Decode a JWT via command line

jwt-decode.sh

# will not work in all cases, see https://gist.github.com/angelo-v/e0208a18d455e2e6ea3c40ad637aac53#gistcomment-3439904

function jwt-decode() {
  sed 's/\./\n/g' <<< $(cut -d. -f1,2 <<< $1) | base64 --decode | jq
}

JWT=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWV9.TJVA95OrM7E2cBab30RMHrHDcEfxjoYZgeFONFh7HgQ

jwt-decode $JWT

why not just cut -d"." -f1,2 <<< $1 | sed 's/\./\n/g' | base64 --decode | jq?

With just jq: jq -R 'split(".") | .[1] | @base64d | fromjson' <<< "$JWT"

@lukaslihotzki, nice work :)

With just jq: jq -R 'split(".") | .[1] | @base64d | fromjson' <<< "$JWT"

JWTs use base64url encoding, which neither jq nor base64 (from GNU coreutils) can handle. So the above examples all work some of the time, not reliably.

Valid point @philpennock. Do you have a reliable solution?

Just replace the characters? jq -R 'gsub("-";"+") | gsub("_";"/") | split(".") | .[1] | @base64d | fromjson'

JWTs use base64url encoding, which neither jq nor base64 (from GNU coreutils) can handle. So the above examples all work some of the time, not reliably.

@philpennock is correct. Here is my solution in a shell script:

#!/bin/bash

# pad base64URL encoded to base64
paddit() {
  input=$1
  l=`echo -n $input | wc -c`
  while [ `expr $l % 4` -ne 0 ]
  do
    input="${input}="
    l=`echo -n $input | wc -c`
  done
  echo $input
}

# read and split the token and do some base64URL translation
read jwt
read h p s <<< $(echo $jwt | tr [-_] [+/] | sed 's/\./ /g')

h=`paddit $h`
p=`paddit $p`

# assuming we have jq installed
echo $h | base64 -d | jq
echo $p | base64 -d | jq

Oh, I never spoke up, sorry: I use shell around Perl because Perl had the easiest access to base64url when I last went looking, but I was impressed by the solution from @lukaslihotzki using gsub inside jq as being the solution with the fewest additional dependencies.

I was also trying to decode a JWT token. I could decode a JWT with:

for line in `echo $JWT | tr "." "\n"`; do echo $line | base64 --decode | jq  && echo;done

@seva-ramin's script is fantastic, but just a small bug: tr [+_] [-/] should be tr [-_] [+/]

@seva-ramin's script is fantastic, but just a small bug: tr [+_] [-/] should be tr [-_] [+/]

Ugh! what a silly mistake! @danmactough, Thank you for catching that. I have updated my post.

@seva-ramin for me, on OSX Mojave I get base64: invalid option -- d
Changing to -D works for last two lines:

# assuming we have jq installed
echo $h | base64 -D | jq
echo $p | base64 -D | jq

@seva-ramin for me, on OSX Mojave I get base64: invalid option -- d
Changing to -D works for last two lines:

# assuming we have jq installed
echo $h | base64 -D | jq
echo $p | base64 -D | jq

Darn it. They broke base64 in Mojave? I am on Catalina and both options work. Here is the man page for base64 on Catalina.

I additionally had to remove empty parts but then it worked perfectly fine

jq -R 'split(".") | select(length > 0) | .[0],.[1] | @base64d | fromjson' <<< $1

With just jq: jq -R 'split(".") | .[1] | @base64d | fromjson' <<< "$JWT"

@lukaslihotzki thanks, very useful 👍

With just jq: jq -R 'split(".") | .[1] | @base64d | fromjson' <<< "$JWT"

Thanks @lukaslihotzki, very useful!

Or an universal GO approach using RawStdEncoding (with temporary file):

cat << EOFT > ./temp.go &&  go run ./temp.go $JWT |jq  '.|select(.type=="wrapping")' ; rm ./temp.go
package main
import (
        "encoding/base64"
        "strings"
        "fmt"
        "os"
)
var  encoded = os.Args[1]
func main() {
split := strings.Split(encoded, ".")
for i := 0; i < len(split); i++ {
        tokenBytes, err := base64.RawStdEncoding.DecodeString(split[i])
        if err != nil {
          return
        }
        var sToken=string(tokenBytes)
        fmt.Printf("%s",sToken)
    }
}
EOFT

With just jq: jq -R 'split(".") | .[1] | @base64d | fromjson' <<< "$JWT"

Excellent solution, thanks @lukaslihotzki

What about echo "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWV9.TJVA95OrM7E2cBab30RMHrHDcEfxjoYZgeFONFh7HgQ" | cut -d '.' -f 2 | base64 -d

what about basenc it's part of coreutils ?

basenc -d --base64url -i <your_file> | jq

Sure; coreutils 8.31 and newer, so was not in stable OS releases at the time of the gist. Today, I'd recommend basenc.

Oh, beware though that basenc complains about missing = signs, even in --base64url mode, so you'll also need to suppress stderr.