SSL functions as a protocol layer beneath the application layers of IMAP, HTTP, and SMTP. If transmission of messages between a Messaging Server and its clients and between the servers and other servers is encrypted, there is little chance for eavesdropping on the communications. If connecting clients and servers are authenticated, there is little chance for intruders to spoof them.
End-to-end encryption of message transmission requires the use of S/MIME. See Requirements for Using S/MIME with Communications Express Mail in Sun Java Communications Suite 5 Installation Guide.
The extra performance overhead in setting up an SSL connection can put a burden on the server. In designing your messaging installation and in analyzing performance, you need to balance security needs against server capacity.
If you use SSL for encryption, you can improve server performance by installing a hardware encryption accelerator. An encryption accelerator typically consists of a hardware board, installed permanently in your server machine, and a software driver. Sun UltraSPARC IV computers have built-in CPU support for SSL encryption but it is not enabled by default.
The SSL connection process between client and server using HTTP/SSL (HTTPS) is as follows:
The client initiates contact using HTTPS. The client specifies which secret-key algorithms it can use.
The server sends its certificate for authentication and specifies which secret-key algorithm should be used. It will specify the strongest algorithm which it has in common with the client. If there is no match (for example, client is 40 bit only, server requires 128 bits), the connection will be refused.
If the server has been configured to require client authentication, it will ask the client for its certificate at this point.
A known signed Certification Authority
A valid signature
A host name in the certificate matches the same of the server in the HTTPS request
A cipher is the algorithm used to encrypt and decrypt data in the encryption process. Some ciphers are stronger than others, meaning that a message encrypted by a stronger cipher is more difficult for an unauthorized person to decrypt.
A cipher operates on data by applying a key to the data. Generally, the longer the key the cipher users during encryption, the more difficult it is to decrypt the data without the proper decryption key.
When a client initiates an SSL connection with Messaging Server, the client lets the server know what ciphers and key lengths it prefers to use for encryption. In any encrypted communication, both parties must use the same ciphers. Because there are a number of cipher-and-key combinations in common use, a server should be flexible in its support for encryption. For more information on ciphers, see Chapter 23, Configuring Security and Access Control, in Sun Java System Messaging Server 6.3 Administration Guide.