From e584076acaa6e44bacfd8263609bf0126fcb3700 Mon Sep 17 00:00:00 2001
From: Bob Mottram <bob@freedombone.net>
Date: Thu, 3 Sep 2020 19:48:32 +0100
Subject: [PATCH] Unit test for constant time string check

---
 auth.py  | 41 +++++++++++++++++++++++------------------
 tests.py | 29 +++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+), 18 deletions(-)

diff --git a/auth.py b/auth.py
index 317ca0bb..021a17b6 100644
--- a/auth.py
+++ b/auth.py
@@ -34,6 +34,28 @@ def getPasswordHash(salt: str, providedPassword: str) -> str:
     return binascii.hexlify(pwdhash).decode('ascii')
 
 
+def constantTimeStringCheck(string1: str, string2: str) -> bool:
+    """Compares two string and returns if they are the same
+    using a constant amount of time
+    See https://sqreen.github.io/DevelopersSecurityBestPractices/
+    timing-attack/python
+    """
+    # strings must be of equal length
+    if len(string1) != len(string2):
+        return False
+    ctr = 0
+    matched = True
+    for ch in string1:
+        if ch != string2[ctr]:
+            matched = False
+        else:
+            # this is to make the timing more even
+            # and not provide clues
+            matched = matched
+        ctr += 1
+    return matched
+
+
 def verifyPassword(storedPassword: str, providedPassword: str) -> bool:
     """Verify a stored password against one provided by user
     """
@@ -44,24 +66,7 @@ def verifyPassword(storedPassword: str, providedPassword: str) -> bool:
     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
-        else:
-            # this is to make the timing more even
-            # and not provide clues
-            matched = matched
-        ctr += 1
-    return matched
+    return constantTimeStringCheck(pwHash, storedPassword)
 
 
 def createBasicAuthHeader(nickname: str, password: str) -> str:
diff --git a/tests.py b/tests.py
index 127d0c79..ccbf216c 100644
--- a/tests.py
+++ b/tests.py
@@ -54,6 +54,7 @@ from person import setBio
 from skills import setSkillLevel
 from roles import setRole
 from roles import outboxDelegate
+from auth import constantTimeStringCheck
 from auth import createBasicAuthHeader
 from auth import authorizeBasic
 from auth import storeBasicCredentials
@@ -2085,8 +2086,36 @@ def testTranslations():
             assert langJson.get(englishStr)
 
 
+def testConstantTimeStringCheck():
+    print('testConstantTimeStringCheck')
+    assert constantTimeStringCheck('testing', 'testing')
+    assert not constantTimeStringCheck('testing', '1234')
+    assert not constantTimeStringCheck('testing', '1234567')
+
+    itterations = 256
+
+    start = time.time()
+    for timingTest in range(itterations):
+        constantTimeStringCheck('nnjfbefefbsnjsdnvbcueftqfeuqfbqefnjeniwufgy',
+                                'nnjfbefefbsnjsdnvbcueftqfeuqfbqefnjeniwufgy')
+    end = time.time()
+    avTime1 = ((end - start) * 1000000 / itterations)
+
+    # change characters and observe timing difference
+    start = time.time()
+    for timingTest in range(itterations):
+        constantTimeStringCheck('nnjfbefefbsnjsdnvbcueftqfeuqfbqefnjeniwufgy',
+                                'nnjfbefefbsnjsdnvbcueftqfeuqfbqeznjeniwufgy')
+    end = time.time()
+    avTime2 = ((end - start) * 1000000 / itterations)
+    timeDiffMicroseconds = abs(avTime2 - avTime1)
+    # time difference should be less than 10uS
+    assert timeDiffMicroseconds < 10
+
+
 def runAllTests():
     print('Running tests...')
+    testConstantTimeStringCheck()
     testTranslations()
     testValidContentWarning()
     testRemoveIdEnding()