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Re: What can an attacker see in an encrypted message?

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[In almost every paragraph of this reply, I found myself writing "if 
it's a strong cipher", so I edited out all those references. Please take 
it as read.]

On 05/10/15 13:02, Anonymous Remailer (austria) wrote:
> After seeing all of the cryptanalysis on the ng recently it occurs
> to me to ask, just how much can an attacker figure out from a
> ciphertext? What if he or she has some of the passphrase? Here I'm
> talking about a ciphertext created using one of the algorithms
> available in gpg.

Every bit of the passphrase will affect around half the bits of the 
ciphertext, so having some of the passphrase is likely to be of little 
help other than to reduce the search space for the rest of the passphrase.

> Let's say, purely for example, that the passphrase is the ever
> powerful "qwertyuiop" that's been brought up here.
>
> If the attacker has figured out that "qwer" is a part of the
> passphrase can that attacker read any of the ciphertext? If so, how
> much? Does that knowledge of some of the passphrase allow the
> attacker to figure out the rest of it?

Well, it certainly can't hurt! Of course, it depends on the algorithm. A 
one-bit difference in the key can affect as many as half the bits of the 
ciphertext. So knowing 32 bits ("qwer") out of 80 ("qwertyuiop") is 
nice, but not sufficient in itself, at least not in that way. On the 
other hand, it reduces the search space from 2^80 to 2^48, which may 
well bring it into the realms of practical brute force.

> And, pretty much on this same subject, could an attacker tell what
> kind of data is encrypted in a file? Can they tell if it's a binary
> or a plaintext file that's encrypted?

No, for the same reason as given above.

> Can they tell much about the contents of the ciphertext at all
> without having part of the passphrase, or a part of the session key?

No.

> Could the attacker figure out a frequency of characters, or tell
> that some particular characters are used in a ciphertext without
> having the passphrase or session key?

No.

> Would a nation state attacker be able to figure out much from a
> ciphertext created using one of the algorithms available in gpg?

Ha! That depends on whether they've found a practical attack for the 
algorithm in question, and that information is classified, which means 
we don't know either way.

-- 
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
Sig line 4 vacant - apply within

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Thread

What can an attacker see in an encrypted message? "Anonymous Remailer (austria)" <mixmaster@remailer.privacy.at> - 2015-10-05 14:02 +0200
  Re: What can an attacker see in an encrypted message? Richard Heathfield <rjh@cpax.org.uk> - 2015-10-05 13:39 +0100
  Re: What can an attacker see in an encrypted message? Paulo Marques <pmarques@grupopie.com> - 2015-10-05 14:19 +0100
  Re: What can an attacker see in an encrypted message? William Unruh <unruh@invalid.ca> - 2015-10-05 20:46 +0000
  Re: What can an attacker see in an encrypted message? Robert Wessel <robertwessel2@yahoo.com> - 2015-10-05 22:39 -0500
    Re: What can an attacker see in an encrypted message? Jakob Bohm <jb-usenet@wisemo.com> - 2015-10-06 08:01 +0200
      Re: What can an attacker see in an encrypted message? PA7PTL <PA7PTL@panteltje.com> - 2015-10-06 07:07 +0000
        Re: What can an attacker see in an encrypted message? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-06 11:22 +0100
          Re: What can an attacker see in an encrypted message? eugene.styer@gmail.com - 2015-10-06 08:48 -0700
    Re: What can an attacker see in an encrypted message? Richard Heathfield <rjh@cpax.org.uk> - 2015-10-06 10:15 +0100
  Re: What can an attacker see in an encrypted message? Jeffrey Goldberg <nobody@goldmark.org> - 2015-10-06 11:22 -0500
    Re: What can an attacker see in an encrypted message? Jakob Bohm <jb-usenet@wisemo.com> - 2015-10-06 20:12 +0200
      Re: What can an attacker see in an encrypted message? Jeffrey Goldberg <nobody@goldmark.org> - 2015-10-07 19:21 -0500
        Re: What can an attacker see in an encrypted message? Richard Heathfield <rjh@cpax.org.uk> - 2015-10-08 06:33 +0100
          Re: What can an attacker see in an encrypted message? Jeffrey Goldberg <nobody@goldmark.org> - 2015-10-08 22:21 -0500
            Re: What can an attacker see in an encrypted message? Richard Heathfield <rjh@cpax.org.uk> - 2015-10-09 07:34 +0100
  Re: What can an attacker see in an encrypted message? Anonymous <anonymous@foto.nl1.torservers.net> - 2015-10-07 09:20 -0400

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