bpo-40791: Use CRYPTO_memcmp() for compare_digest (#20456)

hashlib.compare_digest uses OpenSSL's CRYPTO_memcmp() function
when OpenSSL is available.

Note: The _operator module is a builtin module. I don't want to add
libcrypto dependency to libpython. Therefore I duplicated the wrapper
function and added a copy to _hashopenssl.c.
This commit is contained in:
Christian Heimes 2020-05-27 21:50:06 +02:00 committed by GitHub
parent 210a137396
commit db5aed931f
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7 changed files with 221 additions and 37 deletions

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@ -138,6 +138,11 @@ This module also provides the following helper function:
.. versionadded:: 3.3
.. versionchanged:: 3.10
The function uses OpenSSL's ``CRYPTO_memcmp()`` internally when
available.
.. seealso::

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@ -4,14 +4,15 @@ Implements the HMAC algorithm as described by RFC 2104.
"""
import warnings as _warnings
from _operator import _compare_digest as compare_digest
try:
import _hashlib as _hashopenssl
except ImportError:
_hashopenssl = None
_openssl_md_meths = None
from _operator import _compare_digest as compare_digest
else:
_openssl_md_meths = frozenset(_hashopenssl.openssl_md_meth_names)
compare_digest = _hashopenssl.compare_digest
import hashlib as _hashlib
trans_5C = bytes((x ^ 0x5C) for x in range(256))

