AWS Cognito token validation with Go

// AWS Cognito public keys are available at address:
// https://cognito-idp.{region}.amazonaws.com/{userPoolId}/.well-known/jwks.json
publicKeysURL := "https://cognito-idp.{region}.amazonaws.com/{userPoolId}/.well-known/jwks.json"

// Start with downloading public keys information
// The .Fetch method is used from https://github.com/lestrrat-go/jwx package
publicKeySet, err := jwk.Fetch(publicKeysURL)
if err != nil{
	log.Printf("failed to parse key: %s", err)
}

// Get JWT as string from *principal
// Token is Base64-encoded JSON that contains user details - called "claims".
// ---
// Token is separated into 3 sections - header, payload and signature
// You can test and validate your token with jwt.io
tokenString := fmt.Sprintf("%s", *principal)

// We want to get details from the JWT: client_id and unique user identifier.
// Let's add client_id. We can verify, if it match our App cliet ID in AWS Cognito User Pool
// We can also add user identifier (f.e. "username") to use it with our App
type AWSCognitoClaims struct{
	Client_ID string `json:client_id`
	Username string `json:username`
	jwt.StandardClaims
}

// JWT Parse - it's actually doing parsing, validation and returns back a token.
// Use .Parse or .ParseWithClaims methods from https://github.com/dgrijalva/jwt-go
token, err := jwt.ParseWithClaims(tokenString, &AWSCognitoClaims{}, func(token *jwt.Token) (interface{}, error){
	
	// Verify if the token was signed with correct signing method
	// AWS Cognito is using RSA256 in my case
	_, ok := token.Method.(*jwt.SigningMethodRSA);

	if !ok {
		return nil, fmt.Errorf("Unexpected signing method: %v", token.Header["alg"])
	}

	// Get "kid" value from token header
	// "kid" is shorthand for Key ID
	kid, ok := token.Header["kid"].(string)
	if !ok {
			return nil, errors.New("kid header not found")
	}

	// Check client_id attribute from the token
	claims, ok := token.Claims.(*AWSCognitoClaims)
	if !ok {
		return nil, errors.New("There is problem to get claims")
	}
	log.Printf("client_id: %v", claims.Client_ID)

	// "kid" must be present in the public keys set
	keys := publicKeySet.LookupKeyID(kid);
	if len(keys) == 0 {
			 return nil, fmt.Errorf("key %v not found", kid)
	}

	// In our case, we are returning only one key = keys[0]
	// Return token key as []byte{string} type
	var tokenKey interface{}
	if err := keys[0].Raw(&tokenKey); err != nil {
		return nil, errors.New("failed to create token key")
	}

	return tokenKey, nil
})

if err != nil{
	// This place can throw expiration error
	log.Printf("token problem: %s", err)
}

// Check if token is valid
if !token.Valid {
	log.Println("token is invalid")
}