Alice and Bob post at random intervals inside a thread on an anonymous message board
The thread of posts is interpreted as a string of bits
Posts by Alice represent a 1
Posts by Bob represent a 0
The security of this key in bits is the length of the thread in posts, minus 1
So 100 posts make for 99-bit security
Alice and Bob are the only ones who know the key, even though the posts which comprise it are hosted publically. All Eve sees when she browses their thread is a series of blank posts made by anonymous users. She knows what Alice and Bob are doing, but