What Is TLS (Transport Layer Security)?

Virtually everything we do on the Internet revolves around secure communications. However, all the information we share between computers can fall into unauthorized hands. As a proactive measure, security experts have developed TLS (Transport Layer Security) to protect data sent over the Internet. Learn what TLS is, how it works, and how to use it.

What is TLS?

According to RFC 5246, published by the IETF, Transport Layer Security (TLS) is a cryptographic protocol that safeguards the communication between two computer applications operating on a network. It’s a means of encrypting communications between two endpoints. 

The protocol enables client/server applications to communicate so no third party can eavesdrop, tamper with, or forge a message. As a result, hackers can’t access your sensitive data, such as your financial details.  

Transport Layer Security is a security protocol but not a transport protocol. Although it’s often referred to in the context of the transport layer due to its role in securing communication channels, it’s designed to run on top of a reliable transport protocol, such as TCP. 

In the OSI model, TLS sits between the application and transport layers. The protocol effectively wraps the transport layer's services to provide a secure channel for application layer protocols.

TLS applies to all data exchanged over the Internet, including that from web browsing sessions. Any website that requests personal data from the user or includes transactions needs SSL/TLS.

TLS offers encrypted security for your website and ensures data integrity between a user's web browser and the website's server. A Transport Layer Security certificate triggers HTTPS, and a padlock symbol appears in the URL bar after establishing a secure connection. 

Organizations that wish to secure data are moving to support TLS across all their deployments. Alongside web browsing, other applications that require data privacy over a network often use TLS. These include file transfers, email, instant messaging, VPN connections, Voice over IP, and video conferencing.

The latest version of the Transport Layer Security protocol used for encryption is TLS 1.3. It is much more secure than TLS 1.2. 

History of TLS 

The journey of secure Internet communication began with developing the Secure Sockets Layer protocol (SSL). Back in 1995, Netscape Communications developed SSL to secure web traffic. However, due to significant security flaws, SSL 1.0 never released to the public.

SSL 2.0 was the first version of SSL to be generally released over the web. However, it was quickly replaced by SSL 3.0, which TLS is based on. The creators intended SSL 3.0 to address many of its predecessor's vulnerabilities. 

Recognizing the need for an even more secure protocol led to the creation of Transport Layer Security. The transition from SSL to TLS also signified the protocol's evolution under the guidance of the Internet Engineering Task Force (IETF).

TLS was first defined in RFC 2246 in 1999 as an independent protocol that included several security improvements. It introduced using the HMAC (Hash-Based Message Authentication Code) for better integrity checks and refined the handshake process to enhance security.

Published in RFC 4346, TLS 1.1 solved specific issues in TLS 1.0, including protection against cipher block chaining (CBC) attacks. It also supported IANA registration of parameters to improve compatibility.

Because the earlier versions didn’t offer stronger cipher suites, i.e. Rivest Cipher 4 (RC4), TLS 1.2 was standardized in 2008 as per the RFC 5246 whitepaper. The protocol also introduced authenticated encryption with associated data (AEAD). 

There were also more flexible hashing algorithms. For example, Message-Digest Algorithm 5 (MD5) cryptographic hashes were supported.

Over the years, TLS continued to evolve, and ultimately, the Transport Layer Security standard across the web is TLS version 1.3 as defined by RFC 8446. It’s the most robust protocol. TLS 1.3 simplifies the handshake process, reduces latency, and removes outdated cryptographic algorithms. It is designed to be more resilient against modern security threats.

How does TLS work?

The protocol consists of two layers: the TLS Record Protocol and the TLS Handshake Protocol. The Transport Layer Security handshake is the initial phase where the client and server agree on the parameters of their secure communication. 

During the handshake stage, both elements:

• Exchange information to agree on the version of Transport Layer Security to use
• Select cryptographic algorithms they will use
• Authenticate each other using digital certificates to help prove both parties are who they claim to be and establish a secure session
• Negotiate cryptographic keys before data transmission begins

During the process, the client and server also exchange cryptographic keys. TLS uses public key cryptography, also called asymmetric cryptography. It consists of a public key and a private key.

These keys encrypt and decrypt the data they wish to send to the recipient. Keep in mind that only the recipient's private key can decrypt the data.

Once the keys are exchanged and the session is established, all data transmitted between the devices is encrypted using the agreed-upon encryption algorithm.

When the communication is complete, the Transport Layer Security session is terminated. This involves securely closing the connection and discarding the session keys. 

Understanding the TLS handshake process

Let’s look at what happens with TLS handshake behind the scenes.

