bpo-33770: improve base64 exception message for encoded inputs of invalid length (#7416)

1 files changed, 12 insertions, 1 deletions

diff --git a/Modules/binascii.c b/Modules/binascii.c
index 59e99282..5667018 100644
--- a/Modules/binascii.c
+++ b/Modules/binascii.c
@@ -510,7 +510,18 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data)
     }
 
     if (leftbits != 0) {
-        PyErr_SetString(Error, "Incorrect padding");
+        if (leftbits == 6) {
+            /*
+            ** There is exactly one extra valid, non-padding, base64 character.
+            ** This is an invalid length, as there is no possible input that
+            ** could encoded into such a base64 string.
+            */
+            PyErr_SetString(Error,
+                            "Invalid base64-encoded string: "
+                            "length cannot be 1 more than a multiple of 4");
+        } else {
+            PyErr_SetString(Error, "Incorrect padding");
+        }
         _PyBytesWriter_Dealloc(&writer);
         return NULL;
     }