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@ -8,12 +8,16 @@ import warnings
from test.support import hashlib_helper
from _operator import _compare_digest as operator_compare_digest
try:
from _hashlib import HMAC as C_HMAC
from _hashlib import hmac_new as c_hmac_new
from _hashlib import compare_digest as openssl_compare_digest
except ImportError:
C_HMAC = None
c_hmac_new = None
openssl_compare_digest = None
def ignore_warning(func):
@ -505,87 +509,101 @@ class CopyTestCase(unittest.TestCase):
class CompareDigestTestCase(unittest.TestCase):
def test_compare_digest(self):
def test_hmac_compare_digest(self):
self._test_compare_digest(hmac.compare_digest)
if openssl_compare_digest is not None:
self.assertIs(hmac.compare_digest, openssl_compare_digest)
else:
self.assertIs(hmac.compare_digest, operator_compare_digest)
def test_operator_compare_digest(self):
self._test_compare_digest(operator_compare_digest)
@unittest.skipIf(openssl_compare_digest is None, "test requires _hashlib")
def test_openssl_compare_digest(self):
self._test_compare_digest(openssl_compare_digest)
def _test_compare_digest(self, compare_digest):
# Testing input type exception handling
a, b = 100, 200
self.assertRaises(TypeError, hmac.compare_digest, a, b)
self.assertRaises(TypeError, compare_digest, a, b)
a, b = 100, b"foobar"
self.assertRaises(TypeError, hmac.compare_digest, a, b)
self.assertRaises(TypeError, compare_digest, a, b)
a, b = b"foobar", 200
self.assertRaises(TypeError, hmac.compare_digest, a, b)
self.assertRaises(TypeError, compare_digest, a, b)
a, b = "foobar", b"foobar"
self.assertRaises(TypeError, hmac.compare_digest, a, b)
self.assertRaises(TypeError, compare_digest, a, b)
a, b = b"foobar", "foobar"
self.assertRaises(TypeError, hmac.compare_digest, a, b)
self.assertRaises(TypeError, compare_digest, a, b)
# Testing bytes of different lengths
a, b = b"foobar", b"foo"
self.assertFalse(hmac.compare_digest(a, b))
self.assertFalse(compare_digest(a, b))
a, b = b"\xde\xad\xbe\xef", b"\xde\xad"
self.assertFalse(hmac.compare_digest(a, b))
self.assertFalse(compare_digest(a, b))
# Testing bytes of same lengths, different values
a, b = b"foobar", b"foobaz"
self.assertFalse(hmac.compare_digest(a, b))
self.assertFalse(compare_digest(a, b))
a, b = b"\xde\xad\xbe\xef", b"\xab\xad\x1d\xea"
self.assertFalse(hmac.compare_digest(a, b))
self.assertFalse(compare_digest(a, b))
# Testing bytes of same lengths, same values
a, b = b"foobar", b"foobar"
self.assertTrue(hmac.compare_digest(a, b))
self.assertTrue(compare_digest(a, b))
a, b = b"\xde\xad\xbe\xef", b"\xde\xad\xbe\xef"
self.assertTrue(hmac.compare_digest(a, b))
self.assertTrue(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))
self.assertTrue(compare_digest(a, b))
# Testing bytearrays of different lengths
a, b = bytearray(b"foobar"), bytearray(b"foo")
self.assertFalse(hmac.compare_digest(a, b))
self.assertFalse(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))
self.assertFalse(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))
self.assertTrue(compare_digest(a, b))
self.assertTrue(compare_digest(b, a))
# Testing byte bytearray of different lengths
a, b = bytearray(b"foobar"), b"foo"
self.assertFalse(hmac.compare_digest(a, b))
self.assertFalse(hmac.compare_digest(b, a))
self.assertFalse(compare_digest(a, b))
self.assertFalse(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))
self.assertFalse(compare_digest(a, b))
self.assertFalse(compare_digest(b, a))
# Testing str of same lengths
a, b = "foobar", "foobar"
self.assertTrue(hmac.compare_digest(a, b))
self.assertTrue(compare_digest(a, b))
# Testing str of different lengths
a, b = "foo", "foobar"
self.assertFalse(hmac.compare_digest(a, b))
self.assertFalse(compare_digest(a, b))
# Testing bytes of same lengths, different values
a, b = "foobar", "foobaz"
self.assertFalse(hmac.compare_digest(a, b))
self.assertFalse(compare_digest(a, b))
# Testing error cases
a, b = "foobar", b"foobar"
self.assertRaises(TypeError, hmac.compare_digest, a, b)
self.assertRaises(TypeError, compare_digest, a, b)
a, b = b"foobar", "foobar"
self.assertRaises(TypeError, hmac.compare_digest, a, b)
self.assertRaises(TypeError, compare_digest, a, b)
a, b = b"foobar", 1
self.assertRaises(TypeError, hmac.compare_digest, a, b)
self.assertRaises(TypeError, compare_digest, a, b)
a, b = 100, 200
self.assertRaises(TypeError, hmac.compare_digest, a, b)
self.assertRaises(TypeError, compare_digest, a, b)
a, b = "fooä", "fooä"
self.assertRaises(TypeError, hmac.compare_digest, a, b)
self.assertRaises(TypeError, compare_digest, a, b)
# subclasses are supported by ignore __eq__
class mystr(str):
@ -593,22 +611,22 @@ class CompareDigestTestCase(unittest.TestCase):
return False
a, b = mystr("foobar"), mystr("foobar")
self.assertTrue(hmac.compare_digest(a, b))
self.assertTrue(compare_digest(a, b))
a, b = mystr("foobar"), "foobar"
self.assertTrue(hmac.compare_digest(a, b))
self.assertTrue(compare_digest(a, b))
a, b = mystr("foobar"), mystr("foobaz")
self.assertFalse(hmac.compare_digest(a, b))
self.assertFalse(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))
self.assertTrue(compare_digest(a, b))
a, b = mybytes(b"foobar"), b"foobar"
self.assertTrue(hmac.compare_digest(a, b))
self.assertTrue(compare_digest(a, b))
a, b = mybytes(b"foobar"), mybytes(b"foobaz")
self.assertFalse(hmac.compare_digest(a, b))
self.assertFalse(compare_digest(a, b))
if __name__ == "__main__":

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@ -0,0 +1,2 @@
:func:`hashlib.compare_digest` uses OpenSSL's ``CRYPTO_memcmp()`` function
when OpenSSL is available.

