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Encode / Decode Studio — Base64, URL, JWT & more

Paste %E3%81%82, an eyJ… token or any encoded string — the format is detected and decoded instantly. Errors say exactly which character is wrong and why, and nothing leaves your browser.

On-device・No upload

Your files and input are processed only inside your browser on this device and are never uploaded to our servers.

Input
Input (chars): 00 B
Result
The result appears here as you type

How to use

  1. Pick a format at the top (Base64, URL, HTML entities, …) and choose Encode or Decode.

  2. Paste text into the input; the result appears immediately. For Base64 you can also “Choose file” to turn an image or any file into a data: URL.

  3. Paste an encoded string and the tool suggests the matching format (with a preview) — switch with one click.

  4. If there is an error, read the position and cause, then use “Select error position” to jump to the character.

  5. Copy or download the result. “Swap” moves the result back to the input to check the reverse conversion.

What it does

Nine formats in one place

Base64 (standard, URL-safe, padding, 76-column wrap), URL (component, full URI, form), HTML entities, Unicode escapes (JS, ES2015, Python, CSS), Punycode, hex, binary, Quoted-Printable and JWT.

Format auto-detection

When your input looks like Base64, percent-encoding, a JWT, an xn-- domain and so on, the tool verifies that it actually decodes, then suggests it with a preview of the result.

Errors with position and cause

Messages such as “‘*’ at position 5 is not a Base64 character” or “bytes from position 12 are not valid UTF-8 (possibly Shift_JIS)” tell you what is wrong and how to fix it.

Readable JWTs with expiry check

Header and payload are shown as formatted JSON; exp, iat and nbf are shown in your local time and UTC with an expired / valid badge. The tool states clearly that signatures are not verified and warns about alg: none.

Binary-safe decoding

If the decoded bytes are not UTF-8 text they are shown as hex; PNG, JPEG, PDF and other formats are identified from their content and can be saved as files, with an inline image preview.

Solid IDN handling

Full-width letters and ideographic dots are normalized before Punycode conversion, and you can paste a whole URL or e-mail address — only the host name is converted.

Use cases

  • Read Base64 found in e-mail headers, logs or API responses
  • Turn a logged URL like %E6%9D%B1%E4%BA%AC back into readable text
  • Check a JWT’s expiry (exp), issuer and claims while debugging authentication
  • Convert an internationalized domain to its xn-- form for DNS or e-mail settings
  • Escape <, & and quotes before placing text in HTML

Supported formats

Encode / decode

Base64 / Base64URL / data: URLURL (percent-encoding)HTML entities (named, decimal, hex)Unicode escapes (\uXXXX, \u{…}, \U, \x, CSS)Punycode / IDNHexBinaryQuoted-Printable

Decode only

JWT (JWS: header & payload)

File input

Any file → Base64 / data: URL (up to 10 MB)

Capability matrix

FeatureSupportNotes
Base64 (standard, URL-safe, data: URL)Supported—
URL encode / decode (UTF-8)SupportedURLs encoded in Shift_JIS or EUC-JP are not recovered
HTML entity decodingPartialAll numeric references; named references cover the 253 HTML 4 names, not the 2,000+ HTML5 additions
Punycode / IDNPartialLowercasing and NFKC only; no full UTS #46 mapping or IDNA2008 validity checks
JWT header & payloadSupported—
JWT signature verificationNot supportedRequires the key — this tool only reads the contents
Encrypted JWT (JWE) decryptionNot supported—
Hex, binary, Quoted-PrintableSupported—
Format auto-detectionSupportedOnly candidates that actually decode are suggested
Offline useSupportedWorks without a connection once loaded

Limits & notes

  • Text input up to 5 million characters; files up to 10 MB for Base64.
  • Text is treated as UTF-8. Bytes in other encodings are shown as binary (hex).
  • URL decoding accepts only valid UTF-8 byte sequences; anything else is reported with its position.
  • JWTs are not verified or decrypted. Confirm validity on the server that issued them.
  • Named entities follow HTML 4 (253 names including &apos;).

Privacy & safety

Everything you paste or upload is converted inside your browser. Strings containing tokens or passwords are never sent to or stored on a server. Closing the page discards everything.

FAQ

Why does Base64-decoded text look garbled?
Base64 encodes bytes, not characters, so the original text encoding matters. This tool decodes as UTF-8; if the bytes are not valid UTF-8 it shows them as hex instead of guessing.
What is the difference between encodeURIComponent and encodeURI?
encodeURIComponent also escapes / ? & = and similar characters, so use it for query values. encodeURI keeps URL delimiters and is meant for whole URLs. Form encoding additionally turns spaces into +.
Is it safe to paste a JWT here?
Decoding happens only in your browser and the token is never sent. Remember that anyone can read a JWT, and this tool does not verify signatures — keep live production tokens out of screenshots and shared screens.
What does xn-- mean in a domain?
DNS only supports ASCII, so internationalized domain names are converted with Punycode into labels starting with “xn--”. For example, bücher.de becomes xn--bcher-kva.de.
Can it decode a truncated sequence like “%E3%81”?
Incomplete UTF-8 cannot be turned back into a character, so the tool reports the position where the invalid bytes start. Check that the whole URL was copied.
Is my data sent to a server?
No. Conversion, detection and JWT inspection all run in your browser.

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