diff --git a/Lib/hmac.py b/Lib/hmac.py
index 1911689..6f2ae2e 100644
--- a/Lib/hmac.py
+++ b/Lib/hmac.py
@@ -3,6 +3,8 @@
 Implements the HMAC algorithm as described by RFC 2104.
 """
 
+import warnings as _warnings
+
 trans_5C = bytes((x ^ 0x5C) for x in range(256))
 trans_36 = bytes((x ^ 0x36) for x in range(256))
 
@@ -16,7 +18,7 @@
 _secret_backdoor_key = []
 
 class HMAC:
-    """RFC2104 HMAC class.
+    """RFC 2104 HMAC class.  Also complies with RFC 4231.
 
     This supports the API for Cryptographic Hash Functions (PEP 247).
     """
@@ -52,7 +54,21 @@
         self.inner = self.digest_cons()
         self.digest_size = self.inner.digest_size
 
-        blocksize = self.blocksize
+        if hasattr(self.inner, 'block_size'):
+            blocksize = self.inner.block_size
+            if blocksize < 16:
+                # Very low blocksize, most likely a legacy value like
+                # Lib/sha.py and Lib/md5.py have.
+                _warnings.warn('block_size of %d seems too small; using our '
+                               'default of %d.' % (blocksize, self.blocksize),
+                               RuntimeWarning, 2)
+                blocksize = self.blocksize
+        else:
+            _warnings.warn('No block_size attribute on given digest object; '
+                           'Assuming %d.' % (self.blocksize),
+                           RuntimeWarning, 2)
+            blocksize = self.blocksize
+
         if len(key) > blocksize:
             key = self.digest_cons(key).digest()
 
diff --git a/Lib/test/test_hmac.py b/Lib/test/test_hmac.py
index c654e87..d8c03db 100644
--- a/Lib/test/test_hmac.py
+++ b/Lib/test/test_hmac.py
@@ -1,6 +1,7 @@
 import hmac
-from hashlib import sha1
+import hashlib
 import unittest
+import warnings
 from test import test_support
 
 class TestVectorsTestCase(unittest.TestCase):
@@ -43,7 +44,7 @@
 
     def test_sha_vectors(self):
         def shatest(key, data, digest):
-            h = hmac.HMAC(key, data, digestmod=sha1)
+            h = hmac.HMAC(key, data, digestmod=hashlib.sha1)
             self.assertEqual(h.hexdigest().upper(), digest.upper())
 
         shatest(b"\x0b" * 20,
@@ -75,6 +76,161 @@
                  b"and Larger Than One Block-Size Data"),
                 "e8e99d0f45237d786d6bbaa7965c7808bbff1a91")
 
