PPTP Point-to-Point Tunneling Protocol

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2.4.2 Cryptographic Assaults

It is probably unlikely that a cryptoanalyst will spend a great deal of time cracking private VPN traffic between two different sites. The compromise methodologies used to assault crypto schemes are sometimes complicated and time-consuming. Therefore, the following different attack mechanisms are illustrations of the tools used in analysis work. It is highly likely that interested individuals and organizations will develop applications to conduct these assaults. It is also reasonable to predict that such tools will be made widely available in the foreseeable future.

2.4.2.1 Ciphertext-only attack

The cryptoanalyst uses captured ciphertext, as much as can be obtained, to try to deduce the plaintext or reverse generate the keys if possible. The purpose of this assault is ultimately to have keys to unlock new messages sent across the wire, with the sender and receiver oblivious to the fact that the eavesdropper can capture and read their messages. By using a faster key generation and exchange system, you can multiply the work of the cryptoanalyst by the number of keys used.

2.4.2.2 Known plaintext attack

The cryptoanalyst has access to the plaintext that produced certain ciphertext messages as well as the ciphertext itself. The goal is to either deduce the keys used to encrypt the data or develop an algorithm that produces similar results that can be used in reverse.

2.4.2.3 Chosen plaintext attack

Here, the cryptoanalyst has both the ciphertext and the plaintext as well as the ability to send his own plaintext through the algorithm to produce new ciphertext. This assault could also be used to insert correctly ciphered garbage into a communications stream, but since that provides such a small lever, the goal of the analyst would still be to recover the keys, so that he can either listen to communications as they happen, or intrude upon them by staging a man-in-the-middle attack.

2.4.2.4 Chosen ciphertext attack

Much like the chosen plaintext attack, it is assumed that the cryptoanalyst has somehow gotten a black box that does decryption based on an input of ciphertext. The attacker can pick and choose which ciphertext he wants to decrypt. Chosen ciphertext attacks can be used against public key systems such as RSA. For more technical detail on the weaknesses of RSA as an implemented protocol not necessarily attacks upon the algorithms itself , consult Bruce Schneiers Applied Cryptography John Wiley and Sons, Inc., 2 nd Edition, 1996, probably one of the premier works done in the field.

2.4.2.5 Brute force attacks

A standard attack plan for a cipher cracker is to use what we call the brute force attack upon the algorithm. This assumes that the cracker has intercepted an encrypted message and knows the algorithm used to produce it, but doesnt know the key. Depending on the amount of time he is willing to invest in using a crowbar and whether he started with a key of all zero bits set, he should eventually find the key just by trying to decrypt the data with each successive key