The system can, for example, help trace child pornography on the Internet without infringing on the privacy of Internet users, said researchers at the University of Twente in The Netherlands.
Internet service providers could use the tool to keep their network 'clean', they said.
The police uses a standard database to detect illegal photographs, like child pornography, on equipment they seize, such as computers or USB sticks.
For security reasons, the database cannot be used to scan networks such as the Internet.
If the database were to end up in criminal hands, criminals could use it to make their own illegal photographs untraceable, researchers said.
With the new system, the police database can be used to scan a network as well.
This makes it possible to search for illegal photographs without the risk of the database ending up in wrong hands.
The standard police database does not contain any truly illegal photographs but only features of these photographs expressed in numbers (hash values).
When the police seize equipment, such as a PC, the database is brought into action to compare the known hash values with those of the photographs on the PC.
In this way, the police are able to verify whether the equipment contains any illegal photographs.
"You can compare it with a sort of safe, the contents of which you cannot see but in which you can place photographs," said researcher Andreas Peter.
"Subsequently, it is the 'safe' alone that scans the photographs and then sends a confidential report to the police. The police can open this report with a special key. The report indicates how many illegal photographs are in the safe," Peter said.