semi-preditcable OTPs

Joseph Ashwood ashwood at
Tue Oct 25 16:06:31 EDT 2005

----- Original Message ----- 
From: "Travis H." <solinym at>
Subject: semi-preditcable OTPs
> Despite [flawed OTPs], the NSA wasn't able to crack any messages.

> My question is, why?   I think I know the reason, and that is that any
> predictability in a symbol of the OTP correlated to a predictability
> in only one plaintext symbol.  In other words, there was no "leverage"
> whereby that plaintext could then be used to derive other symbols.
> Can anyone explain this better (or more accurately)?  Is this lack of
> diffusion?  Or does it have something to do with the unicity distance?

You've pretty much got it. In order for a OTP to work you simply need what I 
commonly refer to as an overflow of entropy. The source of this entropy 
doesn't matter and it can be from the plaintext as much as it can be from 
the key. This extends the unicity distance (as you noted) and can render it 
impossible to decrypt.

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