TLS/SSL Certificates (Media Engineering Architecture & Deployment)

# TLS/SSL Certificates

Learn about SSL certificates, how to create your own and how to request some from [Let's Encrypt][letsencrypt].

This material is part of [architecture & deployment course](https://github.com/MediaComem/comem-archidep) for [Media Engineering](https://heig-vd.ch/formations/bachelor/filieres/ingenierie-des-medias).

**You will need**

* A Unix CLI
* A server with an Ubuntu operating system a public IP address
  * A website deployed on that server behind an [nginx][nginx] reverse proxy

**Recommended reading**

* [Reverse Proxying](../reverse-proxy/)

---
class: center, middle
## What is a TLS or SSL certificate?

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---
### Public key certificates

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A [**public key certificate**][pubkey-certificate] is an electronic document
that proves the ownership of a public key using [public key cryptography][pubkey].

A [**TLS or SSL certificate**][tls-certificate] is a type of public key certificate
that allows a computer such as a **web server** to prove that it owns a public key.

> SSL is the original Secure Sockets Layer protocol first published in 1995 and which is now deprecated.
> TLS is the newer and more secure [Transport Layer Security][tls] protocol first published in 1999,
> its latest version being TLS 1.3 published in 2018 (at the time of writing).
> Although TLS is used today, TLS certificates are sometimes still called "SSL certificates".

A TLS certificate is linked to one or multiple domain name:

* For example `google.com` or `www.microsoft.com`.
* It could also be `*.example.com`.
  In this case, it's a **wildcard certificate** valid for all matching subdomains.

---
### What is a TLS certificate good for?

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A TLS certificate is one of the components that allows a server to **communicate securely over HTTPS** using the TLS protocol.

* The client and server agree on a [cipher suite][cipher-suite] (cipher and hash functions) they both support.
* The server provides its TLS certificate and the client confirms its validity.
* A symmetric encryption key is exchanged using the asymmetric [Diffie-Hellman key exchange][dh].
* The client and server can now communicate securely by using symmetric encryption to encrypt all traffic.

---
## Validity of TLS certificates

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Your browser will not simply accept any TLS certificate as valid.
You can generate your own TLS certificate to test this.

On your server, run the following commands:

```bash
$> `mkdir ~/certificate`

$> `cd ~/certificate`

$> `openssl req -newkey rsa:2048 -nodes -keyout key.pem \`
   `-x509 -days 365 -out certificate.pem`

Generating a 2048 bit RSA private key
...
You are about to be asked to enter information that will be incorporated
into your certificate request.
...
Country Name (2 letter code) [AU]:CH
State or Province Name (full name) [Some-State]:Vaud
Locality Name (eg, city) []:Yverdon
Organization Name (eg, company) [Internet Widgits Pty Ltd]:HEIG-VD
Organizational Unit Name (eg, section) []:
Common Name (e.g. server FQDN or YOUR name) []:john-doe.archidep.ch
Email Address []:john.doe@heig-vd.ch
```

---
### What's in a TLS certificate?

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The previous command generated two files:

* A **TLS certificate** in the `certificate.pem` file.
* A **private key** in the `key.pem` file.

```bash
$> `ls`
certificate.pem key.pem
```

The `certificate.pem` file is simply a Base64-encoded plain text file:

```bash
$> `cat certificate/certificate.pem`
-----BEGIN CERTIFICATE-----
MIID7jCCAtagAwIBAgIJAPPUhT7FLeLRMA0GCSqGSIb3DQEBCwUAMIGLMQswCQYD
VQQGEwJDSDENMAsGA1UECAwEVmF1ZDEQMA4GA1UEBwwHWXZlcmRvbjEQMA4GA1UE
...
```

---
#### Decoding the contents of a TLS certificate

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A TLS certificate is not encrypted.
You can decode its contents with the following command:

```bash
$> `openssl x509 -text -noout -in certificate.pem`
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            ef:ea:3a:93:c5:74:a8:e7
    Signature Algorithm: sha256WithRSAEncryption
        Issuer: C = CH, ST = Vaud, L = Yverdon, O = HEIG-VD,
                CN = john-doe.archidep.ch,
                emailAddress = john-doe@heig-vd.ch
        Validity
            Not Before: Jan 15 14:28:11 2019 GMT
            Not After : Jan 15 14:28:11 2020 GMT
...
```

---
### Configuring nginx to use you TLS certificate

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Assuming you already have a website deployed with nginx, add the following lines to its configuration file:

```bash
server {

listen 80;
* listen 443 ssl;
* ssl_certificate /home/john_doe/certificate/certificate.pem;
* ssl_certificate_key /home/john_doe/certificate/key.pem;
* ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
* ssl_ciphers HIGH:!aNULL:!MD5;

server_name john-doe.archidep.ch;
  root /home/john_doe/my-website;
  index index.html;
}
```

Reload nginx's configuration with `sudo nginx -s reload`.

