2 files changed, 32 insertions, 26 deletions

diff --git a/internal/secp256k1/point.go b/internal/secp256k1/point.go
index 1def176..5215590 100644
--- a/internal/secp256k1/point.go
+++ b/internal/secp256k1/point.go
@@ -162,6 +162,18 @@ func (p *Point) Negate() *Point {
         return &Point{x: p.x.Clone(), y: negY, infinity: false}
 }
 
+// XBytes returns the x-coordinate as a 32-byte big-endian slice.
+// Returns nil for the point at infinity.
+func (p *Point) XBytes() []byte {
+        if p.infinity {
+                return nil
+        }
+        b := p.x.value.Bytes()
+        out := make([]byte, 32)
+        copy(out[32-len(b):], b)
+        return out
+}
+
 // String returns a readable representation
 func (p *Point) String() string {
         if p.infinity {
diff --git a/nip04.go b/nip04.go
index 1f1c245..f979bcd 100644
--- a/nip04.go
+++ b/nip04.go
@@ -10,13 +10,13 @@ import (
         "fmt"
         "strings"
 
-        "github.com/btcsuite/btcd/btcec/v2"
+        "code.northwest.io/nostr/internal/secp256k1"
 )
 
-// NIP04Encrypt encrypts a plaintext message for a recipient using NIP-04
-// (ECDH shared secret + AES-256-CBC). The sender's private key and
-// recipient's public key (64-char hex) are required.
-// Returns the ciphertext in NIP-04 format: "<base64>?iv=<base64>"
+// NIP04Encrypt encrypts plaintext for a recipient using NIP-04
+// (ECDH shared secret + AES-256-CBC).
+// recipientPubHex is the recipient's x-only public key as 64-char hex.
+// Returns ciphertext in NIP-04 format: "<base64>?iv=<base64>"
 func (k *Key) NIP04Encrypt(recipientPubHex, plaintext string) (string, error) {
         if k.priv == nil {
                 return "", fmt.Errorf("cannot encrypt: public-only key")
@@ -45,7 +45,7 @@ func (k *Key) NIP04Encrypt(recipientPubHex, plaintext string) (string, error) {
 }
 
 // NIP04Decrypt decrypts a NIP-04 ciphertext using the receiver's private key
-// and sender's public key (64-char hex).
+// and the sender's x-only public key as 64-char hex.
 func (k *Key) NIP04Decrypt(senderPubHex, ciphertext string) (string, error) {
         if k.priv == nil {
                 return "", fmt.Errorf("cannot decrypt: public-only key")
@@ -91,37 +91,31 @@ func (k *Key) NIP04Decrypt(senderPubHex, ciphertext string) (string, error) {
         return string(unpadded), nil
 }
 
-// nip04SharedSecret computes the ECDH shared secret per NIP-04:
-// SHA256(privKey * recipientPubKey).x
-func nip04SharedSecret(priv *btcec.PrivateKey, recipientPubHex string) ([]byte, error) {
+// nip04SharedSecret computes the NIP-04 ECDH shared secret:
+// SHA256( (privKey * recipientPubKey).x )
+func nip04SharedSecret(priv *secp256k1.PrivateKey, recipientPubHex string) ([]byte, error) {
         pubBytes, err := hex.DecodeString(recipientPubHex)
         if err != nil {
                 return nil, fmt.Errorf("decode pubkey: %w", err)
         }
 
-        // NIP-04 pubkeys are x-only (32 bytes, Schnorr). Prefix with 0x02 for
-        // compressed SEC1 parsing.
-        if len(pubBytes) == 32 {
-                pubBytes = append([]byte{0x02}, pubBytes...)
-        }
-
-        pub, err := btcec.ParsePubKey(pubBytes)
+        // Expect 32-byte x-only key
+        pub, err := secp256k1.ParsePublicKeyXOnly(pubBytes)
         if err != nil {
                 return nil, fmt.Errorf("parse pubkey: %w", err)
         }
 
-        // ECDH: scalar multiply
-        var point btcec.JacobianPoint
-        pub.AsJacobian(&point)
-        priv.Key.SetByteSlice(priv.Serialize())
-        btcec.ScalarMultNonConst(&priv.Key, &point, &point)
-        point.ToAffine()
+        // ECDH: scalar multiply — shared point = priv.D * recipientPub
+        shared := pub.Point.ScalarMul(priv.D)
 
-        // Shared secret is SHA256 of the x-coordinate
-        xBytes := point.X.Bytes()
-        shared := sha256.Sum256(xBytes[:])
+        // Shared secret = SHA256(x-coordinate, big-endian, 32 bytes)
+        xBytes := shared.XBytes()
+        if xBytes == nil {
+                return nil, fmt.Errorf("ECDH result is point at infinity")
+        }
 
-        return shared[:], nil
+        h := sha256.Sum256(xBytes)
+        return h[:], nil
 }
 
 func pkcs7Pad(data []byte, blockSize int) []byte {