Pretty Printing JSON Web Tokens (JWTs) on the Command Line using Go
Similar to my previous articles in this series, you may want to create a command-line Go app to decode JWTs.
Let us use the following JWT in example.jwt, via JWT.io:
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c
As well as the following encrypted JWT (JWE) from RFC7516: JSON Web Encryption (JWE):
eyJhbGciOiJSU0EtT0FFUCIsImVuYyI6IkEyNTZHQ00ifQ.OKOawDo13gRp2ojaHV7LFpZcgV7T6DVZKTyKOMTYUmKoTCVJRgckCL9kiMT03JGeipsEdY3mx_etLbbWSrFr05kLzcSr4qKAq7YN7e9jwQRb23nfa6c9d-StnImGyFDbSv04uVuxIp5Zms1gNxKKK2Da14B8S4rzVRltdYwam_lDp5XnZAYpQdb76FdIKLaVmqgfwX7XWRxv2322i-vDxRfqNzo_tETKzpVLzfiwQyeyPGLBIO56YJ7eObdv0je81860ppamavo35UgoRdbYaBcoh9QcfylQr66oc6vFWXRcZ_ZT2LawVCWTIy3brGPi6UklfCpIMfIjf7iGdXKHzg.48V1_ALb6US04U3b.5eym8TW_c8SuK0ltJ3rpYIzOeDQz7TALvtu6UG9oMo4vpzs9tX_EFShS8iB7j6jiSdiwkIr3ajwQzaBtQD_A.XFBoMYUZodetZdvTiFvSkQ
This can be decoded using the following Go program, which takes input from stdin and outputs the decoded (but not verified) contents:
package main
import (
"bytes"
"encoding/base64"
"encoding/json"
"fmt"
"io"
"os"
"strings"
)
func PrettyString(str []byte) (string, error) {
var prettyJSON bytes.Buffer
if err := json.Indent(&prettyJSON, str, "", " "); err != nil {
return "", err
}
return prettyJSON.String(), nil
}
func main() {
data, err := io.ReadAll(os.Stdin)
if err != nil {
panic(err)
}
segments := strings.Split(string(data), ".")
for i, segment := range segments {
if i == 2 {
break
}
bytes, err := base64.RawURLEncoding.DecodeString(segment)
if err != nil {
fmt.Printf("Error decoding string: %s ", err.Error())
return
}
pretty, err := PrettyString(bytes)
if err != nil {
panic(err)
}
fmt.Println(pretty)
if i == 0 {
// give us a generic object to interact with
decoded := make(map[string](interface{}))
err = json.Unmarshal([]byte(bytes), &decoded)
if err != nil {
panic(err)
}
// don't decode further if this is an encrypted JWT (JWE)
if _, ok := decoded["enc"]; ok {
break
}
}
}
}
