Methods, systems, and apparatus, including computer programs encoded on computer storage media, for creating secure browser cookies. One of the methods includes providing an encrypted cookie request that requests encryption of a cookie of the digital component provider and includes a digital component request identifier; receiving an encrypted cookie generated by encrypting the cookie using the digital component request identifier and an encryption key, wherein the encrypted cookie is configured for inclusion in a request for digital components from the digital component provider for presentation on the webpage; generating a digital component request for digital components that includes the encrypted cookie and requests identification of a digital component selected for presentation on the webpage using the encrypted cookie; and transmitting the digital component request that includes the encrypted cookie and requests identification of a digital component selected for presentation on the webpage using the encrypted cookie.