diff --git a/day25.py b/day25.py
new file mode 100644
index 0000000..fca6a0d
--- /dev/null
+++ b/day25.py
@@ -0,0 +1,34 @@
+card_public_key, door_public_key = 12092626, 4707356
+# card_public_key, door_public_key = 5764801, 17807724
+
+
+def loop_size_from(expected):
+    value = 1
+    subject_number = 7
+    count = 0
+    while value != expected:
+        count += 1
+        value = (value * subject_number) % 20201227
+    return count
+
+
+card_loop_size = loop_size_from(card_public_key)
+# door_loop_size = loop_size_from(door_public_key)
+
+print(card_loop_size)
+# print(door_loop_size)
+
+
+def encryption(public_key, loop_size):
+    value = 1
+    subject_number = public_key
+    for _ in range(loop_size):
+        value = (value * subject_number) % 20201227
+    return value
+
+# e1 = encryption(card_public_key, door_loop_size)
+e2 = encryption(door_public_key, card_loop_size)
+
+
+# print(e1)
+print(e2)