Add ElligatorSwift key creation and ECDH logic

Co-authored-by: Dhruv Mehta <856960+dhruv@users.noreply.github.com>

src/key.cpp

@@ -11,6 +11,7 @@
 #include <random.h>
 
 #include <secp256k1.h>
+#include <secp256k1_ellswift.h>
 #include <secp256k1_extrakeys.h>
 #include <secp256k1_recovery.h>
 #include <secp256k1_schnorrsig.h>
@@ -331,6 +332,42 @@ bool CKey::Derive(CKey& keyChild, ChainCode &ccChild, unsigned int nChild, const
     return ret;
 }
 
+EllSwiftPubKey CKey::EllSwiftCreate(Span<const std::byte> ent32) const
+{
+    assert(fValid);
+    assert(ent32.size() == 32);
+    std::array<std::byte, EllSwiftPubKey::size()> encoded_pubkey;
+
+    auto success = secp256k1_ellswift_create(secp256k1_context_sign,
+                                             UCharCast(encoded_pubkey.data()),
+                                             keydata.data(),
+                                             UCharCast(ent32.data()));
+
+    // Should always succeed for valid keys (asserted above).
+    assert(success);
+    return {encoded_pubkey};
+}
+
+ECDHSecret CKey::ComputeBIP324ECDHSecret(const EllSwiftPubKey& their_ellswift, const EllSwiftPubKey& our_ellswift, bool initiating) const
+{
+    assert(fValid);
+
+    ECDHSecret output;
+    // BIP324 uses the initiator as party A, and the responder as party B. Remap the inputs
+    // accordingly:
+    bool success = secp256k1_ellswift_xdh(secp256k1_context_sign,
+                                          UCharCast(output.data()),
+                                          UCharCast(initiating ? our_ellswift.data() : their_ellswift.data()),
+                                          UCharCast(initiating ? their_ellswift.data() : our_ellswift.data()),
+                                          keydata.data(),
+                                          initiating ? 0 : 1,
+                                          secp256k1_ellswift_xdh_hash_function_bip324,
+                                          nullptr);
+    // Should always succeed for valid keys (assert above).
+    assert(success);
+    return output;
+}
+
 bool CExtKey::Derive(CExtKey &out, unsigned int _nChild) const {
     if (nDepth == std::numeric_limits<unsigned char>::max()) return false;
     out.nDepth = nDepth + 1;

src/key.h

@@ -22,6 +22,12 @@
  */
 typedef std::vector<unsigned char, secure_allocator<unsigned char> > CPrivKey;
 
+/** Size of ECDH shared secrets. */
+constexpr static size_t ECDH_SECRET_SIZE = CSHA256::OUTPUT_SIZE;
+
+// Used to represent ECDH shared secret (ECDH_SECRET_SIZE bytes)
+using ECDHSecret = std::array<std::byte, ECDH_SECRET_SIZE>;
+
 /** An encapsulated private key. */
 class CKey
 {
@@ -156,6 +162,27 @@ public:
 
     //! Load private key and check that public key matches.
     bool Load(const CPrivKey& privkey, const CPubKey& vchPubKey, bool fSkipCheck);
+
+    /** Create an ellswift-encoded public key for this key, with specified entropy.
+     *
+     *  entropy must be a 32-byte span with additional entropy to use in the encoding. Every
+     *  public key has ~2^256 different encodings, and this function will deterministically pick
+     *  one of them, based on entropy. Note that even without truly random entropy, the
+     *  resulting encoding will be indistinguishable from uniform to any adversary who does not
+     *  know the private key (because the private key itself is always used as entropy as well).
+     */
+    EllSwiftPubKey EllSwiftCreate(Span<const std::byte> entropy) const;
+
+    /** Compute a BIP324-style ECDH shared secret.
+     *
+     *  - their_ellswift: EllSwiftPubKey that was received from the other side.
+     *  - our_ellswift: EllSwiftPubKey that was sent to the other side (must have been generated
+     *                  from *this using EllSwiftCreate()).
+     *  - initiating: whether we are the initiating party (true) or responding party (false).
+     */
+    ECDHSecret ComputeBIP324ECDHSecret(const EllSwiftPubKey& their_ellswift,
+                                       const EllSwiftPubKey& our_ellswift,
+                                       bool initiating) const;
 };
 
 struct CExtKey {

src/pubkey.cpp

@@ -7,6 +7,7 @@
 
 #include <hash.h>
 #include <secp256k1.h>
+#include <secp256k1_ellswift.h>
 #include <secp256k1_extrakeys.h>
 #include <secp256k1_recovery.h>
 #include <secp256k1_schnorrsig.h>
@@ -335,6 +336,20 @@ bool CPubKey::Derive(CPubKey& pubkeyChild, ChainCode &ccChild, unsigned int nChi
     return true;
 }
 
+CPubKey EllSwiftPubKey::Decode() const
+{
+    secp256k1_pubkey pubkey;
+    secp256k1_ellswift_decode(secp256k1_context_static, &pubkey, UCharCast(m_pubkey.data()));
+
+    size_t sz = CPubKey::COMPRESSED_SIZE;
+    std::array<uint8_t, CPubKey::COMPRESSED_SIZE> vch_bytes;
+
+    secp256k1_ec_pubkey_serialize(secp256k1_context_static, vch_bytes.data(), &sz, &pubkey, SECP256K1_EC_COMPRESSED);
+    assert(sz == vch_bytes.size());
+
+    return CPubKey{vch_bytes.begin(), vch_bytes.end()};
+}
+
 void CExtPubKey::Encode(unsigned char code[BIP32_EXTKEY_SIZE]) const {
     code[0] = nDepth;
     memcpy(code+1, vchFingerprint, 4);

src/pubkey.h

@@ -291,6 +291,28 @@ public:
     SERIALIZE_METHODS(XOnlyPubKey, obj) { READWRITE(obj.m_keydata); }
 };
 
+/** An ElligatorSwift-encoded public key. */
+struct EllSwiftPubKey
+{
+private:
+    static constexpr size_t SIZE = 64;
+    std::array<std::byte, SIZE> m_pubkey;
+
+public:
+    /** Construct a new ellswift public key from a given serialization. */
+    EllSwiftPubKey(const std::array<std::byte, SIZE>& ellswift) :
+        m_pubkey(ellswift) {}
+
+    /** Decode to normal compressed CPubKey (for debugging purposes). */
+    CPubKey Decode() const;
+
+    // Read-only access for serialization.
+    const std::byte* data() const { return m_pubkey.data(); }
+    static constexpr size_t size() { return SIZE; }
+    auto begin() const { return m_pubkey.cbegin(); }
+    auto end() const { return m_pubkey.cend(); }
+};
+
 struct CExtPubKey {
     unsigned char version[4];
     unsigned char nDepth;

src/span.h

@@ -274,6 +274,7 @@ Span<std::byte> MakeWritableByteSpan(V&& v) noexcept
 // Helper functions to safely cast to unsigned char pointers.
 inline unsigned char* UCharCast(char* c) { return (unsigned char*)c; }
 inline unsigned char* UCharCast(unsigned char* c) { return c; }
+inline unsigned char* UCharCast(std::byte* c) { return (unsigned char*)c; }
 inline const unsigned char* UCharCast(const char* c) { return (unsigned char*)c; }
 inline const unsigned char* UCharCast(const unsigned char* c) { return c; }
 inline const unsigned char* UCharCast(const std::byte* c) { return reinterpret_cast<const unsigned char*>(c); }