def METHOD_NAME(self, pk, sk, c):
    s = c['c1'] ** sk['x']
    m = c['c2'] * (s ** -1)
    if group.groupSetting() == 'integer':
        M = group.decode(m % group.p)
    elif group.groupSetting() == 'elliptic_curve':
        M = group.decode(m)
    if debug: print('m => %s' % m)
    if debug: print('dec M => %s' % M)
    return M