diff --git a/2016/Sources/AdventOfCode.swift b/2016/Sources/AdventOfCode.swift
index 3276e89..8278a5a 100644
--- a/2016/Sources/AdventOfCode.swift
+++ b/2016/Sources/AdventOfCode.swift
@@ -5,6 +5,7 @@ let allChallenges: [any AdventDay] = [
   Day01(),
   Day02(),
   Day03(),
+  Day04(),
 ]
 
 @main
diff --git a/2016/Sources/Day04.swift b/2016/Sources/Day04.swift
new file mode 100644
index 0000000..429f343
--- /dev/null
+++ b/2016/Sources/Day04.swift
@@ -0,0 +1,54 @@
+import Foundation
+import Collections
+
+private typealias KeyValuePair = (k: Character, v: Int)
+
+struct Day04: AdventDay {
+  let data: String
+
+  func part1() -> Int {
+    return data.matches(of: /([a-z-]+)-(\d+)\[([a-z]+)]/)
+      .reduce(0) { sum, match in
+        let (_, name, id, checksum) = match.output
+
+        let counts = Dictionary(
+          uniqueKeysWithValues: name.uniqued()
+            .filter { $0.isLetter }
+            .map { c in (c, name.count(where: { $0 == c })) }
+        )
+
+        let actualChecksum = counts.keys
+          .sorted()
+          .sorted(by: { counts[$0]! > counts[$1]! })
+          .prefix(5)
+
+        return if actualChecksum.elementsEqual(checksum) {
+          sum + Int(id)!
+        } else {
+          sum
+        }
+      }
+  }
+
+  func part2() -> Int {
+    let asciiStart = Int(Character("a").asciiValue!)
+
+    return data.matches(of: /([a-z-]+)-(\d+)\[([a-z]+)]/)
+      .map { match in
+        let (_, name, id, _) = match.output
+
+        let decrypted = name.split(separator: "-")
+          .map { word in
+            String(
+              word.map { letter in
+                Character(UnicodeScalar((Int(letter.asciiValue!) - asciiStart + Int(id)!) % 26 + asciiStart)!)
+              }
+            )
+          }
+          .joined(separator: " ")
+
+        return (decrypted, Int(id)!)
+      }
+      .first(where: { $0.0 == "northpole object storage" })!.1
+  }
+}
diff --git a/2016/Tests/Day04Tests.swift b/2016/Tests/Day04Tests.swift
new file mode 100644
index 0000000..b9f3a6c
--- /dev/null
+++ b/2016/Tests/Day04Tests.swift
@@ -0,0 +1,16 @@
+import Testing
+
+@testable import AoC2016
+
+struct Day04Tests {
+  let testData = """
+  aaaaa-bbb-z-y-x-123[abxyz]
+  a-b-c-d-e-f-g-h-987[abcde]
+  not-a-real-room-404[oarel]
+  totally-real-room-200[decoy]
+  """
+
+  @Test func testPart1() throws {
+    #expect(try Day04(data: testData).part1() == 1514)
+  }
+}