How to securely verify an HMAC in Python 2.7?

Question

I'm using Python 2.7 and am creating an HMAC using the `hmac` library. Python 3.3 includes a `compare_digest()` function that will compare two digests and resist timing attacks, but that's not available in 2.7. Prevailing advice is not to roll my own crypto, so are there any mature Python libraries that provide that functionality? PyCrypto does not appear to.

Accepted Answer

For anyone finding this from search, if using Django, then you can also use the `constant_time_compare` function in `django.utils.crypto`.

```
>>> from django.utils.crypto import constant_time_compare
>>> constant_time_compare("foo", "bar")
False
>>> constant_time_compare("foo", "foo")
True
```

That this comes with the same caveat as `hmac.compare_digest` (and actually uses `hmac.compare_digest` if it exists):

Note: If a and b are of different lengths, or if an error occurs, a timing attack could theoretically reveal information about the types and lengths of a and b–but not their values.

How to securely verify an HMAC in Python 2.7?

Solution

Problem