<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def sign(key, qs):
 """Signs the query string using the key."""

    sig = derive_signature(key, qs)
    return "%s&%s" % (qs, urlencode([("sig", sig)]))