Merge expanding and checking Hmac functions to avoid re-creating the Mac instance.

benchmark/src/androidTest/java/com/vitorpamplona/quartz/benchmark/HkdfBenchmark.kt

@@ -46,7 +46,15 @@ class HkdfBenchmark {
     }
 
     @Test
-    fun hkdfExpand() {
+    fun hkdfExpandWithCheck() {
+        val hkdf = Hkdf()
+        benchmarkRule.measureRepeated {
+            hkdf.fastExpand(sharedKey, encrypted.nonce, encrypted.ciphertext, encrypted.mac)
+        }
+    }
+
+    @Test
+    fun hkdfFastExpand() {
         val hkdf = Hkdf()
         benchmarkRule.measureRepeated {
             hkdf.fastExpand(sharedKey, encrypted.nonce)

quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/nip44Encryption/Nip44v2.kt

@@ -113,12 +113,14 @@ class Nip44v2 {
         decoded: EncryptedInfo,
         conversationKey: ByteArray,
     ): String {
-        val messageKey = getMessageKeys(conversationKey, decoded.nonce)
-
-        checkHMacAad(messageKey, decoded)
+        val messageKey = checkMessageKeys(conversationKey, decoded)
 
         return unpad(
-            chaCha.decrypt(decoded.ciphertext, messageKey.chachaNonce, messageKey.chachaKey),
+            chaCha.decrypt(
+                decoded.ciphertext,
+                messageKey.chachaNonce,
+                messageKey.chachaKey,
+            ),
         )
     }
 
@@ -210,6 +212,11 @@ class Nip44v2 {
         nonce: ByteArray,
     ): Hkdf.MessageKey = hkdf.fastExpand(conversationKey, nonce)
 
+    fun checkMessageKeys(
+        conversationKey: ByteArray,
+        decoded: EncryptedInfo,
+    ): Hkdf.MessageKey = hkdf.fastExpand(conversationKey, decoded.nonce, decoded.ciphertext, decoded.mac)
+
     /** @return 32B shared secret */
     fun computeConversationKey(
         privateKey: ByteArray,

quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/nip44Encryption/crypto/Hkdf.kt

@@ -20,6 +20,7 @@
  */
 package com.vitorpamplona.quartz.nip44Encryption.crypto
 
+import com.vitorpamplona.quartz.nip01Core.core.toHexKey
 import java.nio.ByteBuffer
 import javax.crypto.Mac
 import javax.crypto.spec.SecretKeySpec
@@ -88,6 +89,8 @@ class Hkdf(
     fun fastExpand(
         key: ByteArray,
         nonce: ByteArray,
+        ciphertext: ByteArray? = null,
+        ciphertextMac: ByteArray? = null,
     ): MessageKey {
         check(key.size == hashLen)
         check(nonce.size == hashLen)
@@ -112,10 +115,22 @@ class Hkdf(
         mac.update(3)
         val round3 = mac.doFinal()
 
-        val hmacKey = ByteArray(32)
+        val hmacKey = ByteArray(hashLen)
         System.arraycopy(round2, 12, hmacKey, 0, 20)
         System.arraycopy(round3, 0, hmacKey, 20, 12)
 
+        // checks the mac here to avoid building a new Mac.getInstance(algorithm)
+        if (ciphertext != null && ciphertextMac != null) {
+            mac.init(FixedKey(hmacKey, algorithm))
+            mac.update(nonce)
+            mac.update(ciphertext)
+            val calculatedMac = mac.doFinal()
+
+            check(calculatedMac.contentEquals(ciphertextMac)) {
+                "Invalid Mac: Calculated ${calculatedMac.toHexKey()}, decoded: ${ciphertextMac.toHexKey()}"
+            }
+        }
+
         return MessageKey(
             chachaKey = round1,
             chachaNonce = round2.copyOfRange(0, 12),