diff --git a/CMakeLists.txt b/CMakeLists.txt
new file mode 100644
index 0000000..6ad3e34
--- /dev/null
+++ b/CMakeLists.txt
@@ -0,0 +1,17 @@
+cmake_minimum_required(VERSION 3.20)
+
+project(
+    Chaos
+    LANGUAGES CXX
+)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+add_library(Chaos INTERFACE)
+
+target_include_directories(Chaos INTERFACE
+    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/Chaos>
+)
+
+add_subdirectory(ChaosTests)
diff --git a/Chaos/Md4.hpp b/Chaos/Md4.hpp
new file mode 100644
index 0000000..d10e076
--- /dev/null
+++ b/Chaos/Md4.hpp
@@ -0,0 +1,277 @@
+#ifndef CHAOS_MD4HASHER_HPP
+#define CHAOS_MD4HASHER_HPP
+
+#include <cstdint>
+#include <array>
+
+namespace Chaos::Md4
+{
+
+struct Buffer
+{
+    uint32_t Regs_[4] = { 0x67452301, 0xefcdab89, 0x98badcfe, 0x10325476 };
+};
+
+using Block = std::array<uint32_t, 16>;
+
+struct Algorithm
+{
+public:
+    static void UpdateBuffer(Buffer & buffer, const Block & block)
+    {
+        uint32_t a = buffer.Regs_[0];
+        uint32_t b = buffer.Regs_[1];
+        uint32_t c = buffer.Regs_[2];
+        uint32_t d = buffer.Regs_[3];
+
+        FF(a, b, c, d, block[ 0], 3);
+        FF(d, a, b, c, block[ 1], 7);
+        FF(c, d, a, b, block[ 2], 11);
+        FF(b, c, d, a, block[ 3], 19);
+        FF(a, b, c, d, block[ 4], 3);
+        FF(d, a, b, c, block[ 5], 7);
+        FF(c, d, a, b, block[ 6], 11);
+        FF(b, c, d, a, block[ 7], 19);
+        FF(a, b, c, d, block[ 8], 3);
+        FF(d, a, b, c, block[ 9], 7);
+        FF(c, d, a, b, block[10], 11);
+        FF(b, c, d, a, block[11], 19);
+        FF(a, b, c, d, block[12], 3);
+        FF(d, a, b, c, block[13], 7);
+        FF(c, d, a, b, block[14], 11);
+        FF(b, c, d, a, block[15], 19);
+
+        GG(a, b, c, d, block[ 0], 3);
+        GG(d, a, b, c, block[ 4], 5);
+        GG(c, d, a, b, block[ 8], 9);
+        GG(b, c, d, a, block[12], 13);
+        GG(a, b, c, d, block[ 1], 3);
+        GG(d, a, b, c, block[ 5], 5);
+        GG(c, d, a, b, block[ 9], 9);
+        GG(b, c, d, a, block[13], 13);
+        GG(a, b, c, d, block[ 2], 3);
+        GG(d, a, b, c, block[ 6], 5);
+        GG(c, d, a, b, block[10], 9);
+        GG(b, c, d, a, block[14], 13);
+        GG(a, b, c, d, block[ 3], 3);
+        GG(d, a, b, c, block[ 7], 5);
+        GG(c, d, a, b, block[11], 9);
+        GG(b, c, d, a, block[15], 13);
+
+        HH(a, b, c, d, block[ 0], 3);
+        HH(d, a, b, c, block[ 8], 9);
+        HH(c, d, a, b, block[ 4], 11);
+        HH(b, c, d, a, block[12], 15);
+        HH(a, b, c, d, block[ 2], 3);
+        HH(d, a, b, c, block[10], 9);
+        HH(c, d, a, b, block[ 6], 11);
+        HH(b, c, d, a, block[14], 15);
+        HH(a, b, c, d, block[ 1], 3);
+        HH(d, a, b, c, block[ 9], 9);
+        HH(c, d, a, b, block[ 5], 11);
+        HH(b, c, d, a, block[13], 15);
+        HH(a, b, c, d, block[ 3], 3);
+        HH(d, a, b, c, block[11], 9);
+        HH(c, d, a, b, block[ 7], 11);
