Remote Decrypting Guardians and RPC calls

[JohnLCaron]

This is part 2 of the Remote Guardian refactor (also see #84), documenting my take on the minimal state and message exchange needed by the Guardian during decryption.

DecryptingGuardian state that needs to be persisted between the key ceremony and the decryption:

class DecryptingGuardian {
  String id;
  int guardianXCoordinate; // aka sequence_order
  Rsa.PrivateKey rsa_private_key;

  /** The election (ElGamal) secret key */
  public final ElGamal.KeyPair election_keypair;

  /** Other guardians' partial key backups of this guardian's keys, keyed by other guardian id. */
  public final Map<String, KeyCeremony2.ElectionPartialKeyBackup> otherGuardianPartialKeyBackups;

  /** All guardians' public coefficient commitments, keyed by guardian id. */
  public final ImmutableMap<String, ImmutableList<Group.ElementModP>> guardianCommittments;
}

In what follows, I assume it may be advantageous to ask for multiple decryptions in a single message, rather than sending separate messages, eg one for each selection. Its up to the DecryptingGuardian to return these in the order received, though one could add an identifier if needed.

Remote procedure calls that are minimally needed. I'm using grpc/protobuf notation, but I think the ideas should be clear for any implementation:

service DecryptingTrusteeService {
  rpc partialDecrypt (PartialDecryptionRequest) returns (PartialDecryptionResponse) {}
  rpc compensatedDecrypt (CompensatedDecryptionRequest) returns (CompensatedDecryptionResponse) {}
}

message PartialDecryptionRequest {
  ElementModQ extended_base_hash = 1; // The election extended_base_hash.
  repeated ElGamalCiphertext text = 2; // The text(s) to decrypt.
}

message PartialDecryptionResponse {
  RemoteError error = 1; // non empty on error
  repeated PartialDecryptionResult results = 2;
}

message PartialDecryptionResult {
  ElementModP decryption = 1;
  ChaumPedersenProof proof = 2;
}

• Note: its not clear if we need to be able to send a nonce. Im assuming the nonce is only needed for unit testing, which is best done separate from the RPC. I've left it out, so a random nonce is always used.

message CompensatedDecryptionRequest {
  ElementModQ extended_base_hash = 1; // The election extended_base_hash.
  string missing_guardian_id = 2; // The id of the guardian that's missing.
  repeated ElGamalCiphertext text = 3; // The text(s) to decrypt.
}

message CompensatedDecryptionResponse {
  RemoteError error = 1; // non empty on error
  repeated CompensatedDecryptionResult results = 2;
}

message CompensatedDecryptionResult {
  ElementModP decryption = 1;
  ChaumPedersenProof proof = 2;
  ElementModP recoveryPublicKey = 3;
}

• Note: I've combined fetching the recoveryPublicKey into the same call as the compensated decryption, since they are always done together. (This could be placed into CompensatedDecryptionResponse, since its the same for all results in the response).
• Note: I assume a nonce is not needed in the RPC.