How to convert hex value to an integer?

When working on a method defined in a class that controls the lead object. The desired end result is to produce a unique number for each Lead record. I currently have this working, but the out put is a hex value and I need it to be integer. The field to populate is called "Unique_Reference_number__C". An auto-number field is not sufficient, as there are different formats applied contingent on lead field values. How to convert hex to integer?Is there a simple way to do that? Here is my method:

private void GenerateRandomNumber(List lList){ System.debug('*lList*********** '+lList.size()); for(Lead l: lList){ if(l.Unique_Reference_number__c==null || l.Unique_Reference_number__c==''){ System.debug('*1*********** '); Integer len = 7; Blob blobKey = crypto.generateAesKey(128); String key = EncodingUtil.(blobKey); String pwd = 'ID'+key.substring(0,len); l.Unique_Reference_number__c= pwd; System.debug('*1*********** '+pwd); }