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What is Sigmajoin?#

Sigmajoin is a privacy protocol designed for use with UTXO-based blockchains, aiming to enhance user anonymity. It builds on the principles of a previous protocol called Zerojoin but introduces features to make it more practical and efficient.

Key Advantages#

ZeroJoin (ErgoMixer) Sigmajoin
Proofs Used Proofs of knowledge of Diffie-Hellman tuples Proofs of knowledge of Diffie-Hellman tuples
Interaction Level Partially Non-interactive: Requires online presence for remixing Non-interactive
Outsourceability No, mixing cannot be outsourced Yes, mixes can be outsourced to third-parties
Types of Boxes 2 (Half-Mix and Full-Mix): Limited to two boxes, constraining scalability and flexibility More than 2: Offers better scalability and flexibility. Half-mix boxes eliminated as bloat
Stealth Withdraw Supported Supported
Fee Handling Embedded Fee: Includes fees within mix boxes, requiring meticulous calculation Outsourced Fee: Fees paid by third-party "mixer," simplifying the process and adding flexibility

Special Features#

1. Outsourceability#

The mixing process can be outsourced to a third-party service called a mixer. These mixers can perform the mixing operations in a trustless manner, meaning they cannot steal funds.

2. Stealth Withdraw#

Sigmajoin allows for transactions that appear to be mixes but are actually withdrawals. This makes it even more difficult for an observer to trace transactions, enhancing privacy.

3. Outsourced Fee#

In many privacy protocols, how to pay the mining fee is a challenge. Sigmajoin allows the fee to be paid by the third-party mixers, further simplifying the process for users.

How Does Sigmajoin Work?#


  • Mix-box: A special kind of coin (or UTXO) used in Sigmajoin. Each mix-box has two registers a and b which are elements of a mathematical group G.
  • Alice: The generic term for a participant in the protocol.

Core Operations#

  1. Deposit: Users can deposit their coins into a special 'pool' as mix-boxes. These boxes can be in fixed denominations.
  2. Mix: A third-party service or another user can take any two mix-boxes from the pool and mix them. After mixing, two new mix-boxes are added back to the pool. This process conceals the original ownership of the boxes.
  3. Withdraw: Users can withdraw their coins from the pool at any time.

Steps for Mixing Boxes#

  1. Select Two Boxes: Choose any two mix-boxes from the pool.
  2. Re-randomise Public Key: Perform a mathematical operation on the registers a and b of each selected mix-box.
  3. Validation: Prove that the mathematical operations were done correctly. The new mix-boxes should look identical to an observer.

Note: This is a simplified document. For technical details and mathematical proofs, consult the whitepaper.