On Fri, Jul 16, 2021 at 01:04:21PM +0200, Hannes Reinecke wrote:

+/**
+ * base64_decode() - base64-decode some bytes
+ * @src: the base64-encoded string to decode
+ * @len: number of bytes to decode
+ * @dst: (output) the decoded bytes.
+ *
+ * Decodes the base64-encoded bytes @src according to RFC 4648.
+ *
+ * Return: number of decoded bytes
+ */
+int base64_decode(const char *src, int len, u8 *dst)
+{
+        int i, bits = 0, pad = 0;
+        u32 ac = 0;
+        size_t dst_len = 0;
+
+        for (i = 0; i < len; i++) {
+                int c, p = -1;
+
+                if (src[i] == '=') {
+                        pad++;
+                        if (i + 1 < len && src[i + 1] == '=')
+                                pad++;
+                        break;
+                }
+                for (c = 0; c < strlen(lookup_table); c++) {
+                        if (src[i] == lookup_table[c]) {
+                                p = c;
+                                break;
+                        }
+                }
+                if (p < 0)
+                        break;
+                ac = (ac << 6) | p;
+                bits += 6;
+                if (bits < 24)
+                        continue;
+                while (bits) {
+                        bits -= 8;
+                        dst[dst_len++] = (ac >> bits) & 0xff;
+                }
+                ac = 0;
+        }
+        dst_len -= pad;
+        return dst_len;
+}
+EXPORT_SYMBOL_GPL(base64_decode);

This should return an error if the input isn't valid base64.

- Eric