We are using External objects with custom adapter where ExternalId is 36 character unique identifier. When accessing external object Salesforce creates a 18 character long key similar to SObject Id i.e.: x055M0000001AABBA1
Considering that Salesforce uses ExternalId to retrieve data, my first thought is that there should be a map between key and ExternalId stored.
Is there a way to derive ExternalId from Salesforce key?