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@ -29,25 +29,25 @@ The initial phase of Stealth development was proposed and implemented in [BSIP-0
Phase One of Stealth development consisted of the implementation of Confidential Transactions (CT) [[1]](#see-also), which was initially proposed for the Bitcoin network in a paper by Greg Maxwell. CT provides two key features: **value-blinding**, and **stealth addresses**. Value-blinding means that the value amounts of transactions are unknowable except to the parties of the transaction (provided the transaction is sufficiently removed from the operation that initially converted a public balance to a blind balance). Stealth addresses are an address format that the sender can use to derive a "shared secret" public key for which only the recipient can derive a private key, without revealing publicly that the stealth address and the public key used to transmit the funds are related. The use of stealth addresses helps to anonymize transactions, however it creates a burden of communication between the sender and the recipient, who must be made aware of the existence of an inbound transaction.
Stealth addresses also privide partial **unlinkability**, or the inability to "link" balance elements (we'll call them UTXOs for Unspent Transaction Outputs) together to see that they are controlled by the same party. However, wallet implementations may, in order to facilitate discovery by the receiving party, publicly embed the stealth address of the receiving party into the transaction, thus breaking unlinkability and threatening anonymity. Heretofore, no privacy-preserving method has been proposed on this network to enable discovery, other than the sender and receiver being in direct communication off of the blockchain (e.g. sending **transaction receipts**). Furthermore, CT transactions are **traceable**, meaning that a transaction reveals which UTXOs are consumed in the creation of new UTXOs. This has two implications: (1) a transaction history can be constructed, and it may be possible to establish a vary narrow set of originating users who initally "blinded" a previously public balance, thus potentially establishing to a high degree of probability *who* is involved and *how much* value is involved in a particular sequence of transactions, and (2) a user spending a balance consisting of more than one UTXO will of necessity reveal that those UTXOs are controlled by the same party, or "linked," thus again breaking unlinkability.
Stealth addresses also provide partial **unlinkability**, or the inability to "link" balance elements (we'll call them UTXOs for Unspent Transaction Outputs) together to see that they are controlled by the same party. However, wallet implementations may, in order to facilitate discovery by the receiving party, publicly embed the stealth address of the receiving party into the transaction, thus breaking unlinkability and threatening anonymity. Heretofore, no privacy-preserving method has been proposed on this network to enable discovery, other than the sender and receiver being in direct communication off of the blockchain (e.g. sending **transaction receipts**). Furthermore, CT transactions are **traceable**, meaning that a transaction reveals which UTXOs are consumed in the creation of new UTXOs. This has two implications: (1) a transaction history can be constructed, and it may be possible to establish a vary narrow set of originating users who initally "blinded" a previously public balance, thus potentially establishing to a high degree of probability *who* is involved and *how much* value is involved in a particular sequence of transactions, and (2) a user spending a balance consisting of more than one UTXO will of necessity reveal that those UTXOs are controlled by the same party, or "linked," thus again breaking unlinkability.
In what follows we propose methods of enhancing the confidential abilities offered by the BitShares network by integrating specific solutions to achieve unlinkability, untraceability, and automated transaction discovery by the receiving party. Additionally, since BitShares is a multi-asset blockchain, we propse to adopt Confidential Assets (CA). CA is an extension to CT that hides not just the values of a transaction, but also the assets involved. Further, we propose various convenience and user-experience enhancements to ease the backup burden placed on users and reduce the chances of lost funds. Lastly, we propose an ongoing effort to harden wallets and p2p nodes against metadata leaks.
## Rationale
We view "Stealth" as a constellation of features providing robust privacy and "grandma-friendly" usability and reliability. A user of Stealth should be reasonably assured that their privacy is complete, and not underminable by easy-to-make procedural mistakes.
We view "Stealth" as a constellation of features providing robust privacy and "grandma-friendly" usability and reliability. A user of Stealth transactions should be reasonably assured that their privacy is complete, and not underminable by easy-to-make procedural mistakes.
### "Blind" vs. "Stealth"
### "Blinded" vs. "Stealth"
To facilitate an orderly, confusion-free upgrade, and to differentiate between Phase One and Phase Two functionality, we have adopted the convention of referring to the existing CT transactions that only blind transaction amounts as **Blind Transfers**, and the proposed more fully-private transaction features as **Stealth Transfers**. The names convey that Stealth Transfers will be more private than Blind Transfers (although some user education will be needed on this). Upon maturity of the Stealth feature set, the utilization of Blind transfers should be retired.
### Specific features and enhancements
Specific features and enhancments propose by this and associated BSIPs are as follows:
Specific features and enhancments proposed by this and associated BSIPs are as follows:
#### Asset Hiding
Confidential Assets (CA) extends Confidential Transactions (CT) to include asset hiding. It is described in a paper (link) by Greg Maxwell. Existing CT functionality can hide the value amount of a transaction, but the asset being transfered is publically visible in the transaction format. Since BitShares is a multi-asset blockchain, it would be of value to hide not just the value but also the identity of the asset involved. CA provides a method to do this. The proposal to implement CA on the BitShares network in [BSIP-1201](bsip-1201.md).
Confidential Assets (CA) extends Confidential Transactions (CT) to include asset hiding. It is described in a paper (link) by Greg Maxwell. Existing CT functionality can hide the value amount of a transaction, but the asset being transfered is publically visible in the transaction format. Since BitShares is a multi-asset blockchain, it would be of value to hide not just the value but also the identity of the asset involved. CA provides a method to do this. The proposal to implement CA on the BitShares network is in [BSIP-1201](bsip-1201.md).
#### Unlinkability and Untraceability
@ -59,13 +59,13 @@ We propose and discuss the implementation of ring signatures for Stealth transac
#### Scanning for Inbound Stealth Transactions
The current implementation of CT requires that a sender must comunicate to the recipient a "transaction receipt" that contains, in encrypted form, data that the recipient needs in order to take posession of a transaction output. This is burdensome, and implies a high risk of lost funds as a result of lost or uncommunicated receipts.
The current implementation of CT requires that a sender must communicate to the recipient a "transaction receipt" that contains, in encrypted form, data that the recipient needs in order to take possession of a transaction output. This is burdensome, and implies a high risk of lost funds as a result of lost or uncommunicated receipts.
We propose automated, privacy-preserving methods of scanning for inbound transactions in [BSIP-1203](bsip-1203.md).
#### Backups of Stealth Balances
In [BSIP-1204](bsip-1204.md) we propose standardized derivation of Stealth addresses to enable backup seeds or brain keys to be used to securely back up a Stealth account prior to first use, enabling recovery in the event of a lost or corrupted hot wallet.
In [BSIP-1204](bsip-1204.md) we propose standardized derivation of Stealth addresses to enable backup seeds or brainkeys to be used to securely back up a Stealth account prior to first use, enabling recovery in the event of a lost or corrupted hot wallet.
#### Metadata Hiding