+        HH(b, c, d, a, block[15], 15);
+
+        buffer.Regs_[0] += a;
+        buffer.Regs_[1] += b;
+        buffer.Regs_[2] += c;
+        buffer.Regs_[3] += d;
+    }
+
+private:
+    static uint32_t Rotl(uint32_t v, int_fast8_t s)
+    {
+        return (v << s) | (v >> (32 - s));
+    }
+
+    static uint32_t F(uint32_t x, uint32_t y, uint32_t z)
+    {
+        return (x & y) | ((~x) & z);
+    }
+
+    static uint32_t G(uint32_t x, uint32_t y, uint32_t z)
+    {
+        return (x & y) | (x & z) | (y & z);
+    }
+
+    static uint32_t H(uint32_t x, uint32_t y, uint32_t z)
+    {
+        return x ^ y ^ z;
+    }
+
+    static void FF(uint32_t & a, uint32_t b, uint32_t c, uint32_t d,
+                   uint32_t x, int_fast8_t s)
+    {
+        a = Rotl(a + F(b, c, d) + x, s);
+    }
+
+    static void GG(uint32_t & a, uint32_t b, uint32_t c, uint32_t d,
+                   uint32_t x, int_fast8_t s)
+    {
+        constexpr uint32_t C2 = 0x5a827999;
+
+        a = Rotl(a + G(b, c, d) + x + C2, s);
+    }
+
+    static void HH(uint32_t & a, uint32_t b, uint32_t c, uint32_t d,
+                   uint32_t x, int_fast8_t s)
+    {
+        constexpr uint32_t C3 = 0x6ed9eba1;
+
+        a = Rotl(a + H(b, c, d) + x + C3, s);
+    }
+};
+
+struct Hash
+{
+    std::string ToHexString() const
+    {
+        char buf[33];
+
+        std::sprintf(buf,
+                     "%02x%02x%02x%02x%02x%02x%02x%02x"
+                     "%02x%02x%02x%02x%02x%02x%02x%02x",
+                      RawDigest[ 0], RawDigest[ 1], RawDigest[ 2], RawDigest[ 3],
+                      RawDigest[ 4], RawDigest[ 5], RawDigest[ 6], RawDigest[ 7],
+                      RawDigest[ 8], RawDigest[ 9], RawDigest[10], RawDigest[11],
+                      RawDigest[12], RawDigest[13], RawDigest[14], RawDigest[15]);
+
+        return std::string(buf, buf + 32);
+    }
+
+    std::array<uint8_t, 16> RawDigest;
+};
+
+class Hasher
+{
+public:
+    Hasher()
+        : BlockSize_(0)
+        , Word_(0)
+        , WordBytesPacked_(0)
+        , MessageSizeBytes_(0)
+    {
+        Block_.fill(0);
+    }
+
+    template<typename InputIt>
+    void Update(InputIt begin, InputIt end)
+    {
+        MessageSizeBytes_ += UpdateImpl(begin, end);
+    }
+
+    Hash Finish()
+    {
+        uint64_t messageSizeBytesMod64 = MessageSizeBytes_ % 64;
+
+        int_fast8_t paddingNeededBytes;
+
+        if (messageSizeBytesMod64 < 56)
+        {
+            paddingNeededBytes = 56 - messageSizeBytesMod64;
+        }
+        else if (messageSizeBytesMod64 > 56)
+        {
+            paddingNeededBytes = 120 - messageSizeBytesMod64;
+        }
+        else
+        {
+            paddingNeededBytes = 64;
+        }
+
+        UpdateImpl(PAD_, PAD_ + paddingNeededBytes);
+
+        // TODO: does not comply with the standard:
+        // "In the unlikely event that b is greater than pow(2, 64),
+        // then only the low-order 64 bits of b are used."
