| CPC H04L 63/0876 (2013.01) [H04L 63/12 (2013.01); H04L 2463/081 (2013.01)] | 15 Claims |

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1. A method of maintaining a secure relationship between a server and a client device implementing a browser, the method comprising at the client device:
receiving a first message comprising a first challenge from the server and determining and providing a first response to the first challenge, the first challenge comprising at least a set of coordinates and colors of an image to be rendered by the client device, wherein a resulting rendered image is specific to at least a graphics processing unit (GPU) of the client device;
establishing a cookie associated with the secure relationship;
providing the cookie to the server to maintain the secure relationship;
receiving a second message, after providing the first response to the first challenge, the second message comprising the first challenge and a second challenge from the server; and
determining the first response and a second response to the second challenge, and providing a composite response from which the first response and the second response are derivable by the server;
wherein each challenge uses a challenge function adapted to provide a fingerprint of a combination of the client device and the browser.
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