Merge bitcoin-core/secp256k1#1354: Add ellswift to CHANGELOG

7c7467a Refer to ellswift.md in API docs (Pieter Wuille)
c32ffd8 Add ellswift to CHANGELOG (Pieter Wuille)

Pull request description:

  A follow-up with a CHANGELOG entry for bitcoin#1129.

ACKs for top commit:
  real-or-random:
    ACK 7c7467a
  theStack:
    ACK 7c7467a

Tree-SHA512: 4f066e4b8d5e130f2b5bea0ed4c634e9426bc576342aad6c306e0805a8354e27a5e679b15ec869d4e7d36eb5d53174e46b3bf5e15d19a7e165afc82e46ddfcf5

Author: real-or-random

CHANGELOG.md

@@ -7,6 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+#### Added
+ - New module `ellswift` implements ElligatorSwift encoding for public keys and x-only Diffie-Hellman key exchange for them.
+   ElligatorSwift permits representing secp256k1 public keys as 64-byte arrays which cannot be distinguished from uniformly random. See:
+   - Header file `include/secp256k1_ellswift.h` which defines the new API.
+   - Document `doc/ellswift.md` which explains the mathematical background of the scheme.
+   - The [paper](https://eprint.iacr.org/2022/759) on which the scheme is based.
+
 ## [0.3.2] - 2023-05-13
 We strongly recommend updating to 0.3.2 if you use or plan to use GCC >=13 to compile libsecp256k1. When in doubt, check the GCC version using `gcc -v`.
 

include/secp256k1_ellswift.h

@@ -41,6 +41,8 @@ extern "C" {
  * - The paper uses an additional encoding bit for the parity of y. Here the
  *   parity of t is used (negating t does not affect the decoded x coordinate,
  *   so this is possible).
+ *
+ * For mathematical background about the scheme, see the doc/ellswift.md file.
  */
 
 /** A pointer to a function used by secp256k1_ellswift_xdh to hash the shared X