diff --git a/Doc/library/hashlib.rst b/Doc/library/hashlib.rst
index e3b5ebb..e33ef50 100644
--- a/Doc/library/hashlib.rst
+++ b/Doc/library/hashlib.rst
@@ -88,6 +88,24 @@
 
    .. versionadded:: 2.7
 
+.. data:: algorithms_guaranteed
+
+   A set containing the names of the hash algorithms guaranteed to be supported
+   by this module on all platforms.
+
+   .. versionadded:: 2.7.9
+
+.. data:: algorithms_available
+
+   A set containing the names of the hash algorithms that are available in the
+   running Python interpreter.  These names will be recognized when passed to
+   :func:`new`.  :attr:`algorithms_guaranteed` will always be a subset.  The
+   same algorithm may appear multiple times in this set under different names
+   (thanks to OpenSSL).
+
+   .. versionadded:: 2.7.9
+
+
 The following values are provided as constant attributes of the hash objects
 returned by the constructors:
 
diff --git a/Lib/hashlib.py b/Lib/hashlib.py
index 6d69ad2..4a411bc 100644
--- a/Lib/hashlib.py
+++ b/Lib/hashlib.py
@@ -15,8 +15,9 @@
 
 md5(), sha1(), sha224(), sha256(), sha384(), and sha512()
 
-More algorithms may be available on your platform but the above are
-guaranteed to exist.
+More algorithms may be available on your platform but the above are guaranteed
+to exist.  See the algorithms_guaranteed and algorithms_available attributes
+to find out what algorithm names can be passed to new().
 
 NOTE: If you want the adler32 or crc32 hash functions they are available in
 the zlib module.
@@ -58,9 +59,14 @@
 # always available algorithm is added.
 __always_supported = ('md5', 'sha1', 'sha224', 'sha256', 'sha384', 'sha512')
 
+algorithms_guaranteed = set(__always_supported)
+algorithms_available = set(__always_supported)
+
 algorithms = __always_supported
 
-__all__ = __always_supported + ('new', 'algorithms', 'pbkdf2_hmac')
+__all__ = __always_supported + ('new', 'algorithms_guaranteed',
+                                'algorithms_available', 'algorithms',
+                                'pbkdf2_hmac')
 
 
 def __get_builtin_constructor(name):
@@ -128,6 +134,8 @@
     import _hashlib
     new = __hash_new
     __get_hash = __get_openssl_constructor
+    algorithms_available = algorithms_available.union(
+        _hashlib.openssl_md_meth_names)
 except ImportError:
     new = __py_new
     __get_hash = __get_builtin_constructor
diff --git a/Lib/test/test_hashlib.py b/Lib/test/test_hashlib.py
index 3fc172f..76be461 100644
--- a/Lib/test/test_hashlib.py
+++ b/Lib/test/test_hashlib.py
@@ -109,6 +109,15 @@
             tuple([_algo for _algo in self.supported_hash_names if
                                                 _algo.islower()]))
 
+    def test_algorithms_guaranteed(self):
+        self.assertEqual(hashlib.algorithms_guaranteed,
+            set(_algo for _algo in self.supported_hash_names
+                  if _algo.islower()))
+
+    def test_algorithms_available(self):
+        self.assertTrue(set(hashlib.algorithms_guaranteed).
+                            issubset(hashlib.algorithms_available))
+
     def test_unknown_hash(self):
         self.assertRaises(ValueError, hashlib.new, 'spam spam spam spam spam')
         self.assertRaises(TypeError, hashlib.new, 1)
diff --git a/Misc/NEWS b/Misc/NEWS
index 2ad993b..88af5a4 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -19,6 +19,9 @@
 Library
 -------
 
+- Issue #21307: As part of PEP 466, backport hashlib.algorithms_guaranteed and
+  hashlib.algorithms_available.
+
 - Issue #22259: Fix segfault when attempting to fopen a file descriptor
   corresponding to a directory.
 
diff --git a/Modules/_hashopenssl.c b/Modules/_hashopenssl.c
index 7116eeb..df53e96 100644
--- a/Modules/_hashopenssl.c
+++ b/Modules/_hashopenssl.c
@@ -712,6 +712,61 @@
 
 #endif
 
+/* State for our callback function so that it can accumulate a result. */
+typedef struct _internal_name_mapper_state {
+    PyObject *set;
+    int error;
+} _InternalNameMapperState;
+
+
+/* A callback function to pass to OpenSSL's OBJ_NAME_do_all(...) */
+static void
+_openssl_hash_name_mapper(const OBJ_NAME *openssl_obj_name, void *arg)
+{
+    _InternalNameMapperState *state = (_InternalNameMapperState *)arg;
+    PyObject *py_name;
+
+    assert(state != NULL);
+    if (openssl_obj_name == NULL)
+        return;
+    /* Ignore aliased names, they pollute the list and OpenSSL appears to
+     * have a its own definition of alias as the resulting list still
+     * contains duplicate and alternate names for several algorithms.     */
+    if (openssl_obj_name->alias)
+        return;
+
+    py_name = PyString_FromString(openssl_obj_name->name);
+    if (py_name == NULL) {
+        state->error = 1;
+    } else {
+        if (PySet_Add(state->set, py_name) != 0) {
+            state->error = 1;
+        }
+        Py_DECREF(py_name);
+    }
+}
+
+
+/* Ask OpenSSL for a list of supported ciphers, filling in a Python set. */
+static PyObject*
+generate_hash_name_list(void)
+{
+    _InternalNameMapperState state;
+    state.set = PyFrozenSet_New(NULL);
+    if (state.set == NULL)
+        return NULL;
+    state.error = 0;
+
+    OBJ_NAME_do_all(OBJ_NAME_TYPE_MD_METH, &_openssl_hash_name_mapper, &state);
+
+    if (state.error) {
+        Py_DECREF(state.set);
+        return NULL;
+    }
+    return state.set;
+}
+
+
 /*
  *  This macro generates constructor function definitions for specific
  *  hash algorithms.  These constructors are much faster than calling
@@ -792,9 +847,10 @@
 PyMODINIT_FUNC
 init_hashlib(void)
 {
-    PyObject *m;
+    PyObject *m, *openssl_md_meth_names;
 
     OpenSSL_add_all_digests();
+    ERR_load_crypto_strings();
 
     /* TODO build EVP_functions openssl_* entries dynamically based
      * on what hashes are supported rather than listing many
@@ -809,6 +865,14 @@
     if (m == NULL)
         return;
 
+    openssl_md_meth_names = generate_hash_name_list();
+    if (openssl_md_meth_names == NULL) {
+        return;
+    }
+    if (PyModule_AddObject(m, "openssl_md_meth_names", openssl_md_meth_names)) {
+        return;
+    }
+
 #if HASH_OBJ_CONSTRUCTOR
     Py_INCREF(&EVPtype);
     PyModule_AddObject(m, "HASH", (PyObject *)&EVPtype);