🔐 Add an endpoint to encrypt a batch of ballots (#82)

* 🔐 Add an endpoint to encrypt a batch of ballots

* Fix linting error

* Rename `tracking_hash` to `next_seed_hash` in encryption response

The intent is to make it as clear as possible for the caller that this
value should be used as the seed_hash of the next request.

Co-authored-by: Keith Fung <[email protected]>

Author: rkorsak

app/api/v1/mediator/ballot.py

@@ -1,16 +1,20 @@
-from electionguard.ballot import CiphertextBallot
+from electionguard.ballot import CiphertextBallot, PlaintextBallot
 from electionguard.ballot_box import accept_ballot, BallotBoxState
+from electionguard.ballot_store import BallotStore
 from electionguard.election import (
     CiphertextElectionContext,
     ElectionDescription,
     InternalElectionDescription,
 )
-from electionguard.ballot_store import BallotStore
+from electionguard.encrypt import encrypt_ballot
+from electionguard.group import ElementModQ
+from electionguard.serializable import read_json_object, write_json_object
+from electionguard.utils import get_optional
 from fastapi import APIRouter, Body, HTTPException
-from typing import Any
+from typing import Any, Optional
 
-from ..models import AcceptBallotRequest
-from ..tags import CAST_AND_SPOIL
+from ..models import AcceptBallotRequest, EncryptBallotsRequest, EncryptBallotsResponse
+from ..tags import CAST_AND_SPOIL, ENCRYPT_BALLOTS
 
 router = APIRouter()
 
@@ -43,6 +47,40 @@ def spoil_ballot(request: AcceptBallotRequest = Body(...)) -> Any:
     return spoiled_ballot.to_json_object()
 
 
+@router.post("/encrypt", tags=[ENCRYPT_BALLOTS])
+def encrypt_ballots(request: EncryptBallotsRequest = Body(...)) -> Any:
+    """
+    Encrypt one or more ballots
+    """
+    ballots = [PlaintextBallot.from_json_object(ballot) for ballot in request.ballots]
+    description = InternalElectionDescription(
+        ElectionDescription.from_json_object(request.description)
+    )
+    context = CiphertextElectionContext.from_json_object(request.context)
+    seed_hash = read_json_object(request.seed_hash, ElementModQ)
+    nonce: Optional[ElementModQ] = (
+        read_json_object(request.nonce, ElementModQ) if request.nonce else None
+    )
+
+    encrypted_ballots = []
+    current_hash = seed_hash
+
+    for ballot in ballots:
+        encrypted_ballot = encrypt_ballot(
+            ballot, description, context, current_hash, nonce
+        )
+        if not encrypted_ballot:
+            raise HTTPException(status_code=500, detail="Ballot failed to encrypt")
+        encrypted_ballots.append(encrypted_ballot)
+        current_hash = get_optional(encrypted_ballot.tracking_hash)
+
+    response = EncryptBallotsResponse(
+        encrypted_ballots=[ballot.to_json_object() for ballot in encrypted_ballots],
+        next_seed_hash=write_json_object(current_hash),
+    )
+    return response
+
+
 def handle_ballot(request: AcceptBallotRequest, state: BallotBoxState) -> Any:
     ballot = CiphertextBallot.from_json_object(request.ballot)
     description = ElectionDescription.from_json_object(request.description)

app/api/v1/models/ballot.py

@@ -1,14 +1,28 @@
-from typing import Any
+from typing import Any, List, Optional
 
 from .base import Base
 from .election import ElectionDecription, CiphertextElectionContext
 
 
 CiphertextAcceptedBallot = Any
 CiphertextBallot = Any
+PlaintextBallot = Any
 
 
 class AcceptBallotRequest(Base):
     ballot: CiphertextBallot
     description: ElectionDecription
     context: CiphertextElectionContext
+
+
+class EncryptBallotsRequest(Base):
+    ballots: List[PlaintextBallot]
+    seed_hash: str
+    nonce: Optional[str] = None
+    description: ElectionDecription
+    context: CiphertextElectionContext
+
+
+class EncryptBallotsResponse(Base):
+    encrypted_ballots: List[PlaintextBallot]
+    next_seed_hash: str