Entanglement

entangle the data stream, by attacker, using a known source.

Then - either

A you can get a result as the data stream is resolved (whatever that may entail) as the entangled data you kept is resolved as well.

B if the original source data is not resolved - ie they end user thinks that there was a compromise - then you can safely attempt to resolve the data yourself, as the original will have been thrown away. This may not give the exact key, but will/can provide information of the key system