MailChimp offers merge tags which dynamically pull in campaign data. By using combinations of these tags we can generate and pass unique identifiers (UUID) into the key parameter without the need for any javascript. For example:

*|CAMPAIGN_UID|*  pulls in your campaign’s unique ID

*|DATE:y/M/d/H/u|*  pulls the date and time your subscribers receive your email down to the microsecond

By combining the two we can successfully create a dynamic UUID and our key parameter would therefore look something like this:
key=<*|CAMPAIGN_UID|*><*|DATE:y/M/d/H/u|*> 

NOTE: the same UUID must always be passed into the key parameter of both the anchor and image tag

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