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Introduction to SSL/TLS


The Secure Sockets Layer (SSL) protocol was originally developed by Netscape Corporation to provide a mechanism for secure communication over the Internet. Subsequently, the protocol was adopted by the Internet Engineering Task Force (IETF) and renamed to Transport Layer Security (TLS). The latest specification of the TLS protocol is RFC 5246.

The SSL/TLS protocol sits between an application protocol layer and a reliable transport layer (such as TCP/IP). It is independent of the application protocol and can thus be layered underneath many different protocols, for example: HTTP, FTP, SMTP, and so on.

SSL/TLS security features

The SSL/TLS protocol supports the following security featues:

  • Privacy—messages are encrypted using a secret symmetric key, making it impossible for eavesdroppers to read messages sent over the connection.

  • Message integrity—messages are digitally signed, to ensure that they cannot be tampered with.

  • Authentication—the identity of the target (server program) is authenticated and (optionally) the client as well.

  • Immunity to man-in-the-middle attacks—because of the way authentication is performed in SSL/TLS, it is impossible for an attacker to interpose itself between a client and a target.

Cipher suites

To support all of the facets of SSL/TLS security, a number of different security algorithms must be used together. Moreover, for each of the security features (for example, message integrity), there are typically several different algorithms available. To manage these alternatives, the security algorithms are grouped together into cipher suites. Each cipher suite contains a complete collection of security algorithms for the SSL/TLS protocol. />.

Public key cryptography

Public key cryptography (also known as asymmetric cryptography) plays a critically important role in SSL/TLS security. With this form of cryptography, encryption and decryption is performed using a matching pair of keys: a public key and a private key. A message encrypted by the public key can only be decrypted by the private key; and a message encrypted by the private key can only be decrypted by the public key. This basic mathematical property has some important consequences for cryptography:

  • It becomes extremely easy to establish secure communications with people you have never previously had any contact with. Simply publish the public key in some accessible place. Anyone can now download the public key and use it to encrypt a message that only you can decrypt, using your private key.

  • You can use your private key to digitally sign messages. Given a message to sign, simply generate a hash value from the message, encrypt that hash value using your private key, and append it to the message. Now, anyone can use the public key to decrypt the hash value and check that the message has not been tampered with.


Actually, it is not compulsory to use public key cryptography with SSL/TLS. But the SSL/TLS protocol is practically useless (and very insecure) without it.

X.509 certificates

An X.509 certificate provides a way of binding an identity (in the form of an X.500 distinguished name) to a public key. X.509 is a standard specified by the IETF and the most recent specification is RFC 4158. The X.509 certificate consists essentially of an identity concatenated with a public key, with the whole certificate being digitally signed in order to guarantee the association between the identity and the public key.

But who signs the certificate? It has to be someone (or some identity) that you trust. The certificate signer could be one of the following:

  • Self—if the certificate signs itself, it is called a self-signed certificate. If you need to deploy a self-signed certificate, the certificate must be obtained from a secure channel. The only guarantee you have of the certificate's authenticity is that you obtained it from a trusted source.

  • CA certificate—a more scalable solution is to sign certificates using a Certificate Authority (CA) certificate. In this case, you only need to be careful about deploying the original CA certificate (that is, obtaining it through a secure channel). All of the certificates signed by this CA, on the other hand, can be distributed over insecure, public channels. The trusted CA can then be used to verify the signature on the certificates. In this case, the CA certificate is self-signed.

  • Chain of CA certificates—an extension of the idea of signing with a CA certificate is to use a chain of CA certificates. For example, certificate X could be signed by CA foo, which is signed by CA bar. The last CA certificate in the chain (the root certificate) is self-signed.

For more details about managing X.509 certificates, see Managing Certificates.

Target-only authentication

The most common way to configure SSL/TLS is to associate an X.509 certificate with the target (server side) but not with the client. This implies that the client can verify the identity of the target, but the target cannot verify the identity of the client (at least, not through the SSL/TLS protocol). It might seem strange that we worry about protecting clients (by confirming the target identity) but not about protecting the target. Keep in mind, though, that SSL/TLS security was originally developed for the Internet, where protecting clients is a high priority. For example, if you are about to connect to your bank's Web site, you want to be very sure that the Web site is authentic. Also, it is typically easier to authenticate clients using other mechanisms (such as HTTP Basic Authentication), which do not incur the high maintenance overhead of generating and distributing X.509 certificates.

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