This disclosure describes methods and systems for encoding and decoding signals from a host signal such as audio, video or imagery. One claim recites a method comprising: receiving a host signal carrying an auxiliary signal; extracting data representing at least some features of the host signal, said extracting utilizes one or more processors; using the data representing at least some features of the host signal to determine a key; and detecting the auxiliary signal in a transform domain associated with the key, the detecting utilizes one or more processors. Other claims and combinations are provided as well.