+ *
+ * Note: even though EVP_DecodeUpdate attempts to detect and report end of
+ * content, the context doesn't currently remember it and will accept more data
+ * in the next call. Therefore, the caller is responsible for checking and
+ * rejecting a 0 return value in the middle of content.
+ *
+ * Note: even though EVP_DecodeUpdate has historically tried to detect end of
+ * content based on line length, this has never worked properly. Therefore,
+ * we now return 0 when one of the following is true:
+ * - Padding or B64_EOF was detected and the last block is complete.
+ * - Input has zero-length.
+ * -1 is returned if:
+ * - Invalid characters are detected.
+ * - There is extra trailing padding, or data after padding.
+ * - B64_EOF is detected after an incomplete base64 block.