From 6568be91ff8864d0bc55c158e978cc441e17aa0a Mon Sep 17 00:00:00 2001
From: Bob Mottram <bob@freedombone.net>
Date: Thu, 3 Sep 2020 19:07:02 +0100
Subject: [PATCH] Constant time password hash match

---
 auth.py  | 35 ++++++++++++++++++++++++++++-------
 tests.py |  1 -
 2 files changed, 28 insertions(+), 8 deletions(-)

diff --git a/auth.py b/auth.py
index 7ac45cec..07cf91af 100644
--- a/auth.py
+++ b/auth.py
@@ -10,7 +10,6 @@ import base64
 import hashlib
 import binascii
 import os
-import secrets
 
 
 def hashPassword(password: str) -> str:
@@ -24,17 +23,39 @@ def hashPassword(password: str) -> str:
     return (salt + pwdhash).decode('ascii')
 
 
-def verifyPassword(storedPassword: str, providedPassword: str) -> bool:
-    """Verify a stored password against one provided by user
+def getPasswordHash(salt: str, providedPassword: str) -> str:
+    """Returns the hash of a password
     """
-    salt = storedPassword[:64]
-    storedPassword = storedPassword[64:]
     pwdhash = hashlib.pbkdf2_hmac('sha512',
                                   providedPassword.encode('utf-8'),
                                   salt.encode('ascii'),
                                   100000)
-    pwdhash = binascii.hexlify(pwdhash).decode('ascii')
-    return pwdhash == storedPassword
+    return binascii.hexlify(pwdhash).decode('ascii')
+
+def verifyPassword(storedPassword: str, providedPassword: str) -> bool:
+    """Verify a stored password against one provided by user
+    """
+    if not storedPassword:
+        return False
+    if not providedPassword:
+        return False
+    salt = storedPassword[:64]
+    storedPassword = storedPassword[64:]
+    pwHash = getPasswordHash(salt, providedPassword)
+    # check that hashes are of equal length
+    if len(pwHash) != len(storedPassword):
+        return False
+    # Compare all of the characters before returning true or false.
+    # Hence the match should take a constant amount of time.
+    # See https://sqreen.github.io/DevelopersSecurityBestPractices/
+    # timing-attack/python
+    ctr = 0
+    matched = True
+    for ch in pwHash:
+        if ch != storedPassword[ctr]:
+            matched = False
+        ctr += 1
+    return matched
 
 
 def createBasicAuthHeader(nickname: str, password: str) -> str:
diff --git a/tests.py b/tests.py
index 127d0c79..7c5c2865 100644
--- a/tests.py
+++ b/tests.py
@@ -2084,7 +2084,6 @@ def testTranslations():
                 print(englishStr + ' is missing from ' + lang + '.json')
             assert langJson.get(englishStr)
 
-
 def runAllTests():
     print('Running tests...')
     testTranslations()