diff --git a/include/boost/hash2/sha2.hpp b/include/boost/hash2/sha2.hpp
index e5b97db..b014ee9 100644
--- a/include/boost/hash2/sha2.hpp
+++ b/include/boost/hash2/sha2.hpp
@@ -12,9 +12,7 @@
 #include <boost/hash2/detail/rot.hpp>
 #include <boost/hash2/detail/write.hpp>
 #include <boost/assert.hpp>
-#include <boost/cstdint.hpp>
 #include <array>
-#include <cstddef>
 #include <cstdint>
 #include <cstring>
 
@@ -23,10 +21,11 @@ namespace boost
 namespace hash2
 {
 
-class sha2_256
+namespace detail
 {
-private:
 
+struct sha2_256_base
+{
     std::uint32_t state_[ 8 ];
 
     static const int N = 64;
@@ -36,18 +35,8 @@ class sha2_256
 
     std::uint64_t n_;
 
-private:
-
-    void init()
+    sha2_256_base(): m_( 0 ), n_( 0 )
     {
-        state_[ 0 ] = 0x6a09e667;
-        state_[ 1 ] = 0xbb67ae85;
-        state_[ 2 ] = 0x3c6ef372;
-        state_[ 3 ] = 0xa54ff53a;
-        state_[ 4 ] = 0x510e527f;
-        state_[ 5 ] = 0x9b05688c;
-        state_[ 6 ] = 0x1f83d9ab;
-        state_[ 7 ] = 0x5be0cd19;
     }
 
     static std::uint32_t Sigma0( std::uint32_t x ) noexcept
@@ -140,14 +129,6 @@ class sha2_256
         state_[7] += h;
     }
 
-public:
-    using result_type = std::array<unsigned char, 32>;
-
-    sha2_256(): m_( 0 ), n_( 0 )
-    {
-        init();
-    }
-
     void update( void const * pv, std::size_t n )
     {
         unsigned char const* p = static_cast<unsigned char const*>( pv );
@@ -201,6 +182,35 @@ class sha2_256
 
         BOOST_ASSERT( m_ == n_ % N );
     }
+};
+
+} // namespace detail
+
+class sha2_256 : detail::sha2_256_base
+{
+private:
+
+    void init()
+    {
+        state_[ 0 ] = 0x6a09e667;
+        state_[ 1 ] = 0xbb67ae85;
+        state_[ 2 ] = 0x3c6ef372;
+        state_[ 3 ] = 0xa54ff53a;
+        state_[ 4 ] = 0x510e527f;
+        state_[ 5 ] = 0x9b05688c;
+        state_[ 6 ] = 0x1f83d9ab;
+        state_[ 7 ] = 0x5be0cd19;
+    }
+
+public:
+    using result_type = std::array<unsigned char, 32>;
+
+    sha2_256()
+    {
+        init();
+    }
+
+    using detail::sha2_256_base::update;
 
     result_type result()
     {
@@ -223,6 +233,53 @@ class sha2_256
     }
 };
 
+class sha2_224 : detail::sha2_256_base
+{
+private:
+
+    void init()
+    {
+        state_[ 0 ] = 0xc1059ed8;
+        state_[ 1 ] = 0x367cd507;
+        state_[ 2 ] = 0x3070dd17;
+        state_[ 3 ] = 0xf70e5939;
+        state_[ 4 ] = 0xffc00b31;
+        state_[ 5 ] = 0x68581511;
+        state_[ 6 ] = 0x64f98fa7;
+        state_[ 7 ] = 0xbefa4fa4;
+    }
+
+public:
+    using result_type = std::array<unsigned char, 28>;
+
+    sha2_224()
+    {
+        init();
+    }
+
+    using detail::sha2_256_base::update;
+
+    result_type result()
+    {
+        unsigned char bits[ 8 ];
+        detail::write64be( bits, n_ * 8 );
+
+        std::size_t k = m_ < 56 ? 56 - m_ : 64 + 56 - m_;
+        unsigned char padding[ 64 ] = { 0x80 };
+
+        update( padding, k );
+        update( bits, 8 );
+        BOOST_ASSERT( m_ == 0 );
+
+        result_type digest;
+        for( int i = 0; i < 7; ++i ) {
+            detail::write32be( &digest[ i * 4 ], state_[ i ] );
+        }
+
+        return digest;
+    }
+};
+
 } // namespace hash2
 } // namespace boost
 
