diff --git a/lib/hash.js b/lib/hash.js
index f02d570..3044074 100644
--- a/lib/hash.js
+++ b/lib/hash.js
@@ -10,6 +10,8 @@ Hash.sha256 = function(buf) {
   return new Buffer(hash);
 };
 
+Hash.sha256.blocksize = 512;
+
 Hash.sha256sha256 = function(buf) {
   try {
     return Hash.sha256(Hash.sha256(buf));
@@ -39,3 +41,49 @@ Hash.sha512 = function(buf) {
   var hash = sha512(buf);
   return new Buffer(hash);
 };
+
+Hash.sha512.blocksize = 1024;
+
+Hash.hmac = function(hashf, data, key) {
+  if (!Buffer.isBuffer(data) || !Buffer.isBuffer(key))
+    throw new Error('hash: data and key must be buffers');
+
+  //http://en.wikipedia.org/wiki/Hash-based_message_authentication_code
+  //http://tools.ietf.org/html/rfc4868#section-2
+  if (!hashf.blocksize)
+    throw new Error('hash: Blocksize for hash function unknown');
+
+  var blocksize = hashf.blocksize/8;
+  
+  if (key.length > blocksize)
+    key = hashf(key);
+  else if (key < blocksize) {
+    var fill = new Buffer(blocksize);
+    fill.fill(0);
+    key.copy(fill);
+    key = fill;
+  }
+
+  var o_key = new Buffer(blocksize);
+  o_key.fill(0x5c);
+
+  var i_key = new Buffer(blocksize);
+  i_key.fill(0x36);
+
+  var o_key_pad = new Buffer(blocksize);
+  var i_key_pad = new Buffer(blocksize);
+  for (var i = 0; i < blocksize; i++) {
+    o_key_pad[i] = o_key[i] ^ key[i];
+    i_key_pad[i] = i_key[i] ^ key[i];
+  }
+
+  return hashf(Buffer.concat([o_key_pad, hashf(Buffer.concat([i_key_pad, data]))]));
+};
+
+Hash.sha256hmac = function(data, key) {
+  return Hash.hmac(Hash.sha256, data, key);
+};
+
+Hash.sha512hmac = function(data, key) {
+  return Hash.hmac(Hash.sha512, data, key);
+};
diff --git a/test/test.hash.js b/test/test.hash.js
index eea3517..2e8d010 100644
--- a/test/test.hash.js
+++ b/test/test.hash.js
@@ -20,6 +20,22 @@ describe('hash', function() {
 
   });
 
+  describe('#sha256hmac', function() {
+    
+    it('should compute this known empty test vector correctly', function() {
+      var key = new Buffer('');
+      var data = new Buffer('');
+      Hash.sha256hmac(data, key).toString('hex').should.equal('b613679a0814d9ec772f95d778c35fc5ff1697c493715653c6c712144292c5ad');
+    });
+
+    it('should compute this known non-empty test vector correctly', function() {
+      var key = new Buffer('key');
+      var data = new Buffer('The quick brown fox jumps over the lazy dog');
+      Hash.sha256hmac(data, key).toString('hex').should.equal('f7bc83f430538424b13298e6aa6fb143ef4d59a14946175997479dbc2d1a3cd8');
+    });
+
+  });
+
   describe('#sha256sha256', function() {
 
     it('should calculate the hash of this buffer correctly', function() {
@@ -80,4 +96,20 @@ describe('hash', function() {
 
   });
 
+  describe("#sha512hmac", function() {
+
+    it('should calculate this known empty test vector correctly', function() {
+      var hex = 'b936cee86c9f87aa5d3c6f2e84cb5a4239a5fe50480a6ec66b70ab5b1f4ac6730c6c515421b327ec1d69402e53dfb49ad7381eb067b338fd7b0cb22247225d47';
+      Hash.sha512hmac(new Buffer([]), new Buffer([])).toString('hex').should.equal(hex);
+    });
+
+    it('should calculate this known non-empty test vector correctly', function() {
+      var hex = 'c40bd7c15aa493b309c940e08a73ffbd28b2e4cb729eb94480d727e4df577b13cc403a78e6150d83595f3b17c4cc331f12ca5952691de3735a63c1d4c69a2bac';
+      var data = new Buffer("test1");
+      var key = new Buffer("test2");
+      Hash.sha512hmac(data, key).toString('hex').should.equal(hex);
+    });
+
+  });
+
 });