Perfect Forward Secrecy: What It is, How It Works?

Ruheni Mathenge  - Streaming Expert
Last updated: October 28, 2023
Read time: 6 minutes

Perfect forward secrecy is a great way to stay safe online. Check out this guide to know what it is, how it works, its pros and cons, everything.


Perfect Forward Secrecy (PFS) is a unique encryption protocol that provides robust security to online traffic. It ensures no data leak even if the cyber attacker steals the private encryption key to decrypt the information. PFS utilizes a key exchange algorithm that changes the session encryption key after every message, call, and web reload. This way, only a bit of your data can be compromised upon breach, and the major data will stay encrypted and protected. This article sheds light on Perfect Forward Secrecy and its operation to protect online activities.

Any data transmitted online remains vulnerable to unauthorized interception or even download. This can include personal or sensitive data like passwords, credit card numbers, and bank account details. So, finding an effective way to protect this information is essential.

In addition, a hacker may retrieve your past conversations upon getting hold of your private keys. Perfect forward secrecy (PFS) is one of the best ways to prevent this from happening and keep you safe online.

What is Perfect forward secrecy?

Perfect forward secrecy (PFS) is an encryption type that regularly changes the encryption key to protect your online activities. So, only a tiny bit of your data will be compromised in case of a security breach.

It is designed to switch keys after every call, message, and web page load. As a result, the intruder will only manage to get one message or operation because the rest of the data is encrypted with different keys.

The system will also safeguard the data of networks using the SSL/TLS protocols if their assigned keys are compromised.

How perfect forward secrecy works

Let us assume you are chatting with a friend through a secure messaging app that uses perfect forward secrecy. Also, the app has public and private keys that will encrypt your communication and identify the intended sender and receiver. These keys help your friend and you to recognize each other.

The key exchange algorithm will then create a temporary key to encrypt every single message. Therefore, when you send a message to your friend, the key will encrypt it. At the same time, your friend will decrypt the message with the same key. This process repeats with new session keys every time you send a message.

Therefore, even if a hacker manages to intercept your conversation, they will only access a single message and not the whole chat. Moreover, if they obtain your public and private keys, they can’t see your dialogue as every message is encrypted with different keys. Unfortunately, malicious actors can fake your friend or your identity and potentially snoop on future conversations.

Why is perfect forward secrecy hot right now?

A couple of major incidents happened in recent years that made perfect forward secrecy widespread in cyber security.

The first case is when Edward Snowden revealed how the US government collects network traffic secretly. So, if an institution can conduct mass surveillance, then anyone can. Snowden showed that secret surveillance is a reality rather than a possibility.

Even so, the IT community had been facing the risk of data compromise for years. Unfortunately, hiding a secret for long only gave malicious actors more time to figure out how to access it. Thankfully, the long-term SSL keys brought a sigh of relief by introducing advanced security to manage the risk.

The second critical incident was with the Heartbleed vulnerability that showed how OpenSSL could be attacked easily. After accommodating the long-term SSL keys for so long and from Snowden’s revelation, the IT community needed a more transient key exchange method.

In 2016, Apple decided that all apps in the App Store must employ perfect forward secrecy. Fast forward to 2018, the Internet Engineering Task Force (IETF) completed the new TLS 1.3 standard that orders PFS for all TLS sessions.

Unfortunately, the benefit of PFS is also its biggest downside. Hackers cannot decrypt your information unless they use one of two specific decryption approaches. Sadly, in the same way, your team cannot either.

What is perfect forward secrecy used for?

Perfect forward secrecy

Some of the uses of the perfect forward secrecy are:

  • Protecting web data. Perfect forward secrecy will secure your privacy by ensuring that nobody intercepts your web data. It safeguards the network’s transport layer as well as TLS, SSL, and HTTPS protocols. In addition, browsers can introduce PFS with websites compatible with HTTPS.
  • Securing instant messaging data. PFS is one of the most efficient ways to protect online conversation. The Signal messaging app popularized PFS encryption.
  • Safeguarding email communications. Email Services such as utilize PFS to secure messages in transit.

Pros and cons of perfect forward secrecy

Advantages of perfect forward secrecy

  • Secures your past data and communications. Even if hackers compromise your keys and meddle with your private chats, they can’t access your information.
  • Hackers can only access a small amount of data that won’t be of any help if they attack a PFS-protected server.

Disadvantages of perfect forward secrecy

  • Difficult to troubleshoot from the developers’ side.
  • Needs more programming resources and power.
  • Doesn’t secure future communications after public and private keys are compromised.

This digital age needs perfect forward secrecy

The consequences of not using perfect forward secrecy are more severe, especially if a private key is compromised. This can give a malicious actor instant access to all the past information transferred between a client and server using a specific key. Theoretically, anyone with your private key can access your encrypted traffic and decrypt everything.

Let us say you use HTTPS to protect your password when accessing your online bank. After a while, a hacker manages to obtain your bank’s TLS private key by whatever means. If you don’t have perfect secrecy, the hacker can use the private key to decrypt your past data and steal your password.

Hackers manage to do this if PFS is lacking because of the disposition of key exchange between the client and server. Initially, the client will set up a pre-master secret encrypted with the server’s public key. Then, it goes to the server, where the private key decrypts it. At this juncture, the pre-master secret in on both the client and the server.

Henceforth, the generation of session keys will rely on the pre-master secret, which plays a vital role in a back-and-forth conversation. These session keys are known as the master secret.

Unfortunately, a hacker can steal the private key if the server uses it repeatedly in the pre-master encryption process. The repercussion is that they will be able to snoop on and decrypt all the encrypted chats on the server, including past data.

For this to be successful, the attacker will have to acquire two random numbers used in the encryption process for the client and server. Sadly, they are conveyed in plain text, making acquiring them easy. In some cases, hackers can even get the pre-master secret.

It becomes easy to decrypt the pre-master secret because they already have the server’s private key. Now, they have all the puzzle pieces to create the master secret, giving them the power to decrypt all session data.

It is impossible to acquire all the sensitive data with one session data. However, if the hackers watch your traffic long enough, they will get something worthwhile.

A simpler way to stay secure

Perfect forward secrecy is a valuable security tool, but it is not the only way to safeguard your online privacy. A virtual private network (VPN) will keep you safe all the time. For example, NordVPN uses multiple layers of robust encryption. As a result, your online activities remain inaccessible even with a compromised internet connection.

Moreover, a single subscription allows you to connect up to six simultaneous devices, including computers, smartphones, routers, smart TVs, etc. In addition, it will help you bypass geo-restrictions of popular streaming services like Netflix, Amazon Prime Video, Hulu, and more.

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About the Author

Ruheni Mathenge

Ruheni Mathenge

Streaming Expert
201 Posts

Tech researcher and writer with a passion for cybersecurity. Ruheni Mathenge specializes in writing long-form content dedicated to helping individuals and businesses navigate and understand the constantly evolving online security and web freedom worlds. He specializes in VPNs, online anonymity, and encryption. His articles have appeared in many respected technology publications. Ruheni explains complicated technical concepts clearly and simply. He advocates digital freedom and online privacy at every level.

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