diff --git a/2016/src/Day04.kt b/2016/src/Day04.kt
new file mode 100644
index 0000000..67f41eb
--- /dev/null
+++ b/2016/src/Day04.kt
@@ -0,0 +1,43 @@
+fun main() {
+    val regex = "([a-z-]+)+-(\\d+)\\[(\\w+)]".toRegex()
+
+    fun part1(input: List<String>) = input.sumOf {
+        val (name, id, checksum) = regex.find(it)!!.destructured
+
+        val counts = name
+            .replace("-", "")
+            .associateWith { char -> Regex("$char").findAll(name).count() }
+
+        val actualChecksum = counts.keys.sorted()
+            .sortedByDescending(counts::get)
+            .take(5)
+            .joinToString("")
+
+        if (actualChecksum == checksum) {
+            id.toInt()
+        } else {
+            0
+        }
+    }
+
+    fun part2(input: List<String>) = input.map {
+        val (name, id, _) = regex.find(it)!!.destructured
+
+        val decrypted = name
+            .split("-")
+            .joinToString(" ") { word ->
+                word.map { c -> Char(((c.code - 'a'.code) + id.toInt()) % 26 + 'a'.code) }
+                    .joinToString("")
+            }
+
+        "$decrypted: $id"
+    }.find { it.contains("northpole object storage") }
+
+    check(part1(listOf("aaaaa-bbb-z-y-x-123[abxyz]")) == 123)
+    check(part1(listOf("a-b-c-d-e-f-g-h-987[abcde]")) == 987)
+    check(part1(listOf("not-a-real-room-404[oarel]")) == 404)
+    check(part1(listOf("totally-real-room-200[decoy]")) == 0)
+    println(part1(readInput("Day04")))
+
+    println(part2(readInput("Day04")))
+}