Ethereum co-founder Vitalik Buterin published the third installment of his cryptographic obfuscation series on Friday, examining a method known as "local mixing." The release follows Part I on June 29 and Part II, which covered the diamond iO writeup, on July 28.
While previous publications focused on lattice-heavy constructions, local mixing introduces a completely different approach to cryptography. Buterin noted that the method relies on ideas drawn from hash function design, featuring no elliptic curves, no prime factorization, and no lattices. Instead, its closest relative is symmetric cryptography.
The local mixing process utilizes a circuit built from logic gates such as XOR, AND, and NOT. The pipeline maintains identical outputs while scrubbing traces of internal logic through steps including reversibility, hardening, gadgetization, mixing, and obfuscation. During the mixing phase, junk gates are scattered through the circuit, the arrangement is shuffled, and small blocks are swapped out for different gates that compute the same function.
Reversibility serves as a foundational step because it allows a reversible gate to be rewritten as an arbitrary pile of other reversible gates with matching behavior, a task more difficult to achieve with standard AND or OR gates.
Despite exploring the concept, Buterin described local mixing as a "wild and risky bet" that rests upon a "graveyard of failed attempts at white-box cryptography." He added that proponents of the method believe increased effort and a willingness to accept higher overhead could make the approach viable.
Referred to by Buterin as the "final frontier of cryptography," obfuscation allows a program to be converted into an encrypted version that runs on ordinary inputs and returns standard outputs while keeping its code hidden. When combined with a blockchain, obfuscation can help facilitate use cases such as private, collusion-resistant voting without the need to trust an M-of-N committee.
