jwt: signature is invalid: what it means and how to fix it

What the error means

jwt: signature is invalid means a decoder rejected the input as invalid encoding. The fastest path is to identify what format you have, normalize it, then decode again.

Most common real-world causes

• JWT problems are often: not 3 segments, wrong key/algorithm, or option mismatch (aud/iss/sub).
• The input is not actually encoded in the expected format (Base64 vs Base64URL vs plain text).
• You copied only part of the string (truncated token/payload).
• Whitespace/newlines were introduced during copy/paste.
• Wrong character set: URL-safe Base64 uses '-' and '_' instead of '+' and '/'.
• You decoded using the wrong function (decodeURIComponent on non-URL-encoded data, atob on non-Base64).

Fast debugging steps

• If you see a JWT library error, decode the token parts first to confirm structure and claims.
• Confirm what you are decoding (URL encoding, Base64, Base64URL, JWT).
• Trim whitespace and remove line breaks before decoding.
• If it's a JWT, ensure it has 3 dot-separated parts (header.payload.signature).
• If it's Base64URL, convert '-' -> '+' and '_' -> '/' and add padding if needed.

Code example (go)

// Go JWT structure decode (no signature verification)
package main

import (
  "encoding/base64"
  "fmt"
  "strings"
)

func main() {
  parts := strings.Split(token, ".")
  if len(parts) != 3 {
    panic("JWT must have 3 segments")
  }

  decode := func(s string) []byte {
    b, err := base64.RawURLEncoding.DecodeString(s)
    if err != nil {
      panic(err)
    }
    return b
  }

  fmt.Println("header", string(decode(parts[0])))
  fmt.Println("payload", string(decode(parts[1])))
  // Signature verification requires the signing key.
}

Fix without uploading data

Encoded strings often contain secrets (tokens, IDs). Decode locally and share only redacted snippets.

• URL Encode/Decode for percent-encoding.
• Base64 Encode/Decode for Base64/Base64URL payloads.
• JWT Decoder to inspect header/payload without uploads.

FAQ

Is Base64 the same as Base64URL? No. Base64URL uses '-' and '_' and often omits padding. Normalize before decoding.

Does decoding a JWT verify it? No. Decoding shows claims; verification requires the signing key.