
+            .batch_blind_evaluate_finish(&mut rng, blinded_elements.iter(), &prepared_elements)?;
         let messages: Vec<_> = messages.collect();
 
         for (parameter, message) in parameters.evaluation_element.iter().zip(messages) {
@@ -338,7 +345,7 @@ where
     Ok(())
 }
 
-fn test_poprf_evaluate<CS: CipherSuite>(tvs: &[VOPRFTestVectorParameters]) -> Result<()>
+fn test_poprf_blind_evaluate<CS: CipherSuite>(tvs: &[VOPRFTestVectorParameters]) -> Result<()>
 where
     <CS::Hash as OutputSizeUser>::OutputSize:
         IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
@@ -357,10 +364,10 @@ where
         let PoprfServerBatchEvaluatePrepareResult {
             prepared_evaluation_elements,
             prepared_tweak,
-        } = server.batch_evaluate_prepare(blinded_elements.iter(), Some(&parameters.info))?;
+        } = server.batch_blind_evaluate_prepare(blinded_elements.iter(), Some(&parameters.info))?;
         let prepared_evaluation_elements: Vec<_> = prepared_evaluation_elements.collect();
         let PoprfServerBatchEvaluateFinishResult { messages, proof } =
-            PoprfServer::batch_evaluate_finish::<_, _, Vec<_>>(
+            PoprfServer::batch_blind_evaluate_finish::<_, _, Vec<_>>(
                 &mut rng,
                 blinded_elements.iter(),
                 &prepared_evaluation_elements,
@@ -476,3 +483,56 @@ where
     }
     Ok(())
 }
+
+// Tests input, sksm -> output
+fn test_oprf_evaluate<CS: CipherSuite>(tvs: &[VOPRFTestVectorParameters]) -> Result<()>
+where
+    <CS::Hash as OutputSizeUser>::OutputSize:
+        IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
+{
+    for parameters in tvs {
+        for i in 0..parameters.input.len() {
+            let server = OprfServer::<CS>::new_with_key(&parameters.sksm)?;
+
+            let server_evaluate_result = server.evaluate(&parameters.input[i])?;
+
+            assert_eq!(&parameters.output[i], &server_evaluate_result.to_vec());
+        }
+    }
+    Ok(())
+}
+
+fn test_voprf_evaluate<CS: CipherSuite>(tvs: &[VOPRFTestVectorParameters]) -> Result<()>
+where
+    <CS::Hash as OutputSizeUser>::OutputSize:
+        IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
+{
+    for parameters in tvs {
+        for i in 0..parameters.input.len() {
+            let server = VoprfServer::<CS>::new_with_key(&parameters.sksm)?;
+
+            let server_evaluate_result = server.evaluate(&parameters.input[i])?;
+
+            assert_eq!(&parameters.output[i], &server_evaluate_result.to_vec());
+        }
+    }
+    Ok(())
+}
+
+fn test_poprf_evaluate<CS: CipherSuite>(tvs: &[VOPRFTestVectorParameters]) -> Result<()>
+where
+    <CS::Hash as OutputSizeUser>::OutputSize:
+        IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
+{
+    for parameters in tvs {
+        for i in 0..parameters.input.len() {
+            let server = PoprfServer::<CS>::new_with_key(&parameters.sksm)?;
+
+            let server_evaluate_result =
+                server.evaluate(&parameters.input[i], Some(&parameters.info))?;
+
+            assert_eq!(&parameters.output[i], &server_evaluate_result.to_vec());
+        }
+    }
+    Ok(())
+}

src/voprf.rs

@@ -19,8 +19,9 @@ use generic_array::GenericArray;
 use rand_core::{CryptoRng, RngCore};
 
 use crate::common::{
-    derive_keypair, deterministic_blind_unchecked, generate_proof, i2osp_2, verify_proof,
+    derive_keypair, deterministic_blind_unchecked, generate_proof, hash_to_group, i2osp_2,
-    BlindedElement, EvaluationElement, Mode, PreparedEvaluationElement, Proof, STR_FINALIZE,
+    server_evaluate_hash_input, verify_proof, BlindedElement, EvaluationElement, Mode,
+    PreparedEvaluationElement, Proof, STR_FINALIZE,
 };
 #[cfg(feature = "serde")]
 use crate::serialization::serde::{Element, Scalar};
@@ -253,13 +254,13 @@ where
     /// Computes the second step for the multiplicative blinding version of
     /// DH-OPRF. This message is sent from the server (who holds the OPRF key)
     /// to the client.
-    pub fn evaluate<R: RngCore + CryptoRng>(
+    pub fn blind_evaluate<R: RngCore + CryptoRng>(
         &self,
         rng: &mut R,
         blinded_element: &BlindedElement<CS>,
     ) -> VoprfServerEvaluateResult<CS> {
         let mut prepared_evaluation_elements =
-            self.batch_evaluate_prepare(iter::once(blinded_element));
+            self.batch_blind_evaluate_prepare(iter::once(blinded_element));
         let prepared_evaluation_element = [prepared_evaluation_elements.next().unwrap()];
 
