Supported cipher suites & protocol versions

How secure is an HTTPS connection?

This is partially physical considerations such as restricting access to private keys and decrypted traffic (see Offloading vs. inspection). Another part is the encryption.

A secure connection’s protocol version and cipher suite, including encryption bit strength and encryption algorithms, is negotiated between the client and the SSL/TLS terminator during the handshake.

The FortiWeb operation mode determines which device is the SSL terminator. It is either:

• the FortiWeb (if doing SSL offloading)
• the web server (if FortiWeb is doing only SSL inspection)

When FortiWeb is the SSL terminator, FortiWeb controls which ciphers are allowed (see SSL offloading cipher suites and protocols (reverse proxy and true transparent proxy)).

When the web server is the terminator, it controls which ciphers are allowed (see SSL inspection cipher suites and protocols (offline and transparent inspection)). If it selects a cipher that FortiWeb does not support, FortiWeb cannot perform the SSL inspection task.

SSL offloading cipher suites and protocols (reverse proxy and true transparent proxy)

If you have configured SSL offloading for your FortiWeb operating in reverse proxy mode, you can specify which protocols a server policy allows and whether the set of cipher suites it supports is medium-level security, high-level security or a customized set. (See Configuring a server policy.)

In true transparent proxy mode, you can specify these same advanced SSL settings to configure offloading for a server pool member. (See and Creating a server pool.)

Selecting the supported cipher suites using the advanced SSL settings

The SSL/TLS encryption level in the advanced SSL settings provides the following options:

• High — Supports the ciphers listed in High/medium SSL/TLS encryption levels
• Medium — Supports all ciphers supported by the high encryption level, plus the additional ciphers listed in the table Medium-only SSL/TLS encryption levels
• Customized — Allows you to select the ciphers that the policy supports.

High/medium SSL/TLS encryption levels

Medium-only SSL/TLS encryption levels

Generally speaking, for security reasons, SHA-1 is preferable, although you may not be able to use it for client compatibility reasons. Avoid using:

• Older hash algorithms, such as MD5. To disable MD5, for SSL/TLS encryption level, select High.
• Encryption bit strengths less than 128
• Older styles of renegotiation (These are vulnerable to man-in-the-middle (MITM) attacks.)
• Client-initiated renegotiation (Configure Disable Client-Initiated SSL Renegotiation.)

SSL inspection cipher suites and protocols (offline and transparent inspection)

In transparent inspection and offline protection modes, if the client and server communicate using a cipher that FortiWeb does not support, FortiWeb cannot perform the SSL inspection task.

If you are not sure which cipher suites your web server supports, you can use a client-side tool to test. See Checking the SSL/TLS handshake & encryption.

Supported ciphers for offline and transparent inspection

In offline and transparent inspection mode, FortiWeb does not support Ephemeral Diffie-Hellman key exchanges, which may be accepted by clients such as Google Chrome.

See also

• Offloading vs. inspection
• How to offload or inspect HTTPS
• Defeating cipher padding attacks on individually encrypted inputs

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