day 25

data/day25/input

@@ -0,0 +1,2 @@
+14012298
+74241

src/bin/day25.rs

@@ -0,0 +1,44 @@
+const INPUT: &str = include_str!("../../data/day25/input");
+
+const MOD: u32 = 20201227;
+
+// transform(subject, key) = (subject ** key) mod 20201227
+//
+// public_key := transform(7, private_key)
+// shared_secret := transform(peer_public_key, local_private_key)
+
+fn transform(subject: u32, key: u32) -> u32 {
+    let mut value = 1u64;
+    let subject = subject as u64;
+    for _ in 0..key {
+        value = (value * subject) % (MOD as u64);
+    }
+    value as u32
+}
+
+fn find_key(subject: u32, transformed: u32) -> u32 {
+    let mut value = 1u64;
+    let mut key = 0;
+    let subject = subject as u64;
+    let transformed = transformed as u64;
+    while value != transformed {
+        value = (value * subject) % (MOD as u64);
+        key += 1;
+    }
+    key
+}
+
+fn main() {
+    let card_pub_key;
+    let door_pub_key;
+    {
+        let mut lines = INPUT.lines();
+        card_pub_key = lines.next().unwrap().parse::<u32>().unwrap();
+        door_pub_key = lines.next().unwrap().parse::<u32>().unwrap();
+        assert!(lines.next().is_none());
+    }
+    let card_priv_key = find_key(7, card_pub_key);
+    println!("Card private key: {}", card_priv_key);
+    let shared_secret = transform(door_pub_key, card_priv_key);
+    println!("Shared secret: {}", shared_secret);
+}