fix: Handle both plain and base64-encoded public keys in API responses

## Issue
Even after storing keys as base64, the API was returning plain "p:g:h" format
for existing elections that had keys stored as plain UTF-8 bytes, causing:
- Client receives: "23:5:13" (plain text)
- Client tries to decode as base64 (btoa call)
- Results in: "Invalid base64: 23:5:13... - String contains an invalid character"

## Root Cause
1. Old elections have public_key stored as plain UTF-8: b'23:5:13'
2. New elections store as base64: b'MjM6NToxMw=='
3. Both were decoded to string before return, exposing wrong format
4. Also fixed ElGamal class name typo: ElGamal() → ElGamalEncryption()

## Fix
1. Detect public key format before returning:
   - If plain "p:g:h" format (contains ':'), encode to base64
   - If already base64 (starts with 'MjM6'), return as-is
2. Always return base64-encoded string to client
3. Updated both /setup and /public-keys endpoints in votes.py
4. Updated /init-keys endpoint in admin.py
5. Fixed class name in setup_election function

## Files Changed
- backend/routes/votes.py: Lines 502, 509-518, 560-569
- backend/routes/admin.py: Lines 179-197

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

e-voting-system/backend/routes/admin.py

@@ -176,6 +176,17 @@ async def init_election_keys(election_id: int, db: Session = Depends(get_db)):
         else:
             logger.info(f"Election {election_id} already has valid public key")
 
+        # Ensure public key is base64-encoded for client
+        pubkey_to_return = election.public_key
+        if isinstance(pubkey_to_return, bytes):
+            pubkey_str = pubkey_to_return.decode('utf-8')
+            # If it's plain "p:g:h" format, encode it to base64
+            if ':' in pubkey_str and not pubkey_str.startswith('MjM6'):  # Not already base64
+                pubkey_to_return = base64.b64encode(pubkey_str.encode('utf-8')).decode('ascii')
+            else:
+                # Already base64, just decode to string
+                pubkey_to_return = pubkey_str
+
         return {
             "status": "success",
             "election_id": election_id,
@@ -183,7 +194,7 @@ async def init_election_keys(election_id: int, db: Session = Depends(get_db)):
             "elgamal_p": election.elgamal_p,
             "elgamal_g": election.elgamal_g,
             "public_key_generated": True,
-            "public_key": election.public_key.decode('ascii') if isinstance(election.public_key, bytes) else election.public_key
+            "public_key": pubkey_to_return
         }
 
     except HTTPException:

e-voting-system/backend/routes/votes.py

@@ -499,18 +499,29 @@ async def setup_election(
 
     # Générer les clés ElGamal si nécessaire
     if not election.public_key:
-        elgamal = ElGamal()
+        elgamal = ElGamalEncryption()
         # Store as base64-encoded bytes (database column is LargeBinary)
         # public_key_bytes returns UTF-8 "p:g:h", then encode to base64
         election.public_key = base64.b64encode(elgamal.public_key_bytes)
         db.add(election)
         db.commit()
 
+    # Ensure public key is base64-encoded for client
+    pubkey_to_return = election.public_key
+    if isinstance(pubkey_to_return, bytes):
+        pubkey_str = pubkey_to_return.decode('utf-8')
+        # If it's plain "p:g:h" format, encode it to base64
+        if ':' in pubkey_str and not pubkey_str.startswith('MjM6'):  # Not already base64
+            pubkey_to_return = base64.b64encode(pubkey_str.encode('utf-8')).decode('ascii')
+        else:
+            # Already base64, just decode to string
+            pubkey_to_return = pubkey_str
+
     return {
         "status": "initialized",
         "election_id": election_id,
         "public_keys": {
-            "elgamal_pubkey": election.public_key.decode('ascii') if isinstance(election.public_key, bytes) else election.public_key
+            "elgamal_pubkey": pubkey_to_return
         },
         "blockchain_blocks": blockchain.get_block_count()
     }
@@ -546,8 +557,19 @@ async def get_public_keys(
             detail="Election keys not initialized. Call /setup first."
         )
 
+    # Ensure public key is base64-encoded for client
+    pubkey_to_return = election.public_key
+    if isinstance(pubkey_to_return, bytes):
+        pubkey_str = pubkey_to_return.decode('utf-8')
+        # If it's plain "p:g:h" format, encode it to base64
+        if ':' in pubkey_str and not pubkey_str.startswith('MjM6'):  # Not already base64
+            pubkey_to_return = base64.b64encode(pubkey_str.encode('utf-8')).decode('ascii')
+        else:
+            # Already base64, just decode to string
+            pubkey_to_return = pubkey_str
+
     return {
-        "elgamal_pubkey": election.public_key.decode('ascii') if isinstance(election.public_key, bytes) else election.public_key
+        "elgamal_pubkey": pubkey_to_return
     }