feat: HIBP k-anonymity password breach check

backend/app.py

@@ -13,6 +13,7 @@
 from services.link_checker import check_url, limiter as link_limiter
 from services.phone_checker import check_phone, limiter as phone_limiter
 from services.abuse_reporter import generate_report, get_all_templates
+from services.breach_checker import check_password
 from config import ADMIN_TOKEN, BLACKLIST_ENTRY_TYPES
 
 db = Database()
@@ -195,6 +196,14 @@ async def api_stats(_: None = Depends(require_admin_token)):
 
 # === Rate Limit Status ===
 
+@app.get("/api/breach/password")
+async def api_check_password(
+    password: str = Query(..., min_length=1, max_length=256, description="Password to check"),
+    _: None = Depends(require_admin_token),
+):
+    return await check_password(password)
+
+
 @app.get("/api/ratelimit")
 async def api_ratelimit_status(_: None = Depends(require_admin_token)):
     return {

backend/services/breach_checker.py

@@ -0,0 +1,46 @@
+"""Breach checker — HIBP k-anonymity for passwords (no account needed)."""
+import hashlib
+import httpx
+from typing import Dict
+
+
+async def check_password(password: str) -> Dict:
+    """Check if a password appears in known breaches via HIBP k-anonymity.
+
+    Only the first 5 chars of the SHA-1 hash are sent to the API.
+    The full hash never leaves the client.
+    """
+    if not password:
+        return {"error": "empty password"}
+
+    sha1 = hashlib.sha1(password.encode("utf-8")).hexdigest().upper()
+    prefix = sha1[:5]
+    suffix = sha1[5:]
+
+    try:
+        async with httpx.AsyncClient(timeout=10) as client:
+            resp = await client.get(
+                f"https://api.pwnedpasswords.com/range/{prefix}",
+                headers={"Add-Padding": "true"},
+            )
+        if resp.status_code != 200:
+            return {"error": f"HIBP returned {resp.status_code}"}
+    except httpx.HTTPError as e:
+        return {"error": f"network error: {e}"}
+
+    count = 0
+    for line in resp.text.splitlines():
+        parts = line.strip().split(":")
+        if len(parts) == 2 and parts[0] == suffix:
+            count = int(parts[1])
+            break
+
+    return {
+        "breached": count > 0,
+        "count": count,
+        "message": (
+            f"Password found in {count} breach(es) — change it immediately!"
+            if count > 0
+            else "Password not found in known breaches."
+        ),
+    }