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Powers of Tau Operational Writeup
=================================

Round: 2
Date: 2017-11-10
Name: Jason Davies
Location: London, UK

Challenge: 467bc84f6eb98ff956eaf12a1b7ef4dc0aff1093c7a0d5c1dfbdb85bbfffb20a43965d0daefee3fec6c1a47af69100e117b44b74371824ac8af1e33b6f91add5
Response: 2f728af894524f55bda7a3e2c2e2db6a57a992811e90ed57456d62aead5106cdc5c97c86532d14b5185cc74d169f1b0c2c0ef1e582231ffa7936da55047c0cb2

Preparation Steps
=================

Git repository: https://github.com/ebfull/powersoftau
Commit hash: 9e1553c437183540392a7231d0788318a19b18a3
Compiler: rustc 1.23.0-nightly (d6b06c63a 2017-11-09)
Build: cargo build --release --features=u128-support
b2sum(./target/release/compute): be42f68b07c5c857bb6561a9ac2967d671ef412a71c87c2fb31776a6ab38c756736de66e554553021e129ecab45d922092873df8b71bd9a775ec05f189485198

I used a brand new 16GB USB stick and loaded ubuntu-17.04-desktop-amd64.iso; b2sum: 6a1c975b25b4e7f2dbf4fda84fe8b5de3ed6f4532b8c4f17e533ed11a0a8b5b9ad9fb83e8e4b89447c3a427be73f77a5f7c71b7f733fcc4bebf346e9c5c0de43.

I reformatted a second brand new 16GB USB stick to ext4, then copied the
`challenge` file and the `target/release/compute` binary.

Sidechannel Defences
====================

First of all, I lined a large cardboard box with aluminium foil in order to
make a rudimentary faraday cage.  Then, I assembled an airgap compute node
using some relatively cheap parts, putting them all inside the box:

* Motherboard: Asus H81 Pro BTC (no radio, bluetooth or speakers AFAIK)
* CPU: Intel G1840
* Ram: 2x cheap 1GB sticks
* PSU: EVGA SuperNOVA 1300 G2
* Monitor: old Dell TFT display
* Keyboard: generic USB keyboard

No other peripherals or cables were connected.  I placed the compute node in my
cellar (~6ft below ground level) and I remained with the node during the entire
time it was computing, without using any other devices in the vicinity (no
mobile phone etc.)  The only cables coming out of the box were the two power
cables, one for the PSU and one for the monitor.

Image: https://pbs.twimg.com/media/DOT55KUXUAEV44-.jpg:large

Procedure
=========

I booted the node, with "Try Ubuntu" (Live CD mode).  Then, I inserted the
challenge USB stick and ran `./compute` in the USB media directory, entering
some additional entropy as requested by typing randomly on the keyboard.  The
box lid was only partially opened to allow use of the keyboard and to view the
monitor at this point.  After 60 minutes had passed, I looked inside the lid
and saw that the computation had completed, so I wrote down the BLAKE2b hash,
and unmounted and removed the USB stick, and then powered the node down.

Postprocessing
==============

I took the USB stick and transferred the response file to my laptop, and then
uploaded it using the laptop to S3 via Sean Bowe's transcript site.

I did not destroy the compute node but I'm unlikely to use it or plug it in for
some time.
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