Base64 Encoder/Decoder

Encode and decode strings, with URL-safe alphabet for tokens and JWTs.

Runs 100% client-side
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On this page4 sections

HOW TO USE

  1. 01Choose Encode or Decode.
  2. 02Paste your text in the input area — output appears automatically.
  3. 03Toggle URL-safe alphabet for tokens used in URLs and JWTs.

Try it

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WHEN TO USE

Use this when you need to encode binary data or arbitrary text for transport in systems that only handle ASCII — common in HTTP Basic Auth headers, JSON payloads that embed file content, and data URIs. Use Decode whenever you encounter a Base64 string in a log, token, or API response and want to read what it actually contains. Toggle URL-safe alphabet when the Base64 appears in JWT segments or URL parameters, where + and / must be replaced with - and _.

WHAT BUGS THIS FINDS

  • Padding errors in strict decoders

    Missing = padding at the end of a Base64 string causes a decode failure in strict libraries — Encode shows the correct padded form to compare against what the consumer receives.

  • Standard vs URL-safe alphabet confusion

    A JWT or URL token using - and _ instead of + and / fails to decode with a standard Base64 decoder — toggling URL-safe reveals whether the alphabet is the root cause.

  • Binary data sent as plain text

    An API field that should contain Base64-encoded binary gets sent as raw text — Encode shows what the Base64 form should look like, making the missing encoding step obvious.

  • Double-encoding

    A value encoded twice produces a string that decodes to another Base64 string instead of readable text — Decode twice confirms whether double-encoding is the root cause.

QA USE CASES

01

HTTP Basic Auth header inspection

Decode the Base64 segment of an Authorization: Basic header to confirm the username:password pair is correct before debugging a 401 response.

02

JWT segment reading

Decode the header and payload segments of a JWT (split on dots) to read the algorithm, subject, and expiry claims without a full JWT decoder.

03

Binary attachment encoding check

Encode a small test file to Base64 and compare the output against what an API or email client sends to confirm the attachment encoding pipeline is correct.