Base64 to JSON

What the Base64 to JSON tool does

Sometimes a chunk of JSON arrives wrapped in Base64, a solid block of characters with no braces or quotes in sight. This tool unwraps it. Paste the Base64 in, press Convert, and the JSON inside comes back out as readable text you can actually look at.

It is the same decoding step as the plain Base64 decoder, just aimed at the case where you already know, or strongly suspect, that the thing hiding inside is JSON.

How to use it

1. Paste the Base64. Drop the encoded string into the box, or upload a text file holding it.
2. Press Convert. The decoded JSON appears below, ready to Copy or Download.

Clear wipes both boxes for a fresh start.

Why JSON ends up in Base64 so often

JSON is text, so you might wonder why anyone would Base64 it at all. The answer is travel. JSON frequently needs to ride somewhere that is fussy about punctuation, inside a URL, a cookie, an HTTP header, or a database field, and JSON is full of characters like quotes, braces, and colons that those places treat as special. Wrapping the JSON in Base64 flattens it into a plain run of safe characters that slides through any of them untouched, then gets unwrapped at the other end. That is the whole reason this conversion exists.

The most common case: a JWT payload

If you have run into Base64-wrapped JSON, there is a good chance it was a JSON Web Token. A JWT is three parts joined by dots, and the middle part, the payload, is JSON describing who the token is for and when it expires, packed into Base64 so it can sit safely in a header or a URL. Decode that middle part here and you can read the claims inside, things like the subject, the issued and expiry times, and any roles. It is the quickest way to see what a token is actually asserting, which is handy when you are debugging a login that will not behave. One note: tokens usually use the URL-safe Base64url variant, so if the payload has - or _ characters in it, run it through the Base64url tools instead.

How it works

Under the hood it is a straight Base64 decode. The tool reads the encoded string, rebuilds the original bytes, and shows them as text. If that text was JSON going in, JSON is what you see coming out. The mechanics of the decoding, the four-characters-to-three-bytes business, are the same as on the Base64 Decode page, so if you want that walked through in full, it lives there.

After decoding: reading and tidying the JSON

What you get back is the raw JSON, which may well arrive as one long unbroken line, since that is how it is usually stored. That is perfectly valid JSON, just not pretty to read. If you want it laid out with proper indentation, or you want to confirm it is well formed, drop the decoded text into a dedicated JSON formatter or validator. This tool's job is to get the JSON out of its Base64 wrapper. Making it readable or checking it for errors is the next step, and a job better suited to a tool built for it.

Questions people ask

How do I convert a Base64 string to JSON?

Paste the Base64 into the box and press Convert. The tool decodes it and shows the JSON text that was packed inside, ready to copy.

Can I use this to read a JWT?

You can read the payload, the middle section of the token, which is JSON describing the token's claims. Note that tokens usually use the URL-safe Base64url variant, so if you see - or _ characters, use the Base64url tools.

The JSON came out as one long line. How do I format it?

That is normal and still valid. To indent and lay it out nicely, or to check it is well formed, paste the decoded text into a JSON formatter or validator. This tool handles the decoding step only.

What if the result is not JSON?

Then the Base64 was holding something else, plain text, XML, or binary. The tool always decodes the Base64 faithfully, but it can only show JSON if JSON was what went in.

References

1. Josefsson, S. (2006). RFC 4648: The Base16, Base32, and Base64 Data Encodings. Internet Engineering Task Force (IETF). https://www.rfc-editor.org/rfc/rfc4648
2. Bray, T. (Ed.) (2017). RFC 8259: The JavaScript Object Notation (JSON) Data Interchange Format. Internet Engineering Task Force (IETF). https://www.rfc-editor.org/rfc/rfc8259