What crypto suites does Email Security offer (Strong, Normal, Weak) when TLS over SMTP is enabled?
12/20/2019 1048 10793
What crypto suite options does Email Security offer (Strong, Normal, Weak) when TLS over SMTP is enabled?
Perfect Forward Secrecy (PFS) is an added layer of privacy that guaranties that the encryption keys used in a TLS conversation are completely unique. Without PFS, an attacker who successfully steals a mail server's private keys can potentially decrypt intercepted conversations. PFS ensures that no amount of information from the server or from previous conversations can be used towards breaking any future conversations.
The Web UI allows one of three levels of encryption strength:
Strong: Ciphers, in order of preference, are the American AES (128 bits or more), Japanese Camellia (128 bits or more), and American Triple-DES (156). No stream ciphers are supported. The MD5 hash function is not allowed in either certificates or the HMAC. This setting is not the default since it will not interoperate with Exchange 2003 or with sites that decided to stop using block ciphers in order to counter the BEAST TLS attack.
Normal: In addition to the strong ciphers, supports the South Korean SEED (128 bits) and, for TLS v1.0 only, American RC4 (128 bits) ciphers. In addition, the MD5 hash is allowed in the HMAC.
Weak: In addition to all strong and medium ciphers, the 56-bit DES cipher is supported. With modern computers this is essentially clear-text.
The cipher table above applies to the SMTP protocol only; the cipher settings for HTTPS are different because web servers and mail servers are not vulnerable to the same type of threats. For example: SSLv3 is disabled in the Web UI to address the POODLE attack which is a vulnerability that applies specifically to HTTPS, not SMTP. Since some SMTP implementations may still require SSLv3, it remains enabled in SMTP. Due to this setting, some compliance tests will incorrectly report that SMTP is vulnerable to POODLE.
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 disabled with TLS v1.1 and above.
The "Normal" cipherstring deliberately selects 3DES at a lower preference because it is so computationally expensive and to improve interoperability with Exchange Server 2003.