diff --git a/backend/services/main/routes/envelope_messaging.py b/backend/services/main/routes/envelope_messaging.py
index eb1b833..3efa2aa 100644
--- a/backend/services/main/routes/envelope_messaging.py
+++ b/backend/services/main/routes/envelope_messaging.py
@@ -14,7 +14,9 @@ import time
 from datetime import datetime
 from pathlib import Path
 from typing import Optional
+
 import httpx
+from nacl.exceptions import CryptoError
 from fastapi import APIRouter, Depends, HTTPException, status, Request
 from sqlalchemy.orm import Session
 from pydantic import BaseModel, Field
@@ -376,6 +378,20 @@ async def send_encrypted_message(
 
     except HTTPException:
         raise
+    except CryptoError as e:
+        logger.warning(
+            "DM send: transport NaCl decrypt failed (message/file key mismatch or corrupt ciphertext): %s",
+            e,
+        )
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=(
+                "Transport decryption failed: the encrypted message and each attachment must be "
+                "encrypted with the same client ephemeral keypair. For resumable uploads, the "
+                "ciphertext bytes on the server must match the transport fields in this request—"
+                "re-encrypt on the client or abort the upload session and start over."
+            ),
+        ) from e
     except Exception as e:
         logger.exception("Error sending encrypted message: %s", e)
         raise HTTPException(
diff --git a/backend/services/main/service_calls.py b/backend/services/main/service_calls.py
index 70dedb1..408c9f9 100644
--- a/backend/services/main/service_calls.py
+++ b/backend/services/main/service_calls.py
@@ -10,6 +10,9 @@ import os
 import logging
 import json
 
+import httpx
+from fastapi import HTTPException, status
+
 # Import request models for in-process calls
 
 logger = logging.getLogger("uvicorn.error")
@@ -418,11 +421,23 @@ async def process_message_with_files_in_messaging_service(
         "files": transport_files,
     }
     try:
-        import httpx
         async with httpx.AsyncClient(timeout=timeout) as client:
             r = await client.post(url, json=payload)
             r.raise_for_status()
             return r.json()
+    except httpx.HTTPStatusError as e:
+        if e.response.status_code == status.HTTP_400_BAD_REQUEST:
+            try:
+                body = e.response.json()
+                detail = body.get("detail", str(body)) if isinstance(body, dict) else str(body)
+            except Exception:
+                detail = (e.response.text or "").strip() or str(e)
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail=detail,
+            ) from e
+        logger.error("Failed to process message+files in messaging service: %s", e)
+        raise
     except Exception as e:
         logger.error("Failed to process message+files in messaging service: %s", e)
         raise
diff --git a/backend/services/messaging/main.py b/backend/services/messaging/main.py
index d67c886..0988b7f 100644
--- a/backend/services/messaging/main.py
+++ b/backend/services/messaging/main.py
@@ -11,17 +11,21 @@ API Endpoints:
 """
 
 import logging
+import sys
 import time
 import base64
 import os
 from typing import Dict, Any
 from fastapi import FastAPI, HTTPException, status
+from nacl.exceptions import CryptoError
 from fastapi.middleware.cors import CORSMiddleware
 from contextlib import asynccontextmanager
 from pydantic import BaseModel
 
 logger = logging.getLogger("uvicorn.error")
 
+_B64_DECODE_KW = {"validate": True} if sys.version_info >= (3, 11) else {}
+
 # Import encryption modules
 from .encryption import generate_nonce, TRANSPORT_NONCE_SIZE, decrypt_transport_blob, decrypt_transport_message
 from .processor import process_encrypted_message, process_encrypted_message_and_files
@@ -371,16 +375,29 @@ async def process_message_with_files(
 
     plaintext_files: list[bytes] = []
     filenames: list[str] = []
-    for tf in transport_files:
+    for idx, tf in enumerate(transport_files):
         enc_b64 = tf.get("encrypted_file_data_b64", "")
-        transport_blob = base64.b64decode(enc_b64)
-        plaintext_files.append(
-            decrypt_transport_blob(
-                client_public_key_b64=client_public_key_b64,
-                encrypted_blob=transport_blob,
-                ephemeral_private_key=private_key,
+        try:
+            transport_blob = base64.b64decode(enc_b64, **_B64_DECODE_KW)
+        except Exception as e:
+            logger.error("Invalid base64 for transport file index=%s filename=%r: %s", idx, tf.get("filename"), e)
+            raise
+        try:
+            plaintext_files.append(
+                decrypt_transport_blob(
+                    client_public_key_b64=client_public_key_b64,
+                    encrypted_blob=transport_blob,
+                    ephemeral_private_key=private_key,
+                )
             )
-        )
+        except Exception as e:
+            logger.error(
+                "Transport file decrypt failed index=%s filename=%r (check same ephemeral as message): %s",
+                idx,
+                tf.get("filename"),
+                e,
+            )
+            raise
         filenames.append(tf.get("filename", "file"))
 
     return process_encrypted_message_and_files(
@@ -417,6 +434,15 @@ async def process_message_with_files_http(request: ProcessMessageWithFilesReques
             recipient_public_key_b64=request.recipient_public_key_b64,
             transport_files=transport_files,
         )
+    except CryptoError as e:
+        logger.warning("process-with-files: transport CryptoError: %s", e)
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=(
+                "Transport decryption failed: message and files must use the same client ephemeral "
+                "key as when file ciphertext was produced."
+            ),
+        ) from e
     except Exception as e:
         logger.exception("Failed to process message with files: %s", e)
         raise