Tor Denial of Service Defenses for Onion Services

Author: [Valentin Franck] [email protected]

This is just a prototype and should not be used in production, mostly because it was not written with security in mind, but only to proof and evaluate the concept. Currently, the challenges, that clients resolve to obtain tokens, are not yet integrated. In theory, a client would have to solve a hard challenge, before it obtains tokens.

Prerequisites:

The same as in vanilla Tor. Cryptography is written for OpenSSL 1.1.1.

Build

To build with the defenses enabled checkout the branch dos-defenses-master and proceed as usual:

git checkout dos-defenses-master
sh autogen.sh && ./configure && make && make install

Enable the Defenses for an Onion Service

To enable the defenses for an onion service add the following lines to your torrc:

# set the HSDir as in vanilla Tor
HiddenServiceDir /path/to/hs-dir

# ...configure the onion service as in vanilla Tor.

# only v3 onion services are supported
HiddenServiceVersion 3

# enables the defenses
HiddenServiceEnableHsDoSDefense 1

# number of tokens that may be requested per challenge.
# note, there are no challenges implemented!
HiddenServiceEnableHsDoSDefenseTokenNum 100

# set the maximum rate for INTRODUCE2 cells without tokens.
# exceeding cells will be discarded, unless they contain a token.
HiddenServiceEnableHsDoSRatePerSec 10000

# set the maximum burst for INTRODUCE2 cells without tokens.
# exceeding cells will be discarded, unless they contain a token.
HiddenServiceEnableHsDoSBurstPerSec 10000

Enable the Defenses for a Client

To enable the defenses for a client add the following lines to your torrc:

# enable the client to use the defenses, for all services that support it
HsDoSRetrieveTokens 1

# use this directory to store retrieved tokens for future redemptions.
# if not set, tokens are only kept in memory by the tor process.
HsDoSClientDir /path/to/client-dir 

How do the Defenses work?

The DoS defenses are token-based. Service operators define rate limits for introductions. Once these rate limits are exceeded clients may only connect to the onion service, if they include a valid token in their introduction cells. Tokens are only issued by the onion service after the client solved a challenge.

Tokens are based on verifiable oblivious pseudo random functions (V-OPRFs). The cryptographic scheme is borrowed from Privacy Pass: https://blog.cloudflare.com/privacy-pass-the-math/

Evaluation

There is a branch dos-defenses-master-evaluation which by default prints some benchmarks to the CLI, when the DoS defenses are used by either client or onion service. Furthermore, to evaluate the cryptographic operations, the test test_hs_dos_token_benchmark can be run.

Tests

There are some unit tests for the cryptographic operations and for encoding and decoding of service descriptors. The rest of the code is mostly untested.

Further Information

This prototype has been implemented as part of a master thesis, which can be found under the branches dos-defenses-master and dos-defenses-master-evaluation in the file: thesis.pdf