15.7 SSL and TLS – Introduction to Cryptography with Coding Theory, 3rd Edition

15.7 SSL and TLS

If you have ever paid for anything over the Internet, your transactions were probably kept secret by SSL or its close relative TLS. Secure Sockets Layer (SSL) was developed by Netscape in order to perform http communications securely. The first version was released in 1994. Version 3 was released in 1995. Transport Layer Security (TLS) is a slight modification of SSL version 3 and was released by the Internet Engineering Task Force in 1999. These protocols are designed for communications between computers with no previous knowledge of each other’s capabilities.

In the following, we’ll describe SSL version 3. TLS differs in a few minor details such as how the pseudorandom numbers are calculated. SSL consists of two main components. The first component is known as the record protocol and is responsible for compressing and encrypting the bulk of the data sent between two entities. The second component is a collection of management protocols that are responsible for setting up and maintaining the parameters used by the record protocol. The main part of this component is called the handshake protocol.

We will begin by looking at the handshake protocol, which is the most complicated part of SSL. Let us suppose that Alice has bought something online from Gigafirm and wants to pay for her purchase. The handshake protocol performs authentication between Alice’s computer and the server at Gigafirm and is used to allow Alice and Gigafirm to agree upon various cryptographic algorithms. Alice’s computer starts by sending Gigafirm’s computer a message containing the following:

  1. The highest version of SSL that Alice’s computer can support

  2. A random number consisting of a 4-byte timestamp and a 28-byte random number

  3. A Cipher Suite containing, in decreasing order of preference, the algorithms that Alice’s computer wants to use for public key (for example, RSA, Diffie-Hellman, ...), block cipher encryption (3DES, DES, AES, ...), hashing (SHA-1, MD5, ...), and compression (PKZip, ...)

Gigafirm’s computer responds with a random 32-byte number (chosen similarly) and its choices of which algorithms to use; for example, RSA, DES, SHA-1, PKZip.

Gigafirm’s computer then sends its X.509 certificate (and the certificates in its certification chain). Gigafirm can ask for Alice’s certificate, but this is rarely done for two reasons. First, it would impede the transaction, especially if Alice does not have a valid certificate. This would not help Gigafirm accomplish its goal of making sales. Secondly, Alice is going to send her credit card number later in the transaction, and this serves to verify that Alice (or the thief who picked her pocket) has Alice’s card.

We’ll assume from now on that RSA was chosen for the public key method. The protocol differs only slightly for other public key methods.

Alice now generates a 48-byte pre-master secret, encrypts it with Gigafirm’s public key (from its certificate), and sends the result to Gigafirm, who decrypts it. Both Alice and Gigafirm now have the following secret random numbers:

  1. The 32-byte random number rA that Alice sent Gigafirm.

  2. The 32-byte random number rG that Gigafirm sent Alice.

  3. The 48-byte pre-master secret spm.

Note that the two 32-byte numbers were not sent securely. The pre-master secret is secure, however.

Since they both have the same numbers, both Alice and Gigafirm can calculate the master secret as the concatenation of

MD5(spm||SHA1(A||spm||rA||rG))MD5(spm||SHA1(BB||spm||rA||rG))MD5(spm||SHA1(CCC||spm||rA||rG)).

The A, BB, and CCC are strings added for padding. Note that timestamps are built into rA and rG. This prevents Eve from doing replay attacks, where she tries to use information intercepted from one session to perform similar transactions later.

Since MD5 produces a 128-bit (= 16-byte) output, the master secret has 48 bytes. The master secret is used to produce a key block, by the same process that the master secret was produced from the pre-master secret. Enough hashes are concatenated to produce a sufficiently long key block. The key block is then cut into six secret keys, three for communications from Alice to Gigafirm and three for communications from Gigafirm to Alice. For Alice to Gigafirm, one key serves as the secret key in the block cipher (3DES, AES, ...) chosen at the beginning of the communications. The second is a message authentication key. The third is the initial value for the CBC mode of the block cipher. The three other keys are for the corresponding purposes for Gigafirm to Alice.

Now Alice and Gigafirm are ready to communicate using the record protocol. When Alice sends a message to Gigafirm, she does the following:

  1. Compresses the message using the agreed-upon compression method.

  2. Hashes the compressed message together with the message authentication key (the second key obtained from the key block). This yields the hashed message authentication code.

  3. Uses the block cipher in CBC mode to encrypt the compressed message together with the hashed message authentication code, and sends the result to Gigafirm.

Gigafirm now does the following:

  1. Uses the block cipher to decrypt the message received. Gigafirm now has the compressed message and the hashed message authentication code.

  2. Uses the compressed message and the Alice-to-Gigafirm message authentication key to recompute the hashed message authentication code. If it agrees with the hashed message authentication code that was in the message, the message is authenticated.

  3. Decompresses the compressed message to obtain Alice’s message.

Communications from Gigafirm are encrypted and decrypted similarly, using the other three keys deduced from the key block. Therefore, Alice and Gigafirm can exchange information securely.