Traffic Decryption Overview

The following topics provide an overview of Transport Layer Security/Secure Sockets Layer (TLS/SSL) inspection, discuss the prerequisites for TLS/SSL inspection configuration, and detail deployment scenarios.


Note

Because TLS and SSL are often used interchangeably, we use the expression TLS/SSL to indicate that either protocol is being discussed. The SSL protocol has been deprecated by the IETF in favor of the more secure TLS protocol, so you can usually interpret TLS/SSL as referring to TLS only.

The exception is SSL policies. Because the FMC configuration option is Policies > Access Control > SSL, we use the term SSL policies although these policies are used to define rules for TLS and SSL traffic.

For more information about SSL and TLS protocols, see a resource such as SSL vs. TLS - What's the Difference?.

Traffic Decryption Explained

Most traffic on the internet is encrypted and in most cases, you don't want to decrypt it; even if you don't, you can still glean some information about it and block it from your network if necessary.

Your choices are:

  • Decrypt the traffic and subject it to the full array of deep inspection:

    • Advanced Malware Protection

    • Security intelligence

    • Threat Intelligence Director

    • Application detectors

    • URL and category filtering

  • Leave the traffic encrypted and set up your access control and SSL policy to look for and potentially block:

    • Old protocol versions (such as Secure Sockets Layer)

    • Unsecure cipher suites

    • Applications with high risk and low business relevance

    • Untrusted issuer Distinguished Names

Access control policy

An access control policy is the main configuration that invokes subpolicies and other configurations, including an SSL policy. If you associate an SSL policy with access control, the system uses that SSL policy to handle encrypted sessions before it evaluates the sessions with access control rules. If you do not configure TLS/SSL inspection, or your devices do not support it, access control rules handle all encrypted traffic.

Access control rules also handle encrypted traffic when your TLS/SSL inspection configuration allows the traffic to pass. However, some access control rule conditions require unencrypted traffic, so encrypted traffic might match fewer rules. Also, by default, the system disables intrusion and file inspection of encrypted payloads. This helps reduce false positives and improves performance when an encrypted connection matches an access control rule that has intrusion and file inspection configured.

Notes

Set up decrypt rules only if your managed device handles encrypted traffic. TLS/SSL ruless require processing overhead that can impact performance.

The Firepower System does not support mutual authentication; that is, you cannot upload a client certificate to the FMC and use it for either Decrypt - Resign or Decrypt - Known Key TLS/SSL rule actions. For more information, see Decrypt and Resign (Outgoing Traffic). and Known Key Decryption (Incoming Traffic).

The Firepower System does not currently support TLS version 1.3 encryption or decryption. When users visit a web site that negotiates TLS 1.3 encryption, users might see errors similar to the following in their web browser:

  • ERR_SSL_PROTOCOL_ERROR

  • SEC_ERROR_BAD_SIGNATURE

  • ERR_SSL_VERSION_INTERFERENCE

For more information about how to control this behavior, contact Cisco TAC.

If you set the value of TCP maximum segment size (MSS) using FlexConfig, the observed MSS could be less than your setting. For more information, see About the TCP MSS.

TLS/SSL Handshake Processing

In this documentation, the term TLS/SSL handshake represents the two-way handshake that initiates encrypted sessions in both the SSL protocol and its successor protocol, TLS.

In an inline deployment, the system processes the TLS/SSL handshake, potentially modifying the ClientHello message and acting as a TCP proxy server for the session.

The following figure shows an inline deployment.

The Threat Defense managed device is between the client and the server. When configured, it can intercept, decrypt, encrypt, and analyze, and route traffic.

After the client establishes a TCP connection with the server (after it successfully completes the TCP three-way handshake), the managed device monitors the TCP session for any attempt to initiate an encrypted session. The TLS/SSL handshake establishes an encrypted session using the exchange of specialized packets between client and server. In the SSL and TLS protocols, these specialized packets are called handshake messages. The handshake messages communicate which encryption attributes both the client and server support:

  • ClientHello—The client specifies multiple supported values for each encryption attribute.

  • ServerHello—The server specifies a single supported value for each encryption attribute, and the ServerHello response determines which encryption method the system uses during the secure session.

