From b4e25d38821b052390696d37a7c209acdec89006 Mon Sep 17 00:00:00 2001
From: Marcin Chrzanowski <marcin.j.chrzanowski@gmail.com>
Date: Fri, 25 Dec 2020 15:52:55 +0100
Subject: Add day 25

---
 25/a.lua | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)
 create mode 100644 25/a.lua

diff --git a/25/a.lua b/25/a.lua
new file mode 100644
index 0000000..f250565
--- /dev/null
+++ b/25/a.lua
@@ -0,0 +1,33 @@
+card_pkey = 8421034
+door_pkey = 15993936
+modulus = 20201227
+
+function step(value, subject)
+    return (value * subject) % modulus
+end
+
+function reverse(pkey, subject)
+    value = 1
+    steps = 0
+    while value ~= pkey do
+        value = step(value, subject)
+        steps = steps + 1
+    end
+
+    return steps
+end
+
+function transform(subject, loop)
+    value = 1
+    for i = 1, loop do
+        value = step(value, subject)
+    end
+
+    return value
+end
+
+card_steps = reverse(card_pkey, 7)
+
+encryption_key = transform(door_pkey, card_steps)
+
+print(encryption_key)
-- 
cgit v1.2.3