Comment by betterunix

"I bet in other contexts (say insider trading), you'd agree that it doesn't matter whether some CEO tipped off his buddy about insider information by carefully varying load on a server to modulate response times on a web page, thus communicating bits of information."

Ah, but what if the CEO is just taking a long time to reply to emails from friends, because he is very busy preparing for some huge business move -- is it insider trading if one of those friends sets up an options position that profits from increased volatility? This gets down to the difference between a side channel (inadvertent) and a covert channel (deliberate). The distinction does not matter from an information theoretic point of view; the same information is communicated in either case.