A good one to start with would be this paper: https://www.freehaven.net/anonbib/cache/chaum-mix.pdf https://www.freehaven.net/anonbib/cache/chaum-mix.pdf It's

A good one to start with would be this paper: https://www.freehaven.net/anonbib/cache/chaum-mix.pdf https://www.freehaven.net/anonbib/cache/chaum-mix.pdf It's one of the earliest and very well cited.