1. Client “Hello." The client, which is the user’s web browser, sends a "ClientHello" message to the server, which includes the client's supported TLS version, cipher suites, compression methods, and a randomly generated number.
2. Server “Hello." The server responds with a "ServerHello" message, which contains the server's chosen TLS version.
3. Authentication. The server sends its digital certificate to the client for authentication. This certificate contains the server's public key and is signed by a trusted Certificate Authority (CA) to establish its authenticity. 
4. Key exchange. Depending on the suite, the server may send a "ServerKeyExchange" message. For example, an RSA key exchange uses the server's public key directly, while Diffie-Hellman shares additional parameters.
5. Pre-master secret. The client generates a "pre-master secret," encrypts it with the server's public key, and sends it to the server. Both the client and server use this pre-master secret to generate the same session keys for encryption.
6. Private key decryption. The server begins to decrypt the premaster secret.
7. Session-specific keys established. At this stage, both client and server independently generate session keys using the client random, server random, and premaster secret.
8. Client “Thank you." The client sends a "finished" message encrypted with the session key to indicate it's ready for secure communication.
9. Server “Thank you." The server sends a similar "finished" message, also encrypted with the session key.
10. Secure communication occurs. The handshake is complete, and both parties now use the session keys for secure, symmetric encryption during their ongoing communication.

Benefits of TLS 

TLS has become an IETF-approved standard protocol for online security. It accomplishes the following purposes.

• Secure authentication mechanism. Transport Layer Security connection authenticates the peer’s identity using asymmetric cryptography to reduce the risk of impersonation.
• Data encryption. The protocol encrypts the data exchanged between the client and server to ensure privacy. TLS uses symmetric cryptography in this case. Websites that handle financial transactions, such as online shopping, encrypt data exchanged between the user and the server to protect the confidential information of customers. 
• Data integrity. It ensures the integrity of the data transmitted. This means that the data cannot be modified in transit without detection, which is achievable using message authentication codes (MACs).
• Trust and credibility. Having an SSL certificate is crucial for building trust with users. When users see the "https://" and the padlock icon, they are more likely to feel secure in sharing their information with the website.
• Compliance with regulations. Many regulations and standards, such as HIPAA for healthcare in the US, require the use of encryption to safeguard users' data. 
• SEO benefits. Search engines like Google give preference to HTTPS-enabled websites in their search rankings, which can improve a website's visibility.

Security is a critical consideration across various forms of communication on the Internet, as it helps maintain the integrity of the transmitted data. 

What is a TLS certificate? 

A TLS certificate is a type of digital certificate issued by a Certificate Authority (CA) to verify the ownership of a public key and secure communications over the Internet. It is a critical component of TLS. 

Transport Layer Security certificates contain the following information: the subject domain name, the name of the issuing CA, additional subject domain names or subdomains, the date of issue, the expiry date, the public key, and a digital signature.

The security of a TLS certificate is based on the trust model of Public Key Infrastructure (PKI). When a CA issues a certificate, it verifies the identity of the certificate requester.

Challenges of TLS 

Like anything else, Transport Layer Security has challenges. As it becomes more widely used, it expands the attack surface. Some implementations still use outdated cryptographic algorithms that are vulnerable to attacks. 

Unlike protocols like IPSec, which operate at a lower network layer, TLS requires implementation at the application layer. This means that each application needs configuring to support TLS. Furthermore, the encryption and decryption processes involved in TLS can introduce some overhead in applications. 

TLS is a complex protocol with various versions, cipher suites, and configuration options that require technical expertise. 

What is the difference between TLS and SSL?

We often use the terms SSL and TLS interchangeably. However, Transport Layer Security serves as a new upgraded version of SSL. If anything, a TLS certificate is a more accurate name for SSL certificates. However, SSL has become the catch-all industry brand name because it has been in use for a long time and is more familiar to people.

Most certificate authorities (CAs) have continued to use the word SSL. In short, they mean TLS when they say SSL. All SSL and TLS certificates essentially do the same thing.

The key difference between TLS and SSL is in terms of results. In a nutshell, version 1.3 reduces latency to promote faster website load times. 

Almost every SSL version is now deprecated. SSL supports cipher suites with known security vulnerabilities. Transport Layer Security versions (1.2 and 1.3) are active and use advanced encryption algorithms, which lowers the chances of someone intending to compromise your security. 

Finally, the TLS handshake for the latest versions has a faster connection than SSL. 

Frequently asked questions

What are the main attacks on TLS?

The most common TLS attacks include phishing, SSL stripping, and man-in-the-middle (MitM) attacks. 

Is TLS 1.2 still secure?

Yes, TLS 1.2 is still considered secure, but version 1.3 offers improved security and performance. Use version 1.3 in modern applications. 

Is TLS used for email encryption?

Yes, Transport Layer Security works for email encryption and to secure protocols like SMTP to protect email data in transit between servers and clients. 

Can I use TLS with HTTP?

Yes, you can use Transport Layer Security with HTTP, resulting in HTTPS. ‘S’ stands for security for data transmitted between a web server and a client browser.