+        const uint64_t messageSizeBits = MessageSizeBytes_ * 8;
+
+        uint8_t extracted[] =
+        {
+            static_cast<uint8_t>((messageSizeBits >>  0) & 0xFF),
+            static_cast<uint8_t>((messageSizeBits >>  8) & 0xFF),
+            static_cast<uint8_t>((messageSizeBits >> 16) & 0xFF),
+            static_cast<uint8_t>((messageSizeBits >> 24) & 0xFF),
+            static_cast<uint8_t>((messageSizeBits >> 32) & 0xFF),
+            static_cast<uint8_t>((messageSizeBits >> 40) & 0xFF),
+            static_cast<uint8_t>((messageSizeBits >> 48) & 0xFF),
+            static_cast<uint8_t>((messageSizeBits >> 56) & 0xFF),
+        };
+
+        static_assert(std::size(extracted) == 8);
+
+        UpdateImpl(extracted, extracted + std::size(extracted));
+
+        Hash result;
+
+        int_fast8_t i = 0;
+        for (int_fast8_t reg = 0; reg < 4; ++reg)
+        {
+            for (int_fast8_t shift = 0; shift < 32; shift += 8)
+            {
+                result.RawDigest[i++] = (Buffer_.Regs_[reg] >> shift) & 0xFF;
+            }
+        }
+
+        return result;
+    }
+
+private:
+    static constexpr uint8_t PAD_[] =
+    {
+        0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
+    };
+
+    static_assert(std::size(PAD_) == 64);
+
+    Buffer Buffer_;
+
+    Block Block_;
+    int_fast8_t BlockSize_;
+
+    uint32_t Word_;
+    int_fast8_t WordBytesPacked_;
+
+    uint64_t MessageSizeBytes_;
+
+    template<typename InputIt>
+    uint64_t UpdateImpl(InputIt begin, InputIt end)
+    {
+        uint64_t written = 0;
+
+        for (InputIt it = begin; it != end; ++it, ++written)
+        {
+            Word_ |= (static_cast<uint32_t>(*it) << (WordBytesPacked_ * 8));
+            ++WordBytesPacked_;
+
+            if (WordBytesPacked_ == 4)
+            {
+                Block_[BlockSize_++] = Word_;
+                WordBytesPacked_ = 0;
+                Word_ = 0;
+
+                if (BlockSize_ == 16)
+                {
+                    Algorithm::UpdateBuffer(Buffer_, Block_);
+                    BlockSize_ = 0;
+                }
+            }
+        }
+
+        return written;
+    }
+};
+
+} // namespace Chaos
+
+#endif
diff --git a/ChaosTests/CMakeLists.txt b/ChaosTests/CMakeLists.txt
new file mode 100644
index 0000000..0891ef0
--- /dev/null
+++ b/ChaosTests/CMakeLists.txt
@@ -0,0 +1,20 @@
+include(FetchContent)
+
+FetchContent_Declare(
+        googletest
+        GIT_REPOSITORY https://github.com/google/googletest.git
+        GIT_TAG v1.17.0
+)
+
+set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+FetchContent_MakeAvailable(googletest)
+
+add_executable(ChaosTests Md4HasherTests.cpp)
+target_link_libraries(ChaosTests gtest gtest_main)
+target_include_directories(ChaosTests PRIVATE
+    $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/Chaos>
+)
+
+include(GoogleTest)
+
+gtest_discover_tests(ChaosTests)
diff --git a/ChaosTests/MD4HasherTests.cpp b/ChaosTests/MD4HasherTests.cpp
new file mode 100644
index 0000000..cb72ab6
--- /dev/null
+++ b/ChaosTests/MD4HasherTests.cpp
@@ -0,0 +1,177 @@
+#include <gtest/gtest.h>
+
+#include "Md4.hpp"
+
+using namespace Chaos;
+
+TEST(Md4Tests, RFCTest)
+{
+    struct Helper
+    {
+        std::string operator()(const char * in) const
+        {
+            Md4::Hasher hasher;
+            hasher.Update(in, in + strlen(in));
+            return hasher.Finish().ToHexString();
+        }
+    };
+
+    Helper hash;
+
+    ASSERT_EQ("31d6cfe0d16ae931b73c59d7e0c089c0", hash(""));
+    ASSERT_EQ("bde52cb31de33e46245e05fbdbd6fb24", hash("a"));
+    ASSERT_EQ("a448017aaf21d8525fc10ae87aa6729d", hash("abc"));
+    ASSERT_EQ("d9130a8164549fe818874806e1c7014b", hash("message digest"));
+    ASSERT_EQ("d79e1c308aa5bbcdeea8ed63df412da9", hash("abcdefghijklmnopqrstuvwxyz"));
+    ASSERT_EQ("043f8582f241db351ce627e153e7f0e4", hash("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"));
