In one embodiment, a computing device determines a Completely Automated Public Turing Test to Tell Computers and Humans Apart (CAPTCHA). The CAPTCHA includes a first static image that has image sections that are arranged in a first order. Each of the image sections corresponds to a unique identifier. The CAPTCHA further includes a second static image that includes each of the image sections of the first static image that are arranged in a second order. The computing device generates web-browser-executable code for converting the second static image to the first static image based on the first static image, the first order, and the unique identifiers. The computing device sends the second static image and the web-browser-executable code to a client device.