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[liberationtech] Deterministic builds and software trust [was: Help test Tor Browser!]

Mike Perry mikeperry at
Tue Jun 18 20:54:30 PDT 2013

Jacob Appelbaum:
> Hi,
> I'm really excited to say that Tor Browser has had some really important
> changes. Mike Perry has really outdone himself - from deterministic
> builds that allow us to verify that he is honest to actually having
> serious usability improvements. 

First, thanks for the praise, Jake!

But: I've been meaning to clarify this "honesty" point for a few days
now, and Cooper's similar statement in another thread about security
being all about trust reminded me of it.

I actually disagree with the underlying assumptions of both points.

I didn't spend six agonizing weeks (and counting) getting deterministic
builds to work for Tor Browser to prove that I was honest or
trustworthy. I did it because I don't believe that software development
models based on single party trust can actually be secure against
serious adversaries anymore, given the current trends in computer
security and "cyberwar".

For the past several years, we've been seeing a steady increase in the
weaponization, stockpiling, and the use of exploits by multiple
governments, and by multiple *areas* of multiple governments. This
includes weaponized exploits specifically designed to "bridge the air
gap", by attacking software/hardware USB stacks, disconnected Bluetooth
interfaces, disconnected Wifi interfaces, etc. Even if these exploits
themselves don't leak (ha!), the fact that they are known to exist means
that other parties can begin looking for them.

In this brave new world, without the benefit of anonymity to protect
oneself from such targeted attacks, I don't believe it is possible to
keep a software-based GPG key secure anymore, nor do I believe it is
possible to keep even an offline build machine secure from malware
injection anymore, especially against the types of adversaries that Tor
has to contend with.

This means that software development has to evolve beyond the simple
models of "Trust my gpg-signed apt archive from my trusted build
machine", or even projects like Debian going to end up distributing
state-sponsored malware in short order.

This is where deterministic builds come in: any individual can use our
anonymity network to download our source code, verify it against public
signed, audited, and mirrored git repositories, and reproduce our builds
exactly, without being subject to such targeted attacks. If they notice
any differences, they can alert the public builders/signers, hopefully
using a pseudonym or our anonymous trac account.

This also will eventually allow us to create a number of auxiliary
authentication mechanisms for our packages, beyond just trusting the
offline build machine and the gpg key integrity.

I believe it is important for Tor to set an example on this point, and I
hope that the Linux distributions will follow in making deterministic
packaging the norm. (Don't despair: it probably won't take 6 weeks per
package. Firefox is just a bitch).

Otherwise, I really don't think we'll have working computers left in
5-10 years from now :/.

I hope to write a longer blog post about this topic on the Tor Blog in
the next couple weeks, discussing the dangers of exploit weaponization
and the threats it poses to software engineering and software
distribution. I'm still mulling over the exact focus and if I should
split the two ideas apart, or combine them into one post...

Ideas and comments welcome!

Mike Perry

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