fix: Hmac Fixes

Updated handling of hmac to use hkdf keys

Author: Alex Risch

library/src/main/java/org/xmtp/android/library/Conversations.kt

@@ -2,7 +2,6 @@ package org.xmtp.android.library
 
 import android.util.Log
 import com.google.protobuf.kotlin.toByteString
-import com.google.protobuf.kotlin.toByteStringUtf8
 import io.grpc.StatusException
 import kotlinx.coroutines.CancellationException
 import kotlinx.coroutines.channels.awaitClose
@@ -327,7 +326,7 @@ data class Conversations(
                         Crypto.deriveKey(
                             conversation.keyMaterial!!,
                             ByteArray(0),
-                            info.toByteStringUtf8().toByteArray()
+                            info.toByteArray(Charsets.UTF_8),
                         )
                     val hmacKeyData = HmacKeyData.newBuilder()
                     hmacKeyData.hmacKey = hmacKey.toByteString()

library/src/main/java/org/xmtp/android/library/Crypto.kt

@@ -87,12 +87,7 @@ class Crypto {
             salt: ByteArray,
             info: ByteArray,
         ): ByteArray {
-            val keySpec = SecretKeySpec(secret, "HmacSHA256")
-            val hmac = Mac.getInstance("HmacSHA256")
-            hmac.init(keySpec)
-            val derivedKey = hmac.doFinal(salt + info)
-
-            return derivedKey.copyOfRange(0, 32)
+            return Hkdf.computeHkdf("HMACSHA256", secret, salt, info, 32)
         }
 
         fun verifyHmacSignature(