When configuring SSL decryption rules, you can apply the actions described in the following topics. These actions are also available for the default action, which applies to any traffic that does not match an explicit rule.
Note: Any traffic that passes through the SSL decryption policy must then pass through the access control policy. Except for traffic you drop in the SSL decryption policy, the ultimate allow or drop decision rests with the access control policy.
If you elect to decrypt and re-sign traffic, the system acts as a man-in-the-middle.
For example, the user types in https://www.cisco.com in a browser. The traffic reaches the FTD device, the device then negotiates with the user using the CA certificate specified in the rule and builds an SSL tunnel between the user and the FTD device. At the same time the device connects to https://www.cisco.com and creates an SSL tunnel between the server and the FTD device.
Thus, the user sees the CA certificate configured for the SSL decryption rule instead of the certificate from www.cisco.com. The user must trust the certificate to complete the connection. The FTD device then performs decryption/re-encryption in both directions for traffic between the user and destination server.
Note: If the client does not trust the CA used to re-sign the server certificate, it warns the user that the certificate should not be trusted. To prevent this, import the CA certificate into the client trusted CA store. Alternatively, if your organization has a private PKI, you can issue an intermediate CA certificate signed by the root CA which is automatically trusted by all clients in the organization, then upload that CA certificate to the device.
If you configure a rule with the Decrypt Re-Sign action, the rule matches traffic based on the referenced internal CA certificate’s signature algorithm type, in addition to any configured rule conditions. Because you can select a single re-sign certificate for the SSL decryption policy, this can limit traffic matching for resign rules.
For example, outgoing traffic encrypted with an elliptic curve (EC) algorithm matches a Decrypt Re-Sign rule only if the re-sign certificate is an EC-based CA certificate. Similarly, traffic encrypted with an RSA algorithm matches Decrypt Re-Sign rules only if the global re-sign certificate is RSA; outgoing traffic encrypted with an EC algorithm does not match the rule, even if all other configured rule conditions match.
Decrypt Known Key
If you own the destination server, you can implement decryption with a known key. In this case, when the user opens a connection to https://www.cisco.com, the user sees the actual certificate for www.cisco.com, even though it is the FTD device that is presenting the certificate.
Your organization must be the owner of the domain and certificate. For the example of cisco.com the only possible way to have the end user see Cisco’s certificate would be if you actually own the domain cisco.com (i.e. you are Cisco Systems) and have ownership of the cisco.com certificate signed by a public CA. You can only decrypt with known keys for sites that your organization owns.
The main purpose of decrypting with a known key is to decrypt traffic heading to your HTTPS server to protect your servers from external attacks. For inspecting client side traffic to external HTTPS sites, you must use decrypt re-sign as you do not own the servers.
Note: To use known key decryption, you must upload the server’s certificate and key as an internal identity certificate, and then add it to the list of known-key certificates in the SSL decryption policy settings. Then, you can deploy the rule for known-key decryption with the server’s address as the destination address. For information on adding the certificate to the SSL decryption policy, see Configure Certificates for Known Key and Re-Sign Decryption.
If you elect to bypass decryption for certain types of traffic, no processing is done on the traffic. The encrypted traffic proceeds to the access control policy, where it is allowed or dropped based on the access control rule it matches.
You can simply block encrypted traffic that matches an SSL decryption rule. Blocking in the SSL decryption policy prevents the connection from reaching the access control policy.
When you block an HTTPS connection, the user does not see the system default block response page. Instead, the user sees the browser’s default page for a secure connection failure. The error message does not indicate the site was blocked due to policy. Instead, errors might indicate that there are no common encryption algorithms. It will not be obvious from this message that you blocked the connection on purpose.