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+# TrustyHash - A Trustable Hash Calculator
 
-### Features
+TrustyHash is a small [client-side](https://unhosted.org/) web application that
+computes SHA-256 hash values on both local files and on remote URLs, with a
+strong emphasis on a process that will allow you to trust the results. Works
+offline!
 
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+TrustyHash homepage: https://github.com/sprin/TrustyHash
+
+[Use it here](https://sprin.github.io/TrustyHash/)
+
+## How is this useful?
+
+Integrity: "We have in hand the same set of sequences of bits that came into
+existence when the object was created" - [Lynch](http://www.clir.org/pubs/reports/pub92/lynch.html)
+
+"Friends don't let friends use unverified downloads."
+
+This fills a need for a verifiable, web-based hash calculator written in free
+JavaScript. If you already use the command-line hash utilities on your
+system, you should continue to use those. This is targeted towards users who do
+not have or are unable to use the hash utilities on their local systems. While
+universal command-line-literacy is a good goal, the concepts of file integrity and
+authenticity and the ability to use tools for verification are perhaps more
+fundamental.
+
+Integrity is the first link in secure systems, and key to determining
+authenticity. If we trust the association between an author and the hash value
+of a file they created, perhaps because we trust them and they gave us the
+hash in person, we can authenticate whether a file we believe to be the same
+really did come from them. We can achieve the same result if the author had
+used a signing key, and signed and distributed a hash value along with the
+file, and we could trust the association between a particular key and the
+author - albeit with somewhat more complexity and caveats (eg, has the signing
+key been kept private?).
+
+In a few words, this tool aims to enable verification of integrity and
+authenticity claims in an accessible way that depends only on a trusted hash
+value and the correctness and integrity of the TrustyHash app and the browser
+it executes in. See the section "Trust" below for recommendations on
+how to verify integrity of this application.
+
+## Usage
+
+Local files can be opened from a file select dialog, or dragged into the "drop
+area". Remote URLs can be entered, and if the remote server allows cross-origin
+GET requests via
+[CORS](https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS),
+the file will be downloaded to the browser, with the option of saving locally.
+
+It's recommended to save the application, verify the integrity, and use the
+saved copy from then on. To save from the browser, use "Save Page" > "Web Page,
+HTML Only", and use a filename of `TrustyHash.html`. To verify, read the
+section on "Trust" below.