The NIX Ghost Vault became available for use at block #53,000, originally using the Zerocoin Protocol. As of block #232,000, the Zerocoin Protocol was actively replaced with Sigma which significantly reduces on-chain proof sizes from 25kb to 1.5kb, reduces Commitment Key Pack lengths by 73%, and significantly increases anonymity sets by introducing 256-bit elliptic curves which would be roughly equivalent to 3072-bit RSA as opposed to 2048-bit RSA previously used in the Zerocoin Protocol. Massive credit goes to the ZCoin development team for creating the first Sigma cryptographic codebase implementation and their developments on these grounds.
The NIX Ghost Vault provides privacy for the sender and/or the receiver by essentially burning coins and allowing users to re-issue them later with no attached transaction history. The Ghost Vault can be utilized in 4 ways:
Holder Privacy (1-Way Ghosting) By ghosting your public NIX into your Ghost Vault, an outside observer has no proof that your wallet still possess those coins.
Sender Privacy (1-Way Ghosting) By sending your previously ghosted NIX to a receivers public address, the recipient receives newly minted public coins with no attached history.
Receiver Privacy (1-Way Ghosting) For receiver-only privacy, you can ghost your public NIX directly into the recipients Ghost Vault.
Sender and Receiver Privacy (2-Way Ghosting) Sending your previously ghosted NIX from your Ghost Vault to the recipients Ghost Vault results in a full public address-less transaction.