After a TLS/SSL handshake completes, the managed device caches encrypted session data, which allows session resumption without requiring the full handshake. The managed device also caches server certificate data, which allows faster handshake processing in subsequent sessions that use the same certificate.

ClientHello Message Handling

The client sends the ClientHello message to the server that acts as the packet destination if a secure connection can be established. The client sends the message to initiate the TLS/SSL handshake or in response to a ServerHello message from the destination server.

Overview

The following figure shows an example. Also see RFC 8446, sec. 4 RFC 5246, sec. 7.3. You can also consult a resource such as What Happens in a TLS Handshake? at cloudflare.com.

The client initiates secure negotiation with a ClientHello message. The server responds and the negotiations complete when the cipher suite has been agreed on.

The process can be summarized as follows:

  1. ClientHello initiates the process.

    The ClientHello message contains the Server Name Indication (SNI), which has the server's fully qualified domain name.

  2. After a managed device processes a ClientHello message and transmits it to the destination server, the server determines whether it supports the decryption attributes the client specified in the message. If it does not support those attributes, the server sends a handshake failure alert to the client. If it supports those attributes, the server sends the ServerHello message. If the agreed-upon key exchange method uses certificates for authentication, the server certificate message immediately follows the ServerHello message.

    The server certificate contains the Subject Alternative Name (SAN), which can have fully qualified domain names and IP addresses. For more information about the SAN, see Distinguished Name.

  3. When the managed device receives these messages, it attempts to match them with TLS/SSL rules configured on the system. These messages contain information that was absent from either the ClientHello message or the session data cache. Specifically, the system can potentially match these messages on TLS/SSL rules' Distinguished Names, Certificate Status, Cipher Suites, and Versions conditions.

Note that although the data transmitted in the session is encrypted, the handshake messages are not encrypted.

Data exchange

If you configure TLS/SSL decryption, when a managed device receives a ClientHello message, the system attempts to match the message to TLS/SSL rules that have the Decrypt - Resign or Decrypt - Known Key action. The match relies on data from the ClientHello message and from cached server certificate data. Possible data includes:

Table 1. Data Availability for TLS/SSL Rule Conditions

TLS/SSL Rule Condition

Data Present In

Zones

ClientHello

Networks

ClientHello

VLAN Tags

ClientHello

Ports

ClientHello

Users

ClientHello

Applications

ClientHello (Server Name Indicator extension)

Categories

ClientHello (Server Name Indicator extension)

Certificate

Server certificate (potentially cached)

Distinguished Names

Server certificate (potentially cached)

Certificate Status

Server certificate (potentially cached)

Cipher Suites

ServerHello

Versions

ServerHello

Note

Use the Cipher Suite and Version rule conditions only in rules with either the Block or Block with reset rule actions. The use of these conditions in rules with other rule actions can interfere with the system's ClientHello processing, resulting in unpredictable performance.

ClientHello modifications

If the ClientHello message matches a Decrypt - Resign or Decrypt - Known Key rule, the system modifies the ClientHello message as follows:

  • Compression methods—Strips the compression_methods element, which specifies the compression methods the client supports. The system cannot decrypt compressed sessions.

  • Cipher suites—Strips cipher suites from the cipher_suites element if the system does not support them. If the system does not support any of the specified cipher suites, the system transmits the original, unmodified element. This modification reduces the Unknown Cipher Suite and Unsupported Cipher Suite types of undecryptable traffic.

  • Session identifiers—Strips any value from the Session Identifier element and the SessionTicket extension (RFC 5077, sec 3.2) that does not match cached session data. If a ClientHello value matches cached data, an interrupted session can resume without the client and server performing the full TLS/SSL handshake. This modification increases the chances of session resumption and reduces the Session Not Cached type of undecryptable traffic.

  • Elliptic curves—Strips elliptic curves from the Supported Elliptic Curves extension if the system does not support them. If the system does not support any of the specified elliptic curves, the managed device removes the extension and strips any related cipher suites from the cipher_suites element.

  • ALPN extensions—Strips any value from the Application-Layer Protocol Negotiation (ALPN) extension that is unsupported in the system (for example, the HTTP/2 protocol).

