secagg/shared/secagg_messages.proto

/*
 * Copyright 2017 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.
 */

// Definition of the protocol buffers for the SecAgg protocol

syntax = "proto3";

package fcp.secagg;

import "google/protobuf/any.proto";

option java_package = "fcp.secagg.shared";
option java_outer_classname = "SecAggMessages";

// TODO(team): remove container class, using
// option java_multiple_files = true;

// # MESSAGE WRAPPERS EXPOSED TO THE OUTSIDE

// This message is a wrapper (simulating polymorphism) for all messages sent
// from a client to the server during the protocol.
message ClientToServerWrapperMessage {
  // Each message from client to server contains exactly one of these
  // semantic types, depending on the phase of the protocol.
  oneof message_content {
    // Abort the protocol.
    AbortMessage abort = 1;

    // Round 0 message; see details at message definition.
    AdvertiseKeys advertise_keys = 2;

    // Round 1 message; see details at message definition.
    ShareKeysResponse share_keys_response = 3;

    // Round 2 message; see details at message definition.
    MaskedInputCollectionResponse masked_input_response = 4;

    // Round 3 message; see details at message definition.
    UnmaskingResponse unmasking_response = 5;
  }
}

// This message is a wrapper (simulating polymorphism) for all messages sent
// from the server to a client during the protocol.
message ServerToClientWrapperMessage {
  // Each message from server to client contains exactly one of these
  // semantic types, depending on the phase of the protocol.
  oneof message_content {
    // Abort the protocol.
    AbortMessage abort = 1;

    // Round 1 message; see details at message definition.
    ShareKeysRequest share_keys_request = 2;

    // Round 2 message; see details at message definition.
    MaskedInputCollectionRequest masked_input_request = 3;

    // Round 3 message; see details at message definition.
    UnmaskingRequest unmasking_request = 4;
  }
}

// # MESSAGES INTERNAL TO THE LIBRARY

// ## ABORT MESSAGE
// Sent by the server to the client to cause the client to abort and erase all
// the state relate to the current session. Can signify either an abort due to
// some error or an abort because the server needs no more messages.
// Sent by the client to the server to notify it that the sending client
// aborted.
message AbortMessage {
  // Can contain optional logging/diagnostic info.
  string diagnostic_info = 1;
  // If true, the client will halt early but mark the protocol as a success,
  // rather than as aborted.
  bool early_success = 2;
}

// ## ROUND 0 MESSAGES: ADVERTISE KEYS
// AdvertiseKeys is sent by clients who wish to participate in the protocol.

// Part of a ClientToServerWrapperMessage. Contains a pair of public keys.
message AdvertiseKeys {
  // Pair of public keys for this client.
  PairOfPublicKeys pair_of_public_keys = 1;
}

// A pair of public keys. Used as part of AdvertiseKeys and ShareKeysRequest.
// TODO(team): move away from comprossed encoding of ECPoint into
// encoding the complete PublicKey, including the other parameters.
message PairOfPublicKeys {
  // An encoding of the Diffie Hellman public key used to generate correlated
  // noise to be added to ciphertexts.
  // The encoding used is the one returned by the getEncoded(compressed=true)
  // method of the class org.bouncycastle.math.ec.ECPoint
  // TODO(team): figure out the actual encoding used.
  bytes noise_pk = 1;

  // An encoding of the Diffie Hellman public key used to establish private
  // channels between the protocol parties.
  // The encoding used is the one returned by the getEncoded(compressed=true)
  // method of the class org.bouncycastle.math.ec.ECPoint
  // TODO(team): figure out the actual encoding used.
  bytes enc_pk = 2;
}

// ## ROUND 1 MESSAGES: SHARE KEYS
// Clients who are selected to participate in the protocol receive a list of the
// (pairs of) public keys of all other clients. Each client secret-shares its
// own noise_sk and prf_sk with all the other clients (encrypting shares for
// client j with their own enc_pk) and sends all these encrypted shares to the
// server for distribution.

// Part of a ServerToClientWrapperMessage. Contains a list of pairs of public
// keys, as well as the logging ID for the SecAgg execution.
message ShareKeysRequest {
  // List of public keys for all clients, ordered by the clients'
  // logical ids. Each client infers its logical id "i" from the
  // position "i" of its pair of keys in the list.
  // Note that the logical ids are assumed to be 1-indexed (i.e. the first
  // public key in the repeated field corresponds to client with logical id 1)
  repeated PairOfPublicKeys pairs_of_public_keys = 1;
  // The logging ID for the SecAgg execution. All participants in the protocol
  // will use this ID while logging, to allow metrics for the entire execution
  // to be collected.
  int64 sec_agg_execution_logging_id = 2;

  // May be populated with implementation-specific data.
  repeated google.protobuf.Any extra_data = 3;

  // The session ID for the Secagg execution. All clients will use this session
  // ID as part of the seed of PRNGs used during the execution of the protocol,
  bytes session_id = 4 ;
}

