silentpayments: address #1765 follow-ups, add changelog entry #1896

 111 | + *                            within the array, i.e., after the call (including on
 112 | + *                            failure), the index fields of the recipient objects
 113 | + *                            may no longer correspond to the positions in the
 114 | + *                            array. Multiple recipient objects with the same scan
 115 | + *                            public key and/or same spend public key can be passed
 116 | + *                            if they carry a different index.

---

 650 | @@ -648,6 +651,8 @@ int secp256k1_silentpayments_recipient_scan_outputs(
 651 |      }
 652 |      secp256k1_ge_from_bytes(&prevouts_pubkey_sum_ge, &prevouts_summary->data[5]);
 653 |      combined = (int)prevouts_summary->data[4];
 654 | +    /* Note that the "combined" flag can currently only be 0, as we only have support for full-nodes, i.e.

---

  86 | - *             - A hash output is not a valid scalar (negligible probability
  87 | - *               per hash evaluation)
  88 | - *             - Any group (i.e. recipients sharing the same scan public key) exceeds
  89 | - *               the protocol limit SECP256K1_SILENTPAYMENTS_RECIPIENT_GROUP_LIMIT
  90 | + *             - An invalid output public key is created (can only happen for an
  91 | + *               adversarially chosen recipient spend public key)

---

   6 | @@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   7 |  
   8 |  ## [Unreleased]
   9 |  
  10 | +#### Added
  11 | + - New module `silentpayments` implements sending and receiving of Silent Payments according to the [BIP 352 specification](https://github.com/bitcoin/bips/blob/master/bip-0352.mediawiki). See:
  12 | +   - Header file `include/secp256k1_silentpayments.h` which defines the new API.
  13 | +   - Usage example `examples/silentpayments.c`.
  14 | + - The `silentpayments` API currently requires full access to the transaction data (light client support is not implemented).

---

   6 | @@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   7 |  
   8 |  ## [Unreleased]
   9 |  
  10 | +#### Added
  11 | + - New module `silentpayments` implements sending and receiving of Silent Payments according to the [BIP 352 specification](https://github.com/bitcoin/bips/blob/master/bip-0352.mediawiki). See:

---

  95 | @@ -97,16 +96,14 @@ typedef struct secp256k1_silentpayments_recipient {
  96 |   *  In:           recipients: pointer to an array of pointers to Silent Payments

---