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If you look at this page you'll see an monochrome image.

QR code

Above it says

Tip: Use the SHA1 checksum shown to verify file integrity.

However, I don't know if that's related. What are these things?

3 Answers 3

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It's a two-dimensional barcode (or QR Code). They are commonly used to encode a URL, which can be read by, for example, the camera on a mobile phone (with suitable decoding software).

In this case it is used to encode the URL of the zip file that can be downloaded on the same page. This can be confirmed by looking at the URL of the image itself:

http://chart.apis.google.com/chart?chs=150x150&cht=qr&chl=http://recaptcha.googlecode.com/files/recaptcha-dotnet-1.0.3.0-binary.zip&chld=L|1&choe=UTF-8

which uses a Google API to generate the image, based on the file's URL. Or, one could decode the image online.

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  • maybe i should have asked what the point of it was. Why would someone use this?
    – user3109
    May 28, 2010 at 23:18
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    @acidzombie24 - to verify that the file they're getting is the same as the file they want to get and that you're not being duped by some XSS exploit or something of the like.
    – MDMarra
    May 28, 2010 at 23:35
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    I think the idea is that it's easier to take a photo than to type in a URL (or whatever the data happens to be) - especially on a phone keypad. They are sometimes used in magazines and in advertising campaigns, mainly as a novelty though. In this case, I really don't see the point because it's very unlikely anyone needs the recaptcha .NET binary on their phone. It may, however, be quite useful for projects that directly produce mobile phone software.
    – sblair
    May 28, 2010 at 23:52
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The image has nothing to do with the SHA1 checksum of the file. It is an image encoding of the URL.

As explained in other answers, the image could be used to transfer the URL from screen to a device with an optical input mechanism.

The SHA1 checksum of the file is printed below, and should be used to verify the integrity of the downloaded file (that it has not been corrupted in the file transfer).

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On Linux you can generate the SHA1sum with sha1sum filename.zip and then see if your download went correctly.

info shasum