1993-10-22 - Re: Subliminal Channels

Header Data

From: hfinney@shell.portal.com (Hal Finney)
To: cypherpunks@toad.com
Message Hash: b6086f58351d0004cd85a2fdbb5a8201b18ae7790056c15b865bf59e24447208
Message ID: <9310221610.AA00302@jobe.shell.portal.com>
Reply To: N/A
UTC Datetime: 1993-10-22 16:12:56 UTC
Raw Date: Fri, 22 Oct 93 09:12:56 PDT

Raw message

From: hfinney@shell.portal.com (Hal Finney)
Date: Fri, 22 Oct 93 09:12:56 PDT
To: cypherpunks@toad.com
Subject: Re:  Subliminal Channels
Message-ID: <9310221610.AA00302@jobe.shell.portal.com>
MIME-Version: 1.0
Content-Type: text/plain

Steganography, the art of hiding a secret message inside of an openly
readable one, can be thought of as a subliminal channel.  In the prisoner
example, the prisoners could have pre-arranged that, say, every 10th
character in the typed messages they exchange would be used to spell out
a secret message.  Or perhaps word or sentence lengths or spacings could
send a message.

In general, in any system where there is ambiguity, more than one way of
expressing a valid message, there is a subliminal channel.  Since DSS
signatures are apparently not unique for a given message (unlike, say,
RSA signatures as specified in the PKCS standards), they have such a