diff --git a/test/sha2.cpp b/test/sha2.cpp
index 665e958..cc5134f 100644
--- a/test/sha2.cpp
+++ b/test/sha2.cpp
@@ -52,8 +52,10 @@ template<class H> std::string digest( std::vector<char> const & v )
 }
 
 using boost::hash2::sha2_256;
+using boost::hash2::sha2_224;
 
-int main()
+static
+void sha_256()
 {
     // https://en.wikipedia.org/wiki/SHA-2#Test_vectors
 
@@ -106,6 +108,55 @@ int main()
         std::vector<char> buf(1000000, 0x00);
         BOOST_TEST_EQ( digest<sha2_256>( buf ), std::string( "d29751f2649b32ff572b5e0a9f541ea660a50f94ff0beedfb0b692b924cc8025" ) );
     }
+}
+
+static
+void sha_224()
+{
+    // https://en.wikipedia.org/wiki/SHA-2#Test_vectors
+
+    BOOST_TEST_EQ( digest<sha2_224>( "" ), std::string( "d14a028c2a3a2bc9476102bb288234c415a2b01f828ea62ac5b3e42f" ) );
+
+    // https://csrc.nist.gov/CSRC/media/Projects/Cryptographic-Standards-and-Guidelines/documents/examples/SHA224.pdf
+    BOOST_TEST_EQ( digest<sha2_224>( "abc" ), std::string( "23097d223405d8228642a477bda255b32aadbce4bda0b3f7e36c9da7" ) );
+    BOOST_TEST_EQ( digest<sha2_224>( "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq" ), std::string( "75388b16512776cc5dba5da1fd890150b0c6455cb4f58b1952522525" ) );
+
+    // https://csrc.nist.gov/CSRC/media/Projects/Cryptographic-Standards-and-Guidelines/documents/examples/SHA2_Additional.pdf
+    BOOST_TEST_EQ( digest<sha2_224>( "\xff" ), std::string( "e33f9d75e6ae1369dbabf81b96b4591ae46bba30b591a6b6c62542b5" ) );
+    BOOST_TEST_EQ( digest<sha2_224>( "\xe5\xe0\x99\x24" ), std::string( "fd19e74690d291467ce59f077df311638f1c3a46e510d0e49a67062d" ) );
+
+    {
+        char buf[ 56 ] = { 0 };
+        BOOST_TEST_EQ( digest<sha2_224>( buf ), std::string( "5c3e25b69d0ea26f260cfae87e23759e1eca9d1ecc9fbf3c62266804" ) );
+    }
+
+    {
+        char buf[ 1000 ] = {};
+        for( auto& c : buf ) c = 'Q';
+        BOOST_TEST_EQ( digest<sha2_224>( buf ), std::string( "3706197f66890a41779dc8791670522e136fafa24874685715bd0a8a" ) );
+    }
 
+    {
+        char buf[ 1000 ] = {};
+        for( auto& c : buf ) c = 'A';
+        BOOST_TEST_EQ( digest<sha2_224>( buf ), std::string( "a8d0c66b5c6fdfd836eb3c6d04d32dfe66c3b1f168b488bf4c9c66ce" ) );
+    }
+
+    {
+        char buf[ 1005 ] = {};
+        for( auto& c : buf ) c = '\x99';
+        BOOST_TEST_EQ( digest<sha2_224>( buf ), std::string( "cb00ecd03788bf6c0908401e0eb053ac61f35e7e20a2cfd7bd96d640" ) );
+    }
+
+    {
+        std::vector<char> buf(1000000, 0x00);
+        BOOST_TEST_EQ( digest<sha2_224>( buf ), std::string( "3a5d74b68f14f3a4b2be9289b8d370672d0b3d2f53bc303c59032df3" ) );
+    }
+}
+
+int main()
+{
+    sha_256();
+    sha_224();
     return boost::report_errors();
 }