> The `listen 443 ssl` directive instructs nginx to also listen on port 443 (HTTPS) for this site.
> The [`ssl_certificate`][nginx-ssl-certificate-directive] directive makes it serve the specified TLS certificate file to clients.
> The [`ssl_certificate_key`][nginx-ssl-certificate-key-directive] directive makes it
> use the private key in the specified file to perform the asymmetric cryptography in the TLS protocol.

---
### Invalid certificate authority

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If you access your website with this configuration, you will see the following:

In this example, Chrome indicates that there is an error of type `NET::ERR_CERT_AUTHORITY_INVALID` error.
This means that there is no valid **certificate authority** that guarantees that this certificate is valid.

---
#### Self-signed root certificate

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Your TLS certificate is not valid because is is a **self-signed** certificate.

As you can see, the certificate details indicates that it is a **root certificate**,
meaning that no other certificate authority guarantees its validity.
Since you signed it yourself (by running the earlier `openssl req` command),
and you are not a valid certificate authority, it is considered invalid by your browser.

---
### How to make a certificate valid

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To be valid, a TLS certificate must be **signed by a valid certificate authority**.
This signature is a [digital signature][digital-signature] using [public key cryptography][pubkey]:

* The certificate authority has a **private and public key pair**.
* They will **use their private key to create a signature** of your certificate.
* They will **distribute their public key** so that anyone in possession of your certificate and their public key can **verify the signature**.

Of course, the certificate authority that signed your certificate must prove that it owns the public key that is being distributed.
To do this, it provides a public key certificate of its own, similar to your own TLS certificate.

---
### Chain of trust

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Your TLS certificate and various other public key certificates are thus linked together in a **chain of trust**:

Each certificate, starting with your own **end-user certificate**
(or end-entity certificate) is signed by the next certificate authority,
proving its validity to the client.
This is a type of [public key infrastructure][pki].

---
#### Viewing a certificate's chain of trust

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Browsers allow you to view a TLS certificate's chain of trust:

---
#### Intermediate certificates

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In this example, there are 3 certificates in the chain, but there could be more.
All certificates in the middle are **intermediate certificate authorities**:

---
#### Root certificate authorities

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The **root certificate authority** is the one at the top of the chain:

---
### Root certificate validity

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As you can see in the chain of trust diagram, the **root certificate** is self-signed:
there is no inherent difference between it and a certificate you have generated yourself with `openssl req`.

How then does the browser know that a root certificate authority is valid?

---
#### Trusted CA Certificate Lists

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Browsers and operating systems have **hardcoded lists of root certificates** that are considered to be **trusted**.
For example:

* [Trusted Root Certificates in iOS][ios-root-ca-list]
* [Trusted Root Certificates in Mozilla Firefox][mozilla-root-ca-list]

When your browser checks a TLS's certificate chain of trust,
it expects the chain's root certificate to be one of the **already trusted** ones;
otherwise, the TLS certificate is deemed invalid.

In other words, if you want to launch a new company to issue valid TLS certificates,
you must contact Mozilla, Apple, Microsoft, etc. to have them include your new root certificate
in their programs:

* [Apple Root Program][apple-root-ca]
* [Microsoft Root Program][microsoft-root-ca]
* [Mozilla Root Program][mozilla-root-ca]
* [Oracle Root Program][oracle-root-ca]

---
## Obtaining a TLS certificate

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To obtain a valid TLS certificate, you need to request one from a [**certification authority (CA)**][ca].
In 2018, the 3 most popular CAs are:

* [IdenTrust][identrust]
* [Comodo][comodo]
* [DigiCert][digicert]

Before the certification authority will give you a signed, valid TLS certificate,
they will make you go through **domain validation**.
In other words, they will ask you to **prove that you are the legitimate owner of the domain**
indicated in the certificate.

---
### Domain validation

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There are multiple techniques to do that.
For example, assuming that you own the domain `example.com`
and request a TLS certificate for that domain from a CA:

* **HTTP validation:** the CA can ask you to put a file at `http://example.com/abc.txt` containing a random validation token.
  Doing this proves that you control the server which serves the content for the domain.
* **DNS validation:** the CA can ask you to create a custom DNS record for your domain.
  Doing this proves that you control the DNS zone file for the domain.
* **Email validation:** the CA can send you a mail with a validation link at `admin@example.com`.
  Following the link proves that you are the administrator of the domain,
  since only you could manage that email address.