  • Other Extensions—Strips the Next Protocol Negotiation (NPN) and TLS Channel IDs extensions.

    TLS/SSL rules with a Decrypt - Resign or Decrypt - Known Key action now natively support the Extended Master Secret (EMS) extension during ClientHello negotiation, enabling more secure communications. The EMS extension is defined by RFC 7627.

After the system modifies the ClientHello message, it determines whether the message passes access control evaluation (which can include deep inspection). If the message passes, the system transmits it to the destination server.

If the ClientHello message does not match a Decrypt - Resign or Decrypt - Known Key rule, the system does not modify the message. It then determines whether the message passes access control evaluation (which can include deep inspection). If the message passes inspection, the system transmits it to the destination server.

ClientHello is not modified if traffic matches a Monitor rule condition.

Man-in-the-middle

Direct communication between the client and server is no longer possible during the TLS/SSL handshake, because after message modification the Message Authentication Codes (MACs) computed by the client and server no longer match. For all subsequent handshake messages (and for the encrypted session once established), the managed device acts as a man-in-the-middle. It creates two TLS/SSL sessions, one between client and managed device, one between managed device and server. As a result, each session contains different cryptographic session details.


Note

The cipher suites that the system can decrypt are frequently updated and do not correspond directly to the cipher suites you can use in TLS/SSL rule conditions. For the current list of decryptable cipher suites, contact Cisco TAC.

ServerHello and Server Certificate Message Handling

Overview

The following figure shows an example. Also see RFC 8446, sec. 4 RFC 5246, sec. 7.3. You can also consult a resource such as What Happens in a TLS Handshake? at cloudflare.com.

The client initiates secure negotiation with a ClientHello message. The server responds and the negotiations complete when the cipher suite has been agreed on.

The process can be summarized as follows:

  1. ClientHello initiates the process.

    The ClientHello message contains the Server Name Indication (SNI), which has the server's fully qualified domain name.

  2. After a managed device processes a ClientHello message and transmits it to the destination server, the server determines whether it supports the decryption attributes the client specified in the message. If it does not support those attributes, the server sends a handshake failure alert to the client. If it supports those attributes, the server sends the ServerHello message. If the agreed-upon key exchange method uses certificates for authentication, the server certificate message immediately follows the ServerHello message.

    The server certificate contains the Subject Alternative Name (SAN), which can have fully qualified domain names and IP addresses. For more information about the SAN, see Distinguished Name.

  3. When the managed device receives these messages, it attempts to match them with TLS/SSL rules configured on the system. These messages contain information that was absent from either the ClientHello message or the session data cache. Specifically, the system can potentially match these messages on TLS/SSL rules' Distinguished Names, Certificate Status, Cipher Suites, and Versions conditions.

Note that although the data transmitted in the session is encrypted, the handshake messages are not encrypted.

TLS/SSL Rule actions

If the messages do not match any TLS/SSL rules, the managed device performs the SSL Policy Default Actions.

If the messages match a rule that belongs to an SSL policy associated with an access control policy, the managed device continues as appropriate:

Action: Monitor
The TLS/SSL handshake continues to completion. The managed device tracks and logs traffic but does not decrypt encrypted it.
Action: Block or Block with Reset

The managed device blocks the TLS/SSL session and, if configured, resets the TCP connection.

Action: Do Not Decrypt

The TLS/SSL handshake continues to completion. The managed device does not decrypt the application data exchanged during the TLS/SSL session.

Action: Decrypt - Known Key

The managed device attempts to match the server certificate data to an Internal Certificate object previously imported into the FMC. Because you cannot generate an Internal Certificate object, and you must possess its private key, we assume you own the server on which you're using known key decryption.

If the certificate matches a known certificate, the TLS/SSL handshake continues to completion. The managed device uses the uploaded private key to decrypt and re encrypt the application data exchanged during the TLS/SSL session.

If the server changes its certificate between the initial connection with the client and subsequent connections, you must import the new server certificate in the FMC for future connections to be decrypted.

Action: Decrypt - Resign

The managed device processes the server certificate message and re-signs the server certificate with the previously imported or generated certificate authority (CA). The TLS/SSL handshake continues to completion. The managed device then uses the uploaded private key to decrypt and re encrypt the application data exchanged during the TLS/SSL session.