         // This can't fail because we know the size of the inputs.
@@ -267,7 +268,7 @@ where
             mut messages,
             proof,
         } = self
-            .batch_evaluate_finish(
+            .batch_blind_evaluate_finish(
                 rng,
                 iter::once(blinded_element),
                 &prepared_evaluation_element,
@@ -286,7 +287,7 @@ where
     /// [`Error::Batch`] if the number of `blinded_elements` and
     /// `evaluation_elements` don't match or is longer then [`u16::MAX`]
     #[cfg(feature = "alloc")]
-    pub fn batch_evaluate<'a, R: RngCore + CryptoRng, I>(
+    pub fn batch_blind_evaluate<'a, R: RngCore + CryptoRng, I>(
         &self,
         rng: &mut R,
         blinded_elements: &'a I,
@@ -297,10 +298,10 @@ where
         <&'a I as IntoIterator>::IntoIter: ExactSizeIterator,
     {
         let prepared_evaluation_elements = self
-            .batch_evaluate_prepare(blinded_elements.into_iter())
+            .batch_blind_evaluate_prepare(blinded_elements.into_iter())
             .collect();
         let VoprfServerBatchEvaluateFinishResult { messages, proof } = self
-            .batch_evaluate_finish::<_, _, Vec<_>>(
+            .batch_blind_evaluate_finish::<_, _, Vec<_>>(
                 rng,
                 blinded_elements.into_iter(),
                 &prepared_evaluation_elements,
@@ -310,11 +311,11 @@ where
         Ok(VoprfServerBatchEvaluateResult { messages, proof })
     }
 
-    /// Alternative version of `batch_evaluate` without
+    /// Alternative version of `batch_blind_evaluate` without memory allocation.
-    /// memory allocation. Returned [`PreparedEvaluationElement`] have to be
+    /// Returned [`PreparedEvaluationElement`] have to be
     /// [`collect`](Iterator::collect)ed and passed into
-    /// [`batch_evaluate_finish`](Self::batch_evaluate_finish).
+    /// [`batch_blind_evaluate_finish`](Self::batch_blind_evaluate_finish).
-    pub fn batch_evaluate_prepare<'a, I: Iterator<Item = &'a BlindedElement<CS>>>(
+    pub fn batch_blind_evaluate_prepare<'a, I: Iterator<Item = &'a BlindedElement<CS>>>(
         &self,
         blinded_elements: I,
     ) -> VoprfServerBatchEvaluatePreparedEvaluationElements<CS, I>
@@ -328,13 +329,13 @@ where
             })
     }
 
-    /// See [`batch_evaluate_prepare`](Self::batch_evaluate_prepare) for more
+    /// See [`batch_blind_evaluate_prepare`](Self::batch_blind_evaluate_prepare)
-    /// details.
+    /// for more details.
     ///
     /// # Errors
     /// [`Error::Batch`] if the number of `blinded_elements` and
     /// `evaluation_elements` don't match or is longer then [`u16::MAX`]
-    pub fn batch_evaluate_finish<
+    pub fn batch_blind_evaluate_finish<
         'a,
         'b,
         R: RngCore + CryptoRng,
@@ -371,6 +372,22 @@ where
         Ok(VoprfServerBatchEvaluateFinishResult { messages, proof })
     }
 
+    /// Computes the output of the POPRF on the server side
+    ///
+    /// # Errors
+    /// [`Error::Input`]  if the `input` is longer then [`u16::MAX`].
+    pub fn evaluate(&self, input: &[u8]) -> Result<Output<<CS as CipherSuite>::Hash>> {
+        let input_element = hash_to_group::<CS>(input, Mode::Voprf)?;
+        if CS::Group::is_identity_elem(input_element).into() {
+            return Err(Error::Input);
+        };
+        let evaluated_element = input_element * &self.sk;
+
+        let issued_element = CS::Group::serialize_elem(evaluated_element);
+
+        server_evaluate_hash_input::<CS>(input, None, issued_element)
+    }
+
     /// Retrieves the server's public key
     pub fn get_public_key(&self) -> <CS::Group as Group>::Elem {
         self.pk
@@ -431,7 +448,7 @@ where
 }
 
