'Get a digest from a dictionary by looking at keys of the form
\'algo_digest\'.
Returns a 2-tuple (algo, digest) if found, else None. Currently
looks only for SHA256, then MD5.'

def _get_digest(self, info):

    result = None
    for algo in ('sha256', 'md5'):
        key = ('%s_digest' % algo)
        if (key in info):
            result = (algo, info[key])
            break
    return result
