cc/prf/prf_set.h

// Copyright 2020 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
///////////////////////////////////////////////////////////////////////////////

#ifndef TINK_PRF_PRF_SET_H_
#define TINK_PRF_PRF_SET_H_

#include <map>
#include <string>

#include "absl/strings/string_view.h"
#include "tink/util/statusor.h"

namespace crypto {
namespace tink {

// The PRF interface is an abstraction for an element of a pseudo random
// function family, selected by a key. It has the following property:
//   * It is deterministic. PRF.compute(input, length) will always return the
//     same output if the same key is used. PRF.compute(input, length1) will be
//     a prefix of PRF.compute(input, length2) if length1 < length2 and the same
//     key is used.
//   * It is indistinguishable from a random function:
//     Given the evaluation of n different inputs, an attacker cannot
//     distinguish between the PRF and random bytes on an input different from
//     the n that are known.
// Use cases for PRF are deterministic redaction of PII, keyed hash functions,
// creating sub IDs that do not allow joining with the original dataset without
// knowing the key.
// While PRFs can be used in order to prove authenticity of a message, using the
// MAC interface is recommended for that use case, as it has support for
// verification, avoiding the security problems that often happen during
// verification, and having automatic support for key rotation. It also allows
// for non-deterministic MAC algorithms.
class Prf {
 public:
  virtual ~Prf() {}
  // Computes the PRF selected by the underlying key on input and
  // returns the first outputLength bytes.
  // When choosing this parameter keep the birthday paradox in mind.
  // If you have 2^n different inputs that your system has to handle
  // set the output length (in bytes) to at least
  // ceil(n/4 + 4)
  // This corresponds to 2*n + 32 bits, meaning a collision will occur with
  // a probability less than 1:2^32. When in doubt, request a security review.
  // Returns a non ok status if the algorithm fails or if the output of
  // algorithm is less than outputLength.
  virtual util::StatusOr<std::string> Compute(absl::string_view input,
                                              size_t output_length) const = 0;
};

// A Tink Keyset can be converted into a set of PRFs using this primitive. Every
// key in the keyset corresponds to a PRF in the PRFSet.
// Every PRF in the set is given an ID, which is the same ID as the key id in
// the Keyset.
class PrfSet {
 public:
  virtual ~PrfSet() {}
  // The primary ID of the keyset.
  virtual uint32_t GetPrimaryId() const = 0;
  // A map of the PRFs represented by the keys in this keyset.
  // The map is guaranteed to contain getPrimaryId() as a key.
  virtual const std::map<uint32_t, Prf*>& GetPrfs() const = 0;
  // Convenience method to compute the primary PRF on a given input.
  // See PRF.compute for details of the parameters.
  util::StatusOr<std::string> ComputePrimary(absl::string_view input,
                                             size_t output_length) const;
};

}  // namespace tink
}  // namespace crypto

#endif  // TINK_PRF_PRF_SET_H_