simplified solution

.gitignore

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+pwdb.json

auth.py

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+import getpass
+import json
+import pathlib
+import random
+import string
+import sys
+
+# 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: ')
+    # 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, 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):
+    # 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):
+    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):
+    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 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 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)
+        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)
+