my solution

auth.py

@@ -1,57 +1,88 @@
+import getpass
 import json
+import pathlib
+import random
+import string
 import sys
-from getpass import getpass
-from hashlib import sha256
 
-PWDB_PATH = 'pwdb.json'
+# name of the file where we store the pw database
+PWDB_FLNAME = pathlib.Path('pwdb.json')
+
+# list of valid characters for salt (only ASCII letters + digits + punctuation)
+CHARS = string.ascii_letters + string.digits + string.punctuation
+
+# length of salt
+SALT_LENGTH = 5
 
 def get_credentials():
+    # get input from terminal
     username = input('Enter your username: ')
-    hashed_password = pwhash(getpass('Enter your password: '))
-    return (username, hashed_password)
+    # get password using the appropriate module, so that typed characters are not
+    # echoed to the terminal
+    password = getpass.getpass('Enter your password: ')
+    return (username, password)
 
-def authenticate(username, hashed_password, pwdb):
-    return hashed_password == pwdb[username]
+def authenticate(username, pass_text, pwdb):
+    # get the salt from the database
+    salt = pwdb[username][1]
+    # calculate hash and compare with stored hash
+    return pwhash(pass_text, salt) == pwdb[username][0]
 
 def add_user(username, pwdb):
-    pwdb[username] = pwhash(getpass(f'Enter password for {username}: '))
+    # do not try to add a username twice
+    if username in pwdb:
+        raise Exception(f'Username already exists [{username}]!')
+    else:
+        password = getpass.getpass(f'Enter password for {username}: ')
+        salt = get_salt()
+        pwdb[username] = (pwhash(password,salt), salt)
     return pwdb
 
-def read_pwdb(PWDB_PATH):
-    try:
-        pwdb_file = open(PWDB_PATH, 'rt')
-        pwdb = json.load(pwdb_file)
-    except Exception:
+def read_pwdb(pwdb_path):
+    if not pwdb_path.exists():
         pwdb = {}
+    else:
+        with open(pwdb_path, 'rt') as pwdb_file:
+            pwdb = json.load(pwdb_file)
     return pwdb
 
-def write_pwdb(pwdb, PWDB_PATH):
-    pwdb_file = open(PWDB_PATH, 'wt')
-    json.dump(pwdb, pwdb_file)
+def write_pwdb(pwdb, pwdb_path):
+    with open(pwdb_path, 'wt') as pwdb_file:
+        json.dump(pwdb, pwdb_file)
 
+def pwhash(pass_text, salt):
+    # simple additive hash -> very insecure!
+    hash_ = 0
+    full_pass_text = pass_text + salt
+    for idx, char in enumerate(full_pass_text):
+        # use idx as a multiplier, so that shuffling the characters returns a
+        # different hash
+        hash_ += (idx+1)*ord(char)
+    return hash_
 
-def pwhash(pwd):
-    encoded_pwd = pwd.encode("utf-8")
-    m = sha256()
-    m.update(encoded_pwd)
-    return m.hexdigest()
+def get_salt():
+    salt_chars = random.choices(CHARS, k=SALT_LENGTH)
+    return ''.join(salt_chars)
 
+def main(pwdb_path):
+    # load the password database from file
+    pwdb = read_pwdb(pwdb_path)
 
-
-if __name__ == "__main__":
-    PWDB_PATH = 'pwdb.json'
-    pwdb = read_pwdb(PWDB_PATH)
-
+    # if we are passed an argument, we want to add a new user
     if len(sys.argv) > 1:
         pwdb = add_user(sys.argv[1], pwdb)
-        write_pwdb(pwdb, PWDB_PATH)
-    else:
-        username, password = get_credentials()
-        if username not in pwdb:
-            print('Wrong username!')
-        else:
-            if authenticate(username, password, pwdb):
-                print('Successfully authenticated!')
-            else:
-                print('Wrong password!')
+        write_pwdb(pwdb, pwdb_path)
+        return
+
+    # ask for credentials
+    username, password = get_credentials()
+    if username not in pwdb or not authenticate(username, password, pwdb):
+        print('Wrong username or password!')
+    else:
+        print('Successfully authenticated!')
+
+    return
+
+if __name__ == '__main__':
+    main(PWDB_FLNAME)