Securing Custom Domains with SSL

SSL support for your App Engine app goes above and beyond basic SSL by offering globally-distributed SSL endpoints and built-in load balancing to serve your app securely, reliably, and quickly to a worldwide audience.

By default, HTTPS connections on your custom domain will be enabled automatically using managed SSL certificates. Once your custom domain is mapped to your application and you have configured your DNS records, App Engine provisions a managed SSL certificate, handles renewing the certificate before it expires, and revokes it when you remove the custom domain from your application.

Before you begin

Make sure you have already set up your custom domain in your App Engine project.
If you use Cloud Load Balancing and serverless NEGS to route traffic to your App Engine app, we recommend that you map your custom domain to the load balancer instead of directly to your app, and use SSL certificates that are created for the load balancer. This eliminates the need to manage separate SSL certificates for each serverless app. In addition, with Cloud Load Balancing you can set SSL policies that control the features of SSL that your load balancer negotiates with clients. For more information, see the following pages:
Note the following limitation:
- We recommend that you use ingress controls so that your app only receives requests sent from the load balancer (and the VPC if you use it). Otherwise, users can use your app's App Engine URL to bypass the load balancer, Google Cloud Armor security policies, SSL certificates, and private keys that are passed through the load balancer.

Verify a managed certificate has been provisioned

After you set up your custom domain and update the DNS records, a managed SSL certificate is automatically provided within a few minutes. The managed certificate is signed either by Google Trust Services (GTS) or Let's Encrypt.

To verify that the certificate has been provisioned:

In the Google Cloud Console, go to App Engine > Settings > Custom Domains:

Go to the custom domains page
The SSL security displays as Google managed.

Troubleshooting managed SSL certificates

You might need to update the DNS records for your custom domain to verify your domain name. App Engine cannot provision certificates for unverified domains.
You can check the status of your certificate with the Admin API by using an AuthorizedCertificate.GET request.
If a managed certificate has not been provisioned because the DNS records are not available, the ManagedCertificate.ManagementStatus field might be FAILED_RETRYING_NOT_VISIBLE. Verify that your DNS records are up to date, wait a few minutes, then try again. It can take up to 24 hours for DNS records to become available.
If the status is FAILED_PERMANENT, then all renewal attempts have failed. Check your DNS settings then update your custom domain mapping by following the steps to update to managed SSL certificates.

Upgrading to managed SSL certificates

Before you upgrade to Google-managed SSL certificates, note that managed certificates do not support wildcard mappings.

If you are using subdomains, there is a limit of 20 managed certificates per week for each base domain. If you encounter the limit, App Engine keeps trying to issue managed certificates until all requests have been fulfilled.

To move from your own SSL certificates to Google-managed SSL certificates, or to add managed SSL certificates to an existing app with a custom domain, update your domain mapping:

In the Google Cloud Console, go to App Engine > Settings > Custom Domains:

Go to the custom domains page
Select the domain you want to secure, and click Enable managed security.

Disabling managed SSL certificates

To disable managed SSL certificates:

In the Google Cloud Console, go to App Engine > Settings > Custom Domains:

Go to the custom domains page
Select the domain and click Disable managed security.

Using your own SSL certificates

Instead of using managed SSL certificates, you can use your own certificate. If your certificate does not have a transparency proof, your app may show SSL warnings in Chrome due to certificate transparency proof enforcement. For more information on certificate transparency proofs and how to comply, read Enforcing Certificate Transparency.

To use and manage your own SSL certificates instead of Google-managed certificates:

Make sure you have already set up your custom domain in your App Engine project.
Disable the default Google-managed certificates.
Get a certificate for your domain from the certificate authority (CA) of your choice. The exact procedure can vary depending on the authority but see Obtaining a certificate for the typical steps.
Convert your private key and SSL certificate files into formats that are supported by App Engine. Before you can upload your files, your private key must be converted to an RSA private key and your SSL certificates must be concatenated into a single file. For more information, see Converting your private keys and concatenating your SSL certificates.
Ensure you have the right permissions in the Cloud Console and verified ownership (step 3) of all related domains or their parent domains. For example:
- If the certificate is for www.example.com you can verify ownership of either www.example.com or example.com.
- If the certificate is for www.example.com and sub.example.com you can either verify ownership of both www.example.com and sub.example.com, or of example.com.
- If the certificate is for *.example.com you must verify ownership of example.com.
Upload your private key and SSL certificate, and then map your domain to your app:
1. In the Google Cloud Console, go to App Engine > Settings > SSL certificates:
  
