Tuesday, October 29, 2013


In the last few years, we have witnessed a wide range of attacks on the SSL/TLS mechanism. In this article, we will try to cover various attacks that were prominent in the field of cryptography. Transport layer security (TLS) ensures integrity of data transmitted between two parties (server and client) and also provides strong authentication for both parties. The attacks launched in the last few years have exploited various features in the TLS mechanism. We are going to discuss these attacks one by one.

Browser Exploit Against SSL/TLS Attack (BEAST)

This attack was revealed at the Ekoparty Security Conference in 2011. BEAST is based on a type of cryptographic attack called the “chosen plain text attack.” Before I jump into explaining the details of this attack, let us take a look at some of the basic concepts to be understood.

Background Information

There are two types of encryption schemes:

  1. Symmetric key encryption: Encryption and decryption keys are the same.
  2. Asymmetric key encryption: Encryption and decryption keys are not the same.
Symmetric-key encryption can use either stream ciphers or block ciphers. A stream cipher encrypts one bit at a time, while a block cipher encrypts plaintext in chunks. Let’s talk about block cipher. How is a message encrypted using block cipher? You don’t use the block cipher on the message directly but instead you first need to choose the “mode of operation.” CBC (cipher block chaining) is one such mode used by the block ciphers.

In CBC mode, to make each message unique, an initialization vector (IV) is used in the first block. An IV is a random string that is XORed with the plaintext message prior to encryption. Each block of plaintext is XORed with the previous cipher text block before being encrypted. In other words, each cipher text block depends on all plaintext blocks processed up to that point as shown in the figure below. It’s important to note that here IV is not a secret; it only adds randomness to the output. IV is sent along with the message in clear text format. With this background information, let us know focus on how the BEAST attack is accomplished.

How Is the Attack Accomplished?

It was noticed that TLS 1.0, when dealing with multiple packets, allows the following packets to use an IV that is the last cipher text block of the previous packet. In other words, an attacker who can see the encrypted traffic can note the IV used for session cookie (Why? Because the cookie’s location is predictable). Simply put, an active attacker will be able to gather the IVs for each record just by sniffing the network. So if the attacker can “guess” a plaintext message, he can make a guess at the session cookie and see if the cipher text matches. [Note that, since this is a MITM attack, the attacker can mix his traffic with the victim traffic to see the results].

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