Question

I would like to allow people to provide the name of a hash function as a means of digitally fingerprinting some object:

def create_ref(obj, hashfn='sha256'):
    """
    Returns a tuple of hexdigest and the method used to generate
    the digest.

    >>> create_ref({}, 'sha1')
    ('bf21a9e8fbc5a3846fb05b4fa0859e0917b2202f', 'sha1')
    >>> create_ref({}, 'md5')
    ('99914b932bd37a50b983c5e7c90ae93b', 'md5')
    """
    return (eval('hashlib.%s' % hashfn)(unicode(obj)).hexdigest(), hashfn)

Is hard coding hashlib sufficently robust to prevent abuse of eval?

Answer 1

instead of eval, try this code:

def create_ref(obj, hashfn='sha256'):
    """
    Returns a tuple of hexdigest and the method used to generate
    the digest.

    >>> create_ref({}, 'sha1')
    ('bf21a9e8fbc5a3846fb05b4fa0859e0917b2202f', 'sha1')
    >>> create_ref({}, 'md5')
    ('99914b932bd37a50b983c5e7c90ae93b', 'md5')
    """
    allowed = hashlib.algorithms
    if hashfn in allowed:
        return (getattr(hashlib,hashfn)(unicode(obj)).hexdigest(), hashfn)
    else:
        raise NameError('Not a valid algorithm')

This will guarantee that the algorithm provided is a valid algorithm. (Note that hashlib.algorithms is new in 2.7, so if you use an older version, replace hashlib.algorithms with a tuple of allowed algorithms.