  Go to the SSL certificates page
2. Click Upload a new certificate.
3. Upload your concatenated SSL certificate under PEM encoded X.509 public key certificate, for example concat.crt, and then upload your RSA private key under Unencrypted PEM encoded RSA private key, for example myserver.key.pem.
4. Click Upload. Each SSL certificate that you upload is visible and available for use by all of your other Cloud projects so you don't have to upload the same certificate repeatedly.
  Note: If you upload an SSL certificate but never assign it to a domain, that certificate is automatically deleted after 30 days.
5. Select the certificate that you want to assign to a domain and then click Save to use SSL for that domain.
Test your changes by visiting your domain in your browser, using https, for example, https://www.example.com.

Note: Once the private key is uploaded, it is stored securely at Google and cannot be viewed.

Transferring mappings from a serving certificate to a new certificate

When a certificate nears its expiration date, you'll need to upload a new certificate and transfer the old certificate's existing mappings to that new certificate. The following procedure assumes that the existing certificate has not yet expired and is currently serving your custom domain.

To transfer mappings from an actively serving certificate:

Get a new certificate for your domain from the certificate authority (CA) of your choice. See Obtaining a certificate for the typical steps.
Convert your private key and SSL certificate files into formats that are supported by App Engine. For details, see Converting your private keys and concatenating your SSL certificates.
Upload your RSA private key and concatenated SSL certificate:
1. Upload the SSL certificate in the SSL certificates page.
  Go to the SSL certificates page
  1. Click Upload a new certificate.
  2. Upload your concatenated SSL certificate under PEM encoded X.509 public key certificate, for example concat.crt, and then upload your RSA private key under Unencrypted PEM encoded RSA private key, for example myserver.key.pem.
  3. Click Upload.
2. Select the new certificate you just added from the certificate list, then select the domain being served by the old certificate.
3. Click Save to transfer the mappings from the old certificate to the new one.

Obtaining a certificate

The process for getting an SSL certificate will vary depending on the certificate authority that you use. The instructions provided here might need to be adjusted slightly. Typically, each certificate authority provides instructions to assist you through the process.

To obtain a certificate for use with your App Engine app:

Generate your private key and a certificate signing request (CSR) by using the openssl tool:
1. Run the following command from a directory where you want to create the server.csr file:
```
openssl req -nodes -newkey rsa:2048 -keyout [MY_PRIVATE_KEY].key -out [MY_CSR].csr
```
  where:
  - [MY_PRIVATE_KEY].key is the generated file where your private key is stored. Example: myserver.key
  - [MY_CSR].csr is the generated file for your certificate signing request. Example: server.csr
2. When prompted, enter the following information:
  - Your 2 digit country code, for example, US for United States.
  - Your city name.
  - Your company name. You can use your own name if you don't have a company.
  - Your organizational unit or NA if you don't have this.
  - A common name that represents your domain, for example: www.example.com
  - Your email address.
  You don't need to provide any of the other values, they are all optional.
Determine which certificate authority works for you and then purchase a certificate. For example, you can use: SSLMate, Thawte, Comodo, or any other certificate authority.

For details about the types of supported certificates, see App Engine support for SSL certificates.
When your CA requests the contents of your CSR file, follow their instructions for copying and pasting contents from your .csr file that you generated earlier, for example server.csr.
Follow the prompts when your CA requests domain owner approval.

Tip: You might find it easiest to use the email approval method. You will need to configure an email address in your domain account, for example admin@example.com, so that you can receive and respond to the CA's approval request.

Note: After you submit the request for your certificate, it can take a few days before you receive the actual certificate from your CA.
After you provide domain owner approval, the CA sends the certificate to you, which is typically in the .zip file format. Unzip that file to a working directory so that you can concatenate those certificates for upload to App Engine.

