The discussion on this thread inspired me to think about this problem and write up a solution. My approach: Use base 26 to convert a set of unique integers into a set of unique strings. If you expect to assign IDs to N subjects, you can use strings that have k characters, where N < 26^k.
How to generate the ID values (strings) from integers: "Base 26: A mapping from integers to strings"
How to randomly select N unique IDs: "Generate random ID values for subjects in SAS"
The primary advantage of this technique over some of the other proposals is that it ensures uniqueness of the ID values. You don't have to check whether a random string has already been assigned.
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