Around 1995, Netscape Navigator was dominating the browser market with around a 70 percent share. When Netscape created SSL in 1994, it became an instant standard. Microsoft tried to compete, releasing a technology equivalent, Private Communication Technology (PCT), but it had no chance due to Internet Explorer's small market share. It was not until 1996, when Microsoft released Internet Explorer 3, that Netscape's position was challenged.
The first commercial SSL implementation to be released was SSLv2, which appeared in 1994. Version 3 followed in 1995. Netscape also released the SSLv3 reference implementation and worked with the Internet Engineering Task Force (IETF) to turn SSL into a standard. The official name of the standard is Transport Layer Security (TLS), and it is defined in RFC 2246 (http://www.ietf.org/rfc/rfc2246.txt). TLS is currently at version 1.0, but that version is practically the same as SSLv3.1. In spite of the official standard having a different name everyone continues to call the technology SSL, so that is what I will do, too.
SSL lives above TCP and below HTTP in the Open Systems Interconnection (OSI) model, as illustrated in Figure 4-6. Though initially implemented to secure HTTP, SSL now secures many connection-oriented protocols. Examples are SMTP, POP, IMAP, and FTP.
Figure 4-6. SSL belongs to level 6 of the OSI model
In the early days, web hosting required exclusive use of one IP address per hosted web site. But soon hosting providers started running out of IP addresses as the number of web sites grew exponentially. To allow many web sites to share the same IP address, a concept called name-based virtual hosting was devised. When it is deployed, the name of the target web site is transported in the Host request header. However, SSL still requires one exclusive IP address per web site. Looking at the OSI model, it is easy to see why. The HTTP request is wrapped inside the encrypted channel, which can be decrypted with the correct server key. But without looking into the request, the web server cannot access the Host header and, therefore, cannot use that information to choose the key. The only information available to the server is the incoming IP address.
Because only a small number of web sites require SSL, this has not been a major problem. Still, a way of upgrading from non-SSL to SSL communication has been designed (see RFC2817 at http://www.ietf.org/rfc/rfc2817.txt).
4.2.1. SSL Communication Summary
SSL is a hybrid protocol. It uses many of the cryptographic techniques described earlier to make communication secure. Every SSL connection consists of essentially two phases:
4.2.2. Is SSL Secure?
The answer is yes and no. From a technical point of view, transmission can be made secure provided proper encryption algorithms are used together with key lengths of sufficiently large sizes. For example, bulk encryption using the RC4 algorithm and a key length of 128 bits, with an initial handshake using 1024-bit RSA, is considered to be reasonably secure for the moment. But SSL can be a complex protocol to configure and use. Some level of knowledge is required to deploy a reasonably safe installation. (See Eric Murray's study, "SSL Security Survey," at http://www.meer.net/~ericm/papers/ssl_servers.html.) Learn the cryptography and SSL basics and read the complete product documentation related to SSL before you make your first configuration attempt.
18.104.22.168 Man in the middle attacks
Looking at the issue of SSL security from the point of view of a client who wishes to participate in an SSL session, there is a problem known as the man-in-the-middle (MITM) attack. MITM attacks refer to the situation where an attacker can intercept communication between two parties. Each party believes that it is talking to the other party but, in fact, everything goes through the attacker first. MITM attacks can be performed with little difficulty provided the attacker is on the same local network as the victim. (It is far more difficult for an attacker not on the same local network to execute an MITM attack.) There is a collection of tools that help automate such attacks; it's called dsniff (http://www.monkey.org/~dugsong/dsniff/).
When a client application is preparing to establish communication with an SSL server it starts with a domain name and resolves it to the numerical IP address first. This is the weakest point of the process. Using dsniff, it is trivial to intercept domain name resolution requests and send a fake IP address (one the attacker controls) in response. Believing the given IP address is correct, the client will send all traffic for that domain name to the attacker. The attacker will talk to the real server on the victim's behalf. This is all the work required to intercept nonencrypted protocols. But since the SSL protocol specifies server authentication in the handshake phase, the attacker needs to put in more effort when that protocol is used. The attacker cannot successfully pose as the target server since he is not in the possession of its private key. He can attempt to send some other certificate to the client, one for which he has the private key. There are four things the attacker can do:
The only solution to MITM attacks is to enable both server and client authentication. In this case, the attacker will not be able to prove himself to the server as being the genuine client, and as a result the handshake phase of the session fails. Please note: the MITM problem presented here is not a weakness of SSL but rather a weakness of the domain name resolution system that is currently in widespread use. An extension to DNS, Domain Name System Security Extensions (DNSSEC), is being developed to allow for secure DNS resolution and avoidance of the MITM problem. More information is available at http://www.dnssec.net.
22.214.171.124 Nontechnical issues
Some nontechnical issues related to how SSL is used make the end result not as secure as it could be:
The solution to this is to change the way browsers behave, and make them refuse connections to sites with invalid certificates. Unfortunately, this will not happen soon. Until then, the only thing we can do is to try to educate our users.