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Re: Introducing TGP...
From: Jeffrey Walton <noloader () gmail com>
Date: Mon, 14 Jun 2010 04:22:41 -0400

Hi Thor,

I'm probably splitting too fine a hair here...

The SHA256 hashing of the private key may not result in authenticity
assurances on the key (if I'm reading it correctly). I believe that's
an Athenticate-then-Encrypt scheme, and the details of the
interactions in AtE can be tricky. Hugo Krawczyk evaluated similar AtE
systems (for example, SSL) in The Order of Encryption and
Authentication for Protecting Communications. The two AtE schemes
which are provably secure are (1) a block cipher operated in CBC mode,
and (2) stream ciphers which XORs data with a pseudorandom pad.

I can see where the hash might satisfy the psuedo random pad, but I
don't see the stream cipher in the equation. Perhaps a more
traditional Encrpyt-then-Authenticate (for example, IPSec) might be
useful for TGP.  [At least TGP is not using Authenticate-and-Encrypt,
which Krawczyk proved insecure (for example, early SSH)].

If your using SHA-256 as the PRF of a KDF, then TGP might be reducing
the security of the system protecting the private assymmetric key (I'm
presuming AES-256 was chosen for a reason). AES-256 provides a
security level of ~2^255, while SHA-256 provides ~2^128. Its mostly a
theoretical observation: I'd attack the password/passphrase before
attempting pre-image attacks on the hash. [After all these years,
SHA-160 has only been reduced to ~2^50 from a theoretical 2^80, and
2^50  is still beyond my reach].

Jeff

On Sun, Jun 13, 2010 at 5:44 PM, Thor (Hammer of God)
<Thor () hammerofgod com> wrote:

This is what I’ve been talking about... Here is the first part of the docs I wrote up - make sure you see that I'm 
not yet supporting huge files unless you have huge RAM.  **.Net 4.0 Client profile is required to run this.**

Right now the install bits are only available on the pilot site at: http://www.owa.hammerofgod.com in the downloads 
section.   I have to wait on Raging Haggis to return from Canada before posting on www.hammerofgod.com .

Here's a bit from the TGP Overview document included with the install and on the web site.  Please read through it 
before asking silly questions. :)

Also, feel free to hack it up as much as you would like.  I know this is full disclosure, so feel free to zing them 
at me, or if you prefer, I can work with you on any issues you might have

Remember, this is totally free, so my ability to handle custom requests will be limited.  For those looking to break 
it, I would look at fuzzing the XML documents and the "drag and drop public XML" parsing feature.

If you have questions or challenges about any of the security, I would ask to keep it on the list so that everyone 
can get the full benefit of productive security development.   The read-me should pretty much lay everything out for 
you.  If not, we'll take it up from there.

t





TGP – “Thor’s Godly Privacy”

06/13/10 v1.1.06



TGP is a small yet very powerful encryption utility.  With all eyes on “the cloud,” I decided to write an encryption 
application better suited to an environment where portability and security were, at the least, challenging.   In 
cloud computing, not only is the use of file structures becoming more abstract, but the very concept of a “file 
server” is becoming more and more ubiquitous.



As such, I designed TGP with “encryption for the cloud” in mind.  That means that not only does TGP do everything 
your normal PGP-type applications do, but it does things a bit differently – differently in a way that can change the 
way you work with your encrypted data.  At the simplest level, this is done by encrypting data into byte arrays, and 
then converting those byte arrays into Base64 encoded text wrapped inside XML tags.  In this way, not only do you get 
your typical file-based encrypted representation of your data, but you also get data that you can copy and paste 
directly into any email, mailing list, blog-page, or social networking site.



What I think is interesting about this is that if we choose to, we no longer have to be the custodians of our 
encrypted data – we don’t have to worry about actually housing the files: we can just post them to the internet and 
let someone else assume the burden of storing the files for us.



If I want to share encrypted files with someone or secure my own files, all I have to do is TGP encrypt the data I 
want, and post it to a mailing list somewhere.  In the case of a list like Bugtraq or Full Disclosure, the data is 
actually automatically replicated out to any number of archive sites, thus distributing my data for me.  I can 
literally be anywhere in the world and just do a quick search for my post to retrieve my data.  And since the TGP 
public key files are also text representations of encrypted key data, I can do the same with my keys.



Normally, you want to keep your private keys as safe as possible.  This is still the case with TGP.  However, it is 
trivial to build as many private keys as you wish to use for anything you want to use them for.  TGP Private Key 
files are password protected and individually salted, so with a strong passphrase you have very reasonable assurance 
that no one is going to get to your key any time soon.  So, you can create a private key with a strong password, post 
that, and then, say, encrypt a scan of your passport and post that.  Then if you are ever in a pinch while travelling 
or something like that, you can simply use Google or Bing to access your data wherever you are.



Of course, that’s just an example, but I think it illustrates the power of encrypted file structures like this.  You 
can literally use Facebook to post encrypted documents that you don’t have to maintain.



That’s really the main different between TGP and an application like PGP.  That and of course, TGP is free, and 
personally, I think PGP is tardware.  It’s bloated, it’s far too expensive, it’s hard to use, and if you don’t watch 
your licensing, you can get screwed hard like I did when I didn’t want to buy the extended support and one day my 
encrypted drives stopped working until I paid them.  That doesn’t fly.  TGP also doesn’t require that you are an 
admin to install.  However, the .NET installer for the 4.0 client profile does – that’s not my doing.  Regardless, 
here are the file structures TGP uses:



Things that still suck about TGP

Currently TGP uses a memory stream for the destination of the AES cryptostream.  This sucks because it makes the 
maximum file one can encrypt based on available memory.  It’s not a huge deal, but it does keep you from encrypting a 
gigabyte file.  I’ll be changing that soon.



Timothy “Thor” Mullen

Hammer of God

thor () hammerofgod com

www.hammerofgod.com



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_______________________________________________
Full-Disclosure - We believe in it.
Charter: http://lists.grok.org.uk/full-disclosure-charter.html
Hosted and sponsored by Secunia - http://secunia.com/


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