Status Documentation

Whisper - Comparison with the Postal Service over Swarm (PSS)

Overview

Whisper and PSS are two messaging procotols currently being developed in the Ethereum stack - here’s a side-by-side comparison that can help guide your choice between them.

Similarities

Online-first protocols - no built-in offline capability / expectation
Symmetric and asymmetric encryption available
Implemented on top of DevP2P
- DevP2P offers encryption between directly connected peers in the underlying overlay network, but this encryption is broken
- A second layer of encryption is done in an end-to-end fashion at the Whisper/PSS level
4-byte topic acts as hint for decryption
- clients have to agree on topics out-of-band
Recipient anonymity: ability to decrypt makes you a recipient
- Bloom filters and addressing limit anonymity in Whisper and PSS respectively
Sender anonymity: peers generally can’t tell if neighbour peer is message originator
- Being fully surrounded by cooperating adverserial nodes breaks this (similar to an eclipse attack)
- Light clients that don’t repeat traffic will leave more obvious metadata trail
- Global Passive Adversary can possibly detect origin of envelope
When used without bloom filters or without addressing, routing is equivalent for Whisper and PSS respectively

Differences

Feature	Whisper	PSS
Routing	Gossip	Kademlia
DoS protection	Proof of work	Swap incentives
Bandwidth vs darkness	Topic-based bloom filters - intermediary / recipient decides	Partial addressing - sender decides
Encryption in API	Mandatory	Optional
Key exchange	N/A	Optional Diffie-Hellman
Message lifetime	Time-to-live (sender decides)	One pass through each intended node

Whisper

Whisper is a gossip-based combination of a messaging protocol and ephemeral value store - values circulate in the system until expiry. The value store view is perhaps more apt - you throw messages into the cloud of participating nodes and they keep getting replicated until expiry, even when new nodes join.

Gossip

Connected peers form a “random” subset of all nodes in network
Each message is sent to all connected peers
Each time a message is sent to a peer, it’s also added to a seen list for that peer
Multiple peers will send you the same message - only propagated to those that don’t have it on the seen list already
Scalability issues:
- each node sees each message multiple times
- too many restricted nodes will cause low propagation probability
- see https://hackmd.io/B-W4T1SzSgCAbYS2CfoIPg for further analysis

Proof of work

PoW calculated over expiry, ttl, topic, data, nonce meaning that you need both a timestamp and contents of message to create the PoW
pow = (2^BestBit) / (size * TTL) meaning that large messages or those with long TTL are penalized
Tricky for heterogenous devices

Bloom filtering

Topics are compressed to a bloom filter that is sent to directly connected peers
Peers only forward messages to you that match filter
- Violators are disconnected
Recipient decides how “dark” it wants to be by revealing a more or less accurate bloom filter
Aggressive bloom filtering can cause network partitioning, makes nodes less valuable in network
Nodes repeat messages to peers in order to achive sender metadata safety
- Works as long as you’re not surrounded by coordinated malicious peers
- Bloom filters limit efficiency

Key exchange

Must be done out-of-band

Time to live

Sender decides on a time-to-live for messages, and pays PoW for it
Messages are kept/cached in every node until their TTL expires
If a new node connects, it will receive all messages whose TTL has not yet expired
Together with topics and capability to decrypt, this can be seen as an ephemeral key-value store of sorts

PSS

PSS wraps whisper and adds the swarm routing protocol to add deteministic routing. With PSS, routing is done along the kademlia path up to the partial address that was supplied - then gossip takes over.
Unlike Whisper, there is no DHT-like functionality where new nodes are updated on the “current” database of messages.

https://swarm-guide.readthedocs.io/en/latest/pss.html
https://swarm-guide.readthedocs.io/en/latest/resources.html

Kademlia

Overlay networked addressed by ethereum public key hash
Kademlia defines a measure of distance between arbirary nodes
You connect to a subset of all nodes based on these distances so as to reach a good balance between connectivity and distance
Each hop gets you closer to final destination, similar to a binary search

Swap incentives

TODO - not implemented

Partial addressing

Place in kademlia overlay determined by ethereum key
by specifying part of key, we can narrow down the number of hosts that “could” be recipients of message and route towards them - the neighbourhood
Gossip the remaining bit, among neighbourhood peers
Path through network depends on source address and the partial address supplied
- Kademlia assumed to consist of mostly stable nodes with long-term connections - thus one can assume a deterministic path up to the partial addressing point

Key exchange

Rudimentary ephemeral diffie hellman exchange part of the API

Other resources

Code

Random links

Appendix: Examples

Whisper message propagation

graph nodes {
00 – 01; 00 – 10 ; 10 – 11; 01 – 11;
01 – 10;
{rank=same; 00 01} {rank=same; 10 11}
}

digraph step1 {
00 -> 01; 00 -> 10;
{rank=same; 00 01} {rank=same; 10 11}
}

digraph step2 {
00; 01 -> 10; 10 -> 11; 01 -> 11; 10 -> 01;
{rank=same; 00 01} {rank=same; 10 11}
}

Notice how 10, 01 potentially receive message twice (depending on timing) and 11 is guaranteed to recieve twice (in this setup, due to locally limited knowledge in 01, 10 nodes)

PSS message propagation

Example propagation to address 1*:

graph nodes {
00 – 01; 00 – 10 ; 10 – 11; 01 – 11;
01 – 10;
{rank=same; 00 01} {rank=same; 10 11}
}

digraph step1 {
00 -> 10;
{rank=same; 00 01} {rank=same; 10 11}
}

digraph step2 {
00; 10 -> 11;
{rank=same; 00 01} {rank=same; 10 11}
}

Either of 10, 11 could be recipient, message reaches both!