Note

The Firepower System does not support mutual authentication; that is, you cannot upload a client certificate to the FMC and use it for either Decrypt - Resign or Decrypt - Known Key TLS/SSL rule actions. For more information, see Decrypt and Resign (Outgoing Traffic). and Known Key Decryption (Incoming Traffic).

TLS/SSL Best Practices

This section discusses information you should keep in mind when creating your SSL policies and rules.


Note

Because TLS and SSL are often used interchangeably, we use the expression TLS/SSL to indicate that either protocol is being discussed. The SSL protocol has been deprecated by the IETF in favor of the more secure TLS protocol, so you can usually interpret TLS/SSL as referring to TLS only.

The exception is SSL policies. Because the FMC configuration option is Policies > Access Control > SSL, we use the term SSL policies although these policies are used to define rules for TLS and SSL traffic.

For more information about SSL and TLS protocols, see a resource such as SSL vs. TLS - What's the Difference?.

The Case for Decryption

Traffic that is encrypted when it passes through the system can be allowed or blocked only but it cannot be subjected to deep inspection or the full range of policy enforcement (such as intrusion prevention).

All encrypted connections:

  • Are sent through the SSL policy to determine if they should be decrypted or blocked.

    You can also configure TLS/SSL rules to block encrypted traffic of types you know you do not want on your network, such as traffic that uses the nonsecure SSL protocol or traffic with an expired or invalid certificate.

  • If unblocked, whether or not decrypted, traffic goes through the access control policy for a final allow or block decision.

Only decrypted traffic takes advantage of the system's threat defense and policy enforcement features, such as:

  • Advanced Malware Protection

  • Security intelligence

  • Threat Intelligence Director

  • Application detectors

  • URL and category filtering

Keep in mind that decrypting and then re-encrypting traffic adds a processing load on the device, which can reduce overall system performance.

We recommend selectively decrypting traffic to make the best use of access control policies and deep inspection.

In summary:

  • Encrypted traffic can be allowed or blocked by policy; encrypted traffic cannot be inspected

  • Decrypted traffic is subject to threat defense and policy enforcement; decrypted traffic can be allowed or blocked by policy

When to Decrypt Traffic, When Not to Decrypt

This section provides guidelines on when you should decrypt traffic and when you should allow it to pass through the firewall encrypted.

When not to decrypt traffic

You should not decrypt traffic if doing so is forbidden by:

  • Law; for example, some jurisdictions forbid decrypting financial information

  • Company policy; for example, your company might forbid decrypting privileged communications

  • Privacy regulations

  • Traffic that uses certificate pinning (also referred to as TLS/SSL pinning) must remain encrypted to prevent breaking the connection

(Snort 2.) If you elect to bypass decryption for certain types of traffic, no processing is done on the traffic. The encrypted traffic is first evaluated by SSL policy and then proceeds to the access control policy, where a final allow or block decision is made.

(Snort 3.) SSL policy is not bypassed for any connections that match access control rules with actions of Trust, Block, or Block with reset, unless the traffic is prefiltered. The encrypted traffic is first evaluated by SSL policy and then proceeds to the access control policy, where a final allow or block decision is made.

Encrypted traffic can be allowed or blocked on any TLS/SSL rule condition, including, but not limited to:

  • Certificate status (for example, expired or invalid certificate)

  • Protocol (for example, the nonsecure SSL protocol)

  • Network (security zone, IP address, VLAN tag, and so on)

  • Exact URL or URL category

  • Port

  • User group

TLS/SSL rules provide a Do Not Decrypt action for this traffic; for more information, see TLS/SSL Rule Do Not Decrypt Action.


Note

The related information links at the end of this topic explain how some aspects of rule evaluation work. Conditions such as URL and application filtering have limitations with respect to encrypted traffic. Make sure you understand those limitations.

For more information about using URL filtering in Do Not Decrypt rules, see TLS/SSL Rule Do Not Decrypt Action.