// Part of a ClientToServerWrapperMessage. Contains a list of encrypted pairs of
// key shares (one for each other client).
message ShareKeysResponse {
  // The (1-indexed) j'th element of the repeated field (of a ShareKeysResponse
  // message sent from client i) contains the j'th shares of each of client i's
  // noise_sk and prf_sk, encrypted under client j's enc_pk and intended to be
  // sent to him. The i'th share (i.e. the share the a client would send to
  // himself) will be empty.
  // The client indexes above refer to their logical ids.
  // This field is opaque, as it will be the output of an AES/GCM encryption.
  // However, once decrypted, each entry should be interpreted as the
  // serialization of a PairOfKeyShares message.
  repeated bytes encrypted_key_shares = 1;
}

// Each of the encrypted_key_shares bytes in ShareKeysResponse and
// MaskedInputCollectionRequest is an encryption of the serialization of this
// message.
message PairOfKeyShares {
  // The two shares are encodings of BigIntegers which represents elements of
  // the group Z_p, where p is the prime denoting the field over which the
  // elliptic curve prime256v1 is defined. See the ShamirSecretSharing
  // class for more details on the sharing.
  //
  // The following comes from the BigInteger documentation (the encoding is
  // computed using the toByteArray() method):
  // [The encoding consists of] a byte array containing the two's-complement
  // representation of the BigInteger and is in big-endian byte-order: the most
  // significant byte is in the zeroth element.
  // The array will contain the minimum number of bytes required to represent
  // this BigInteger, including at least one sign bit, which is
  // (ceil((this.bitLength() + 1)/8)).
  bytes noise_sk_share = 1;
  bytes prf_sk_share = 2;
}

// ## ROUND 2 MESSAGES: MASKED INPUT COLLECTION
// The server gives each client his shares of the keys of each other client who
// responded in the previous round. Each client computes a masked version of its
// own input and sends it to the server.

// Part of a ServerToClientWrapperMessage. Contains a list of shares of other
// clients' keys encrypted and intended for the client who receives this
// message.
message MaskedInputCollectionRequest {
  // Each bytes field of this message (intended for party with logical id i)
  // contains an encryption of a PairOfKeyShares message.
  // The bytes are ordered according to the clients' logical ids. If a client
  // j did not send the ShareKeysResponse in the previous round, the
  // corresponding encrypted_key_shares bytes field will be empty (the receiving
  // client i will therefore consider such a client j dead from now on).
  repeated bytes encrypted_key_shares = 1;
}

// Part of a ClientToServerWrapperMessage. Contains a map of masked input
// vectors (each identified by a name string).
message MaskedInputCollectionResponse {
  // The string key of the map represents the name of the input vector.
  map<string, MaskedInputVector> vectors = 1;
}

// Part of a MaskedInputCollectionResponse message. Contains an encoding of an
// input vector masked by the addition of pseudo-random values (which will be
// removed later).
message MaskedInputVector {
  // The vector contains a packed representation of a masked SecAggVector,
  // where each element is packed in little-endian order.
  bytes encoded_vector = 1;

  // May be populated with implementation-specific data.
  repeated google.protobuf.Any extra_data = 2;
}

// ## ROUND 3 MESSAGES: UNMASKING
// The server communicates to the clients the list of clients which did not
// complete the previous round (and therefore are considered dead). Each client
// i, for each other client j, responds with either the share of noise_sk (if
// client j is dead) OR share of prf_sk (if client j is still alive).

// Part of a ServerToClientWrapperMessage.
message UnmaskingRequest {
  // Clients which were alive at round 2, but did not send a ROUND 2 response,
  // so they are considered dead from round 3 onwards.
  // NOTE: to minimize bandwidth, this DOES NOT include clients which dropped
  // before round 2 (i.e. those who did not send a round 1 response).
  repeated uint32 dead_3_client_ids = 1;
}

// Part of a ClientToServerWrapperMessage.
message UnmaskingResponse {
  // These shares are NOT encrypted. For each other client j, client i sends to
  // the server his share of noise_sk (if client j is dead) OR share of prf_sk
  // (if client j is still alive).
  // The entries corresponding to clients who died before round 2 (i.e. the
  // ones for which the client sending the message never got any key shares) are
  // empty. The entry corresponding to the client sending the message must be
  // a prf_sk_share (otherwise the client would be dead and would not be sending
  // this response.
  repeated NoiseOrPrfKeyShare noise_or_prf_key_shares = 1;
}

// Part of an UnmaskingResponse. Contains either a share of a noise secret key
// or a share of a prf secret key.
message NoiseOrPrfKeyShare {
  // Either a share of a noise secrete key or a share of a prf secret key.
  // An honest client never reveals both, therefore oneof is appropriate.
  oneof oneof_shares {
    // A share of a noise secret key.
    bytes noise_sk_share = 1;

    // A share of a prf secret key.
    bytes prf_sk_share = 2;
  }
}

// Version(s) supported on each client.
enum ClientVariant {
  // No SecAgg versions are supported on this client.
  SECAGG_CLIENT_VARIANT_NONE = 0;
  // The Java implementation of the SegAgg client.
  SECAGG_CLIENT_VARIANT_JAVA = 1 [deprecated = true];
  // The native (C++) implementation of the SegAgg client.
  SECAGG_CLIENT_VARIANT_NATIVE_V1 = 2;
}