So , how does the police find/monitor criminals' actions (eavesdropping) that do cyber crime(pornography , drug trafficking etc..) without breaking their privacy, and succesfully catch em and provide evidence for the court.
closed as too broad by Lucas Kauffman, TildalWave, Adi, Xander, AJ Henderson Apr 7 '14 at 14:10
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The police in most (if not all) countries obeys to various investigation laws and clear legal frameworks that determine what they are authorized to do, how, when, etc.
Usually to wiretap a communication, a police officer has to obtain the approval of a judiciary authority (e.g. an independant judge).
There might be exceptions to those rules in case of specific investigations and/or when there is urgency to act (e.g. terrorism, or a crime that is going to be commited soon), but again those cases are well defined in the law to prevent abuse/mis-use by the authorities.
All the administrative procedures, paperwork, approvals, authorizations etc. have to be stricly followed in order for the evidence to be received by the court.