A summary report with a cross-filter is a type of report that allows you to summarize data by grouping rows and columns and applying filters based on related objects and fields. A cross-filter is a filter that lets you include or exclude records in your report based on related objects and their fields. In this case, you can create a summary report that includes a cross-filter to the Account object with a sub-filter for credit card number equal to null. This will show you all B2C accounts that are missing the credit card number, regardless of whether it is encrypted or not. References:
https://help.salesforce.com/s/articleView?id=sf.reports_summary_charts.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.reports_cross_filters.htm&type=5