Sign media URLs to avoid becoming an open proxy

Signatures are valid for ~1 week.

app/media.py

@@ -1,15 +1,40 @@
 import base64
+import time
 
 from app.config import BASE_URL
+from app.config import hmac_sha256
 
 SUPPORTED_RESIZE = [50, 740]
+EXPIRY_PERIOD = 86400
+EXPIRY_LENGTH = 7
+
+
+class InvalidProxySignatureError(Exception):
+    pass
+
+
+def proxied_media_sig(expires: int, url: str) -> str:
+    hm = hmac_sha256()
+    hm.update(f'{expires}'.encode())
+    hm.update(b'|')
+    hm.update(url.encode())
+    return base64.urlsafe_b64encode(hm.digest()).decode()
+
+
+def verify_proxied_media_sig(expires: int, url: str, sig: str) -> None:
+    now = int(time.time() / EXPIRY_PERIOD)
+    expected = proxied_media_sig(expires, url)
+    if now > expires or sig != expected:
+        raise InvalidProxySignatureError("invalid or expired media")
 
 
 def proxied_media_url(url: str) -> str:
     if url.startswith(BASE_URL):
         return url
+    expires = int(time.time() / EXPIRY_PERIOD) + EXPIRY_LENGTH
+    sig = proxied_media_sig(expires, url)
 
-    return BASE_URL + "/proxy/media/" + base64.urlsafe_b64encode(url.encode()).decode()
+    return BASE_URL + f"/proxy/media/{expires}/{sig}/" + base64.urlsafe_b64encode(url.encode()).decode()
 
 
 def resized_media_url(url: str, size: int) -> str: