Katzenpost Mix Network

Post Quantum Anonymous Communication Network

Katzenpost is a software project dedicated to designing and implementing mix network protocols. A mix network is a type of anonymous communication network. An anonymous communication network is also known as a traffic analysis resistant network; that is, it's protocols are designed to resist statistical analysis by passive global adversaries.

Traffic analysis typically refers to the statistical analysis of encrypted traffic. Traffic analysis is worth defending against given that common network protocols leak lots of information such as source and destination IP addresses, message size, message sequence, message delay pattern, geographical locations, social graph etc. Mere end to end encryption alone cannot protect against this type of information leakage.

At the most basic level, mixnets are composed of mix nodes. These are a cryptographic packet switching routers which protect our privacy by mixing many messages together and cryptographically transforming them before routing them on to the next hop. Mix nodes also use shuffling or added latency to create uncertainty for network observers. This uncertainty is in regards to trying to link incoming messages with the outgoing messages.

Project Status

Many excited new changes are forthcoming! You can watch our progress being tracked, here:

https://github.com/orgs/katzenpost/projects/6/views/5

There are essentially two types of interaction with a Katzenpost mixnet:

1. clients talk to mixnet services and their traffic stays in the mixnet
2. clients talk to Internet services; proxying through the mixnet onto the Internet.

Building Katzenpost

To build all server related components, type "make" when inside this repo:

cd katzenpost
make

Client:

Currently, Katzen is the only client available for use with Katzenpost. However a SOCKS proxy client is forthcoming and you'll be able to use that with many existing applications.

Server Side Usage/Configuration

Our docker configuration is the most comprehensive and up to date place to learn about how to configure a Katzenpost mix network. Run the makefile in the docker directory to get a usage menu:

$ cd katzenpost/docker; make 
These make targets allow you to control the test network:
 run                - run the testnet in the foreground, until ctrl-C
 start              - start the testnet in the background
 stop               - stop the testnet
 wait               - wait for testnet to have consensus
 watch              - tail -F all logs
 status             - show testnet consensus status
 show-latest-vote   - does what it says
 run-ping           - send a ping over the testnet
 clean-local        - stop, and delete data and binaries
 clean-local-dryrun - show what clean-local would delete
 clean              - the above, plus cleans includes go_deps images

You can run a docker mixnet locally and then inspect the configuration files to learn how to configure a Katzenpost mixnet.

Documentation is a work in progress:

• mix server docs

• dirauth server docs

Expert's Corner

Katzenpost is an unverified decryption mix network that uses a continuous time mixing strategy with client selected exponential delays and a stratified (layered) topology.

Our documentation is in progress, but we have some resources for experts:

• Our threat model document, work-in-progress, can be found here.

• Our design specification documents are available here.

Wire protocol based on Noise

Every component in a Katzenpost mix network uses our "wire" protocol, the protocol that sits on top of either TCP or QUIC, is a cryptographic protocol based on Noise:

The Noise Protocol Framework

We believe in the Noise cryptographic protocol framework, that it is good to use it instead of TLS, whenever possible. Noise places all of the protocol decision making during the design phase of the protocol instead of during protocol runtime. This means there are no protocol downgrade attacks, no dynamic selection of ciphersuite and so on.

However, we use a variation of Noise called Post Quantum Noise, from the paper:

@misc{cryptoeprint:2022/539,
      author = {Yawning Angel and Benjamin Dowling and Andreas Hülsing and Peter Schwabe and Florian Weber},
      title = {Post Quantum Noise},
      howpublished = {Cryptology ePrint Archive, Paper 2022/539},
      year = {2022},
      doi = {10.1145/3548606.3560577},
      note = {\url{https://eprint.iacr.org/2022/539}},
      url = {https://eprint.iacr.org/2022/539}
}

The precise Noise protocol descriptor string we use is:

Noise_pqXX_Kyber768X25519_ChaChaPoly_BLAKE2s

However the hybrid KEM Kyber768X25519 is constructed using a security preserving KEM combiner and a NIKE to KEM adapter with semantic security so that the resulting hybrid KEM is IND-CCA2 in QROM.

Here's a diagram of the pqXX pattern which we use:

sequenceDiagram
    Client-)Server: e
    Server-)Client: ekem, s
    Client-)Server: skem, s
    Server-)Client: skem

Sphinx

We use the Sphinx cryptographic packet format and allow it's geometry to be completely configurable to accomodate various networking requirements. Additionally the Sphinx can use any NIKE (non-interactive key exchange). We also developed a novel post quantum variation called KEM Sphinx. KEM (key encapsulation mechanism) Sphinx is twice as fast on the server side as the original NIKE Sphinx because it only requires one public key operation per hop instead of two. However it has the packet header overhead size penalty that grows linearly with the number of hops.

And here are some Sphinx benchmarks using different KEMs and NIKEs:

Primitive	Sphinx type	nanoseconds/op	seconds/op
X25519	KEM	80093	8.009×10−5
X25519	NIKE	160233	0.000160233
Kyber512	KEM	43758	4.3758e-5
Kyber768	KEM	57049	5.7049e-5
Kyber1024	KEM	72173	7.2173e-5
Kyber768 X25519 Hybrid	KEM	87816	8.7816e-5
CTIDH512	NIKE	336995975	0.336995975
CTIDH1024	NIKE	18599579037	18.599579037
CTIDH2048	NIKE	17056742100	17.0567421
CTIDH1024	KEM	11408217346	11.408217346

We can draw several conclusions from this table of benchmarks:

1. KEM Sphinx is about twice as fast as NIKE Sphinx
2. Kyber768 is faster than X25519
3. Kyber768 X25519 Hybrid KEM Sphinx is almost as fast as X25519 NIKE Sphinx but probably a lot more secure given that it's a post quantum hybrid construction which still uses the classically secure X25519 NIKE.
4. CTIDH is very slow and we probably don't want to use it for Sphinx. We instead think it very useful for application level encryption.

Please also note that hybrid KEMs referred to above are constructed using a security preserving KEM combiner and a NIKE to KEM adapter with semantic security so that the resulting hybrid KEM is IND-CCA2 in QROM.

PKI/Directory Authority

Mix network key management and distribution is handled by the directory authority system, a decentralized voting protocol that can tolerate (1/2 * n)-1 node outages. Clients and mix nodes can talk to the dirauth (directory authority) system to get a published PKI document which is essentially a view of the network which contains public cryptographic keys and network connection information.

The mix descriptors are signed by the mix nodes. Each dirauth also signs their interactions in the voting protocol and the final published PKI document.

Mix nodes and dirauth (directory authority) nodes use our hybrid post quantum signature scheme which is currently:

• ed25519 + Sphincs+

Note that we could have just used Sphincs+ since hash based signature schemes have a solid theoretical foundation, however there can still be mistakes in the hyper tree design and so pairing it with ed25519 is prudent defense in dept.

Clients also use this signature scheme to verify PKI documents.

We are using the SHAKE-256f parameterization of Sphincs+ via the reference implementation.

License

AGPLv3

Donations

Your donations are welcomed and can be made through Open Collective here.

Supported By

This project has received funding from:

• European Union’s Horizon 2020 research and innovation programme under the Grant Agreement No 653497, Privacy and Accountability in Networks via Optimized Randomized Mix-nets (Panoramix).
• The Samsung Next Stack Zero grant.
• NGI0 PET Fund, a fund established by NLnet with financial support from the European Commission's Next Generation Internet programme, under the aegis of DG Communications Networks, Content and Technology under grant agreement No 825310.
• NGI Assure Fund, a fund established by NLnet with financial support from the European Commission's Next Generation Internet programme, under the aegis of DG Communications Networks, Content and Technology under grant agreement No 957073.