Implement stricter password policy

Fixes #22

src/app/views/register.py

@@ -1,6 +1,6 @@
 import web
 from views.forms import register_form
-from views.utils import get_nav_bar, csrf_protected
+from views.utils import get_nav_bar, csrf_protected, password_weakness
 import models.register
 import models.user
 import bcrypt
@@ -41,6 +41,11 @@ class Register:
         if models.user.get_user_id_by_name(data.username):
             return render.register(nav, register, "Invalid user, already exists.")
 
+        # Check password security
+        weakness = password_weakness(data.password, data.username)
+        if weakness is not None:
+            return render.register(nav, register, weakness)
+
         password_hash = bcrypt.hashpw(data.password.encode('UTF-8'), bcrypt.gensalt())
 
         models.register.set_user(data.username, password_hash, data.full_name, data.company,

src/app/views/utils.py

@@ -78,3 +78,61 @@ def csrf_protected(f):
         return f(*args, **kwargs)
 
     return decorated
+
+
+def is_common_password(password):
+    """Helper function that checks various common passwords."""
+    def common_sequences(n):
+        # Check sequences of the same number
+        for i in range(n):
+            for j in range(n):
+                yield ''.join([str(i) for _ in range(j)])
+
+        # Check incrementing sequences
+        for i in range(n):
+            # Starting at 0
+            seq = ''.join([str(j) for j in range(i)])
+            yield seq
+            # Starting at 1
+            yield seq[1:]
+
+            # Decrementing
+            # Starting at 0
+            yield seq[::-1]
+            # Starting at 1
+            yield seq[1::-1]
+
+    common_passwords = [
+        'password', 'qwerty', 'iloveyou', '123123', 'abc123', 'admin',
+        'passwrod', 'password1', 'beelance', 'beelance2'
+    ]
+
+    if password in common_passwords or password in common_sequences(12):
+        return True
+
+    return False
+
+
+def password_weakness(password, username):
+    """
+    Check if the password fulfills the password policy.
+
+    The policy is:
+    - At least 8 characters, but not more than 70 (due to bcrypt)
+    - Does not overlap with the username
+    - Not a common password
+
+    :param password: The password to check
+    :param username: The username of the user (used to check similarity)
+    :return: The most important weakness of the password, or None if it fulfills the policy
+    """
+    if len(password) < 8:
+        return "The password must be at least 5 characters long."
+    elif len(password) > 70:
+        return "The password can't be longer than 70 characters."
+    elif password in username or username in password:
+        return "The password can't overlap with your username."
+    elif is_common_password(password):
+        return "The password is too common. Choose something more unique."
+
+    return None