server/posts: file sha1 in filenames and prevent offline secret cracking

Imagine if we had a bunch of simple strings encrypted with the same key
we use to salt passwords, publicly accessible, which would undermine
our salting model by removing the requirement of filesystem access to
crack our users' passwords, requiring only database access and offline
cracking of our secret.
Wouldn't that be fun?

server/szurubooru/func/posts.py

@@ -97,20 +97,12 @@ FLAG_MAP = {
 }
 
 
-def get_post_security_hash(id: int) -> str:
-    return hmac.new(
-        config.config["secret"].encode("utf8"),
-        msg=str(id).encode("utf-8"),
-        digestmod="md5",
-    ).hexdigest()[0:16]
-
-
 def get_post_content_url(post: model.Post) -> str:
     assert post
     return "%s/posts/%d_%s.%s" % (
         config.config["data_url"].rstrip("/"),
         post.post_id,
-        get_post_security_hash(post.post_id),
+        post.checksum,
         mime.get_extension(post.mime_type) or "dat",
     )
 
@@ -120,7 +112,7 @@ def get_post_thumbnail_url(post: model.Post) -> str:
     return "%s/generated-thumbnails/%d_%s.jpg" % (
         config.config["data_url"].rstrip("/"),
         post.post_id,
-        get_post_security_hash(post.post_id),
+        post.checksum,
     )
 
 
@@ -129,7 +121,7 @@ def get_post_content_path(post: model.Post) -> str:
     assert post.post_id
     return "posts/%d_%s.%s" % (
         post.post_id,
-        get_post_security_hash(post.post_id),
+        post.checksum,
         mime.get_extension(post.mime_type) or "dat",
     )
 
@@ -138,7 +130,7 @@ def get_post_thumbnail_path(post: model.Post) -> str:
     assert post
     return "generated-thumbnails/%d_%s.jpg" % (
         post.post_id,
-        get_post_security_hash(post.post_id),
+        post.checksum,
     )
 
 
@@ -146,7 +138,7 @@ def get_post_thumbnail_backup_path(post: model.Post) -> str:
     assert post
     return "posts/custom-thumbnails/%d_%s.dat" % (
         post.post_id,
-        get_post_security_hash(post.post_id),
+        post.checksum,
     )