More clarity edits in Rationale section.

bsip-1203.md

@@ -25,12 +25,12 @@ A confidential transaction (cTX) does not identify the recipient.  As such, ther
 
 _Keys involved in a cTX output (cTXO):_
 * **One-Time Key (OTK)** — The sender generates this key (public and private) from randomness and uses it to generate a shared-secret between the OTK and the recipient's Address ViewKey.  The OTK PubKey will be clear-text encoded in the Tx receipt, and optionally also recorded in the transaction output to enable automated discovery.
-* **Address Key(s) (ViewKey and SpendKey)** — These are public keys encoded in the recipient's stealth address.  The goal of a stealth address scheme is to _not_ identify these public keys in any transaction output.  A long-form address encodes _two_ public keys, referred to as ViewKey and SpendKey. The SpendKey serves as a base point from which individual Tx output AuthKeys are computed as an offset, and the ViewKey is used with the OTK to compute the offset.  A short-form address encodes only a single public key, which serves as both the ViewKey and SpendKey.
+* **Address Keys (ViewKey and SpendKey)** — These are public keys encoded in the recipient's stealth address.  The goal of a stealth address scheme is to _not_ identify these public keys in any transaction output.  A long-form address encodes _two_ public keys, referred to as ViewKey and SpendKey. The SpendKey serves as a base point from which individual Tx output AuthKeys are computed as an offset, and the ViewKey is used with the OTK to compute that offset.  A short-form address encodes only a single public key, which serves as both the ViewKey and SpendKey.
 * **Tx Output Authorization Key (AuthKey)** — This public key will be recorded in the confidential transaction output (cTXO) as the key which is authorized to spend the commitment.  This key is offset from the address SpendKey by a secret offset that only the sender and recipient can calculate (from the shared secret between OTK and ViewKey).  The sender can only know the offset, but not the full secret key to the AuthKey.  The recipient, knowing the private key behind the SpendKey, can compute the private key to AuthKey and therefore can spend the commitment.
 
 Automated discovery could be enabled if the receipt were embedded within the transaction data structure and if an aspect of that data structure supported a challenge condition which the recipient could recognize.
 
-Current network rules allow for a receipt to be embedded in each Tx output via a `stealth_memo` field which is formatted in a similar way as the encrypted memos that may accompany regular (non-Stealth) transfer operations. These memos are composed of a header specifying the OTK PubKey and the "message PubKey" for which the recipient holds the corresponding private key, followed by cipher text which is AES encrypted with a shared-secret key between the OTK and the message PubKey.  For this `stealth_memo` field, the current behavior of the CLI reference wallet is to use the recipient's Address PubKey as the message PubKey.  Although this is a reasonable choice for encrypting message text generally, it has the severe downside of identifying the recipient's Address PubKey in the memo header, and therefor breaks anonymity and negates the unlinkability provided by using a stealth address scheme.  For this reason, the CLI reference wallet currently does _NOT_ actually embed the memo in the Tx ouput but instead Base58 encodes it and prints it to the screen, calling it a "transaction receipt." The sender must manually, and secretly, transmit this to the recipient via a side channel.
+Current network rules allow for a receipt to be embedded in each Tx output via a `stealth_memo` field which is formatted in a similar way as the encrypted memos that may accompany regular (non-Stealth) transfer operations. These memos are composed of a header specifying the OTK PubKey and the "message PubKey" for which the recipient holds the corresponding private key, followed by cipher text which is AES encrypted with a shared-secret key between the OTK and the message PubKey.  For this `stealth_memo` field, the current behavior of the CLI reference wallet is to use the recipient's Address PubKey as the message PubKey.  Although this is a reasonable choice for encrypting message text generally, it has the severe downside of identifying the recipient's Address PubKey in the memo header, and therefore breaks anonymity and negates the unlinkability provided by using a stealth address scheme.  For this reason, the CLI reference wallet currently does _NOT_ actually embed the memo in the Tx ouput but instead Base58 encodes it and prints it to the screen, calling it a "transaction receipt." The sender must manually, and secretly, transmit this to the recipient via a side channel.
 
 **Stealth Memo structure: (Stealth I)**
 
@@ -42,7 +42,17 @@ Current network rules allow for a receipt to be embedded in each Tx output via a
 
 _Current Stealth I behavior is to use the Address PubKey as the message PubKey, which reveals intended recipient!!_
 
-A very simple solution would be to change the behavior of embedding the Address PubKey in the message PubKey field, and to instead record the Tx output AuthKey in this slot.  Because the recipient is also able to derive this AuthKey through knowledge of her own address private keys (in combination with the OTK recorded in the header), the recipient would simply need to test the OTK against each of their own Address Keys to see if the resulting AuthKey matches the one in the header.  If it does, then the output is recognized as destined to the recipient, even though the recipient's Address PubKeys are not identified in the memo header.  The computational cost of this is one Diffie Hellman round, a hash operation, and a child key derivation.  It should be noted that the AES encryption key should still be computed from the shared secret between the OTK and the address ViewKey, however, as this will still allow view-only wallets which cannot compute the private key behind the AuthKey to decrypt the memo and tally the incoming transaction.
+A very simple solution would be to change the behavior of embedding the Address PubKey in the message PubKey field, and to instead record the Tx output AuthKey in this slot.  Because the recipient is also able to derive this AuthKey through knowledge of her own address private keys (in combination with the OTK recorded in the header), the recipient would simply need to test the OTK against each of their own Address Keys to see if the resulting AuthKey matches the one in the header.  If it does, then the output is recognized as destined to the recipient, even though the recipient's Address PubKeys are not identified in the memo header.  The computational cost of this is one Diffie Hellman round, a hash operation, and a child key derivation.  It should be noted that the AES encryption key should still be computed from the shared secret between the OTK and the address ViewKey, however, as this will allow view-only wallets which cannot compute the private key behind the AuthKey to decrypt the memo and tally the incoming transaction.
+
+**Stealth Memo structure: (Proposed: Stealth II)**
+
+<span></span> | <span></span>
+-----: | :---
+**One-time PubKey:** | Chosen from randomness by sender &nbsp; **_(33 bytes)_**
+**cTXO AuthKey:** |  Child public key of the stealth address and the OTK. &nbsp; **_(33 bytes)_**<br>
+**Cipher Text:** | AES encrypted message, using _key &leftarrow; Shared(OTK,ViewKey)_
+
+_Proposed Stealth II behavior is to embed the AuthKey in the second slot, while still encrypting the message data with a shared key between the OTK and the Address key (specifically, the ViewKey so that watch-only wallets can read the commitment data)._
 
 To support this strategy, a wallet will need to inspect all cTX activity on the network and test the challenge conditions on each transaction.  This could be achieved if API nodes are extended to provide an API call to retrieve `stealth_memo` fields from all cTXOs appearing in a specified block range.  The wallet could simply download the memos, test the challenge on each one, and identify and decrypt the ones that are destined to the wallet.  No need would remain to manually transmit transaction receipts.  The receipts would be embedded, compactly and unlinkably, in the Tx outputs.