Use secrets module instead of random for security-sensitive token generation

Replace random.sample() with secrets.choice() for generating PKCE code
verifiers, OAuth2 state parameters, and OIDC nonces. The random module
uses Mersenne Twister which is not cryptographically secure. The secrets
module uses os.urandom(), providing a CSPRNG suitable for security tokens.

This also fixes a subtle entropy reduction caused by random.sample()
drawing without replacement, which prevented character repetition.

Author: 4gust

msal/oauth2cli/oauth2.py

@@ -13,7 +13,7 @@
 import base64
 import sys
 import functools
-import random
+import secrets
 import string
 import hashlib
 
@@ -277,8 +277,9 @@ def _scope_set(scope):
 
 def _generate_pkce_code_verifier(length=43):
     assert 43 <= length <= 128
+    alphabet = string.ascii_letters + string.digits + "-._~"
     verifier = "".join(  # https://tools.ietf.org/html/rfc7636#section-4.1
-        random.sample(string.ascii_letters + string.digits + "-._~", length))
+        secrets.choice(alphabet) for _ in range(length))
     code_challenge = (
         # https://tools.ietf.org/html/rfc7636#section-4.2
         base64.urlsafe_b64encode(hashlib.sha256(verifier.encode("ascii")).digest())
@@ -488,7 +489,7 @@ def initiate_auth_code_flow(
             raise ValueError('response_type="token ..." is not allowed')
         pkce = _generate_pkce_code_verifier()
         flow = {  # These data are required by obtain_token_by_auth_code_flow()
-            "state": state or "".join(random.sample(string.ascii_letters, 16)),
+            "state": state or "".join(secrets.choice(string.ascii_letters) for _ in range(16)),
             "redirect_uri": redirect_uri,
             "scope": scope,
             }

msal/oauth2cli/oidc.py

@@ -1,7 +1,7 @@
 import json
 import base64
 import time
-import random
+import secrets
 import string
 import warnings
 import hashlib
@@ -238,7 +238,7 @@ def initiate_auth_code_flow(
             # Here we just automatically add it. If the caller do not want id_token,
             # they should simply go with oauth2.Client.
             _scope.append("openid")
-        nonce = "".join(random.sample(string.ascii_letters, 16))
+        nonce = "".join(secrets.choice(string.ascii_letters) for _ in range(16))
         flow = super(Client, self).initiate_auth_code_flow(
             scope=_scope, nonce=_nonce_hash(nonce), **kwargs)
         flow["nonce"] = nonce