SSL/TLS Certificates — Complete Guide

Everything you need to know about HTTPS encryption, certificate types, and proper configuration

1. What is SSL/TLS and how it works

SSL (Secure Sockets Layer) and its successor TLS (Transport Layer Security) are cryptographic protocols that ensure secure communication over the internet. When you visit a site starting with https://, your browser and the server use TLS to encrypt all exchanged data.

TLS provides three key guarantees:

• Encryption — Data is encrypted so third parties cannot read it during transit. Symmetric algorithms like AES-256-GCM are used.
• Authentication — The certificate proves you are communicating with the real server, not an attacker (man-in-the-middle protection).
• Integrity — MAC (Message Authentication Code) guarantees that data has not been altered during transit.

Reference: RFC 8446 — The Transport Layer Security (TLS) Protocol Version 1.3

2. TLS Handshake — step by step

Before data is encrypted, the browser and server must "negotiate" through a process known as the TLS handshake. In TLS 1.3, this process is optimized to just 1 round-trip (1-RTT), while TLS 1.2 required 2 round-trips.

TLS 1.3 Handshake (1-RTT)

1. ClientHello — The browser sends a list of supported cipher suites, key share, and supported TLS versions.
2. ServerHello — The server selects a cipher suite, sends its Key Share, certificate, and Finished message — all in one response.
3. Client verifies — The browser verifies the certificate, generates a session key, and sends a Finished message.
4. Encrypted communication — All further data is encrypted using the agreed session key (AES-256-GCM or ChaCha20-Poly1305).

TLS 1.2 vs TLS 1.3 — Comparison

3. History: from SSL 1.0 to TLS 1.3

The protocol has gone through significant evolution from its origins at Netscape to modern TLS 1.3:

4. Why HTTPS is mandatory

HTTPS is no longer optional — it is the standard for every website, whether it is a blog, web store, or corporate portal.

Google ranking (SEO)

Google announced in 2014 that HTTPS is a ranking signal. Since 2018, Chrome marks all HTTP sites as "Not Secure". According to Google Transparency Report, 95% of pages loaded in Chrome use HTTPS (2024 data).

Browser warnings

Modern browsers actively warn users when accessing HTTP sites:

• Chrome — Displays "Not Secure" next to the URL, and for forms with passwords shows a full-screen warning.
• Firefox — Displays a crossed-out padlock and warning for login forms.
• Safari — Displays "Not Secure" text in the address bar.

GDPR and legal obligation

Article 32 of GDPR requires "appropriate technical measures" to protect personal data. Encryption in transit (HTTPS) is explicitly mentioned as an example. Sites without HTTPS that process personal data risk fines up to 20 million EUR or 4% of global revenue.

HTTP/2 and performance

HTTP/2, which brings significant performance improvements (multiplexing, header compression, server push), in practice requires HTTPS — no browser supports HTTP/2 over plain HTTP.

5. Certificate types: DV, OV, EV

SSL/TLS certificates are divided into three categories based on the level of validation:

DV certificates

The simplest type — the CA only verifies you control the domain. Verification is done via DNS TXT record, HTTP file, or email. Let's Encrypt issues exclusively DV certificates.

OV certificates

In addition to domain ownership, the CA also verifies the organization's identity (name, address, registration). This requires submitting legal documentation. Users can view organization information by clicking the padlock in the browser.

EV certificates

The strictest validation — the CA verifies the legal status, physical address, and operational status of the organization. Browsers used to display a green bar with the company name, but Chrome (from version 77, Sep 2019) and Firefox (from version 70) removed this visual distinction.

6. Let's Encrypt: how to get a free certificate

Let's Encrypt is a free, automated, and open Certificate Authority run by the Internet Security Research Group (ISRG). Launched in April 2016, it has issued over 300 million active certificates covering more than 360 million domains.

Step 1: Install Certbot

# Ubuntu/Debian
sudo apt update
sudo apt install certbot python3-certbot-nginx

# CentOS/RHEL
sudo dnf install certbot python3-certbot-nginx

Step 2: Obtain a certificate

# For Nginx
sudo certbot --nginx -d yourdomain.com -d www.yourdomain.com

# For Apache
sudo certbot --apache -d yourdomain.com -d www.yourdomain.com

# Certificate only (no automatic configuration)
sudo certbot certonly --standalone -d yourdomain.com

Step 3: Automatic renewal

Let's Encrypt certificates are valid for 90 days. Certbot automatically sets up a cron job for renewal:

# Test automatic renewal
sudo certbot renew --dry-run

# Cron job (automatically added)
0 0,12 * * * root certbot renew --quiet

7. Common mistakes and how to fix them

Mixed Content

Occurs when an HTTPS page loads resources (images, scripts, styles) over HTTP. The browser blocks "active mixed content" (scripts, iframes) and shows a warning for "passive mixed content" (images).