---
### Purchasing TLS certificates

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Some certificate authorities sell you TLS certificates.
What you pay for is:

* **Compatibility:** Not all root certificates are as widespread.
  Some may be present in more browsers and operating systems.
  By paying more, you may get a certificate that is certified to be compatible with more clients.
* **Warranty:** Many certificate authorities will pay you a given sum of money
  if your security is compromised because of a weakness in their TLS certificate.
  You may increase that warranty by purchasing a more expensive certificate.
* **[Extended Validation Certificate (EV)][ev-certificate]:**
  Certificate authorities can validate that a legal entity is the owner of a domain,
  enabling the browser to display a so-called "green-bar certificate".

![Extended Validation Certificate](images/ev-certificate.png)

---
### Let's Encrypt

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[Let's Encrypt][letsencrypt] is a **free, automated, and open certificate authority (CA)**, run for the public's benefit.
It is a service provided by the [Internet Security Research Group (ISRG)][isrg].
The key principles behind Let's Encrypt are:

* **Free:** Anyone who owns a domain name can use Let's Encrypt to obtain a valid TLS certificate at zero cost.
* **Automatic:** Software running on a web server can painlessly obtain a certificate,
  securely configure it for use, and automatically take care of renewal.
* **Secure:** Let's Encrypt will serve as a platform for advancing TLS security best practices,
  both on the CA side and by helping site operators properly secure their servers.
* **Transparent:** All certificates issued or revoked will be publicly recorded and available for anyone to inspect.
* **Open:** The automatic issuance and renewal protocol will be published as an open standard that others can adopt.
* **Cooperative:** Much like the underlying Internet protocols themselves,
  Let's Encrypt is a joint effort to benefit the community, beyond the control of any one organization.

---
#### Certbot

.breadcrumbs[<a href="#1">TLS/SSL Certificates</a> > <a href="#18">Obtaining a TLS certificate</a> > <a href="#21">Let's Encrypt</a>]

[Certbot][certbot] is a tool made by the [Electronic Frontier Foundation (EFF)][eff].
It can be installed on any Linux server and will:

* Help you **obtain Let's Encrypt certificates**.
* **Configure your web server,** such as [Apache][apache] or [nginx][nginx],
  to use the certificates.
* Set up **automatic renewal** for these certificates.

---
## References

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* [Public-key Certificate][pubkey-certificate]
  * [Public Key Cryptography][pubkey]
  * [Chain of Trust][chain-of-trust]
* [Certificate Authority (CA)][ca]
  * [Let's Encrypt][letsencrypt]
* [Generating a self-signed certificate using OpenSSL][self-signed]
* [Nginx - Configuring HTTPS servers][nginx-ssl]

[apache]: https://www.apache.org
[apple-root-ca]: https://www.apple.com/certificateauthority/ca_program.html
[ca]: https://en.wikipedia.org/wiki/Certificate_authority
[certbot]: https://certbot.eff.org/
[chain-of-trust]: https://en.wikipedia.org/wiki/Chain_of_trust
[cipher-suite]: https://en.wikipedia.org/wiki/Cipher_suite
[comodo]: https://www.comodo.com
[dh]: https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange
[digicert]: https://www.digicert.com
[digital-signature]: https://en.wikipedia.org/wiki/Digital_signature
[eff]: https://www.eff.org
[ev-certificate]: https://en.wikipedia.org/wiki/Extended_Validation_Certificate
[identrust]: https://www.identrust.com
[ios-root-ca-list]: https://support.apple.com/en-gb/HT204132
[isrg]: https://letsencrypt.org/isrg/
[letsencrypt]: https://letsencrypt.org/
[microsoft-root-ca]: https://docs.microsoft.com/en-us/previous-versions//cc751157(v=technet.10)
[mozilla-root-ca]: https://www.mozilla.org/en-US/about/governance/policies/security-group/certs/policy/
[mozilla-root-ca-list]: https://wiki.mozilla.org/CA/Included_Certificates
[nginx]: http://nginx.org/
[nginx-ssl]: http://nginx.org/en/docs/http/configuring_https_servers.html
[nginx-ssl-certificate-directive]: http://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_certificate
[nginx-ssl-certificate-key-directive]: http://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_certificate_key
[oracle-root-ca]: https://www.oracle.com/technetwork/java/javase/javasecarootcertsprogram-1876540.html
[pki]: https://en.wikipedia.org/wiki/Public_key_infrastructure
[pubkey]: https://en.wikipedia.org/wiki/Public-key_cryptography
[pubkey-certificate]: https://en.wikipedia.org/wiki/Public_key_certificate
[self-signed]: https://www.ibm.com/support/knowledgecenter/SSMNED_5.0.0/com.ibm.apic.cmc.doc/task_apionprem_gernerate_self_signed_openSSL.html
[tls]: https://en.wikipedia.org/wiki/Transport_Layer_Security
[tls-certificate]: https://en.wikipedia.org/wiki/Public_key_certificate#TLS/SSL_server_certificate
[tls-procedure]: https://en.wikipedia.org/wiki/Transport_Layer_Security#Description