When to decrypt traffic

All encrypted traffic must be decrypted to take advantage of the system's threat protection and policy enforcement features. To the extent your managed device allows traffic to be decrypted (subject to its memory and processing power), you should decrypt traffic that is not prohibited by law or regulation. If you must decide what traffic to decrypt, base your decision on the risk of allowing the traffic on your network. The system provides a flexible framework for classifying traffic using rule conditions, which include URL reputation, cipher suite, protocol, and many other factors.

Decrypt and Resign (Outgoing Traffic)

The Decrypt - Resign TLS/SSL rule action enables the system to act as a man in the middle, intercepting, decrypting, and (if the traffic is allowed to pass) inspecting and re-encrypting it. The Decrypt - Resign rule action is used with outgoing traffic; that is, the destination server is outside your protected network.

The FTD device negotiates with the client using an internal Certificate Authority (CA) object specified in the rule and builds a TLS/SSL tunnel between the client and the FTD device. At the same time, the device connects to the destination web site and creates an SSL tunnel between the server and the FTD device.

Thus, the client sees the CA certificate configured for the TLS/SSL rule instead of the certificate from the destination server. The client must trust the firewall's certificate to complete the connection. The FTD device then performs decryption/re-encryption in both directions for traffic between the client and the destination server.

Prerequisite

To use the Decrypt - Resign rule action, you must create an internal CA object using a CA file and paired private key file. You can generate a CA and private key in the system if you don't already have them.


Note

The Firepower System does not support mutual authentication; that is, you cannot upload a client certificate to the FMC and use it for either Decrypt - Resign or Decrypt - Known Key TLS/SSL rule actions. For more information, see Decrypt and Resign (Outgoing Traffic). and Known Key Decryption (Incoming Traffic).

Known Key Decryption (Incoming Traffic)

The Decrypt - Known Key TLS/SSL rule action uses a server's private key to decrypt traffic. The Decrypt - Known Key rule action is used with incoming traffic; that is, the destination server is inside your protected network.

The main purpose of decrypting with a known key is to protect your servers from external attacks.

Prerequisite

To use the Decrypt - Known Key rule action, you must create an internal certificate object using the server’s certificate file and paired private key file.


Note

The Firepower System does not support mutual authentication; that is, you cannot upload a client certificate to the FMC and use it for either Decrypt - Resign or Decrypt - Known Key TLS/SSL rule actions. For more information, see Decrypt and Resign (Outgoing Traffic). and Known Key Decryption (Incoming Traffic).

TLS/SSL Rule Components

Each TLS/SSL rule has the following components.

State

By default, rules are enabled. If you disable a rule, the system does not use it to evaluate network traffic, and stops generating warnings and errors for that rule.

Position

Rules in an SSL policy are numbered, starting at 1. The system matches traffic to rules in top-down order by ascending rule number. With the exception of Monitor rules, the first rule that traffic matches is the rule that handles that traffic.

Conditions

Conditions specify the specific traffic the rule handles. Conditions can match traffic by security zone, network or geographical location, VLAN, port, application, requested URL, user, certificate, certificate subject or issuer, certificate status, cipher suite, or encryption protocol version. The use of conditions can depend on target device licenses.

Action

A rule’s action determines how the system handles matching traffic. You can monitor, allow, block, or decrypt encrypted matching traffic. Decrypted and allowed encrypted traffic is subject to further inspection. Note that the system does not perform inspection on blocked encrypted traffic.

Logging

A rule’s logging settings govern the records the system keeps of the traffic it handles. You can keep a record of traffic that matches a rule. You can log a connection when the system blocks an encrypted session or allows it to pass without decryption, according to the settings in an SSL policy. You can also force the system to log connections that it decrypts for further evaluation by access control rules, regardless of how the system later handles or inspects the traffic. You can log connections to the Firepower Management Center database, as well as to the system log (syslog) or to an SNMP trap server.

For more information about logging, see Best Practices for Connection Logging in the Firepower Management Center Administration Guide.


Tip
Properly creating and ordering TLS/SSL rule is a complex task. If you do not plan your policy carefully, rules can preempt other rules, require additional licenses, or contain invalid configurations. To help ensure that the system handles traffic as you expect, the SSL policy interface has a robust warning and error feedback system for rules.

