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While working with remote files every now and then, you probably want to check file

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integrity.

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Checksums are the solution.

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A checksum is a number that serves as a sum of correct digits in data, and which can be

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used to detect errors that have occurred during file storage or transmission.

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The idea is that based on the content of your file, you should always be able to calculate

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the exact same checksum.

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MD5 sums or SHA sums are commonly used as a checksum on Linux.

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Even if MD5 is considered broken, for simply verifying file downloads it can still be used.

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And if more security is needed, you use either SHA-256 or SHA-512, where SHA-512 is giving

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the better security.

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Use the MD5 sum on any file to calculate that file's MD5 checksum, and SHA sum to calculate

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the file's SHA checksum.

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The tab represents the number that you want to use in the command.

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And to use a checksum for verification, store it in a separate file and distribute it with

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a file.

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Let me show you.

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Okay, let me demonstrate.

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To start it, I'm using MD5 sum on etcpasswd.

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That's showing the checksum.

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And I want to write that checksum to passwd.md5.

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Then I'm going to use my verification command.

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So md5 sum minus cpasswd md5.

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And I can see that etcpasswd is okay.

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And then I'm using user add lisa.

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And I'm running my MD5 sum again.

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And oh boy, what do we see?

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One computed checksum did not match.

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Why is that?

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Because the content of the file has changed.

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And that's exactly what the checksum is doing.

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It's observing changed content.