AESKey is the encrypted value which I got using apex crypto class.
    String TargetUrl = 'https://mysandbox.cs88.force.com/redirectpage';
            String hyperlink  = '<a href="'+TargetUrl+'?uniqueId='+AESKey+'">Link to Join</a>';
            mail.setHtmlBody(htmlbdy);

String KeyValue= ApexPages.currentPage().getParameters().get('uniqueId');
System.debug('RequestKey:-'+KeyValue);

But after doing getParameters().get('uniqueId'), I get
E f7X10FDW3ExD1+jgfcRdHD1Helxk vk=