Invalid Character Errors

Invalid character errors occur when the input contains characters outside the Base64 alphabet. Common causes: using standard Base64 (with + and /) where URL-safe (- and _) is expected, or vice versa; padding characters in unexpected positions; whitespace in the middle of an encoded string. Strip whitespace, normalize to the correct alphabet, and re-decode.

Incorrect Output Length

If decoded output is truncated or has extra bytes, the cause is usually incorrect padding. Base64 output length must be divisible by 4; if it's not, add = padding characters until it is. Conversely, extra = padding is invalid and some decoders reject it.

Corrupted Binary Output

Corrupted binary after decoding is usually caused by text encoding issues. If the Base64 string was stored in a database or transmitted through a system that modified its encoding, characters may have been altered. Store Base64 strings as ASCII or UTF-8 and ensure all transmission paths preserve exact character values.

Unicode Encoding Mismatch

When encoding text strings to Base64, the text must first be encoded to bytes using a consistent encoding — always UTF-8. If the encoder uses UTF-8 and the decoder uses Latin-1, multi-byte characters will decode incorrectly.

Key Takeaway

Most Base64 errors trace to one of three causes: wrong alphabet variant (standard vs URL-safe), incorrect padding, or text encoding mismatch. Check these three things first when debugging Base64 issues.