Authenticating with GitHub Apps

Authenticating as a GitHub App

Authenticating as a GitHub App is required for server-to-server API calls, which let your GitHub App do a couple things:

• Retrieve high-level management information about your GitHub App.
• Request access tokens for an installation of the app, allowing you to make API calls without a signed-in user.

To authenticate as a GitHub App, generate a private key in PEM format and download it to your local machine. You'll use this key to sign a JSON Web Token (JWT) and encode it using the RS256 algorithm. GitHub validates your app's identity by verifying the token with the app's stored public key. You'll exchange this JWT for an installation token, used to authenticate your app as a specific installation.

Listing the installations for an app

To list the installations for your app, include the JWT in the Authorization header in an API request to GET /app/installations. For more information about generating a JWT, see "JWT payload."

$ curl -i -X GET \
-H "Authorization: Bearer YOUR_JWT" \
-H "Accept: application/vnd.github+json" \
https://api.github.com/app/installations

The response will include a list of installations where each installation's id can be used for creating an installation access token. For more information about the response format, see "List installations for the authenticated app."

Authenticating as an installation

Authenticating as an installation lets your app access that organization or user via the API, as well as public resources on GitHub. To authenticate as an installation, you must use an installation access token, which you get by sending a JWT to GitHub to prove your app's identity. Ensure that you have already installed your GitHub App to at least one organization or user account; it is impossible to create an installation token without an installation. For more information, see "Installing GitHub Apps."

By default, installation access tokens are scoped to all the repositories that an installation was granted access to. You can further limit the scope of the installation access token to specific repositories by using the repository_ids parameter. Installation access tokens have the permissions configured by the GitHub App, and like repository access, can also be scoped down using the permissions parameter. For more information, see the Create an installation access token for an app endpoint documentation. All installation tokens expire after 1 hour.

To create an installation access token, include the JWT in the Authorization header in the API request, replacing :installation_id with the installation's id. For more information about generating a JWT, see "JWT payload."

$ curl -i -X POST \
-H "Authorization: Bearer YOUR_JWT" \
-H "Accept: application/vnd.github+json" \
https://api.github.com/app/installations/:installation_id/access_tokens

The response will include your installation access token, the expiration date, the token's permissions, and the repositories that the token can access. For more information about the response format, see the Create an installation access token for an app endpoint.

To authenticate with an installation access token, include it in the Authorization header in the API request. Replace YOUR_INSTALLATION_ACCESS_TOKEN with an installation access token:

$ curl -i \
-H "Authorization: Bearer YOUR_INSTALLATION_ACCESS_TOKEN" \
-H "Accept: application/vnd.github+json" \
https://api.github.com/installation/repositories

Accessing API endpoints as an installation

For a list of REST API endpoints that are available for use by GitHub Apps using an installation access token, see "Endpoints available for GitHub Apps."

For a list of endpoints related to installations, see "Installations."

HTTP-based Git access by an installation

Installations with permissions on contents of a repository, can use their installation access tokens to authenticate for Git access. Use the installation access token as the HTTP password:

git clone https://x-access-token:<token>@github.com/owner/repo.git

Generating a JSON Web Token (JWT)

The JWT that's used to authenticate your application is made up of several claims, as well as a signature that's used to validate its authenticity. Those claims are:

Tokens must be signed using the RS256 algorithm, with a matching alg claim of RS256.

Using Ruby

Here's a Ruby script you can use to generate a JWT. Note you'll have to run gem install jwt before using it. YOUR_PATH_TO_PEM and YOUR_APP_ID are the values you must replace. Make sure to enclose the values in double quotes.

require 'openssl'
require 'jwt'  # https://rubygems.org/gems/jwt

# Private key contents
private_pem = File.read("YOUR_PATH_TO_PEM")
private_key = OpenSSL::PKey::RSA.new(private_pem)

# Generate the JWT
payload = {
  # issued at time, 60 seconds in the past to allow for clock drift
  iat: Time.now.to_i - 60,
  # JWT expiration time (10 minute maximum)
  exp: Time.now.to_i + (10 * 60),
  # GitHub App's identifier
  iss: "YOUR_APP_ID"
}

jwt = JWT.encode(payload, private_key, "RS256")
puts jwt

Using Python

Here is a similar script for generating a JWT in Python. Note you will have to use pip install jwt in order to use this script. This script will prompt you for the location of your PEM file and your app's ID, or you can pass them as inline arguments when you execute the script.