+    def _rfc4231_test_cases(self, hashfunc):
+        def hmactest(key, data, hexdigests):
+            h = hmac.HMAC(key, data, digestmod=hashfunc)
+            self.assertEqual(h.hexdigest().lower(), hexdigests[hashfunc])
+
+        # 4.2.  Test Case 1
+        hmactest(key = b'\x0b'*20,
+                 data = b'Hi There',
+                 hexdigests = {
+                   hashlib.sha224: '896fb1128abbdf196832107cd49df33f'
+                                   '47b4b1169912ba4f53684b22',
+                   hashlib.sha256: 'b0344c61d8db38535ca8afceaf0bf12b'
+                                   '881dc200c9833da726e9376c2e32cff7',
+                   hashlib.sha384: 'afd03944d84895626b0825f4ab46907f'
+                                   '15f9dadbe4101ec682aa034c7cebc59c'
+                                   'faea9ea9076ede7f4af152e8b2fa9cb6',
+                   hashlib.sha512: '87aa7cdea5ef619d4ff0b4241a1d6cb0'
+                                   '2379f4e2ce4ec2787ad0b30545e17cde'
+                                   'daa833b7d6b8a702038b274eaea3f4e4'
+                                   'be9d914eeb61f1702e696c203a126854',
+                 })
+
+        # 4.3.  Test Case 2
+        hmactest(key = b'Jefe',
+                 data = b'what do ya want for nothing?',
+                 hexdigests = {
+                   hashlib.sha224: 'a30e01098bc6dbbf45690f3a7e9e6d0f'
+                                   '8bbea2a39e6148008fd05e44',
+                   hashlib.sha256: '5bdcc146bf60754e6a042426089575c7'
+                                   '5a003f089d2739839dec58b964ec3843',
+                   hashlib.sha384: 'af45d2e376484031617f78d2b58a6b1b'
+                                   '9c7ef464f5a01b47e42ec3736322445e'
+                                   '8e2240ca5e69e2c78b3239ecfab21649',
+                   hashlib.sha512: '164b7a7bfcf819e2e395fbe73b56e0a3'
+                                   '87bd64222e831fd610270cd7ea250554'
+                                   '9758bf75c05a994a6d034f65f8f0e6fd'
+                                   'caeab1a34d4a6b4b636e070a38bce737',
+                 })
+
+        # 4.4.  Test Case 3
+        hmactest(key = b'\xaa'*20,
+                 data = b'\xdd'*50,
+                 hexdigests = {
+                   hashlib.sha224: '7fb3cb3588c6c1f6ffa9694d7d6ad264'
+                                   '9365b0c1f65d69d1ec8333ea',
+                   hashlib.sha256: '773ea91e36800e46854db8ebd09181a7'
+                                   '2959098b3ef8c122d9635514ced565fe',
+                   hashlib.sha384: '88062608d3e6ad8a0aa2ace014c8a86f'
+                                   '0aa635d947ac9febe83ef4e55966144b'
+                                   '2a5ab39dc13814b94e3ab6e101a34f27',
+                   hashlib.sha512: 'fa73b0089d56a284efb0f0756c890be9'
+                                   'b1b5dbdd8ee81a3655f83e33b2279d39'
+                                   'bf3e848279a722c806b485a47e67c807'
+                                   'b946a337bee8942674278859e13292fb',
+                 })
+
+        # 4.5.  Test Case 4
+        hmactest(key = bytes(x for x in range(0x01, 0x19+1)),
+                 data = b'\xcd'*50,
+                 hexdigests = {
+                   hashlib.sha224: '6c11506874013cac6a2abc1bb382627c'
+                                   'ec6a90d86efc012de7afec5a',
+                   hashlib.sha256: '82558a389a443c0ea4cc819899f2083a'
+                                   '85f0faa3e578f8077a2e3ff46729665b',
+                   hashlib.sha384: '3e8a69b7783c25851933ab6290af6ca7'
+                                   '7a9981480850009cc5577c6e1f573b4e'
+                                   '6801dd23c4a7d679ccf8a386c674cffb',
+                   hashlib.sha512: 'b0ba465637458c6990e5a8c5f61d4af7'
+                                   'e576d97ff94b872de76f8050361ee3db'
+                                   'a91ca5c11aa25eb4d679275cc5788063'
+                                   'a5f19741120c4f2de2adebeb10a298dd',
+                 })
+
+        # 4.7.  Test Case 6
+        hmactest(key = b'\xaa'*131,
+                 data = b'Test Using Larger Than Block-Siz'
+                        b'e Key - Hash Key First',
+                 hexdigests = {
+                   hashlib.sha224: '95e9a0db962095adaebe9b2d6f0dbce2'
+                                   'd499f112f2d2b7273fa6870e',
+                   hashlib.sha256: '60e431591ee0b67f0d8a26aacbf5b77f'
+                                   '8e0bc6213728c5140546040f0ee37f54',
+                   hashlib.sha384: '4ece084485813e9088d2c63a041bc5b4'
+                                   '4f9ef1012a2b588f3cd11f05033ac4c6'
+                                   '0c2ef6ab4030fe8296248df163f44952',
+                   hashlib.sha512: '80b24263c7c1a3ebb71493c1dd7be8b4'
+                                   '9b46d1f41b4aeec1121b013783f8f352'
+                                   '6b56d037e05f2598bd0fd2215d6a1e52'
+                                   '95e64f73f63f0aec8b915a985d786598',
+                 })
+
+        # 4.8.  Test Case 7
+        hmactest(key = b'\xaa'*131,
+                 data = b'This is a test using a larger th'
+                        b'an block-size key and a larger t'
+                        b'han block-size data. The key nee'
+                        b'ds to be hashed before being use'
+                        b'd by the HMAC algorithm.',
+                 hexdigests = {
+                   hashlib.sha224: '3a854166ac5d9f023f54d517d0b39dbd'
+                                   '946770db9c2b95c9f6f565d1',
+                   hashlib.sha256: '9b09ffa71b942fcb27635fbcd5b0e944'
+                                   'bfdc63644f0713938a7f51535c3a35e2',
+                   hashlib.sha384: '6617178e941f020d351e2f254e8fd32c'
+                                   '602420feb0b8fb9adccebb82461e99c5'
+                                   'a678cc31e799176d3860e6110c46523e',
+                   hashlib.sha512: 'e37b6a775dc87dbaa4dfa9f96e5e3ffd'
+                                   'debd71f8867289865df5a32d20cdc944'
+                                   'b6022cac3c4982b10d5eeb55c3e4de15'
+                                   '134676fb6de0446065c97440fa8c6a58',
+                 })
+
+    def test_sha224_rfc4231(self):
+        self._rfc4231_test_cases(hashlib.sha224)
+
+    def test_sha256_rfc4231(self):
+        self._rfc4231_test_cases(hashlib.sha256)
+
+    def test_sha384_rfc4231(self):
+        self._rfc4231_test_cases(hashlib.sha384)
+
+    def test_sha512_rfc4231(self):
+        self._rfc4231_test_cases(hashlib.sha512)
+
+    def test_legacy_block_size_warnings(self):
+        class MockCrazyHash(object):
+            """Ain't no block_size attribute here."""
+            def __init__(self, *args):
+                self._x = hashlib.sha1(*args)
+                self.digest_size = self._x.digest_size
+            def update(self, v):
+                self._x.update(v)
+            def digest(self):
+                return self._x.digest()
+
+        warnings.simplefilter('error', RuntimeWarning)
+        try:
+            try:
+                hmac.HMAC(b'a', b'b', digestmod=MockCrazyHash)
+            except RuntimeWarning:
+                pass
+            else:
+                self.fail('Expected warning about missing block_size')
+
+            MockCrazyHash.block_size = 1
+            try:
+                hmac.HMAC(b'a', b'b', digestmod=MockCrazyHash)
+            except RuntimeWarning:
+                pass
+            else:
+                self.fail('Expected warning about small block_size')
+        finally:
+            warnings.resetwarnings()
+
+
 
 class ConstructorTestCase(unittest.TestCase):
 
@@ -95,9 +251,8 @@
 
     def test_withmodule(self):
         # Constructor call with text and digest module.
-        from hashlib import sha1
         try:
-            h = hmac.HMAC(b"key", b"", sha1)
+            h = hmac.HMAC(b"key", b"", hashlib.sha1)
         except:
             self.fail("Constructor call with hashlib.sha1 raised exception.")
 
@@ -106,7 +261,6 @@
     def test_default_is_md5(self):
         # Testing if HMAC defaults to MD5 algorithm.
         # NOTE: this whitebox test depends on the hmac class internals
-        import hashlib
         h = hmac.HMAC(b"key")
         self.assertEqual(h.digest_cons, hashlib.md5)