# HG changeset patch

# User Benjamin Peterson <benjamin@python.org>

# Date 1399849904 25200

# Node ID b40f1a00b13460cc089450028280c4e52dd24a64

# Parent  951775c68b1b7782750c213b0fce1f61d46b2f51

backport hmac.compare_digest to partially implement PEP 466 (closes #21306)

Backport from Alex Gaynor.

diff --git a/Doc/library/hmac.rst b/Doc/library/hmac.rst

--- a/Doc/library/hmac.rst

+++ b/Doc/library/hmac.rst

@@ -38,6 +38,13 @@ An HMAC object has the following methods

    This string will be the same length as the *digest_size* of the digest given to

    the constructor.  It may contain non-ASCII characters, including NUL bytes.

+   .. warning::

+

+      When comparing the output of :meth:`digest` to an externally-supplied

+      digest during a verification routine, it is recommended to use the

+      :func:`compare_digest` function instead of the ``==`` operator

+      to reduce the vulnerability to timing attacks.

+

 .. method:: HMAC.hexdigest()

@@ -45,6 +52,13 @@ An HMAC object has the following methods

    containing only hexadecimal digits.  This may be used to exchange the value

    safely in email or other non-binary environments.

+   .. warning::

+

+      When comparing the output of :meth:`hexdigest` to an externally-supplied

+      digest during a verification routine, it is recommended to use the

+      :func:`compare_digest` function instead of the ``==`` operator

+      to reduce the vulnerability to timing attacks.

+

 .. method:: HMAC.copy()

@@ -52,6 +66,25 @@ An HMAC object has the following methods

    compute the digests of strings that share a common initial substring.

+This module also provides the following helper function:

+

+.. function:: compare_digest(a, b)

+

+   Return ``a == b``.  This function uses an approach designed to prevent

+   timing analysis by avoiding content-based short circuiting behaviour,

+   making it appropriate for cryptography.  *a* and *b* must both be of the

+   same type: either :class:`unicode` or a :term:`bytes-like object`.

+

+   .. note::

+

+      If *a* and *b* are of different lengths, or if an error occurs,

+      a timing attack could theoretically reveal information about the

+      types and lengths of *a* and *b*--but not their values.

+

+

+   .. versionadded:: 2.7.7

+

+

 .. seealso::

    Module :mod:`hashlib`

diff --git a/Lib/hmac.py b/Lib/hmac.py

--- a/Lib/hmac.py

+++ b/Lib/hmac.py

@@ -5,6 +5,9 @@ Implements the HMAC algorithm as describ

 import warnings as _warnings

+from operator import _compare_digest as compare_digest

+

+

 trans_5C = "".join ([chr (x ^ 0x5C) for x in xrange(256)])

 trans_36 = "".join ([chr (x ^ 0x36) for x in xrange(256)])

diff --git a/Lib/test/test_hmac.py b/Lib/test/test_hmac.py

--- a/Lib/test/test_hmac.py

+++ b/Lib/test/test_hmac.py

@@ -302,12 +302,122 @@ class CopyTestCase(unittest.TestCase):

         self.assertTrue(h1.hexdigest() == h2.hexdigest(),

             "Hexdigest of copy doesn't match original hexdigest.")

+

+class CompareDigestTestCase(unittest.TestCase):

+

+    def test_compare_digest(self):

+        # Testing input type exception handling

+        a, b = 100, 200

+        self.assertRaises(TypeError, hmac.compare_digest, a, b)

+        a, b = 100, b"foobar"

+        self.assertRaises(TypeError, hmac.compare_digest, a, b)

+        a, b = b"foobar", 200

+        self.assertRaises(TypeError, hmac.compare_digest, a, b)

+        a, b = u"foobar", b"foobar"

+        self.assertRaises(TypeError, hmac.compare_digest, a, b)

+        a, b = b"foobar", u"foobar"

+        self.assertRaises(TypeError, hmac.compare_digest, a, b)

+

+        # Testing bytes of different lengths

+        a, b = b"foobar", b"foo"

+        self.assertFalse(hmac.compare_digest(a, b))

+        a, b = b"\xde\xad\xbe\xef", b"\xde\xad"

+        self.assertFalse(hmac.compare_digest(a, b))

+

+        # Testing bytes of same lengths, different values

+        a, b = b"foobar", b"foobaz"

+        self.assertFalse(hmac.compare_digest(a, b))

+        a, b = b"\xde\xad\xbe\xef", b"\xab\xad\x1d\xea"

+        self.assertFalse(hmac.compare_digest(a, b))

