Verify a challenging hmac signature against a string / shared-secret
    Returns a boolean if the verification succeeded or failed.

def hmac_signature(string, shared_secret, challenge_hmac):
    '''
    Verify a challenging hmac signature against a string / shared-secret
    Returns a boolean if the verification succeeded or failed.
    '''
    msg = salt.utils.stringutils.to_bytes(string)
    key = salt.utils.stringutils.to_bytes(shared_secret)
    challenge = salt.utils.stringutils.to_bytes(challenge_hmac)
    hmac_hash = hmac.new(key, msg, hashlib.sha256)
    valid_hmac = base64.b64encode(hmac_hash.digest())
    return valid_hmac == challenge