What is SSL Decryption?

SSL decryption is the process of intercepting and decrypting encrypted SSL/TLS sessions so that security services – such as URL filtering, antivirus, data loss prevention (DLP), and Cloud Access Security Broker (CASB) – can inspect the underlying traffic for threats and policy violations before re-encrypting it and forwarding it to its destination.

Key takeaways

• Decrypts SSL/TLS traffic so security tools can inspect its contents.
• Required for deep inspection: DLP, CASB, antivirus, and URL filtering.
• Traffic is re-encrypted and forwarded after inspection completes.
• Decrypt bypass exempts trusted or sensitive traffic from inspection.
• Depends on a trusted root CA certificate installed on client devices.

On this page: Why it matters · How it works · Types · SSL Decryption & Versa · FAQ

Why is SSL Decryption important?

Most enterprise traffic today is encrypted with SSL/TLS, which protects data in transit but also blinds traditional security tools. A firewall or gateway that cannot see inside an encrypted session cannot tell whether it carries a legitimate download, a data-exfiltration attempt, or malware retrieving a payload. SSL decryption closes that blind spot by decrypting the session at a policy enforcement point, inspecting it, then re-encrypting the traffic before forwarding it on.

Without decryption, services such as antivirus, DLP, and CASB only ever see ciphertext – they act on metadata like destination IP and SNI, never the payload. As encrypted traffic approaches nearly all web and SaaS use, decryption becomes the difference between a policy that inspects content and one that only guesses at it. Most organizations balance this with selective decryption, exempting categories such as banking or healthcare where privacy outweighs the benefit.

What is SSL Decryption?

SSL decryption – more precisely TLS decryption, since SSL is the deprecated predecessor of TLS – works by terminating the encrypted session at an inspection point instead of letting it pass straight through. A gateway sits between the client and the server, presents its own certificate – signed by a CA the client trusts – to the client, and separately negotiates a normal TLS session with the real server. Holding both sets of session keys, it decrypts the request, inspects it, re-encrypts it, and forwards it on, then does the same in reverse for the response.

This requires the inspecting device’s root CA certificate to be installed in the client’s trust store, so decrypted sessions don’t trigger certificate warnings. Decryption policies then decide which sessions to open – some are deliberately left alone.

Types of SSL Decryption

SSL decryption implementations generally fall into three approaches. Forward (outbound) decryption inspects traffic that internal users send to the internet or SaaS applications – the most common deployment, letting a gateway apply URL filtering, DLP, and antivirus before traffic leaves the network. Inbound (reverse proxy) decryption protects internally hosted servers, decrypting client requests before they reach the application.

Selective (policy-based) decryption, or decrypt bypass, exempts specific traffic from inspection by category, destination, or certificate attributes – leaving banking or healthcare sessions encrypted for compliance, for example. Most deployments combine all three: broad forward decryption, targeted inbound decryption, and a bypass list for traffic that should never be opened.

SSL Decryption & Versa

In the Versa platform, SSL decryption is a capability that URL filtering and other inspection features use to see inside encrypted sessions. URL filtering works with or without decryption enabled, but enabling it lets the feature categorize traffic using the actual host and URI of the decrypted request rather than the SNI field alone, producing more granular filtering. The same option exists for NGFW-based URL filtering. A Decrypt Bypass setting lets administrators exempt traffic, such as captive-portal redirects or trusted destinations, from decryption.

On the Versa SASE Client, decryption depends on a root CA certificate installed in the endpoint’s trust store; browsers verify decrypted sessions against that internal CA rather than a public one. A missing certificate, or validation by a public CA instead, means decryption isn’t working.

Frequently asked questions

What is the difference between SSL decryption and TLS inspection?

They refer to the same practice. SSL is the deprecated predecessor to TLS, but “SSL decryption” remains the common industry term even though almost all traffic decrypted today is TLS rather than the older SSL protocol. TLS inspection is simply the more technically precise name for the same decrypt-inspect-re-encrypt process.

Why do security tools need to decrypt SSL/TLS traffic?

Encryption that protects data from eavesdroppers also hides malicious payloads and data exfiltration from security tools. Without decryption, a gateway can only inspect metadata such as destination IP and SNI – not the actual content, where threats and sensitive data travel.

Does SSL decryption break end-to-end encryption?

Within the inspected segment, yes – the inspecting device terminates one TLS session and originates another, so it can read plaintext between those legs. Both legs stay individually encrypted, but true end-to-end encryption between client and server no longer holds once decrypted.

What is SSL decrypt bypass?

Decrypt bypass is a policy setting that exempts specific traffic from decryption – for example, sensitive-category destinations, or captive-portal redirects that only need enough visibility to display a login page. It balances inspection coverage against privacy, compliance, and compatibility concerns.

Is SSL decryption the same as a man-in-the-middle attack?

Technically, yes – the mechanism is identical to a man-in-the-middle interception. What makes it legitimate is consent: the deploying organization controls the endpoints, installs its own root CA certificate with the device owner’s knowledge, and applies the practice under a documented policy.

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