+    ASSERT_EQ("e33b4ddc9c38f2199c3e7b164fcc0536", hash("12345678901234567890123456789012345678901234567890123456789012345678901234567890"));
+}
+
+TEST(Md4Tests, PartialUpdateTest)
+{
+    {
+        // "a"
+        Md4::Hasher hasher;
+
+        {
+            const char * in = "a";
+            hasher.Update(in, in + strlen(in));
+        }
+
+        {
+            const char * in = "";
+            hasher.Update(in, in + strlen(in));
+        }
+
+        ASSERT_EQ("bde52cb31de33e46245e05fbdbd6fb24", hasher.Finish().ToHexString());
+    }
+
+    {
+        // "abc"
+        Md4::Hasher hasher;
+
+        {
+            const char * in = "ab";
+            hasher.Update(in, in + strlen(in));
+        }
+
+        {
+            const char * in = "c";
+            hasher.Update(in, in + strlen(in));
+        }
+
+        ASSERT_EQ("a448017aaf21d8525fc10ae87aa6729d", hasher.Finish().ToHexString());
+    }
+
+    {
+        // "message digest"
+        Md4::Hasher hasher;
+
+        {
+            const char * in = "me";
+            hasher.Update(in, in + strlen(in));
+        }
+
+        {
+            const char * in = "ssage ";
+            hasher.Update(in, in + strlen(in));
+        }
+
+        {
+            const char * in = "diges";
+            hasher.Update(in, in + strlen(in));
+        }
+
+        {
+            const char * in = "t";
+            hasher.Update(in, in + strlen(in));
+        }
+
+        ASSERT_EQ("d9130a8164549fe818874806e1c7014b", hasher.Finish().ToHexString());
+    }
+
+    {
+        // "12345678901234567890123456789012345678901234567890123456789012345678901234567890"
+        Md4::Hasher hasher;
+
+        {
+            const char * in = "12345678901234567890";
+            hasher.Update(in, in + strlen(in));
+        }
+
+        {
+            const char * in = "12345678901234567890";
+            hasher.Update(in, in + strlen(in));
+        }
+
+        {
+            const char * in = "12345678901234567890";
+            hasher.Update(in, in + strlen(in));
+        }
+
+        {
+            const char * in = "12345678901234567890";
+            hasher.Update(in, in + strlen(in));
+        }
+
+        ASSERT_EQ("e33b4ddc9c38f2199c3e7b164fcc0536", hasher.Finish().ToHexString());
+    }
+}
+
+TEST(Md4Tests, LongInputTest)
+{
+    struct Helper
+    {
+        std::string operator()(const char * in) const
+        {
+            Md4::Hasher hasher;
+            hasher.Update(in, in + strlen(in));
+            return hasher.Finish().ToHexString();
+        }
+    };
+
+    Helper hash;
+
+    // 2500 zeros ('0').
+    ASSERT_EQ("20de61fc6dc2134f7a7bbcf43fd923e6", hash(std::string(2500, '0').c_str()));
+    // 1000 'a' followed by 1000 'b'.
+    ASSERT_EQ("cbabb47a57b10e0028ec7f519c66f229", hash((std::string(1000, 'a') +
+                                                        std::string(1000, 'b')).c_str()));
+}
+
+TEST(Md4Tests, LongInputPartialUpdateTest)
+{
+    {
+        // 2500 zeros ('0').
+        Md4::Hasher hasher;
+
+        std::string in(750, '0');
+
+        hasher.Update(in.begin(), in.begin() + 250);
+        hasher.Update(in.begin(), in.begin() + 500);
+        hasher.Update(in.begin(), in.begin() + 500);
+        hasher.Update(in.begin(), in.begin() + 750);
+        hasher.Update(in.begin(), in.begin() + 333);
+        hasher.Update(in.begin(), in.begin() + 167);
+
+        ASSERT_EQ("20de61fc6dc2134f7a7bbcf43fd923e6", hasher.Finish().ToHexString());
+    }
+
+    {
+        // 1000 'a' followed by 1000 'b'.
+        Md4::Hasher hasher;
+
+        std::string inA(1000, 'a');
+        std::string inB(1000, 'b');
+
+        hasher.Update(inA.begin(), inA.begin() + 100);
+        hasher.Update(inA.begin(), inA.begin() + 255);
+        hasher.Update(inA.begin(), inA.begin() + 645);
+
+        hasher.Update(inB.begin(), inB.begin() + 33);
+        hasher.Update(inB.begin(), inB.begin() + 701);
+        hasher.Update(inB.begin(), inB.begin() + 266);
+
+        ASSERT_EQ("cbabb47a57b10e0028ec7f519c66f229", hasher.Finish().ToHexString());
+    }
+}