Internet users can download software for their computers from app stores (e.g., Mac App Store and Windows Store) or from other sources, such as the developers' websites. Most Internet users in the US rely on the latter, according to our representative study, which makes them directly responsible for the content they download. To enable users to detect if the downloaded files have been corrupted, developers can publish a checksum together with the link to the program file; users can then

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... verify that the checksum matches the one they obtain from the downloaded file. In this paper, we assess the prevalence of such behavior among the general Internet population in the US (N = 2,000), and we develop easyto-use tools for users and developers to automate both the process of checksum verification and generation. Specifically, we propose an extension to the recent W3C specification for sub-resource integrity in order to provide integrity protection for download links. Also, we develop an extension for the popular Chrome browser that computes and verifies checksums of downloaded files automatically, and an extension for the WordPress CMS that developers can use to easily attach checksums to their remote content. Our in situ experiments with 40 participants demonstrate the usability and effectiveness issues of checksums verification, and shows user desirability for our extension. CCS CONCEPTS • Security and privacy → Web protocol security; Usability in security and privacy; Hash functions and message authentication codes;