diff --git a/25/input.txt b/25/input.txt
new file mode 100644
index 0000000..dd9c49a
--- /dev/null
+++ b/25/input.txt
@@ -0,0 +1,2 @@
+2069194
+16426071
diff --git a/25/main.py b/25/main.py
new file mode 100644
index 0000000..01c6493
--- /dev/null
+++ b/25/main.py
@@ -0,0 +1,30 @@
+
+def read_input(filename: str) -> tuple[int, int]:
+    with open(filename, "r") as f:
+        cards_pub, doors_pub = f.read().splitlines()
+        return int(cards_pub), int(doors_pub)
+
+def reverse_loop_size(public_key: int, subject_number: int) -> int:
+    key = 1
+    loop_size = 0
+    while key != public_key:
+        loop_size += 1
+        key *= subject_number
+        key %= 20201227
+    return loop_size
+
+def transform_key(subject_number: int, loop_size: int) -> int:
+    key = 1
+    for _ in range(loop_size):
+        key *= subject_number
+        key %= 20201227
+    return key
+
+def part1(cards_pub: int, doors_pub: int) -> int:
+    subject_number = 7
+    card_loop_size = reverse_loop_size(cards_pub, subject_number)
+    return transform_key(doors_pub, card_loop_size)
+
+if __name__ == "__main__":
+    cards_pub, doors_pub = read_input("input.txt")
+    print("part1: ", part1(cards_pub, doors_pub))
diff --git a/25/test.txt b/25/test.txt
new file mode 100644
index 0000000..9cbfc23
--- /dev/null
+++ b/25/test.txt
@@ -0,0 +1,2 @@
+5764801
+17807724