// Copyright 2023 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.
syntax = "proto3";

package cobalt;

option java_multiple_files = true;
option java_package = "com.google.cobalt";

////////////////////////////////////////////////////////////////////////////////
// NOTE: This file is used by the Cobalt client and the Cobalt servers.
// The source-of-truth of this file is located in Google's internsl code
// repository, and the file is copied to Android where it is used by the Cobalt
// client. Do not edit the copy of this file in this Android repo as those edits
// will be overwritten when the file is next copied.
////////////////////////////////////////////////////////////////////////////////

// An EncryptedMessage carries the encrypted bytes of another proto message,
// along with information about how it is encrypted.
//
// Observations collected via Cobalt are doubly encrypted. First each individual
// message is encrypted to the Analyzer that will process it. Second each
// Envelope containing many observations is encrypted to the Shuffler. We use
// the EncryptedMessage proto to carry the ciphertext in both cases.
//
message EncryptedMessage {

  // The different schemes used in Cobalt to encrypt a message.
  enum EncryptionScheme {
    // The message is not encrypted. |ciphertext| contains plaintext bytes of a
    // serialized protocol buffer message. This scheme must only be used in
    // tests.
    NONE = 0;

    // Hybrid Cipher using elliptic curve Diffie-Hellman, version 1.
    HYBRID_ECDH_V1 = 1;

    // Hybrid cipher compatible with Tink hybrid encryption/decryption
    // primitives declared in
    // third_party/tink/cc/hybrid/hybrid_key_templates.h
    // Multiple hybrid encryption schemes are supported and indicated by the
    // type of key used.
    HYBRID_TINK = 2;
  }
  // Which scheme was used to encrypt this message?
  EncryptionScheme scheme = 1;

  // Which key was used to encrypt this message?
  // This key is mutually exclusive with |scheme| being set.
  uint32 key_index = 4;

  // 32-byte fingerprint (SHA256) of the recipient’s public key.
  // This is used to facilitate key rotation.
  bytes public_key_fingerprint = 2;

  //  The |contribution_id| field is a cryptographically-secure random number
  //  generated and attached by the Cobalt client. The shuffler counts the
  //  number of unique ids to determine the contribution count per report.
  //
  //  This field should only be set when the |ciphertext| contains a
  //  cobalt.Observation that should be counted towards the shuffler threshold.
  //  All other encrypted messages should not receive a |contribution_id|.
  //
  //  Once an observation is encrypted and assigned a |contribution_id| it
  //  should never be given another id or stored unencrypted.
  bytes contribution_id = 5;

  // The |ciphertext| field contains the bytes of the encryption of the standard
  // serialization of one of the following types of proto messages:
  //
  // - A cobalt.Envelope, as defined in Cobalt's envelope.proto.
  //   EncryptedMessages containing Envelopes are the input to the Shuffler.
  //
  // - A cobalt.Observation, as defined in Cobalt's observation.proto.
  //   An ObservationBatch (defined in observation_batch.proto) contains
  //   EncryptedMessages of this type. ObservationBatches are output from the
  //   Shuffler.
  bytes ciphertext = 3;
}