How to Perform Private Transactions

Private transactions are a change of ownership of tokens where no one but the involved parties can learn who sent how many tokens to whom.

Substrate blockchains are usually account-based and pseudonymous by nature: The whole world can see all transactions and their source, destination and amount. substraTEE offers confidentiality in a similar way that Zcash does: it offers users a way to move their tokens into a dark pool (shielding process) where they can transact privately and later retrieve tokens on their public accounts (unshielding). In the case of Zcash, privacy is guaranteed by zk-SNARKS, in substraTEE it is guaranteed by TEEs.

The detailed design of the shielding and unshielding process is described in the chapter Token Shielding.

In the following demo we show how Alice can send tokens to Bob privately. The demo will run in our docker container so you don't need to setup a complete SGX development machine (although the Intel SGX driver and SDK needs to be installed making /dev/isgx and the aesm service available).

Setup

You'll need SGX enabled hardware and to register with Intel and obtain your own SPID and KEY.

Build worker, client and node in our docker:

# get the docker image
# check for updates on https://hub.docker.com/repository/docker/scssubstratee/substratee_dev
docker pull scssubstratee/substratee_dev:18.04-2.9.1-1.1.2

# create a dedicated demo directory and start the docker container
mkdir demo && cd demo
docker run -it -v $(pwd):/root/work -v /var/run/aesm:/var/run/aesm --device /dev/isgx scssubstratee/substratee_dev:18.04-2.9.1-1.1.2 /bin/bash

# now you are inside the container
# clone and build the worker and the client
# info: change the tag to the latest
cd work
git clone https://github.com/scs/substraTEE-worker.git
cd substraTEE-worker
git checkout v0.6.12-sub2.0.0
make
# this might take 10min+ on a fast machine

# use your SPID and KEY from Intel
echo "<YOUR SPID>" > bin/spid.txt
echo "<YOUR KEY>" > bin/key.txt

# clone and build the node
# info: change the tag to the latest
cd ..
git clone https://github.com/scs/substraTEE-node.git
cd substraTEE-node
git checkout v0.6.12-sub2.0.0
cargo build --release
# another 10min

For a nicer overview of the demo, let's install tmux and split our docker console into multiple terminals

apt update
apt install -y tmux
tmux
tmux split-window -v
tmux split-window -h

You should now see three terminals

Launch node in terminal 1

cd ~/work/substraTEE-node/
./target/release/substratee-node --dev -lruntime=debug

blocks should be produced...

Launch worker in terminal 2

use Ctrl-B + cursors to move between terminals

cd ~/work/substraTEE-worker/bin
./substratee-worker init-shard
./substratee-worker shielding-key
./substratee-worker signing-key
./substratee-worker run

Play in terminal 3

cd ~/work/substraTEE-worker/client
./demo_shielding_unshielding.sh

Now you can watch the process of

Alice creating a new incognito account. This account is never disclosed to the public.
Alice shielding funds onto her incognito account
Alice privately sending funds to Bobs incognito account
Alice unshielding some funds back onto her public account

Cleanup

The files created in the docker container belong to root. This makes it hard to delete them on your normal system. We now give them back to your standard user.

Note: This step is optional.

cd /root/work
ls -la

# write down the numbers on the line containing '.'
# example output: drwxrwxr-x 17 1002 1002   4096 Nov  2 15:10 .
#  where the numbers are 1002 (NUMBER1) and 1002 (NUMBER2)

# give all files back to the external user
chown -R <NUMBER1>:<NUMBER2> substraTEE-worker substraTEE-node

The substraTEE Book