We haven't announced this little upgrade to Spin Rewriter 4.0 yet, but it's also on the list of the upcoming upgrades:

From now on, you'll have the option for our server to remember a securely encrypted variation of your CopyScape username and API key — this means you'll no longer have to enter these credentials whenever you want to check your articles against CopyScape... and this is just one of the many improvements to the user interface that are coming out on October 18th...

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