reject negative sizes in SHAKE digests

Author: picnixz

Lib/test/test_hashlib.py

@@ -207,6 +207,11 @@ def hash_constructors(self):
         constructors = self.constructors_to_test.values()
         return itertools.chain.from_iterable(constructors)
 
+    @property
+    def shake_constructors(self):
+        for shake_name in self.shakes:
+            yield from self.constructors_to_test.get(shake_name, ())
+
     @property
     def is_fips_mode(self):
         return get_fips_mode()
@@ -376,21 +381,50 @@ def test_hexdigest(self):
                 self.assertIsInstance(h.digest(), bytes)
                 self.assertEqual(hexstr(h.digest()), h.hexdigest())
 
-    def test_digest_length_overflow(self):
-        # See issue #34922
-        large_sizes = (2**29, 2**32-10, 2**32+10, 2**61, 2**64-10, 2**64+10)
-        for cons in self.hash_constructors:
-            h = cons(usedforsecurity=False)
-            if h.name not in self.shakes:
-                continue
-            if HASH is not None and isinstance(h, HASH):
-                # _hashopenssl's take a size_t
-                continue
-            for digest in h.digest, h.hexdigest:
-                self.assertRaises(ValueError, digest, -10)
-                for length in large_sizes:
-                    with self.assertRaises((ValueError, OverflowError)):
-                        digest(length)
+    def test_shakes_zero_digest_length(self):
+        for constructor in self.shake_constructors:
+            with self.subTest(constructor=constructor):
+                h = constructor(b'abcdef', usedforsecurity=False)
+                self.assertEqual(h.digest(0), b'')
+                self.assertEqual(h.hexdigest(0), '')
+
+    def test_shakes_invalid_digest_length(self):
+        # See https://github.com/python/cpython/issues/79103.
+        for constructor in self.shake_constructors:
+            with self.subTest(constructor=constructor):
+                h = constructor(usedforsecurity=False)
+                # Note: digest() and hexdigest() take a signed input and
+                # raise if it is negative; the rationale is that we use
+                # internally PyBytes_FromStringAndSize() and _Py_strhex()
+                # which both take a Py_ssize_t.
+                for negative_size in (-1, -10, -(1 << 31), -sys.maxsize):
+                    self.assertRaises(ValueError, h.digest, negative_size)
+                    self.assertRaises(ValueError, h.hexdigest, negative_size)
+
+    def test_shakes_overflow_digest_length(self):
+        # See https://github.com/python/cpython/issues/135759.
+
+        exc_types = (OverflowError, ValueError)
+        # HACL* accepts an 'uint32_t' while OpenSSL accepts a 'size_t'.
+        openssl_overflown_sizes = (sys.maxsize + 1, 2 * sys.maxsize)
+        # https://github.com/python/cpython/issues/79103 restricts
+        # the accepted built-in lengths to 2 ** 29, even if OpenSSL
+        # accepts such lengths.
+        builtin_overflown_sizes = openssl_overflown_sizes + (
+            2 ** 29, 2 ** 32 - 10, 2 ** 32, 2 ** 32 + 10,
+            2 ** 61, 2 ** 64 - 10, 2 ** 64, 2 ** 64 + 10,
+        )
+
+        for constructor in self.shake_constructors:
+            with self.subTest(constructor=constructor):
+                h = constructor(usedforsecurity=False)
+                if HASH is not None and isinstance(h, HASH):
+                    overflown_sizes = openssl_overflown_sizes
+                else:
+                    overflown_sizes = builtin_overflown_sizes
+                for invalid_size in overflown_sizes:
+                    self.assertRaises(exc_types, h.digest, invalid_size)
+                    self.assertRaises(exc_types, h.hexdigest, invalid_size)
 
     def test_name_attribute(self):
         for cons in self.hash_constructors:

Misc/NEWS.d/next/Library/2025-06-20-16-28-47.gh-issue-135759.jne0Zi.rst

@@ -0,0 +1,2 @@
+:mod:`hashlib`: correctly reject negative digest lengths in SHAKE objects.
+Patch by Bénédikt Tran.