TLS/SSL Rule Order Evaluation

When you create the TLS/SSL rule in an SSL policy, you specify its position using the Insert list in the rule editor. TLS/SSL rules in an an SSL policy are numbered, starting at 1. The system matches traffic to TLS/SSL rules in top-down order by ascending rule number.

In most cases, the system handles network traffic according to the first TLS/SSL rule where all the rule’s conditions match the traffic. Except in the case of Monitor rules (which log traffic but do not affect traffic flow), the system does not continue to evaluate traffic against additional, lower-priority rules after that traffic matches a rule. Conditions can be simple or complex; you can control traffic by security zone, network or geographical location, VLAN, port, application, requested URL, user, certificate, certificate distinguished name, certificate status, cipher suite, or encryption protocol version.

Each rule also has an action, which determines whether you monitor, block, or inspect matching encrypted or decrypted traffic with access control. Note that the system does not further inspect encrypted traffic it blocks. It does subject encrypted and undecryptable traffic to access control. However, access control rule conditions require unencrypted traffic, so encrypted traffic matches fewer rules.

Rules that use specific conditions (such as network and IP addresses) should be ordered before rules that use general conditions (such as applications). If you're familiar with the Open Systems Interconnect (OSI) model, use similar numbering in concept. Rules with conditions for layers 1, 2, and 3 (physical, data link, and network) should be ordered first in your rules. Conditions for layers 5, 6, and 7 (session, presentation, and application) should be ordered later in your rules. For more information about the OSI model, see this Wikipedia article.


Tip

Proper TLS/SSL rule order reduces the resources required to process network traffic, and prevents rule preemption. Although the rules you create are unique to every organization and deployment, there are a few general guidelines to follow when ordering rules that can optimize performance while still addressing your needs.

In addition to ordering rules by number, you can group rules by category. By default the system provides three categories: Administrator, Standard, and Root. You can add custom categories, but you cannot delete the system-provided categories or change their order.

Multi-Rule Example

The following scenario summarizes the ways that TLS/SSL rules handle traffic in an inline deployment.

The administrator has many options to handle encrypted traffic, and, depending on the option chosen, the traffic could pass through the firewall with no inspection or traffic could be decrypted and re-encrypted

In this scenario, traffic is evaluated as follows:

  • Undecryptable Traffic Action evaluates encrypted traffic first. For traffic the system cannot decrypt, the system either blocks it without further inspection or passes it for access control inspection. Encrypted traffic that does not match continues to the next rule.

  • TLS/SSL Rule 1: Monitor evaluates encrypted traffic next. Monitor rules track and log encrypted traffic but do not affect traffic flow. The system continues to match traffic against additional rules to determine whether to permit or deny it.

  • TLS/SSL Rule 2: Do Not Decrypt evaluates encrypted traffic third. Matching traffic is not decrypted; the system inspects this traffic with access control, but not file or intrusion inspection. Traffic that does not match continues to the next rule.

  • TLS/SSL Rule 3: Block evaluates encrypted traffic fourth. Matching traffic is blocked without further inspection. Traffic that does not match continues to the next rule.

  • TLS/SSL Rule 4: Decrypt - Known Key evaluates encrypted traffic fifth. Matching traffic incoming to your network is decrypted using a private key you upload. The decrypted traffic is then evaluated against access control rules. Access control rules handle decrypted and unencrypted traffic identically. The system can block traffic as a result of this additional inspection. All remaining traffic is reencrypted before being allowed to the destination. Traffic that does not match the TLS/SSL rule continues to the next rule.

  • TLS/SSL Rule 5: Decrypt - Resign is the final rule. If traffic matches this rule, the system re-signs the server certificate with an uploaded CA certificate, then acts as a man-in-the-middle to decrypt traffic. The decrypted traffic is then evaluated against access control rules. Access control rules treat decrypted and unencrypted traffic identically. The system can block traffic as a result of this additional inspection. All remaining traffic is reencrypted before being allowed to the destination. Traffic that does not match the SSL rule continues to the next rule.

  • SSL policy Default Action handles all traffic that does not match any of the TLS/SSL rules. The default action either blocks encrypted traffic without further inspection or does not decrypt it, passing it for access control inspection.

