Base64 does not provide secrecy. Anyone who receives the encoded value can recover its bytes without a password or key. Hiding credentials in a long string changes their appearance, not who can read them. Apply the same handling rules to encoded secrets as to the originals.
Encoding, encryption and signatures answer different questions
Encoding asks how bytes can travel through a text-oriented system. Encryption protects confidentiality under a cryptographic key. An authentication tag or digital signature can establish that data was produced by an authorized party and has not been altered, provided the verification process and keys are correct. A plain checksum mainly helps detect accidental corruption. None of these guarantees follows from Base64 alone.
A decoded JWT is a useful example. Its header and payload are readable structures, but reading them does not verify the signature, issuer, audience or expiration policy. Treat an unverified claim as untrusted input. Do not let a viewer's formatted JSON become an authorization decision.
Keep decoded content inert
A payload can decode to HTML, JavaScript, an SVG containing active content, or a malformed document. Display text with textContent, not innerHTML. Do not run decoded code or insert an arbitrary SVG into your page. A declared Data URI MIME type is supplied by the sender and can be false. File signatures help identify a likely format, but a matching header does not establish that a file is complete or safe.
// decodedText comes from an untrusted payload.
const output = document.createElement('pre');
output.textContent = decodedText;
document.body.append(output); // displays markup as textSet limits before allocating memory
Encoded text expands data, and a browser may hold the input string, decoded bytes and preview at the same time. Enforce limits before decoding, restrict accepted document types, and let users cancel long operations. Malformed input should produce a separate error state instead of an output that can be copied as if it were successful data.
For sensitive work, inspect the tool's network behavior and retention policy. Browser processing and server API processing have different boundaries. Avoid putting tokens in page URLs, where they can enter browser history, screenshots or access logs. When testing, use synthetic data with the same structure. RFC 4648 discusses encoding ambiguities; the security of a larger protocol still depends on its complete design.