Weak-Signal RadioCommunications forBitcoin Network Resilience

Nick Szabo, Elaine Ou
globalfinancialaccess.com
Scaling Bitcoin 2017

What is Weak-Signal HF Radio?
• Radio transmission using shortwave frequencies (1.6-30MHz)• Radio waves in this band can refract off the ionosphere• Popular for international broadcasting of government
propaganda

Cold War Shortwave RadioBroadcasts• Office of War Information broadcast anti-communist
propaganda during “Campaign of Truth”

Secure Consensus and Broadcast• Most proofs of secure consensus (in general) andof Bitcoin-like formal protocols (in particular)
assume trust-minimized fair broadcast
o Every full node broadcasting directly to every other fullnode – no intermediaries• Real-world consensus
implementations fallshort of provablesecurityEclipse and Routing Attacks• Eclipse attacks(Heilman et al. 2015)
o Attacker directly
connected to thevictim

Internet topology & routing attacks (Apostolaki et al. 2017)• “For 67.9% of nodes, there is at least one AS other than their provider thatintercepts more than 50% of their connections.”• “Delay attackers intercepting 50% of a node’s connection[s] can waste 63%of its mining power.”• “Even a small amount of multi-homing is enough to protect Bitcoin [as awhole] from powerful attackers”

Broadcast andTrust• Pulsar• Inaccessible natural
phenomenon => trustlessbroadcaster• Attack structure isnobody• Beam covers everybody onearth• Access structure iseverybody on earth
with a big radio dish• Blockstream Satellite?

• Clock towero Bell in tower rings at the topof each hour, can hear formiles aroundo Systemically trusted• Attack structure is thebell-ringero Isotropic broadcast• Access structure is everybody
within hearing range of the bello Abstract vs. particularnature of information isimportantBeam Width and Gain
• Narrow beacon provides increases range and bitrate at the expense oftrust-minimized fairness and need for prior knowledge
o Broadcaster can choose direction of beacon but not who is whereo Allows broadcaster to more choices over the access structure• A wider beacon – ideally isotropic – gives trust-minimized fairness atthe expense of range and bitrateByzantine Narrow-Beamer

Yes???

oNYesNoInternet Routing Attacks
Some recommendations from Apostolaki et. al.
2017:• Increase the diversity of node connections
o Eg. Ensure that all Bitcoin nodes are multi-homed.

• Select Bitcoin peers in a route-aware way, adding
extra random connections if the same AS appears
in all paths.

HF Radio vs. Routing Attacks
Before HF: 2 AS bottlenecks to choose from
AS1

AS2

After HF: 4 AS to choose from
AS3

AS1

AS2
AS4

Goals of Weak-Signal HF RadioCommunication• Longer range broadcast
o Avoid injection/modification problems of mesh relay• Allow censorship-resistant participation in thenetworko Chinese firewalls cannot stop radioo Soviet Union needed over 1000 broadcasting stations
to jam American radio• Internet-free participation for SPV nodes• More diverse multi-homing
o Ability to choose a more diverse AS set

Skywave:Using the Ionosphere
F layer (150-800 km) – acts like a mirror

D layer (typical 50-100 km) – fog in front of the mirror

Near-Vertical and Medium-RangeRadio• Near-vertical incidence skywave (NVIS)o 50-650 km (30-400 miles)
o Most reliable frequencies are between wavelengths of40 and 80 meterso Antenna near-horizontal• 1/20th to ¼ wavelength off the ground• Medium-range
o 500-2500 km (300-1500 miles)
o Less reliable than NVIS

Daytime D-layer Attenuation at NVISand Medium Ranges
F layer (average 400 km)

70o

44o

2*(-10) dB

30o2*(-14) dB

2*(-20) dB

D layer (typical 50-100 km)

320 km

900 km

1,400 km

VOA Propagation Map
South Korea

80M, 3.7 MHz,
4W

40M, 7.1 MHz,
4W

VOA Propagation Map
40M, 7MHz 5W TX

Digital Mode Over Radio• Any radio can be a modemo Modulator/Demodulator• Airchat radio mesh network by Anonymous• PSKmailModulation• Ideal conditions: BPSK500
o 500 bps, 1000 Hz bandwidth• Noisy conditions: BPSK500R
o Convolutional encoding• Rate R=1/2, Constraint length K=7o Interleaved datao 250 bps, 1000Hz bandwidth• Awful conditions: MFSKo 62.5 bps, 1260 Hz bandwidthImplementation (TX)
• Binary PSK
o Low bandwidth, decent bit-error rate

Implementation (TX)

Implementation (RX)Constellation Diagram – Output of Phase Locked Loop

Output of Binary Slicer

Bit-Error Rate vs Eb/N0

Messaging Protocol: Frames
Frame

Frame

Header

Payload

CRC<SOH> +3 bytes

0-512 bytes

4 bytes• Headero <SOH> 0x01
o Version 0x30
o Stream ID 0x30
o Block type
• Connection Request,
acknowledge, data, etc

Frame• Payloado Callsign: source socket
o Destination port
(8333)o Stream ID
o Max Payload size
• 2nConnection request

Data Transmission• Header• Counter (block num)o <SOH> 0x01
o Version 0x30
o Stream ID
o Block numbero 6 bits: 0-63
o Counter wraps aroundto 0
o Sender will not allow
counter to get morethan 62 ahead of last
acknowledged frame• DataData Frames

Data Acknowledge,Retransmission Request• Ack / Retransmit Payloado Last block number transmittedo Last block number correctly received, with no gaps
o Last block number received• Frame size adjusted dynamically based on
number of retransmit requests

PollAck / Retransmission Request FrameEffective Data Rate vs SNR

Expected Latency

Latency in seconds

VOA Propagation Map
South Korea

3.7 MHz, 4W

7.1 MHz, 4W

Border Simulation• 5 stations randomly placed oneach side of the border; 900 km
range
AS
3,000 km

AS750 kmo 40m or 75m for medium range
skywave & near-vertical incidence
skywave (NVIS)o 10m dipole antenna• Stations in left country last only
long enough to transmit & confirm
a transaction• Stations in right country relatively
permanent

Design of Portable Temporary V-Dipole
Antenna for NVIS & Medium Range, 40-80m
rem

Balancingtransformer

ireaw
nnte
an

6m mast

2m mastanborder1010mwianten

coax feed
Reflector wires
2m mastFuture Work• Improve noise rejection with MFSKo Better performance in low power longdistance linkso SNR target: -10 dB
o Dynamic modulation based on conditions• Custom Messaging Protocolo Reduce overhead• Electrical shortening for antennas• Run long-distance tests with antennarigs
o Volunteers needed!