TLS Crypto Acceleration

TLS crypto acceleration accelerates the following:

  • TLS/SSL encryption and decryption

  • VPN, including TLS/SSL and IPsec

Supported Hardware

The following hardware models support TLS crypto acceleration:

  • Secure Firewall 3100

  • Firepower 2100

  • Firepower 4100/9300

    For information about TLS crypto acceleration support on Firepower 4100/9300 FTD container instances, see the FXOS Configuration Guide.

TLS crypto acceleration is not supported on any virtual appliances or on any hardware except for the preceding.


Note

For more information about TLS crypto acceleration and the 4100/9300, see the FXOS Configuration Guide.

Features Not Supported by TLS crypto acceleration

Features not supported by TLS crypto acceleration include the following:

  • Managed devices where FTD container instance is enabled.

  • If the inspection engine is configured to preserve connections and the inspection engine fails unexpectedly, TLS/SSL traffic is dropped until the engine restarts.

    This behavior is controlled by the configure snort preserve-connection {enable | disable} command.

TLS Crypto Acceleration Guidelines and Limitations

Keep the following in mind if your managed device has TLS crypto acceleration enabled.

HTTP-only performance

Using TLS crypto acceleration on a managed device that is not decrypting traffic can affect performance.

Federal Information Processing Standards (FIPS)

If TLS crypto acceleration and Federal Information Processing Standards (FIPS) are both enabled, connections with the following options fail:

  • RSA keys less than 2048 bytes in size

  • Rivest cipher 4 (RC4)

  • Single Data Encryption Standard (single DES)

  • Merkle–Damgard 5 (MD5)

  • SSL v3

FIPS is enabled when you configure the FMC and managed devices to operate in a security certifications compliance mode. To allow connections when operating in those modes, you can configure web browsers to accept more secure options.

For more information:

TLS heartbeat

Some applications use the TLS heartbeat extension to the Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) protocols defined by RFC6520. TLS heartbeat provides a way to confirm the connection is still alive—either the client or server sends a specified number of bytes of data and requests the other party echo the response. If this is successful, encrypted data is sent.

When a managed device with TLS crypto acceleration enabled encounters a packet that uses the TLS heartbeat extension, the managed device takes the action specified by the setting for Decryption Errors in the SSL policy's Undecryptable Actions:

  • Block

  • Block with reset

For more information, see Default Handling Options for Undecryptable Traffic.

To determine whether applications are using TLS heartbeat, see Troubleshoot TLS Heartbeat.

You can configure a Max Heartbeat Length in a Network Analysis Policy (NAP) to determine how to handle TLS heartbeats. For more information, see The SSL Preprocessor.

TLS/SSL oversubscription

TLS/SSL oversubscription is a state where a managed device is overloaded with TLS/SSL traffic. Any managed device can experience TLS/SSL oversubscription but only managed devices that support TLS crypto acceleration provide a configurable way to handle it.

When a managed device with TLS crypto acceleration enabled is oversubscribed, any packet received by the managed device is acted on according to the setting for Handshake Errors in the SSL policy's Undecryptable Actions:

  • Inherit default action

  • Do not decrypt

  • Block

  • Block with reset

If the setting for Handshake Errors in the SSL policy's Undecryptable Actions is Do Not decrypt and the associated access control policy is configured to inspect the traffic, inspection occurs; decryption does not occur.

If a significant amount of oversubscription is occurring, you have the following options:

  • Upgrade your managed devices to increase TLS/SSL processing capacity.

  • Change your SSL policies to add Do Not Decrypt rules for traffic that is not a high priority to decrypt.

View the Status of TLS Crypto Acceleration

This topic discusses how you can determine if TLS crypto acceleration is enabled.

Perform the following task in the FMC.

Procedure

Step 1

Log in to the FMC.

Step 2

Click Devices > Device Management.

Step 3

Click Edit (edit icon) to edit a managed device.

Step 4

Click Device page. TLS crypto acceleration status is displayed in the General section.

History for SSL Policy

Feature

Minimum FMC

Minimum FTD

Details

TLS 1.3 decryption.

