Base64 final check: decode and compare exact bytes

Before sharing Base64 output, decode it back and compare with the original bytes. For text, confirm the character encoding, especially non-ASCII characters; for binary data, compare file size or a digest. Check that line wrapping and padding are accepted by the receiving system. Base64 is an encoding, not encryption, so do not treat encoded secrets as safe to publish. A successful round trip proves the encoding survived, but not that the underlying data is valid for its intended application.