lib: base64: add support for base64url

base64url has some small differences to our current base64 implementation:

  - Instead of encoding to `+', `-' is used
  - Instead of encoding to `/', `_' is used
  - Padding with = to reach four byte boundary is optional
  - Invalid characters aren't silently skipped

Everything else is the same though, so let's reuse the code and have an
optional url bool parameter control whether to decode base64 or
base64url.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.barebox.org/20231023143122.1760217-2-a.fatoum@pengutronix.de
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 55 insertions, 5 deletions

diff --git a/lib/base64.c b/lib/base64.c
index ac165ab168..d5ab217528 100644
--- a/lib/base64.c
+++ b/lib/base64.c
@@ -25,6 +25,25 @@ static const char uuenc_tbl_base64[65 + 1] = {
 	'\0' /* needed for uudecode.c only */
 };
 
+static char base64_trchr(char ch, bool url)
+{
+	if (!url)
+		return ch;
+
+	switch (ch) {
+	case '+':
+		return '-';
+	case '/':
+		return '_';
+	case '-':
+		return '+';
+	case '_':
+		return '/';
+	default:
+		return ch;
+	}
+}
+
 /*
  * Encode bytes at S of length LENGTH to uuencode or base64 format and place it
  * to STORE.  STORE will be 0-terminated, and must point to a writable
@@ -68,13 +87,14 @@ EXPORT_SYMBOL(uuencode);
  * Decode base64 encoded string. Stops on '\0'.
  *
  */
-int decode_base64(char *p_dst, int dst_len, const char *src)
+static int __decode_base64(char *p_dst, int dst_len, const char *src, bool url)
 {
 	const char *src_tail;
 	char *dst = p_dst;
 	int length = 0;
+	bool end_reached = false;
 
-	while (dst_len > 0) {
+	while (dst_len > 0 && !end_reached) {
 		unsigned char six_bit[4];
 		int count = 0;
 
@@ -101,13 +121,23 @@ int decode_base64(char *p_dst, int dst_len, const char *src)
 					 * because we did fully decode
 					 * the string (to "ABC").
 					 */
-					if (count == 0)
+					if (count == 0) {
 						src_tail = src;
+					} else if (url) {
+						end_reached = true;
+						goto out;
+					}
+
 					goto ret;
 				}
 				src++;
-				table_ptr = strchr(uuenc_tbl_base64, ch);
-			} while (!table_ptr);
+				table_ptr = strchr(uuenc_tbl_base64, base64_trchr(ch, url));
+			} while (!table_ptr && !url);
+
+			if (!table_ptr) {
+				end_reached = true;
+				goto out;
+			}
 
 			/* Convert encoded character to decimal */
 			ch = table_ptr - uuenc_tbl_base64;
@@ -119,6 +149,7 @@ int decode_base64(char *p_dst, int dst_len, const char *src)
 			six_bit[count] = ch;
 			count++;
 		}
+out:
 
 		/*
 		 * Transform 6-bit values to 8-bit ones.
@@ -151,4 +182,23 @@ ret:
 
 	return length;
 }
+
+/*
+ * Decode base64 encoded string. Stops on '\0'.
+ *
+ */
+int decode_base64(char *p_dst, int dst_len, const char *src)
+{
+	return __decode_base64(p_dst, dst_len, src, false);
+}
 EXPORT_SYMBOL(decode_base64);
+
+/*
+ * Decode base64url encoded string. Stops on '\0'.
+ *
+ */
+int decode_base64url(char *p_dst, int dst_len, const char *src)
+{
+	return __decode_base64(p_dst, dst_len, src, true);
+}
+EXPORT_SYMBOL(decode_base64url);