implement Loopix

[david415]

i have not read the Loopix paper yet but heard a description of it from one of the paper's authors. it sounds like we could implement loopix using txmix. one problem that needs to be fixed for this to happen is: our sphinx implementation needs to have a value in it's header Beta component that marks the message on the final hop as either a dummy traffic message or not. this is needed so that final hop can discard the dummy messages.

https://arxiv.org/abs/1703.00536
https://arxiv.org/pdf/1703.00536.pdf

[david415]

the sphinxmixcrypto ticket is applied-mixnetworks/sphinxmixcrypto#17