How to parse the expiration date of a JWT to a time.Time() in Go?

Question

I'd like to parse the expiration date (exp) from a JSON Web Token (JWT) without verifying it. I've tried the following script (in an attempt to follow How to parse unix timestamp to time.Time):

package main

import (
    "fmt"
    "log"
    "strconv"
    "time"

    jwt "github.com/dgrijalva/jwt-go"
)

func main() {
    tokenString := "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c"

    token, _, err := new(jwt.Parser).ParseUnverified(tokenString, jwt.MapClaims{})
    if err != nil {
        log.Fatal(err)
    }

    if claims, ok := token.Claims.(jwt.MapClaims); ok {
        fmt.Println(claims["exp"])
        i, err := strconv.ParseInt(claims["exp"].(string), 10, 64)
        if err != nil {
            log.Fatal(err)
        }

        tm := time.Unix(i, 0)
        fmt.Println(tm)
    }
}

However, I get this error:

> go run main.go
<nil>
panic: interface conversion: interface {} is nil, not string

goroutine 1 [running]:
main.main()
    /Users/kurt/go/src/github.com/khpeek/mygoproject/jwt_validation/main.go:23 +0x34a
exit status 2

Is there a way I could get a string for exp out of the jwt library? How could I make this give me the expiration date (or the iat) of the JWT?

Answer 1

I ended up calling int64 on claims["exp"] directly rather than first trying to convert it to a string:

package main

import (
    "encoding/json"
    "fmt"
    "log"
    "time"

    jwt "github.com/dgrijalva/jwt-go"
)

func main() {
    tokenString := "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c"

    token, _, err := new(jwt.Parser).ParseUnverified(tokenString, jwt.MapClaims{})
    if err != nil {
        log.Fatal(err)
    }

    claims, ok := token.Claims.(jwt.MapClaims)
    if !ok {
        log.Fatalf("Can't convert token's claims to standard claims")
    }

    var tm time.Time
    switch iat := claims["iat"].(type) {
    case float64:
        tm = time.Unix(int64(iat), 0)
    case json.Number:
        v, _ := iat.Int64()
        tm = time.Unix(v, 0)
    }

    fmt.Println(tm)
}

which prints

> go run main.go
2018-01-17 17:30:22 -0800 PST

Note that I've replaced exp with iat since the simplified example (obtained from jwt.io) does not have an exp claim, but it does have an iat one.