In version 8.3, the Web UI allows the administrator to select one of three levels of SMTP encryption strength:
Strong: American AES (128 bits or higher) and Japanese Camellia (128 bits or higher). This setting is not the default since it will not inter-operate with Exchange 2003. This is the recommended setting when mandatory TLS is enabled on the same path.
Normal: In addition to the strong ciphers, supports the American Triple-DES (3DES) and South Korean SEED (128 bits) ciphers. This is the recommended setting for public-facing paths that must interoperate with older SMTP servers.
Weak: In addition to all strong and medium ciphers, the American RC4 (128 bits) cipher is supported, and Discrete Logarithm Ephemeral Diffie-Hellman (EDH) key exchange is supported when the proxy is acting as a client. In addition, the MD5 hash is allowed in the HMAC. This setting should only be used when the only alternative is clear text.
The OpenSSL Cipherstring selectors are:
To display the actual ciphers, shell into an appliance and use the openssl ciphers command with one of the above strings. For example, to list all the strong ciphers:
TLS v1.2 Galois/Counter Mode (GCM), Authenticated Encryption with Associated Data (AEAD), and SHA-2 hashes are only available when the client uses TLS v1.2. All TLS v1 ciphers are available when the client uses TLS v1.2, except for RC4, which is always disabled with TLS v1.1 and above.
The changes from Release 8.2 to 8.3 are:
All ciphers using less than 128-bit encryption (the former “weak” ciphers) have been removed and are no longer available.
The RC4 cipher has been moved to “weak” only.
The DHE authenticator has been moved to “weak” only.
3DES is no longer included in the "strong" set; it is included in “normal” and “weak.”