Code review:

- enforce op equality by class and pin the tag-uniqueness invariant
- hoist crypto op equals/hashCode onto OpCrypto

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip03Timestamp/ots/VerifyResult.kt

@@ -40,5 +40,10 @@ data class VerifyResult(
         return "block $height attests data existed as of unix timestamp of $timestamp"
     }
 
+    /**
+     * Orders by [height] only, intentionally ignoring [timestamp] — NOT
+     * consistent with [equals], which compares both fields. Fine for picking
+     * the earliest attestation; do not rely on it for sorted-set dedup.
+     */
     override fun compareTo(other: VerifyResult): Int = this.height - other.height
 }

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip03Timestamp/ots/op/OpBinary.kt

@@ -53,7 +53,11 @@ abstract class OpBinary(
         return this.tag() - other.tag()
     }
 
-    override fun equals(other: Any?): Boolean = other is OpBinary && this.tag() == other.tag() && this.arg.contentEquals(other.arg)
+    // Class-scoped equality: never conflates two ops as Timestamp.ops map keys
+    // even if a subclass reused a tag by mistake. hashCode stays tag-based
+    // (collisions are legal). Tag uniqueness itself is a protocol invariant
+    // (serialization dispatches on it), pinned by OtsEqualsContractTest.
+    override fun equals(other: Any?): Boolean = other is OpBinary && this::class == other::class && this.arg.contentEquals(other.arg)
 
     override fun hashCode(): Int = this.tag().toInt() xor this.arg.contentHashCode()
 

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip03Timestamp/ots/op/OpCrypto.kt

@@ -64,6 +64,14 @@ abstract class OpCrypto internal constructor() : OpUnary() {
         return hashFd(ctx)
     }
 
+    // Crypto ops carry no identity state — class-scoped equality with a
+    // tag-based hashCode (never conflates ops even if a subclass reused a
+    // tag by mistake). Tag uniqueness itself is a protocol invariant
+    // (serialization dispatches on it), pinned by OtsEqualsContractTest.
+    override fun equals(other: Any?): Boolean = other is OpCrypto && this::class == other::class
+
+    override fun hashCode(): Int = this.tag().toInt()
+
     companion object {
         fun deserializeFromTag(
             ctx: StreamDeserializationContext,

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip03Timestamp/ots/op/OpKECCAK256.kt

@@ -49,10 +49,6 @@ class OpKECCAK256 : OpCrypto() {
         return hash
     }
 
-    override fun equals(other: Any?): Boolean = (other is OpKECCAK256)
-
-    override fun hashCode(): Int = TAG.toInt()
-
     companion object {
         val TAG: Byte = 103.toByte()
 

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip03Timestamp/ots/op/OpRIPEMD160.kt

@@ -48,10 +48,6 @@ class OpRIPEMD160 : OpCrypto() {
         return hash
     }
 
-    override fun equals(other: Any?): Boolean = (other is OpRIPEMD160)
-
-    override fun hashCode(): Int = TAG.toInt()
-
     companion object {
         val TAG: Byte = 0x03
 

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip03Timestamp/ots/op/OpSHA1.kt

@@ -46,10 +46,6 @@ class OpSHA1 : OpCrypto() {
 
     override fun call(msg: ByteArray): ByteArray = super.call(msg)
 
-    override fun equals(other: Any?): Boolean = (other is OpSHA1)
-
-    override fun hashCode(): Int = TAG.toInt()
-
     companion object {
         val TAG: Byte = 0x02
 

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip03Timestamp/ots/op/OpSHA256.kt

@@ -40,10 +40,6 @@ class OpSHA256 : OpCrypto() {
 
     override fun call(msg: ByteArray): ByteArray = super.call(msg)
 
-    override fun equals(other: Any?): Boolean = (other is OpSHA256)
-
-    override fun hashCode(): Int = TAG.toInt()
-
     companion object {
         val TAG: Byte = 0x08
 

quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip03Timestamp/ots/OtsEqualsContractTest.kt

@@ -24,11 +24,13 @@ import com.vitorpamplona.quartz.nip03Timestamp.ots.op.Op
 import com.vitorpamplona.quartz.nip03Timestamp.ots.op.OpAppend
 import com.vitorpamplona.quartz.nip03Timestamp.ots.op.OpKECCAK256
 import com.vitorpamplona.quartz.nip03Timestamp.ots.op.OpPrepend
+import com.vitorpamplona.quartz.nip03Timestamp.ots.op.OpRIPEMD160
+import com.vitorpamplona.quartz.nip03Timestamp.ots.op.OpSHA1
+import com.vitorpamplona.quartz.nip03Timestamp.ots.op.OpSHA256
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertFalse
 import kotlin.test.assertNotEquals
-import kotlin.test.assertTrue
 
 /**
  * Equals/hashCode contract tests for the types keying [Timestamp.ops]
@@ -36,6 +38,23 @@ import kotlin.test.assertTrue
  * lookups produce duplicate branches and failed upgrades.
  */
 class OtsEqualsContractTest {
+    @Test
+    fun allOpTagsAreUnique() {
+        // The protocol invariant everything leans on: serialization dispatches
+        // on the tag byte and hashCode is tag-based. A new op MUST use a fresh tag.
+        val tags =
+            listOf(
+                OpAppend.TAG,
+                OpPrepend.TAG,
+                OpSHA1.TAG,
+                OpSHA256.TAG,
+                OpRIPEMD160.TAG,
+                OpKECCAK256.TAG,
+            )
+
+        assertEquals(tags.size, tags.toSet().size)
+    }
+
     // --- OpKECCAK256: equals existed without hashCode; equal instances hashed by identity ---
 
     @Test
@@ -54,6 +73,12 @@ class OtsEqualsContractTest {
         assertEquals("branch", map[OpKECCAK256()])
     }
 
+    @Test
+    fun cryptoOpsWithDifferentTagsAreNotEqual() {
+        // The shared OpCrypto equals is tag-keyed — distinct ops must not conflate.
+        assertNotEquals<Op>(OpSHA256(), OpKECCAK256())
+    }
+
     // --- OpBinary: equals is defined once on the superclass, keyed on tag() + arg content ---
 
     @Test
@@ -105,6 +130,6 @@ class OtsEqualsContractTest {
         val result = VerifyResult(null, 100)
 
         result.hashCode()
-        assertTrue(result == VerifyResult(null, 100))
+        assertEquals(VerifyResult(null, 100), result)
     }
 }