Fix sloppy base64 acceptance of some multibyte characters

The base64 encoding functions incorrectly accepted some multibyte
characters; stop doing that (bug#52670).

* src/fns.c (base64_encode_1): Reject all multibyte characters.
* test/src/fns-tests.el (fns-tests-base64-encode-string)
(fns-test-base64url-encode-region)
(fns-test-base64url-encode-string): Add tests.
* doc/lispref/text.texi (Base 64): Rephrase outdated manual text.
* etc/NEWS: Add a notice.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/fns.c b/src/fns.c
index 76c76c92ba9..23721334f76 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -3653,7 +3653,7 @@ base64_encode_1 (const char *from, char *to, ptrdiff_t length,
           c = string_char_and_length ((unsigned char *) from + i, &bytes);
           if (CHAR_BYTE8_P (c))
             c = CHAR_TO_BYTE8 (c);
-          else if (c >= 256)
+          else if (c >= 128)
             return -1;
           i += bytes;
         }
@@ -3696,7 +3696,7 @@ base64_encode_1 (const char *from, char *to, ptrdiff_t length,
           c = string_char_and_length ((unsigned char *) from + i, &bytes);
           if (CHAR_BYTE8_P (c))
             c = CHAR_TO_BYTE8 (c);
-          else if (c >= 256)
+          else if (c >= 128)
             return -1;
           i += bytes;
         }
@@ -3721,7 +3721,7 @@ base64_encode_1 (const char *from, char *to, ptrdiff_t length,
           c = string_char_and_length ((unsigned char *) from + i, &bytes);
           if (CHAR_BYTE8_P (c))
             c = CHAR_TO_BYTE8 (c);
-          else if (c >= 256)
+          else if (c >= 128)
             return -1;
           i += bytes;
         }