How do I identify the program that called a stored procedure?

As part of an application database design spec, we've been asked to block execution of a set of stored procedures if they have been fired directly from SSMS / osql / sqlcmd and so on; that is, they must be permitted to run only from within the application itself.

When our team questioned if this is even possible, we were shown a demo that limited queries to specific systems (actually it let the query run, but logged to a table where it was running from). However, the procedure that did that is encrypted, so we could not find out how it was done.

How do we accomplish this?