+

+        # Testing bytes of same lengths, same values

+        a, b = b"foobar", b"foobar"

+        self.assertTrue(hmac.compare_digest(a, b))

+        a, b = b"\xde\xad\xbe\xef", b"\xde\xad\xbe\xef"

+        self.assertTrue(hmac.compare_digest(a, b))

+

+        # Testing bytearrays of same lengths, same values

+        a, b = bytearray(b"foobar"), bytearray(b"foobar")

+        self.assertTrue(hmac.compare_digest(a, b))

+

+        # Testing bytearrays of diffeent lengths

+        a, b = bytearray(b"foobar"), bytearray(b"foo")

+        self.assertFalse(hmac.compare_digest(a, b))

+

+        # Testing bytearrays of same lengths, different values

+        a, b = bytearray(b"foobar"), bytearray(b"foobaz")

+        self.assertFalse(hmac.compare_digest(a, b))

+

+        # Testing byte and bytearray of same lengths, same values

+        a, b = bytearray(b"foobar"), b"foobar"

+        self.assertTrue(hmac.compare_digest(a, b))

+        self.assertTrue(hmac.compare_digest(b, a))

+

+        # Testing byte bytearray of diffeent lengths

+        a, b = bytearray(b"foobar"), b"foo"

+        self.assertFalse(hmac.compare_digest(a, b))

+        self.assertFalse(hmac.compare_digest(b, a))

+

+        # Testing byte and bytearray of same lengths, different values

+        a, b = bytearray(b"foobar"), b"foobaz"

+        self.assertFalse(hmac.compare_digest(a, b))

+        self.assertFalse(hmac.compare_digest(b, a))

+

+        # Testing str of same lengths

+        a, b = "foobar", "foobar"

+        self.assertTrue(hmac.compare_digest(a, b))

+

+        # Testing str of diffeent lengths

+        a, b = "foo", "foobar"

+        self.assertFalse(hmac.compare_digest(a, b))

+

+        # Testing bytes of same lengths, different values

+        a, b = "foobar", "foobaz"

+        self.assertFalse(hmac.compare_digest(a, b))

+

+        # Testing error cases

+        a, b = u"foobar", b"foobar"

+        self.assertRaises(TypeError, hmac.compare_digest, a, b)

+        a, b = b"foobar", u"foobar"

+        self.assertRaises(TypeError, hmac.compare_digest, a, b)

+        a, b = b"foobar", 1

+        self.assertRaises(TypeError, hmac.compare_digest, a, b)

+        a, b = 100, 200

+        self.assertRaises(TypeError, hmac.compare_digest, a, b)

+        a, b = "fooä", "fooä"

+        self.assertTrue(hmac.compare_digest(a, b))

+

+        # subclasses are supported by ignore __eq__

+        class mystr(str):

+            def __eq__(self, other):

+                return False

+

+        a, b = mystr("foobar"), mystr("foobar")

+        self.assertTrue(hmac.compare_digest(a, b))

+        a, b = mystr("foobar"), "foobar"

+        self.assertTrue(hmac.compare_digest(a, b))

+        a, b = mystr("foobar"), mystr("foobaz")

+        self.assertFalse(hmac.compare_digest(a, b))

+

+        class mybytes(bytes):

+            def __eq__(self, other):

+                return False

+

+        a, b = mybytes(b"foobar"), mybytes(b"foobar")

+        self.assertTrue(hmac.compare_digest(a, b))

+        a, b = mybytes(b"foobar"), b"foobar"

+        self.assertTrue(hmac.compare_digest(a, b))

+        a, b = mybytes(b"foobar"), mybytes(b"foobaz")

+        self.assertFalse(hmac.compare_digest(a, b))

+

+

 def test_main():

     test_support.run_unittest(

         TestVectorsTestCase,

         ConstructorTestCase,

         SanityTestCase,

-        CopyTestCase

+        CopyTestCase,

+        CompareDigestTestCase,

     )

 if __name__ == "__main__":

diff --git a/Modules/operator.c b/Modules/operator.c

--- a/Modules/operator.c

+++ b/Modules/operator.c

@@ -235,6 +235,132 @@ op_delslice(PyObject *s, PyObject *a)

 #define spam2o(OP,ALTOP,DOC) {#OP, op_##OP, METH_O, PyDoc_STR(DOC)}, \

                            {#ALTOP, op_##OP, METH_O, PyDoc_STR(DOC)},

+

+

+/* compare_digest **********************************************************/

+

+/*

+ * timing safe compare

+ *

+ * Returns 1 of the strings are equal.