<!-- BAD: mixed content -->
<img src="http://example.com/image.jpg">
<script src="http://example.com/script.js"></script>

<!-- GOOD: use HTTPS or protocol-relative URLs -->
<img src="https://example.com/image.jpg">
<script src="//example.com/script.js"></script>

Expired certificate (ERR_CERT_DATE_INVALID)

Certificates have an expiration date. When they expire, the browser displays a full-screen warning. Solution: automate renewal with Certbot or use monitoring services (UptimeRobot, SSL Labs) that alert before expiration.

Self-signed certificate (ERR_CERT_AUTHORITY_INVALID)

Self-signed certificates are not signed by a trusted CA. Browsers do not trust them and display a warning. Use them only for development environments, never for production.

Name mismatch (ERR_CERT_COMMON_NAME_INVALID)

Occurs when the domain in the URL does not match the domain in the certificate. For example, a certificate for www.example.com will not work for example.com without a SAN (Subject Alternative Name) field.

Incomplete certificate chain

The server must send intermediate certificates, not just the leaf certificate. Use SSL Labs Server Test to verify your chain.

8. HSTS header and preload list

HTTP Strict Transport Security (HSTS) is a mechanism that instructs the browser to always use HTTPS for a given domain. It is defined in RFC 6797.

How HSTS works

The server sends the Strict-Transport-Security header with the HTTPS response. The browser records this and for future requests automatically converts HTTP to HTTPS, without any redirect.

# Recommended configuration
Strict-Transport-Security: max-age=63072000; includeSubDomains; preload

• max-age=63072000 — HSTS is valid for 2 years (in seconds)
• includeSubDomains — Applies to all subdomains as well
• preload — You allow inclusion in the HSTS preload list

HSTS Preload list

The HSTS preload list is built into Chrome, Firefox, Safari, Edge, and other browsers. Sites on the list are HTTPS-only from the first visit, without the possibility for users to bypass the protection. Apply at hstspreload.org.

9. Configuration: Nginx and Apache

Nginx — recommended SSL configuration

server {
    listen 443 ssl http2;
    server_name yourdomain.com www.yourdomain.com;

    ssl_certificate /etc/letsencrypt/live/yourdomain.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/yourdomain.com/privkey.pem;

    # TLS versions — only 1.2 and 1.3
    ssl_protocols TLSv1.2 TLSv1.3;

    # Cipher suites (Mozilla Intermediate)
    ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384;
    ssl_prefer_server_ciphers off;

    # HSTS
    add_header Strict-Transport-Security "max-age=63072000; includeSubDomains; preload" always;

    # OCSP Stapling
    ssl_stapling on;
    ssl_stapling_verify on;
    ssl_trusted_certificate /etc/letsencrypt/live/yourdomain.com/chain.pem;
    resolver 1.1.1.1 8.8.8.8 valid=300s;

    # DH parameters
    ssl_dhparam /etc/nginx/dhparam.pem;

    # Session cache
    ssl_session_cache shared:SSL:10m;
    ssl_session_timeout 1d;
    ssl_session_tickets off;
}

# HTTP to HTTPS redirect
server {
    listen 80;
    server_name yourdomain.com www.yourdomain.com;
    return 301 https://$server_name$request_uri;
}

Apache — recommended SSL configuration

<VirtualHost *:443>
    ServerName yourdomain.com
    ServerAlias www.yourdomain.com

    SSLEngine on
    SSLCertificateFile /etc/letsencrypt/live/yourdomain.com/fullchain.pem
    SSLCertificateKeyFile /etc/letsencrypt/live/yourdomain.com/privkey.pem

    # TLS versions
    SSLProtocol all -SSLv3 -TLSv1 -TLSv1.1

    # Cipher suites (Mozilla Intermediate)
    SSLCipherSuite ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384
    SSLHonorCipherOrder off

    # HSTS
    Header always set Strict-Transport-Security "max-age=63072000; includeSubDomains; preload"

    # OCSP Stapling
    SSLUseStapling on
    SSLStaplingCache shmcb:/var/run/ocsp(128000)
</VirtualHost>

# HTTP to HTTPS redirect
<VirtualHost *:80>
    ServerName yourdomain.com
    Redirect permanent / https://yourdomain.com/
</VirtualHost>

Reference: Mozilla SSL Configuration Generator — generates optimal configuration for your server.

10. References and resources

• RFC 8446 — TLS 1.3 — Official TLS 1.3 protocol specification
• RFC 6797 — HSTS — HTTP Strict Transport Security specification
• OWASP TLS Cheat Sheet — TLS configuration recommendations
• Mozilla SSL Config Generator — Server configuration generator
• Qualys SSL Labs — Free SSL configuration test
• Google Transparency Report — HTTPS — HTTPS adoption statistics
• Let's Encrypt statistics — Number of active certificates
• MDN — Transport Layer Security