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