Generate a JWT from GitHub App PEM private key in Go - go

Question

I'm trying to use GitHub App and I need to generate a JWT for authenticating (https://docs.github.com/en/developers/apps/building-github-apps/authenticating-with-github-apps#generating-a-private-key)
I'm trying to do that using Goland.
How can I generate a JWT from PEM private key in Go??

Answer 1

The jwt-go library has all the tools you need, and is fairly well documented. You can find it at https://github.com/golang-jwt/jwt.
Assuming you understand what JWTs are and how they're structured, and that you can get that PEM key as a []byte, the process is roughly:
Add "github.com/golang-jwt/jwt/v4" to your imports.
Create a set of claims, which can include the RegisteredClaims type and any custom claims you may need.
Create the token with jwt.NewWithClaims() - you'll need to provide the appropriate signing method. I've primarily used RS256.
Create the JWT string from the token with token.SignedString().
In practice, it will look something like this:
imports "github.com/golang-jwt/jwt/v4"
type MyCustomClaims struct {
*jwt.RegisteredClaims
FooClaim int
BarClaim string
}
func CreateJWT(pemKey []byte) string {
// expires in 60 minutes
expiration := time.Now().Add(time.Second * 3600)
claims := MyCustomClaims{
RegisteredClaims: &jwt.RegisteredClaims{
Issuer: "Example Code Inc.",
ExpiresAt: jwt.NewNumericDate(expiration),
Subject: "JWT Creation",
},
FooClaim: 123,
BarClaim: "bar",
}
token := jwt.NewWithClaims(jwt.SigningMethodRS256, claims)
privateKey, _ := jwt.ParseRSAPrivateKeyFromPEM(pemKey)
myJWT, _ := jwt.SignedString(privateKey)
return myJWT
}

Answer 2

I suggest reading code from this repository:
https://github.com/bradleyfalzon/ghinstallation
I don't know why, but the code in the answer from #JesseB above didn't work for me - it always throws: 401 Unauthorized. Although this repository does use golang-jwt package internally