7.2.0

7.2.0

You can now enable TLS 1.3 decryption in an SSL policy's advanced actions. TLS 1.3 decryption requires the managed device run Snort 3.

Other options are available as well; for more information, see SSL Policy Advanced Options.

New/changed screen: SSL Policy > Advanced Settings

SSL policy advanced settings.

7.2.0

7.1.0

SSL policy advanced settings

New/changed screen: SSL Policy > Advanced Settings

Ability to specify handling of URLs having unknown reputation.

6.7.0

6.7.0

For details, see About URL Filtering with Category and Reputation.

ClientHello modification for Decrypt - Known key rules.

6.7.0

6.7.0

For details, see ClientHello Message Handling.

Ability to extract the certificate in TLS 1.3 traffic to enable traffic to match URL and application criteria in access control rules.

6.7.0

6.7.0

New/modified screen: Policies > Access Control > (edit an access control policy) > Advanced link.

For details, see SSL Policy Advanced Options.

Changes to category and reputation-based URL Filtering.

6.7.0

6.5.0

For details, see About URL Filtering with Category and Reputation.

TLS crypto acceleration cannot be disabled.

6.4.0

6.4.0

TLS crypto acceleration is enabled on all supported devices.

On a managed device with native interfaces, TLS crypto acceleration cannot be disabled.

Support for TLS crypto acceleration on FTD container instances is limited as discussed in the next row of this table.

Removed commands:

system support ssl-hw-accel enable

system support ssl-hw-accel disable

system support ssl-hw-status

Support for TLS crypto acceleration on one FTD container instance on a Firepower 4100/9300 module/security engine.

6.4.0

6.4.0

You can now enable TLS crypto acceleration for one FTD container instance on a module/security engine. TLS crypto acceleration is disabled for other container instances, but enabled for native instances.

New/Modified commands:

config hwCrypto enable

show crypto accelerator status replaces system support ssl-hw-status )

TLS/SSL hardware acceleration is now referred to as TLS crypto acceleration.

6.4.0

6.4.0

The name change reflects that TLS/SSL encryption and decryption acceleration is supported on more devices. Depending on the device, acceleration might be performed in software or in hardware.

Affected screen: To view the status of TLS crypto acceleration, Devices > Device Management > Device, General page.

Extended Master Secret extension supported (see RFC 7627).

6.3.0.1

6.3.0.1

The TLS Extended Master Secret extension is supported for SSL policies; specifically, policies with a rule action of Decrypt - Resign or Decrypt - Known Key.

Extended Master Secret extension not supported.

6.3.0

6.3.0

The extension is stripped during ClientHello modification for Decrypt - Resign rules.

TLS/SSL hardware acceleration enabled by default.

6.3.0

6.3.0

TLS/SSL hardware acceleration is enabled by default on all supported devices but can be disabled if desired.

Extended Master Secret extension supported (see RFC 7627).

6.2.3.9

6.2.3.9

The TLS Extended Master Secret extension is supported for SSL policies; specifically, policies with a rule action of Decrypt - Resign or Decrypt - Known Key.

Aggressive TLS 1.3 downgrade.

6.2.3.7

6.2.3.7

Using the system support ssl-client-hello-enabled aggressive_tls13_downgrade {true|false} CLI command, you can determine the behavior for downgrading TLS 1.3 traffic to TLS 1.2. For details, see the Cisco Secure Firewall Threat Defense Command Reference.

TLS/SSL hardware acceleration introduced.

6.2.3

6.2.3

Certain managed device models perform TLS/SSL encryption and decryption in hardware, improving performance. By default, the feature is enabled.

Affected screen: To view the status of TLS/SSL hardware acceleration, Devices > Device Management > Device, General page.

Category and reputation conditions supported.

6.2.2

6.2.2

Access control rules or SSL rules with category/reputation conditions.

SafeSearch supported.

6.1.0

6.1.0

The system displays an HTTP response page for connections decrypted by the SSL policy, then blocked (or interactively blocked) either by access control rules or by the access control policy default action. In these cases, the system encrypts the response page and sends it at the end of the reencrypted SSL stream.

SafeSearch filters objectionable content and stops people from searching adult sites.

TLS/SSL policy.

6.0.0

6.0.0

Feature introduced.