diff --git a/quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.android.kt b/quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.android.kt
index 96c7119792..33bdb090fc 100644
--- a/quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.android.kt
+++ b/quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.android.kt
@@ -33,3 +33,8 @@ internal actual fun multiplyHigh(
     } else {
         multiplyHighFallback(a, b)
     }
+
+internal actual fun unsignedMultiplyHigh(
+    a: Long,
+    b: Long,
+): Long = unsignedMultiplyHighFallback(a, b)
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.kt
index a7af7dea1c..bf9b6f3ced 100644
--- a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.kt
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.kt
@@ -34,9 +34,20 @@ internal expect fun multiplyHigh(
 
 /**
  * Returns the upper 64 bits of the UNSIGNED 128-bit product of two Long values.
- * Built on top of the signed multiplyHigh with a correction for sign bits.
+ *
+ * On JVM 18+, delegates to Math.unsignedMultiplyHigh (single UMULH instruction).
+ * On older JVMs and other platforms, uses signed multiplyHigh with sign-bit correction.
+ * The correction adds 4 instructions per call; eliminating it saves ~64 insns per field mul.
  */
-internal fun unsignedMultiplyHigh(
+internal expect fun unsignedMultiplyHigh(
+    a: Long,
+    b: Long,
+): Long
+
+/**
+ * Fallback: unsigned multiply high from signed multiply high + correction.
+ */
+internal fun unsignedMultiplyHighFallback(
     a: Long,
     b: Long,
 ): Long = multiplyHigh(a, b) + (a and (b shr 63)) + (b and (a shr 63))
diff --git a/quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.jvm.kt b/quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.jvm.kt
index 8541f81ac6..bc0a32242f 100644
--- a/quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.jvm.kt
+++ b/quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.jvm.kt
@@ -29,3 +29,50 @@ internal actual fun multiplyHigh(
     a: Long,
     b: Long,
 ): Long = Math.multiplyHigh(a, b)
+
+/**
+ * JVM: uses Math.unsignedMultiplyHigh (Java 18+) if available, else fallback.
+ * The HAS_UNSIGNED_MULTIPLY_HIGH check is evaluated once at class init and the
+ * JIT will devirtualize the hot branch after a few invocations.
+ */
+internal actual fun unsignedMultiplyHigh(
+    a: Long,
+    b: Long,
+): Long =
+    if (HAS_UNSIGNED_MULTIPLY_HIGH) {
+        unsignedMultiplyHighNative(a, b)
+    } else {
+        unsignedMultiplyHighFallback(a, b)
+    }
+
+/**
+ * Tries to resolve Math.unsignedMultiplyHigh (Java 18+) as a MethodHandle.
+ * If available, invoking it compiles to a single UMULH instruction, saving
+ * 4 correction instructions per product vs the signed multiplyHigh + fixup path.
+ * This saves ~64 instructions per field multiplication (16 products × 4 insns).
+ *
+ * MethodHandle.invokeExact is JIT-inlined to the same cost as a direct call.
+ */
+private val UNSIGNED_MUL_HIGH: java.lang.invoke.MethodHandle? =
+    try {
+        java.lang.invoke.MethodHandles.lookup().findStatic(
+            Math::class.java,
+            "unsignedMultiplyHigh",
+            java.lang.invoke.MethodType.methodType(
+                java.lang.Long.TYPE,
+                java.lang.Long.TYPE,
+                java.lang.Long.TYPE,
+            ),
+        )
+    } catch (_: Throwable) {
+        null
+    }
+
+/** True if the native unsigned multiply high is available (Java 18+). */
+internal val HAS_UNSIGNED_MULTIPLY_HIGH: Boolean = UNSIGNED_MUL_HIGH != null
+
+/** Call Math.unsignedMultiplyHigh via MethodHandle (only when HAS_UNSIGNED_MULTIPLY_HIGH is true). */
+internal fun unsignedMultiplyHighNative(
+    a: Long,
+    b: Long,
+): Long = UNSIGNED_MUL_HIGH!!.invokeExact(a, b) as Long
diff --git a/quartz/src/nativeMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.native.kt b/quartz/src/nativeMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.native.kt
index 24109c9efb..590dd8188c 100644
--- a/quartz/src/nativeMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.native.kt
+++ b/quartz/src/nativeMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.native.kt
@@ -25,3 +25,8 @@ internal actual fun multiplyHigh(
     a: Long,
     b: Long,
 ): Long = multiplyHighFallback(a, b)
+
+internal actual fun unsignedMultiplyHigh(
+    a: Long,
+    b: Long,
+): Long = unsignedMultiplyHighFallback(a, b)