python: API token generation with itsdangerous

Question

I'm following the "Flask Web Development" book to implement token based authentication. Basically, user authenticate his/herself with HTTP basic auth and a token is generated for it:

s = Serializer(app.config['SECRET_KEY'], expires_in = 3600)
token = s.dumps({ 'id': user.id })

But it looks like this doesn't change as long as the id and SECRET_KEY stay the same. I understand that the transaction will be through HTTPS but still think a dynamic token would be better. How to achieve a dynamic token?

Answer 1

If you need a token that is time sensitive, use the TimedSerializer class instead.

Not only does it used a timestamp to form the signature (thus producing a new signature each time you use it), but you can also limit the token lifetime using that timestamp:

>>> from itsdangerous import TimedSerializer
>>> s = TimedSerializer('sekrit')
>>> token = s.dumps({'id': 'foobar'})
>>> token
'{"id": "foobar"}.COWWsA.dect1vZLaDdgFQUA1G_iTpPY3Hg'
>>> s.loads(token, max_age=3600)
{'id': 'foobar'}
>>> s.loads(token, max_age=0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/site-packages/itsdangerous.py", line 643, in loads
    .unsign(s, max_age, return_timestamp=True)
  File "/Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/site-packages/itsdangerous.py", line 463, in unsign
    date_signed=self.timestamp_to_datetime(timestamp))
itsdangerous.SignatureExpired: Signature age 18 > 0 seconds

Note that the Serializer class doesn't actually support a expires_in keyword argument, so I suspect the code you are quoting actually uses the (undocumented) TimedJSONWebSignatureSerializer class instead (imported with an alias), which does take that keyword argument, and which includes a timestamp as well:

>>> from itsdangerous import TimedJSONWebSignatureSerializer as Serializer
>>> s = Serializer('sekrit', expires_in=3600)
>>> token = s.dumps({'id': 'foobar'})
>>> token
'eyJhbGciOiJIUzI1NiIsImV4cCI6MTQ0MzEwODAyMywiaWF0IjoxNDQzMTA0NDIzfQ.eyJpZCI6ImZvb2JhciJ9.eCD3zKK1lYT8cZ9w8g0YVpaF-1rR-k6UNCYq9dHmvGo'
>>> s.loads(token)
{'id': 'foobar'}
>>> s = Serializer('sekrit', expires_in=0)
>>> token = s.dumps({'id': 'foobar'})
>>> token
'eyJhbGciOiJIUzI1NiIsImV4cCI6MTQ0MzEwNDUwMCwiaWF0IjoxNDQzMTA0NTAwfQ.eyJpZCI6ImZvb2JhciJ9.Eiw3Eco7p61X-ikMxXS5dDVmjYmRSThcsMCxMyuA-r0'
>>> s.loads(token)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/site-packages/itsdangerous.py", line 807, in loads
    date_signed=self.get_issue_date(header))
itsdangerous.SignatureExpired: Signature expired

Because the timestamp is included, the generated token is not static:

>>> s = Serializer('sekrit', expires_in=3600)
>>> token1 = s.dumps({'id': 'foobar'})
>>> token2 = s.dumps({'id': 'foobar'})
>>> from difflib import ndiff
>>> print '\n'.join(ndiff(token1.split('.'), token2.split('.')))
- eyJhbGciOiJIUzI1NiIsImV4cCI6MTQ0MzEwODIwMywiaWF0IjoxNDQzMTA0NjAzfQ
?                                        ^ ^                    ^^

+ eyJhbGciOiJIUzI1NiIsImV4cCI6MTQ0MzEwODIxMSwiaWF0IjoxNDQzMTA0NjExfQ
?                                        ^ ^                    ^^

  eyJpZCI6ImZvb2JhciJ9
- YmrKQTvZEWw4_JOOPn5uEk9QlZNla4o3Gvo09H1MXfM
+ ApeLrwT_R60pkvCYe4ihzJFPG55tGiJK6VSi6BKxAXM

These two tokens differ materially even though they were produced by the same serializer.