letsencrypt-aws

letsencrypt-aws is a program that can be run in the background which automatically provisions and updates certificates on your AWS infrastructure using the AWS APIs and Let's Encrypt.

How it works

letsencrypt-aws takes a list of ELBs, and which hosts you want them to be able to serve. It runs in a loop and every day does the following:

It gets the certificate for that ELB. If the certificate is going to expire soon (in less than 45 days), it generates a new private key and CSR and sends a request to Let's Encrypt. It takes the DNS challenge and creates a record in Route53 for that challenge. This completes the Let's Encrypt challenge and we receive a certificate. It uploads the new certificate and private key to IAM and updates your ELB to use the certificate.

In theory all you need to do is make sure this is running somewhere, and your ELBs' certificates will be kept minty fresh.

How to run it

Before you can use letsencrypt-aws you need to have created an account with the ACME server (you only need to do this the first time). You can register using (if you already have an account you can skip this step):

$ # If you're trying to register for a server besides the Let's Encrypt
$ # production one, see the configuration documentation below.
$ python letsencrypt-aws.py register email@host.com
2016-01-09 19:56:19 [acme-register.generate-key]
2016-01-09 19:56:20 [acme-register.register] email=u'email@host.com'
2016-01-09 19:56:21 [acme-register.agree-to-tos]
-----BEGIN RSA PRIVATE KEY-----
[...]
-----END RSA PRIVATE KEY-----

You'll need to put the private key somewhere that letsencrypt-aws can access it (either on the local filesystem or in S3).

You will also need to have your AWS credentials configured. You can use any of the mechanisms documented by boto3, or use IAM instance profiles (which are supported, but not mentioned by the boto3 documentation). See below for which AWS permissions are required.

letsencrypt-aws takes it's configuration via the LETSENCRYPT_AWS_CONFIG environment variable. This should be a JSON object with the following schema:

{
    "domains": [
        {
            "elb": {
                "name": "ELB name (string)",
                "port": "optional, defaults to 443 (integer)"
            },
            "hosts": ["list of hosts you want on the certificate (strings)"],
            "key_type": "rsa or ecdsa, optional, defaults to rsa (string)"
        }
    ],
    "acme_account_key": "location of the account private key (string)",
    "acme_directory_url": "optional, defaults to Let's Encrypt production (string)"
}

The acme_account_key can either be located on the local filesystem or in S3. To specify a local file you provide "file:///path/to/key.pem", for S3 provide "s3://bucket-nam/object-name". The key should be a PEM formatted RSA private key.

Then you can simply run it: python letsencrypt-aws.py update-certificates.

If you add the --persistent flag it will run forever, rather than just once, sleeping for 24 hours between each check for certificate expiration. This is useful for production environments.

If your certificate is not expiring soon, but you need to issue a new one anyways, the --force-issue flag can be provided.

If you're into Docker, there is an automatically built image of letsencrypt-aws available as alexgaynor/letsencrypt-aws.

Operational Security

Keeping the source of your certificates secure is, for obvious reasons, important. letsencrypt-aws relies heavily on the AWS APIs to do its business, so we recommend running this code from EC2, so that you can use the Metadata service for managing credentials. You can give your EC2 instance an IAM instance profile with permissions to manage the relevant services (see below for complete details).

You need to make sure that the ACME account private key is kept secure. The best choice is probably in an S3 bucket with encryption enabled and access limited with IAM.

Finally, wherever you're running letsencrypt-aws needs to be trusted. letsencrypt-aws generates private keys in memory and uploads them to IAM immediately, they are never stored on disk.

IAM Policy

The minimum set of permissions needed for letsencrypt-aws to work is:

• route53:ChangeResourceRecordSets
• route53:GetChange
• route53:ListHostedZones
• elasticloadbalancing:DescribeLoadBalancers
• elasticloadbalancing:SetLoadBalancerListenerSSLCertificate
• iam:ListServerCertificates
• iam:UploadServerCertificate

If your acme_account_key is provided as an s3:// URI you will also need:

• s3:GetObject

It's likely possible to restrict these permissions by ARN, though this has not been fully explored.

An example IAM policy is:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "",
            "Effect": "Allow",
            "Action": [
                "route53:ChangeResourceRecordSets",
                "route53:GetChange",
                "route53:GetChangeDetails",
                "route53:ListHostedZones"
            ],
            "Resource": [
                "*"
            ]
        },
        {
            "Sid": "",
            "Effect": "Allow",
            "Action": [
                "elasticloadbalancing:DescribeLoadBalancers",
                "elasticloadbalancing:SetLoadBalancerListenerSSLCertificate"
            ],
            "Resource": [
                "*"
            ]
        },
        {
            "Sid": "",
            "Effect": "Allow",
            "Action": [
                "iam:ListServerCertificates",
                "iam:UploadServerCertificate"
            ],
            "Resource": [
                "*"
            ]
        }
    ]
}