Verify token signature in constant time, Run cheap checks first in token validation process (#1032)

* Verify token signature in constant time

To prevent timing side channel attacks

* Run cheap checks first in token validation process

Expensive checks such as the nonce lookup on the database or the
signature check can be run after cheap/fast checks.

src/invidious/helpers/tokens.cr

@@ -1,3 +1,5 @@
+require "crypto/subtle"
+
 def generate_token(email, scopes, expire, key, db)
   session = "v1:#{Base64.urlsafe_encode(Random::Secure.random_bytes(32))}"
   PG_DB.exec("INSERT INTO session_ids VALUES ($1, $2, $3)", session, email, Time.utc)
@@ -76,14 +78,25 @@ def validate_request(token, session, request, key, db, locale = nil)
     raise translate(locale, "Hidden field \"token\" is a required field")
   end
 
-  if token["signature"] != sign_token(key, token)
+  expire = token["expire"]?.try &.as_i
-    raise translate(locale, "Invalid signature")
+  if expire.try &.< Time.utc.to_unix
+    raise translate(locale, "Token is expired, please try again")
   end
 
   if token["session"] != session
     raise translate(locale, "Erroneous token")
   end
 
+  scopes = token["scopes"].as_a.map { |v| v.as_s }
+  scope = "#{request.method}:#{request.path.lchop("/api/v1/auth/").lstrip("/")}"
+  if !scopes_include_scope(scopes, scope)
+    raise translate(locale, "Invalid scope")
+  end
+
+  if !Crypto::Subtle.constant_time_compare(token["signature"].to_s, sign_token(key, token))
+    raise translate(locale, "Invalid signature")
+  end
+
   if token["nonce"]? && (nonce = db.query_one?("SELECT * FROM nonces WHERE nonce = $1", token["nonce"], as: {String, Time}))
     if nonce[1] > Time.utc
       db.exec("UPDATE nonces SET expire = $1 WHERE nonce = $2", Time.utc(1990, 1, 1), nonce[0])
@@ -92,18 +105,6 @@ def validate_request(token, session, request, key, db, locale = nil)
     end
   end
 
-  scopes = token["scopes"].as_a.map { |v| v.as_s }
-  scope = "#{request.method}:#{request.path.lchop("/api/v1/auth/").lstrip("/")}"
-
-  if !scopes_include_scope(scopes, scope)
-    raise translate(locale, "Invalid scope")
-  end
-
-  expire = token["expire"]?.try &.as_i
-  if expire.try &.< Time.utc.to_unix
-    raise translate(locale, "Token is expired, please try again")
-  end
-
   return {scopes, expire, token["signature"].as_s}
 end