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@ -21,6 +21,7 @@
/* EVP is the preferred interface to hashing in OpenSSL */
#include <openssl/evp.h>
#include <openssl/hmac.h>
#include <openssl/crypto.h>
/* We use the object interface to discover what hashes OpenSSL supports. */
#include <openssl/objects.h>
#include "openssl/err.h"
@ -1833,6 +1834,120 @@ _hashlib_get_fips_mode_impl(PyObject *module)
#endif // !LIBRESSL_VERSION_NUMBER
static int
_tscmp(const unsigned char *a, const unsigned char *b,
Py_ssize_t len_a, Py_ssize_t len_b)
{
/* loop count depends on length of b. Might leak very little timing
* information if sizes are different.
*/
Py_ssize_t length = len_b;
const void *left = a;
const void *right = b;
int result = 0;
if (len_a != length) {
left = b;
result = 1;
}
result |= CRYPTO_memcmp(left, right, length);
return (result == 0);
}
/* NOTE: Keep in sync with _operator.c implementation. */
/*[clinic input]
_hashlib.compare_digest
a: object
b: object
/
Return 'a == b'.
This function uses an approach designed to prevent
timing analysis, making it appropriate for cryptography.
a and b must both be of the same type: either str (ASCII only),
or any 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.
[clinic start generated code]*/
static PyObject *
_hashlib_compare_digest_impl(PyObject *module, PyObject *a, PyObject *b)
/*[clinic end generated code: output=6f1c13927480aed9 input=9c40c6e566ca12f5]*/
{
int rc;
/* 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);
}
return PyBool_FromLong(rc);
}
/* List of functions exported by this module */
static struct PyMethodDef EVP_functions[] = {
@ -1840,6 +1955,7 @@ static struct PyMethodDef EVP_functions[] = {
PBKDF2_HMAC_METHODDEF
_HASHLIB_SCRYPT_METHODDEF
_HASHLIB_GET_FIPS_MODE_METHODDEF
_HASHLIB_COMPARE_DIGEST_METHODDEF
_HASHLIB_HMAC_SINGLESHOT_METHODDEF
_HASHLIB_HMAC_NEW_METHODDEF
_HASHLIB_OPENSSL_MD5_METHODDEF

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@ -785,6 +785,8 @@ _operator_length_hint_impl(PyObject *module, PyObject *obj,
return PyObject_LengthHint(obj, default_value);
}
/* NOTE: Keep in sync with _hashopenssl.c implementation. */
/*[clinic input]
_operator._compare_digest = _operator.eq

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@ -1338,6 +1338,46 @@ exit:
#endif /* !defined(LIBRESSL_VERSION_NUMBER) */
PyDoc_STRVAR(_hashlib_compare_digest__doc__,
"compare_digest($module, a, b, /)\n"
"--\n"
"\n"
"Return \'a == b\'.\n"
"\n"
"This function uses an approach designed to prevent\n"
"timing analysis, making it appropriate for cryptography.\n"
"\n"
"a and b must both be of the same type: either str (ASCII only),\n"
"or any bytes-like object.\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.");
#define _HASHLIB_COMPARE_DIGEST_METHODDEF \
{"compare_digest", (PyCFunction)(void(*)(void))_hashlib_compare_digest, METH_FASTCALL, _hashlib_compare_digest__doc__},
static PyObject *
_hashlib_compare_digest_impl(PyObject *module, PyObject *a, PyObject *b);
static PyObject *
_hashlib_compare_digest(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
{
PyObject *return_value = NULL;
PyObject *a;
PyObject *b;
if (!_PyArg_CheckPositional("compare_digest", nargs, 2, 2)) {
goto exit;
}
a = args[0];
b = args[1];
return_value = _hashlib_compare_digest_impl(module, a, b);
exit:
return return_value;
}
#ifndef EVPXOF_DIGEST_METHODDEF
#define EVPXOF_DIGEST_METHODDEF
#endif /* !defined(EVPXOF_DIGEST_METHODDEF) */
@ -1377,4 +1417,4 @@ exit:
#ifndef _HASHLIB_GET_FIPS_MODE_METHODDEF
#define _HASHLIB_GET_FIPS_MODE_METHODDEF
#endif /* !defined(_HASHLIB_GET_FIPS_MODE_METHODDEF) */
/*[clinic end generated code: output=d8dddcd85fb11dde input=a9049054013a1b77]*/
/*[clinic end generated code: output=95447a60132f039e input=a9049054013a1b77]*/