Base64 Is Trivially Reversible
Decoding Base64 requires no key, no password, and no secret of any kind. Any Base64-encoded string can be decoded in seconds by any online tool. If you can see a Base64 string, you can see the data it contains. There is no security value in encoding alone.
Obfuscation Is Not Security
Some developers use Base64 to 'hide' sensitive values in source code or configuration files. This is security through obscurity at its worst. Any attacker sophisticated enough to target your application knows to decode Base64 immediately. It adds zero resistance to any attack.
Where Base64 Appears Legitimately in Security
HTTP Basic Authentication encodes 'username:password' in Base64 for transit compatibility — the actual security comes from TLS encryption of the transport layer. JWT tokens use Base64 to encode JSON payloads — security comes from HMAC or RSA signature verification, not encoding.
Encoding Sensitive Data Correctly
Sensitive data requires encryption, not encoding. Passwords must be hashed with a purpose-built algorithm (bcrypt, Argon2). API keys require cryptographically random generation and secure storage. Base64 these things if format requires it, but the encoding itself provides nothing.
Base64 is a data format transformation, not a security mechanism. In any context where data security matters, encryption and hashing are required — Base64 is at most a transport-layer convenience on top of proper security.