diff --git a/Cargo.lock b/Cargo.lock
index 58e58a1..38d0058 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -9,6 +9,7 @@ name = "aoc2015"
 version = "0.1.0"
 dependencies = [
  "aoc 0.1.0",
+ "md-5 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -25,3 +26,78 @@ dependencies = [
  "aoc 0.1.0",
 ]
 
+[[package]]
+name = "block-buffer"
+version = "0.7.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "block-padding 0.1.5 (registry+https://github.com/rust-lang/crates.io-index)",
+ "byte-tools 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "byteorder 1.3.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "generic-array 0.12.3 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "block-padding"
+version = "0.1.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "byte-tools 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "byte-tools"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "byteorder"
+version = "1.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "digest"
+version = "0.8.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "generic-array 0.12.3 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "generic-array"
+version = "0.12.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "typenum 1.11.2 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "md-5"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "block-buffer 0.7.3 (registry+https://github.com/rust-lang/crates.io-index)",
+ "digest 0.8.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "opaque-debug 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "opaque-debug"
+version = "0.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "typenum"
+version = "1.11.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[metadata]
+"checksum block-buffer 0.7.3 (registry+https://github.com/rust-lang/crates.io-index)" = "c0940dc441f31689269e10ac70eb1002a3a1d3ad1390e030043662eb7fe4688b"
+"checksum block-padding 0.1.5 (registry+https://github.com/rust-lang/crates.io-index)" = "fa79dedbb091f449f1f39e53edf88d5dbe95f895dae6135a8d7b881fb5af73f5"
+"checksum byte-tools 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)" = "e3b5ca7a04898ad4bcd41c90c5285445ff5b791899bb1b0abdd2a2aa791211d7"
+"checksum byteorder 1.3.2 (registry+https://github.com/rust-lang/crates.io-index)" = "a7c3dd8985a7111efc5c80b44e23ecdd8c007de8ade3b96595387e812b957cf5"
+"checksum digest 0.8.1 (registry+https://github.com/rust-lang/crates.io-index)" = "f3d0c8c8752312f9713efd397ff63acb9f85585afbf179282e720e7704954dd5"
+"checksum generic-array 0.12.3 (registry+https://github.com/rust-lang/crates.io-index)" = "c68f0274ae0e023facc3c97b2e00f076be70e254bc851d972503b328db79b2ec"
+"checksum md-5 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)" = "a18af3dcaf2b0219366cdb4e2af65a6101457b415c3d1a5c71dd9c2b7c77b9c8"
+"checksum opaque-debug 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)" = "2839e79665f131bdb5782e51f2c6c9599c133c6098982a54c794358bf432529c"
+"checksum typenum 1.11.2 (registry+https://github.com/rust-lang/crates.io-index)" = "6d2783fe2d6b8c1101136184eb41be8b1ad379e4657050b8aaff0c79ee7575f9"
diff --git a/aoc2015/Cargo.toml b/aoc2015/Cargo.toml
index 6749987..279ebf8 100644
--- a/aoc2015/Cargo.toml
+++ b/aoc2015/Cargo.toml
@@ -9,3 +9,4 @@ edition = "2018"
 [dependencies]
 
 aoc = { path = "../aoc" }
+md-5 = "0.8"
diff --git a/aoc2015/input/day04.txt b/aoc2015/input/day04.txt
new file mode 100644
index 0000000..d75e0dc
--- /dev/null
+++ b/aoc2015/input/day04.txt
@@ -0,0 +1 @@
+yzbqklnj
diff --git a/aoc2015/src/day04.rs b/aoc2015/src/day04.rs
new file mode 100644
index 0000000..222129f
--- /dev/null
+++ b/aoc2015/src/day04.rs
@@ -0,0 +1,48 @@
+use md5::{Digest, Md5};
+
+use aoc::err;
+use aoc::Result;
+
+const INPUT: &str = include_str!("../input/day04.txt");
+
+pub fn run() -> Result<()> {
+    println!("part 1: {}", part1(INPUT)?);
+
+    Ok(())
+}
+
+fn part1(input: &str) -> Result<u64> {
+    let input = input.trim_end();
+
+    let mut hasher = Md5::new();
+
+    for i in 0.. {
+        let content = format!("{}{}", input, i);
+
+        hasher.input(content);
+        let res = format!("{:x}", hasher.result_reset());
+        if &res[..5] == "00000" {
+            return Ok(i);
+        }
+    }
+
+    Err(err!("couldn't find a suitable number"))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    #[ignore] // takes too long!
+    fn part1_provided() {
+        assert_eq!(part1("abcdef").unwrap(), 609043);
+        assert_eq!(part1("pqrstuv").unwrap(), 1048970);
+    }
+
+    #[test]
+    #[ignore] // takes too long!
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 282749);
+    }
+}
diff --git a/aoc2015/src/lib.rs b/aoc2015/src/lib.rs
index 89345b1..e051fbd 100644
--- a/aoc2015/src/lib.rs
+++ b/aoc2015/src/lib.rs
@@ -1,3 +1,4 @@
 pub mod day01;
 pub mod day02;
 pub mod day03;
+pub mod day04;
diff --git a/aoc2015/src/main.rs b/aoc2015/src/main.rs
index 68fad4b..fee1657 100644
--- a/aoc2015/src/main.rs
+++ b/aoc2015/src/main.rs
@@ -3,9 +3,10 @@ use aoc::Result;
 use aoc2015::day01;
 use aoc2015::day02;
 use aoc2015::day03;
+use aoc2015::day04;
 
 fn main() -> Result<()> {
-    let days: &[fn() -> Result<()>] = &[day01::run, day02::run, day03::run];
+    let days: &[fn() -> Result<()>] = &[day01::run, day02::run, day03::run, day04::run];
 
     aoc::run(days)
 }