How to share a password in plain sight


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  • 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 can't distinguish Alice's posts from Bob's.

But Alice knows which posts belong to her, as does Bob, and so they can interpret the thread as a string of bits.

True, Mallory could interrupt the process by anonymously spamming the thread.  That would crash the protocol.  However, it would not enable Mallory to decrypt anything, so anything encrypted by Alice or Bob using that corrupted thread could not by decrypted by Mallory (or anyone except the person who encrypted it).

As long as Alice and Bob are able to make themselves indistinguishable from each other from the perspective of eavesdroppers, this protocol could be modified to work with other mediums of communication.  I can't actually think of a realistic use for this, given asymmetric cryptography, but maybe you will find it interesting nonetheless. 😄

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