#!/usr/bin/env python3
import jwt
import time
import sys

# Get PEM file path
if len(sys.argv) > 1:
    pem = sys.argv[1]
else:
    pem = input("Enter path of private PEM file: ")

# Get the App ID
if len(sys.argv) > 2:
    app_id = sys.argv[2]
else:
    app_id = input("Enter your APP ID: ")

# Open PEM
with open(pem, 'rb') as pem_file:
    signing_key = jwt.jwk_from_pem(pem_file.read())

payload = {
    # Issued at time
    'iat': int(time.time()),
    # JWT expiration time (10 minutes maximum)
    'exp': int(time.time()) + 600,
    # GitHub App's identifier
    'iss': app_id
}

# Create JWT
jwt_instance = jwt.JWT()
encoded_jwt = jwt_instance.encode(payload, signing_key, alg='RS256')

print(f"JWT:  ", encoded_jwt)

Use your GitHub App's identifier (YOUR_APP_ID) as the value for the JWT iss (issuer) claim. You can obtain the GitHub App identifier via the initial webhook ping after creating the app, or at any time from the app settings page in the GitHub.com UI.

After creating the JWT, set it in the Header of the API request:

$ curl -i -H "Authorization: Bearer YOUR_JWT" -H "Accept: application/vnd.github+json" https://api.github.com/app

YOUR_JWT is the value you must replace.

The examples above uses the maximum expiration time of 10 minutes, after which the API will start returning a 401 error:

{
  "message": "'Expiration' claim ('exp') must be a numeric value representing the future time at which the assertion expires.",
  "documentation_url": "https://docs.github.com/rest"
}

You'll need to create a new JWT after the time expires.

Accessing API endpoints as a GitHub App

For a list of REST API endpoints you can use to get high-level information about a GitHub App, see "GitHub Apps."

Generating a private key

After you create a GitHub App, you'll need to generate one or more private keys in order to make requests to the GitHub API as the application itself. You'll use the private key to sign the JWTs used to request an installation access token.

You can create multiple private keys and rotate them to prevent downtime if a key is compromised or lost. To verify that a private key matches a public key, see Verifying private keys.

To generate a private key:

1. In the upper-right corner of any page, click your profile photo, then click Settings.

   

2. In the left sidebar, click Developer settings.

3. In the left sidebar, click GitHub Apps.

4. To the right of the GitHub App you want to modify, click Edit.

5. In "Private keys", click Generate a private key.

6. You will see a private key in PEM format downloaded to your computer. Make sure to store this file because GitHub only stores the public portion of the key.

Verifying private keys

GitHub generates a fingerprint for each private and public key pair using the SHA-256 hash function. You can verify that your private key matches the public key stored on GitHub by generating the fingerprint of your private key and comparing it to the fingerprint shown on GitHub.

To verify a private key:

1. Find the fingerprint for the private and public key pair you want to verify in the "Private keys" section of your GitHub App's developer settings page. For more information, see Generating a private key.
2. Generate the fingerprint of your private key (PEM) locally by using the following command:

   $ openssl rsa -in PATH_TO_PEM_FILE -pubout -outform DER | openssl sha256 -binary | openssl base64

3. Compare the results of the locally generated fingerprint to the fingerprint you see in GitHub.

Deleting private keys

You can remove a lost or compromised private key by deleting it, but you must always have at least one private key registered for your GitHub App. When your GitHub App has only one key, you will need to generate a new one before deleting the old one.