Converting private keys and concatenating SSL certificates

You must convert your private key into an RSA private key and concatenate all of your SSL certificates, before uploading your private key and SSL certificates to App Engine.

Convert the private key file that you generated earlier, into an unencrypted RSA private key. For example, you can run the following openssl rsa command:
```
openssl rsa -in [MY_PRIVATE_KEY].key -out [MY_RSA_KEY].key.pem
```
where:
- [MY_PRIVATE_KEY].key is the generated file that contains your private key is stored. Example: myserver.key
- [MY_RSA_KEY].key is the generated file that contains unencrypted RSA private key. Example: myserver.key.pem
  
  Example:
```
openssl rsa -in myserver.key -out myserver.key.pem
```
Concatenate all of the .crt files from your CA into one file, using the following command:
```
cat [MY_DOMAIN_CERT].crt [MY_SecureServerCA].crt [MY_TrustCA].crt [MY_TrustExternalCARoot].crt > [MY_CONCAT_CERT].crt
```
where
- [MY_DOMAIN_CERT].crt is the certificate for your domain. Example: www_example_com.crt
- [MY_SecureServerCA].crt, [MY_TrustCA].crt, and [MY_TrustExternalCARoot].crt are the other certificate files that are provided by your CA.
- [MY_CONCAT_CERT].crt is the concatenated file that contains all of your .crt certificate files from your CA. Example: concat.crt
  
  Example:
```
cat www_example_com.crt AddTrustExternalCARoot.crt RSADomainValidationSecureServerCA.crt RSAAddTrustCA.crt > concat.crt
```
Verify your SSL certificate and private key:
1. To verify that the private key and certificate match, you can use the openssl x509 and openssl rsa commands. Examples:
```
openssl x509 -noout -modulus -in concat.crt | openssl md5
openssl rsa -noout -modulus -in myserver.key.pem | openssl md5
```
  Both the openssl x509 and openssl rsa commands should return the same output.
2. To verify that a certificate and its CA chain are valid, you can use the openssl verify command. For example:
```
openssl verify -verbose -CAfile concat.crt concat.crt
```
When you are ready, you can upload your RSA private key and concatenated certificates to App Engine.

App Engine support for SSL certificates

App Engine supports the following certificate types:

Single Domain/Hostname
Self-signed
Wildcard
Subject Alternative Name (SAN) / Multi Domain

It requires some things of your certificates and keys:

Private Key and Certificate should be uploaded in PEM format.
Private Keys must not be encrypted.
A certificate file can contain at most five certificates; this number includes chained and intermediate certificates.
All subject names on the host certificate should match or be subdomains of the user's verified domains.
Private keys must use RSA encryption.
Maximum allowed key modulus: 2048 bits

If the host certificate requires an intermediate or chained certificate (as many Certificate Authorities (CAs) issue), you will need to append the intermediate or chained certificates to the end of the public certificate file.

Some App Engine features use special subdomains. For example, an application can use subdomains to address application services, or to address different versions of your application. To use these with SSL, it makes sense to set up a SAN or wildcard certificate. Wildcard certificates only support one level of subdomain.

Using Strict-Transport-Security headers

For security reasons, all applications should encourage clients to use https connections. To instruct the browser to prefer https over http, use the Strict-Transport-Security header.

Viewing enabled TLS versions and ciphers

Install nmap Network Mapper on your computer if it isn't already available. See https://nmap.org/ for installation instructions.
To see which TLS versions and ciphers are enabled for your app, enter the following command:

nmap -sV --script ssl-enum-ciphers -p 443 hostname

Replace hostname with the hostname for your app. You can use either your custom domain or the appspot.com hostname that App Engine created for your app. For example:

nmap -sV --script ssl-enum-ciphers -p 443 example.uc.r.appspot.com

Disabling TLS versions and ciphers

If you use Cloud Load Balancing and serverless NEGS to route traffic to your App Engine app, you can disable a TLS version or cipher by defining a SSL security policy that specifies which TLS versions and ciphers can be used for HTTPS or SSL connections.