What crypto suites does Email Security offer (Strong, Normal, Weak) when TLS over SMTP is enabled?

12/20/2019 1,049 People found this article helpful 483,671 Views

Description

What crypto suite options does Email Security offer (Strong, Normal, Weak) when TLS over SMTP is enabled?

Resolution

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 OpenSSL Cipherstring selectors are:

Strong	`HIGH:!MD5:!SSLv2:!aNULL:!eNULL:@STRENGTH`
Normal	`HIGH:MEDIUM:-3DES:!SSLv2:!aNULL:!eNULL:@STRENGTH:3DES`
Weak	`ALL:!EXPORT:!SSLv2:!aNULL:!eNULL:@STRENGTH`

In versions 8.3, the complete set of ciphers are:

OpenSSL Cipherstring Name	TLS	Key Exchange	Authenticator	Cipher	HMAC	PFS?
Strong
ECDHE-RSA-AES256-GCM-SHA384	v1.2	ECDH	RSA	AESGCM(256)	AEAD	Yes
ECDHE-ECDSA-AES256-GCM-SHA384	v1.2	ECDH	ECDSA	AESGCM(256)	AEAD	Yes
ECDHE-RSA-AES256-SHA384	v1.2	ECDH	RSA	AES(256)	SHA384	Yes
ECDHE-ECDSA-AES256-SHA384	v1.2	ECDH	ECDSA	AES(256)	SHA384	Yes
ECDHE-RSA-AES256-SHA	v1	ECDH	RSA	AES(256)	SHA1	Yes
ECDHE-ECDSA-AES256-SHA	v1	ECDH	ECDSA	AES(256)	SHA1	Yes
ECDH-RSA-AES256-GCM-SHA384	v1.2	ECDH/RSA	ECDH	AESGCM(256)	AEAD
ECDH-ECDSA-AES256-GCM-SHA384	v1.2	ECDH/ECDSA	ECDH	AESGCM(256)	AEAD
ECDH-RSA-AES256-SHA384	v1.2	ECDH/RSA	ECDH	AES(256)	SHA384
ECDH-ECDSA-AES256-SHA384	v1.2	ECDH/ECDSA	ECDH	AES(256)	SHA384
ECDH-RSA-AES256-SHA	v1	ECDH/RSA	ECDH	AES(256)	SHA1
ECDH-ECDSA-AES256-SHA	v1	ECDH/ECDSA	ECDH	AES(256)	SHA1
AES256-GCM-SHA384	v1.2	RSA	RSA	AESGCM(256)	AEAD
AES256-SHA256	v1.2	RSA	RSA	AES(256)	SHA256
AES256-SHA	v1	RSA	RSA	AES(256)	SHA1
CAMELLIA256-SHA	v1	RSA	RSA	Camellia(256)	SHA1
ECDHE-RSA-AES128-GCM-SHA256	v1.2	ECDH	RSA	AESGCM(128)	AEAD	Yes
ECDHE-ECDSA-AES128-GCM-SHA256	v1.2	ECDH	ECDSA	AESGCM(128)	AEAD	Yes
ECDHE-RSA-AES128-SHA256	v1.2	ECDH	RSA	AES(128)	SHA256	Yes
ECDHE-ECDSA-AES128-SHA256	v1.2	ECDH	ECDSA	AES(128)	SHA256	Yes
ECDHE-RSA-AES128-SHA	v1	ECDH	RSA	AES(128)	SHA1	Yes
ECDHE-ECDSA-AES128-SHA	v1	ECDH	ECDSA	AES(128)	SHA1	Yes
ECDH-RSA-AES128-GCM-SHA256	v1.2	ECDH/RSA	ECDH	AESGCM(128)	AEAD
ECDH-ECDSA-AES128-GCM-SHA256	v1.2	ECDH/ECDSA	ECDH	AESGCM(128)	AEAD
ECDH-RSA-AES128-SHA256	v1.2	ECDH/RSA	ECDH	AES(128)	SHA256
ECDH-ECDSA-AES128-SHA256	v1.2	ECDH/ECDSA	ECDH	AES(128)	SHA256
ECDH-RSA-AES128-SHA	v1	ECDH/RSA	ECDH	AES(128)	SHA1
ECDH-ECDSA-AES128-SHA	v1	ECDH/ECDSA	ECDH	AES(128)	SHA1
AES128-GCM-SHA256	v1.2	RSA	RSA	AESGCM(128)	AEAD
AES128-SHA256	v1.2	RSA	RSA	AES(128)	SHA256
AES128-SHA	v1	RSA	RSA	AES(128)	SHA1
CAMELLIA128-SHA	v1	RSA	RSA	Camellia(128)	SHA1
ECDHE-RSA-DES-CBC3-SHA	v1	ECDH	RSA	3DES(168)	SHA1	Yes
ECDHE-ECDSA-DES-CBC3-SHA	v1	ECDH	ECDSA	3DES(168)	SHA1	Yes
EDH-RSA-DES-CBC3-SHA	v1	DH	RSA	3DES(168)	SHA1	Yes
EDH-DSS-DES-CBC3-SHA	v1	DH	DSS	3DES(168)	SHA1	Yes
ECDH-RSA-DES-CBC3-SHA	v1	ECDH/RSA	ECDH	3DES(168)	SHA1
ECDH-ECDSA-DES-CBC3-SHA	v1	ECDH/ECDSA	ECDH	3DES(168)	SHA1
DES-CBC3-SHA	v1	RSA	RSA	3DES(168)	SHA1
Normal
SEED-SHA	v1	RSA	RSA	SEED(128)	SHA1
ECDHE-RSA-RC4-SHA	v1	ECDH	RSA	RC4(128)	SHA1	Yes
ECDHE-ECDSA-RC4-SHA	v1	ECDH	ECDSA	RC4(128)	SHA1	Yes
ECDH-RSA-RC4-SHA	v1	ECDH/RSA	ECDH	RC4(128)	SHA1
ECDH-ECDSA-RC4-SHA	v1	ECDH/ECDSA	ECDH	RC4(128)	SHA1
Weak
EDH-RSA-DES-CBC-SHA	v1	DH	RSA	DES(56)	SHA1	Yes
EDH-DSS-DES-CBC-SHA	v1	DH	DSS	DES(56)	SHA1	Yes
DES-CBC-SHA	v1	RSA	RSA	DES(56)	SHA1
RC4-SHA1	v1	RSA	RSA	RC4(128)	SHA1
RC4-MD5	v1	RSA	RSA	RC4(128)	MD5

Notes:

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.

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