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README.md Fix "Private TLS Cert" Markdown markup Dec 9, 2018
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variables.tf feature: upgrade private-tls-cert to terraform 0.12 Jun 25, 2019

README.md

Private TLS Cert

This module can be used to generate a Certificate Authority (CA) public key and the public and private keys of a TLS certificate signed by this CA. This certificate is meant to be used with private services, such as a Vault cluster accessed solely within your AWS account. For publicly-accessible services, especially services you access through a web browser, you should NOT use this module, and instead get certificates from a commercial Certificate Authority, such as Let's Encrypt.

If you're unfamiliar with how TLS certificates work, check out the Background section.

Quick start

  1. Copy this module to your computer.

  2. Open variables.tf and fill in the variables that do not have a default.

  3. DO NOT configure Terraform remote state storage for this code. You do NOT want to store the state files as they will contain the private keys for the certificates.

  4. Run terraform apply. The output will show you the paths to the generated files:

    Outputs:
    
    ca_public_key_file_path = ca.key.pem
    private_key_file_path = vault.key.pem
    public_key_file_path = vault.crt.pem
    
  5. Delete your local Terraform state:

    rm -rf terraform.tfstate*
    

    The Terraform state will contain the private keys for the certificates, so it's important to clean it up!

  6. To inspect a certificate, you can use OpenSSL:

    openssl x509 -inform pem -noout -text -in vault.crt.pem
    

Now that you have your TLS certs, check out the next section for how to use them.

Using TLS certs

Distributing TLS certs to your servers

Distribute the private and public keys (the files at private_key_file_path and public_key_file_path) to the servers that will use them to handle TLS connections (e.g. Vault). For example, to run Vault with the run-vault module, you need to pass it the TLS certs:

/opt/vault/bin/run-vault --tls-cert-file /opt/vault/tls/vault.crt.pem --tls-key-file /opt/vault/tls/vault.key.pem

We strongly recommend encrypting the private key file while it's in transit to the servers that will use it. Here are some of the ways you could do this:

  • Encrypt the certificate using KMS and include the encrypted files in the AMI for your Vault servers. Give those servers an IAM role that lets them access the same KMS key and decrypt their certs just before booting.
  • Put your TLS cert in a secure S3 Bucket with encryption enabled. Give your Vault servers an IAM role that allows them to download the certs from the S3 bucket just before booting.
  • Manually upload the certificate to each EC2 Instance with scp.

Distributing TLS certs to your clients

Distribute the CA public key (the file at ca_public_key_file_path) to any clients of those services so they can validate the server's TLS cert. Without the CA public key, the clients will reject any TLS connections:

vault read secret/foo

Error initializing Vault: Get https://127.0.0.1:8200/v1/secret/foo: x509: certificate signed by unknown authority

Most TLS clients offer a way to explicitly specify extra public keys that you want to trust. For example, with Vault, you do this via the -ca-cert argument:

vault read -ca-cert=ca.crt.pem secret/foo

Key                 Value
---                 -----
refresh_interval    768h0m0s
value               bar

As an alternative, you can configure the certificate trust on your server so that all TLS clients trust your CA public key by running the update-certificate-store module on your server. Once you do that, your system will trust the public key without having to pass it in explicitly:

update-certificate-store --cert-file /opt/vault/tls/ca.crt.pem
vault read secret/foo

Key                 Value
---                 -----
refresh_interval    768h0m0s
value               bar

Background

How TLS/SSL Works

The industry-standard way to add encryption for data in motion is to use TLS (the successor to SSL). There are many examples online explaining how TLS works, but here are the basics:

  • Some entity decides to be a "Certificate Authority" ("CA") meaning it will issue TLS certificates to websites or other services

  • An entity becomes a Certificate Authority by creating a public/private key pair and publishing the public portion (typically known as the "CA Cert"). The private key is kept under the tightest possible security since anyone who possesses it could issue TLS certificates as if they were this Certificate Authority!

  • In fact, the consequences of a CA's private key being compromised are so disastrous that CA's typically create an "intermediate" CA keypair with their "root" CA key, and only issue TLS certificates with the intermediate key.

  • Your client (e.g. a web browser) can decide to trust this newly created Certificate Authority by including its CA Cert (the CA's public key) when making an outbound request to a service that uses the TLS certificate.

  • When CAs issue a TLS certificate ("TLS cert") to a service, they again create a public/private keypair, but this time the public key is "signed" by the CA. That public key is what you view when you click on the lock icon in a web browser and what a service "advertises" to any clients such as web browsers to declare who it is. When we say that the CA signed a public key, we mean that, cryptographically, any possessor of the CA Cert can validate that this same CA issued this particular public key.

  • The public key is more generally known as the TLS cert.

  • The private key created by the CA must be kept secret by the service since the possessor of the private key can "prove" they are whoever the TLS cert (public key) claims to be as part of the TLS protocol.

  • How does that "proof" work? Well, your web browser will attempt to validate the TLS cert in two ways:

    • First, it will ensure this public key (TLS cert) is in fact signed by a CA it trusts.
    • Second, using the TLS protocol, your browser will encrypt a message with the public key (TLS cert) that only the possessor of the corresponding private key can decrypt. In this manner, your browser will be able to come up with a symmetric encryption key it can use to encrypt all traffic for just that one web session.
  • Now your client/browser has:

    • declared which CA it will trust
    • verified that the service it's connecting to possesses a certificate issued by a CA it trusts
    • used that service's public key (TLS cert) to establish a secure session

Commercial or Public Certificate Authorities

For public services like banks, healthcare, and the like, it makes sense to use a "Commercial CA" like Verisign, Thawte, or Digicert, or better yet a widely trusted but free service like Let's Encrypt. That's because every web browser comes pre-configured with a set of CA's that it trusts. This means the client connecting to the bank doesn't have to know anything about CA's at all. Instead, their web browser is configured to trust the CA that happened to issue the bank's certificate.

Connecting securely to private services is similar to connecting to your bank's website over TLS, with one primary difference: We want total control over the CA.

Imagine if we used a commercial CA to issue our private TLS certificate and that commercial or public CA--which we don't control--were compromised. Now the attackers of that commercial or public CA could impersonate our private server. And indeed, it has happened multiple times.

How We'll Generate a TLS Cert for Private Services

One option is to be very selective about choosing a commercial CA, but to what benefit? What we want instead is assurance that our private service really was launched by people we trust. Those same people--let's call them our "operators"--can become their own CA and generate their own TLS certificate for the private service.

Sure, no one else in the world will trust this CA, but we don't care because we only need our organization to trust this CA.

So here's our strategy for issuing a TLS Cert for a private service:

  1. Create our own CA.

    • If a client wishes to trust our CA, they need only reference this CA public key.
    • We'll deal with the private key in a moment.
  2. Using our CA, issue a TLS Certificate for our private service.

    • Create a public/private key pair for the private service, and have the CA sign the public key.
    • This means anyone who trusts the CA will trust that the possessor of the private key that corresponds to this public key is who they claim to be.
    • We will be extremely careful with the TLS private key since anyone who obtains it can impersonate our private service! For this reason, we recommend immediately encrypting the private key with KMS.
  3. Freely advertise our CA's public key to all internal services.

    • Any service that wishes to connect securely to our private service will need our CA's public key so it can declare that it trusts this CA, and thereby the TLS cert it issued to the private service.
  4. Throw away the CA private key.

    • By erasing a CA private key it's impossible for the CA to be compromised, because there's no private key to steal!
    • Future certs can be generated with a new CA.
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