Issue #15061: Re-implemented hmac.compare_digest() in C

Modules/operator.c

@@ -159,6 +159,146 @@ is_not(PyObject *s, PyObject *a)
 #undef spam2
 #undef spam1o
 #undef spam1o
+
+/* 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 the equivalent of 'a == b', but avoid any short circuiting to\n"
+"counterfeit timing analysis of input data. The function should be used to\n"
+"compare cryptographic secrets. a and b must both either support the buffer\n"
+"protocol (e.g. bytes) or be ASCII only str instances at the same time.\n"
+"\n"
+"Note: In case of an error or different lengths the function may disclose\n"
+"some timing information about the types and lengths of a and b.\n");
+
+
+static PyObject*
+compare_digest(PyObject *self, PyObject *args)
+{
+    PyObject *a, *b;
+    int rc;
+    PyObject *result;
+
+    if (!PyArg_ParseTuple(args, "OO:compare_digest", &a, &b)) {
+        return NULL;
+    }
+
+    /* ASCII unicode string */
+    if(PyUnicode_Check(a) && PyUnicode_Check(b)) {
+        if (PyUnicode_READY(a) == -1 || PyUnicode_READY(b) == -1) {
+            return NULL;
+        }
+        if (!PyUnicode_IS_ASCII(a) || !PyUnicode_IS_ASCII(b)) {
+            PyErr_SetString(PyExc_TypeError,
+                            "comparing strings with non-ASCII characters is "
+                            "not supported");
+            return NULL;
+        }
+
+        rc = _tscmp(PyUnicode_DATA(a),
+                    PyUnicode_DATA(b),
+                    PyUnicode_GET_LENGTH(a),
+                    PyUnicode_GET_LENGTH(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);
+    }
+
+    result = PyBool_FromLong(rc);
+    Py_INCREF(result);
+    return result;
+}
+
+/* operator methods **********************************************************/
+
 #define spam1(OP,DOC) {#OP, OP, METH_VARARGS, PyDoc_STR(DOC)},
 #define spam2(OP,ALTOP,DOC) {#OP, op_##OP, METH_VARARGS, PyDoc_STR(DOC)}, \
                            {#ALTOP, op_##OP, METH_VARARGS, PyDoc_STR(DOC)},
@@ -227,6 +367,8 @@ spam2(ne,__ne__, "ne(a, b) -- Same as a!=b.")
 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 */
 
 };