 /// Concrete type of [`EvaluationElement`]s returned by
-/// [`VoprfServer::batch_evaluate_prepare`].
+/// [`VoprfServer::batch_blind_evaluate_prepare`].
 pub type VoprfServerBatchEvaluatePreparedEvaluationElements<CS, I> = Map<
     Zip<I, Repeat<<<CS as CipherSuite>::Group as Group>::Scalar>>,
     fn(
@@ -608,7 +625,7 @@ mod tests {
         let mut rng = OsRng;
         let client_blind_result = VoprfClient::<CS>::blind(input, &mut rng).unwrap();
         let server = VoprfServer::<CS>::new(&mut rng).unwrap();
-        let server_result = server.evaluate(&mut rng, &client_blind_result.message);
+        let server_result = server.blind_evaluate(&mut rng, &client_blind_result.message);
         let client_finalize_result = client_blind_result
             .state
             .finalize(
@@ -642,10 +659,10 @@ mod tests {
         }
         let server = VoprfServer::<CS>::new(&mut rng).unwrap();
         let prepared_evaluation_elements: Vec<_> = server
-            .batch_evaluate_prepare(client_messages.iter())
+            .batch_blind_evaluate_prepare(client_messages.iter())
             .collect();
         let VoprfServerBatchEvaluateFinishResult { messages, proof } = server
-            .batch_evaluate_finish(
+            .batch_blind_evaluate_finish(
                 &mut rng,
                 client_messages.iter(),
                 &prepared_evaluation_elements,
@@ -690,10 +707,10 @@ mod tests {
         }
         let server = VoprfServer::<CS>::new(&mut rng).unwrap();
         let prepared_evaluation_elements: Vec<_> = server
-            .batch_evaluate_prepare(client_messages.iter())
+            .batch_blind_evaluate_prepare(client_messages.iter())
             .collect();
         let VoprfServerBatchEvaluateFinishResult { messages, proof } = server
-            .batch_evaluate_finish(
+            .batch_blind_evaluate_finish(
                 &mut rng,
                 client_messages.iter(),
                 &prepared_evaluation_elements,
@@ -720,7 +737,7 @@ mod tests {
         let mut rng = OsRng;
         let client_blind_result = VoprfClient::<CS>::blind(input, &mut rng).unwrap();
         let server = VoprfServer::<CS>::new(&mut rng).unwrap();
-        let server_result = server.evaluate(&mut rng, &client_blind_result.message);
+        let server_result = server.blind_evaluate(&mut rng, &client_blind_result.message);
         let wrong_pk = {
             let dst = GenericArray::from(STR_HASH_TO_GROUP)
                 .concat(create_context_string::<CS>(Mode::Oprf));
@@ -736,6 +753,39 @@ mod tests {
         assert!(client_finalize_result.is_err());
     }
 
+    fn verifiable_server_evaluate<CS: CipherSuite>()
+    where
+        <CS::Hash as OutputSizeUser>::OutputSize:
+            IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
+    {
+        let input = b"input";
+        let mut rng = OsRng;
+        let client_blind_result = VoprfClient::<CS>::blind(input, &mut rng).unwrap();
+        let server = VoprfServer::<CS>::new(&mut rng).unwrap();
+        let server_result = server.blind_evaluate(&mut rng, &client_blind_result.message);
+
+        let client_finalize = client_blind_result
+            .state
+            .finalize(
+                input,
+                &server_result.message,
+                &server_result.proof,
+                server.get_public_key(),
+            )
+            .unwrap();
+
+        // We expect the outputs from client and server to be equal given an identical
+        // input
+        let server_evaluate = server.evaluate(input).unwrap();
+        assert_eq!(client_finalize, server_evaluate);
+
+        // We expect the outputs from client and server to be different given different
+        // inputs
+        let wrong_input = b"wrong input";
+        let server_evaluate = server.evaluate(wrong_input).unwrap();
+        assert!(client_finalize != server_evaluate);
+    }
+
     fn zeroize_voprf_client<CS: CipherSuite>()
     where
         <CS::Hash as OutputSizeUser>::OutputSize:
@@ -769,7 +819,7 @@ mod tests {
         let mut rng = OsRng;
         let client_blind_result = VoprfClient::<CS>::blind(input, &mut rng).unwrap();
         let server = VoprfServer::<CS>::new(&mut rng).unwrap();
-        let server_result = server.evaluate(&mut rng, &client_blind_result.message);
+        let server_result = server.blind_evaluate(&mut rng, &client_blind_result.message);
 
         let mut state = server;
         unsafe { ptr::drop_in_place(&mut state) };
@@ -796,6 +846,7 @@ mod tests {
             verifiable_batch_retrieval::<Ristretto255>();
             verifiable_bad_public_key::<Ristretto255>();
             verifiable_batch_bad_public_key::<Ristretto255>();
+            verifiable_server_evaluate::<Ristretto255>();
 
             zeroize_voprf_client::<Ristretto255>();
             zeroize_voprf_server::<Ristretto255>();
@@ -805,6 +856,7 @@ mod tests {
         verifiable_batch_retrieval::<NistP256>();
         verifiable_bad_public_key::<NistP256>();
         verifiable_batch_bad_public_key::<NistP256>();
+        verifiable_server_evaluate::<NistP256>();
 
         zeroize_voprf_client::<NistP256>();
         zeroize_voprf_server::<NistP256>();