diff --git a/aoc2020/aoc2020_bench/benches/aoc2020_bench.rs b/aoc2020/aoc2020_bench/benches/aoc2020_bench.rs
index 6e04bee..26eb1e2 100644
--- a/aoc2020/aoc2020_bench/benches/aoc2020_bench.rs
+++ b/aoc2020/aoc2020_bench/benches/aoc2020_bench.rs
@@ -24,6 +24,7 @@ use aoc2020::day21;
 use aoc2020::day22;
 use aoc2020::day23;
 use aoc2020::day24;
+use aoc2020::day25;
 
 fn aoc2020_all(c: &mut Criterion) {
     c.bench_function("day01", |b| b.iter(|| day01::run().unwrap()));
@@ -50,6 +51,7 @@ fn aoc2020_all(c: &mut Criterion) {
     c.bench_function("day22", |b| b.iter(|| day22::run().unwrap()));
     c.bench_function("day23", |b| b.iter(|| day23::run().unwrap()));
     c.bench_function("day24", |b| b.iter(|| day24::run().unwrap()));
+    c.bench_function("day25", |b| b.iter(|| day25::run().unwrap()));
 }
 
 criterion_group! {
diff --git a/aoc2020/input/day25.txt b/aoc2020/input/day25.txt
new file mode 100644
index 0000000..1e40343
--- /dev/null
+++ b/aoc2020/input/day25.txt
@@ -0,0 +1,2 @@
+12090988
+240583
diff --git a/aoc2020/input/day25_provided.txt b/aoc2020/input/day25_provided.txt
new file mode 100644
index 0000000..9cbfc23
--- /dev/null
+++ b/aoc2020/input/day25_provided.txt
@@ -0,0 +1,2 @@
+5764801
+17807724
diff --git a/aoc2020/src/day25.rs b/aoc2020/src/day25.rs
new file mode 100644
index 0000000..81539d8
--- /dev/null
+++ b/aoc2020/src/day25.rs
@@ -0,0 +1,77 @@
+use std::fmt::Write;
+
+use anyhow::{Context, Result};
+
+const INPUT: &str = include_str!("../input/day25.txt");
+
+pub fn run() -> Result<String> {
+    let mut res = String::with_capacity(128);
+
+    writeln!(res, "part 1: {}", part1(INPUT)?)?;
+
+    Ok(res)
+}
+
+const PUB_KEY_SUBJECT: u64 = 7;
+
+// - Set the value to itself multiplied by the subject number.
+// - Set the value to the remainder after dividing the value by 20201227.
+fn transform(mut value: u64, subject: u64) -> u64 {
+    value *= subject;
+
+    value % 20201227
+}
+
+fn find_loop_size(pub_key: u64) -> usize {
+    let mut value = 1;
+    for i in 1.. {
+        value = transform(value, PUB_KEY_SUBJECT);
+
+        if value == pub_key {
+            return i;
+        }
+    }
+
+    unreachable!()
+}
+
+fn part1(input: &str) -> Result<u64> {
+    let mut lines = input.lines();
+
+    let card_pub_key: u64 = lines
+        .next()
+        .context("no public key found for card")?
+        .parse()?;
+    let door_pub_key: u64 = lines
+        .next()
+        .context("no public key found for door")?
+        .parse()?;
+
+    let card_loop_size = find_loop_size(card_pub_key);
+
+    let mut encryption_key = 1;
+    for _ in 0..card_loop_size {
+        encryption_key = transform(encryption_key, door_pub_key);
+    }
+
+    Ok(encryption_key)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const PROVIDED: &str = include_str!("../input/day25_provided.txt");
+
+    #[test]
+    fn part1_provided() {
+        assert_eq!(find_loop_size(5764801), 8);
+        assert_eq!(find_loop_size(17807724), 11);
+        assert_eq!(part1(PROVIDED).unwrap(), 14897079);
+    }
+
+    #[test]
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 3015200);
+    }
+}
diff --git a/aoc2020/src/lib.rs b/aoc2020/src/lib.rs
index 0ec7fd3..b611f39 100644
--- a/aoc2020/src/lib.rs
+++ b/aoc2020/src/lib.rs
@@ -24,3 +24,4 @@ pub mod day21;
 pub mod day22;
 pub mod day23;
 pub mod day24;
+pub mod day25;
diff --git a/aoc2020/src/main.rs b/aoc2020/src/main.rs
index 6b1ca8e..38d5dbe 100644
--- a/aoc2020/src/main.rs
+++ b/aoc2020/src/main.rs
@@ -26,6 +26,7 @@ use aoc2020::day21;
 use aoc2020::day22;
 use aoc2020::day23;
 use aoc2020::day24;
+use aoc2020::day25;
 
 fn main() -> Result<()> {
     let days: &[DayFunc] = &[
@@ -53,6 +54,7 @@ fn main() -> Result<()> {
         day22::run,
         day23::run,
         day24::run,
+        day25::run,
     ];
 
     aoc::run(days)