Modules/_hashopenssl.c

@@ -938,8 +938,14 @@ _hashlib_HASHXOF_digest_impl(HASHobject *self, Py_ssize_t length)
 /*[clinic end generated code: output=dcb09335dd2fe908 input=3eb034ce03c55b21]*/
 {
     EVP_MD_CTX *temp_ctx;
-    PyObject *retval = PyBytes_FromStringAndSize(NULL, length);
+    PyObject *retval;
+
+    if (length < 0) {
+        PyErr_SetString(PyExc_ValueError, "negative digest length");
+        return NULL;
+    }
 
+    retval = PyBytes_FromStringAndSize(NULL, length);
     if (retval == NULL) {
         return NULL;
     }
@@ -986,9 +992,14 @@ _hashlib_HASHXOF_hexdigest_impl(HASHobject *self, Py_ssize_t length)
     EVP_MD_CTX *temp_ctx;
     PyObject *retval;
 
+    if (length < 0) {
+        PyErr_SetString(PyExc_ValueError, "negative digest length");
+        return NULL;
+    }
+
     digest = (unsigned char*)PyMem_Malloc(length);
     if (digest == NULL) {
-        PyErr_NoMemory();
+        (void)PyErr_NoMemory();
         return NULL;
     }
 

Modules/clinic/sha3module.c.h

@@ -473,16 +473,25 @@ SHA3_TYPE_SLOTS(sha3_512_slots, sha3_512__doc__, SHA3_methods, SHA3_getseters);
 SHA3_TYPE_SPEC(sha3_512_spec, "sha3_512", sha3_512_slots);
 
 static PyObject *
-_SHAKE_digest(PyObject *op, unsigned long digestlen, int hex)
+_SHAKE_digest(SHA3object *self, Py_ssize_t digestlen, int hex)
 {
     unsigned char *digest = NULL;
     PyObject *result = NULL;
-    SHA3object *self = _SHA3object_CAST(op);
 
-    if (digestlen >= (1 << 29)) {
-        PyErr_SetString(PyExc_ValueError, "length is too large");
+    if (digestlen < 0) {
+        PyErr_SetString(PyExc_ValueError, "negative digest length");
+        return NULL;
+    }
+    if ((size_t)digestlen >= (1 << 29)) {
+        /*
+         * Raise OverflowError to match the semantics of OpenSSL SHAKE
+         * when the digest length exceeds the range of a 'Py_ssize_t';
+         * the exception message will however be different in this case.
+         */
+        PyErr_SetString(PyExc_OverflowError, "digest length is too large");
         return NULL;
     }
+
     digest = (unsigned char*)PyMem_Malloc(digestlen);
     if (digest == NULL) {
         return PyErr_NoMemory();
@@ -509,32 +518,32 @@ _SHAKE_digest(PyObject *op, unsigned long digestlen, int hex)
 /*[clinic input]
 _sha3.shake_128.digest
 
-    length: unsigned_long
+    length: Py_ssize_t
 
 Return the digest value as a bytes object.
 [clinic start generated code]*/
 
 static PyObject *
-_sha3_shake_128_digest_impl(SHA3object *self, unsigned long length)
-/*[clinic end generated code: output=2313605e2f87bb8f input=93d6d6ff32904f18]*/
+_sha3_shake_128_digest_impl(SHA3object *self, Py_ssize_t length)
+/*[clinic end generated code: output=6c53fb71a6cff0a0 input=be03ade4b31dd54c]*/
 {
-    return _SHAKE_digest((PyObject *)self, length, 0);
+    return _SHAKE_digest(self, length, 0);
 }
 
 
 /*[clinic input]
 _sha3.shake_128.hexdigest
 
-    length: unsigned_long
+    length: Py_ssize_t
 
 Return the digest value as a string of hexadecimal digits.
 [clinic start generated code]*/
 
 static PyObject *
-_sha3_shake_128_hexdigest_impl(SHA3object *self, unsigned long length)
-/*[clinic end generated code: output=bf8e2f1e490944a8 input=562d74e7060b56ab]*/
+_sha3_shake_128_hexdigest_impl(SHA3object *self, Py_ssize_t length)
+/*[clinic end generated code: output=a27412d404f64512 input=0d84d05d7a8ccd37]*/
 {
-    return _SHAKE_digest((PyObject *)self, length, 1);
+    return _SHAKE_digest(self, length, 1);
 }
 
 static PyObject *