http://7fa6xlti5joarlmkuhjaifa47ukgcwz6tfndgax45ocyn4rixm632jid.onion/anonbib/cache/chaum-mix.html
The following indicates how y uses this untraceable return
address to form a response to x, via a new kind of mix:
K1( R1, Ax ), Kx( R0, M ) --> Ax, R1( Kx( R0, M )).
This mix uses the string of bits R1 that it finds after decrypting the
address part K1( R1, Ax ) as a key to re-encrypt the message part
Kx( R0, M ).