diff --git a/2020/inputs/25.txt b/2020/inputs/25.txt
new file mode 100644
index 0000000..ae79f64
--- /dev/null
+++ b/2020/inputs/25.txt
@@ -0,0 +1,2 @@
+2084668
+3704642
diff --git a/2020/samples/25.txt b/2020/samples/25.txt
new file mode 100644
index 0000000..9cbfc23
--- /dev/null
+++ b/2020/samples/25.txt
@@ -0,0 +1,2 @@
+5764801
+17807724
diff --git a/2020/src/day25.rs b/2020/src/day25.rs
index 2590562..7671d52 100644
--- a/2020/src/day25.rs
+++ b/2020/src/day25.rs
@@ -1,12 +1,75 @@
 use std::io::Read;
 
+use crate::common::from_lines;
 use crate::Solution;
 
+const MOD_BASE: u32 = 20201227;
+const SUBJECT_NUMBER: u32 = 7;
+
+fn loop_count(public_key: u32) -> u64 {
+    let mut value = 1;
+    let mut loops = 0;
+
+    while value != public_key {
+        value *= SUBJECT_NUMBER;
+        value %= MOD_BASE;
+
+        loops += 1;
+    }
+
+    loops as u64
+}
+
+#[inline]
+fn mod_exp(base: u64, mut power: u64, mod_base: u64) -> u64 {
+    let mut result = 1;
+    let mut cur = base;
+
+    while power > 0 {
+        if power % 2 == 1 {
+            result *= cur;
+            result %= mod_base;
+        }
+
+        cur *= cur;
+        cur %= mod_base;
+
+        power /= 2;
+    }
+
+    result
+}
+
 #[derive(Default)]
 pub struct Day25;
 
 impl Solution for Day25 {
-    fn part1(&mut self, _input: &mut dyn Read) -> String {
-        todo!()
+    fn part1(&mut self, input: &mut dyn Read) -> String {
+        let nums: Vec<_> = from_lines(input);
+
+        let exponent: u64 = nums.into_iter().map(loop_count).product();
+        let result = mod_exp(SUBJECT_NUMBER as u64, exponent, MOD_BASE as u64);
+
+        result.to_string()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::test_implementation;
+
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("../samples/25.txt");
+
+    #[test]
+    fn test_loop_count() {
+        assert_eq!(8, loop_count(5764801));
+        assert_eq!(11, loop_count(17807724));
+    }
+
+    #[test]
+    fn sample_part1() {
+        test_implementation!(Day25, 1, SAMPLE, 14897079);
     }
 }