+ * In case of len(a) != len(b) the function tries to keep the timing

+ * dependent on the length of b. CPU cache locally may still alter timing

+ * a bit.

+ */

+static int

+_tscmp(const unsigned char *a, const unsigned char *b,

+        Py_ssize_t len_a, Py_ssize_t len_b)

+{

+    /* The volatile type declarations make sure that the compiler has no

+     * chance to optimize and fold the code in any way that may change

+     * the timing.

+     */

+    volatile Py_ssize_t length;

+    volatile const unsigned char *left;

+    volatile const unsigned char *right;

+    Py_ssize_t i;

+    unsigned char result;

+

+    /* loop count depends on length of b */

+    length = len_b;

+    left = NULL;

+    right = b;

+

+    /* don't use else here to keep the amount of CPU instructions constant,

+     * volatile forces re-evaluation

+     *  */

+    if (len_a == length) {

+        left = *((volatile const unsigned char**)&a);

+        result = 0;

+    }

+    if (len_a != length) {

+        left = b;

+        result = 1;

+    }

+

+    for (i=0; i < length; i++) {

+        result |= *left++ ^ *right++;

+    }

+

+    return (result == 0);

+}

+

+PyDoc_STRVAR(compare_digest__doc__,

+"compare_digest(a, b) -> bool\n"

+"\n"

+"Return 'a == b'.  This function uses an approach designed to prevent\n"

+"timing analysis, making it appropriate for cryptography.\n"

+"a and b must both be of the same type: either str (ASCII only),\n"

+"or any type that supports the buffer protocol (e.g. bytes).\n"

+"\n"

+"Note: If a and b are of different lengths, or if an error occurs,\n"

+"a timing attack could theoretically reveal information about the\n"

+"types and lengths of a and b--but not their values.\n");

+

+static PyObject*

+compare_digest(PyObject *self, PyObject *args)

+{

+    PyObject *a, *b;

+    int rc;

+

+    if (!PyArg_ParseTuple(args, "OO:compare_digest", &a, &b)) {

+        return NULL;

+    }

+

+    /* Unicode string */

+    if (PyUnicode_Check(a) && PyUnicode_Check(b)) {

+        rc = _tscmp(PyUnicode_AS_DATA(a),

+                    PyUnicode_AS_DATA(b),

+                    PyUnicode_GET_DATA_SIZE(a),

+                    PyUnicode_GET_DATA_SIZE(b));

+    }

+    /* fallback to buffer interface for bytes, bytesarray and other */

+    else {

+        Py_buffer view_a;

+        Py_buffer view_b;

+

+        if ((PyObject_CheckBuffer(a) == 0) & (PyObject_CheckBuffer(b) == 0)) {

+            PyErr_Format(PyExc_TypeError,

+                         "unsupported operand types(s) or combination of types: "

+                         "'%.100s' and '%.100s'",

+                         Py_TYPE(a)->tp_name, Py_TYPE(b)->tp_name);

+            return NULL;

+        }

+

+        if (PyObject_GetBuffer(a, &view_a, PyBUF_SIMPLE) == -1) {

+            return NULL;

+        }

+        if (view_a.ndim > 1) {

+            PyErr_SetString(PyExc_BufferError,

+                            "Buffer must be single dimension");

+            PyBuffer_Release(&view_a);

+            return NULL;

+        }

+

+        if (PyObject_GetBuffer(b, &view_b, PyBUF_SIMPLE) == -1) {

+            PyBuffer_Release(&view_a);

+            return NULL;

+        }

+        if (view_b.ndim > 1) {

+            PyErr_SetString(PyExc_BufferError,

+                            "Buffer must be single dimension");

+            PyBuffer_Release(&view_a);

+            PyBuffer_Release(&view_b);

+            return NULL;

+        }

+

+        rc = _tscmp((const unsigned char*)view_a.buf,

+                    (const unsigned char*)view_b.buf,

+                    view_a.len,

+                    view_b.len);

+

+        PyBuffer_Release(&view_a);

+        PyBuffer_Release(&view_b);

+    }

+

+    return PyBool_FromLong(rc);

+}

+

 static struct PyMethodDef operator_methods[] = {

 spam1o(isCallable,

@@ -318,6 +444,8 @@ spam2(ne,__ne__, "ne(a, b) -- Same as a!

 spam2(gt,__gt__, "gt(a, b) -- Same as a>b.")

 spam2(ge,__ge__, "ge(a, b) -- Same as a>=b.")

+    {"_compare_digest", (PyCFunction)compare_digest, METH_VARARGS,

+     compare_digest__doc__},

     {NULL,              NULL}           /* sentinel */

 };