Title: Attacks on Cryptosystems
Course: IN 2001 Principle of Information Assurance > Section: Module 8: Chapter Eight

Attacks on Cryptosystems
1. Historically, attempts to gain unauthorized access to secure communications have used brute force attacks.
2. Ciphertext attacks involve a hacker searching for a common text structure, wording, or syntax in the encrypted message that can enable him or her to calculate the number of each type of letter used in the message.
3. Frequency analysis can be used along with published frequency of occurrence patterns of various languages and can allow an experienced attacker to crack almost any code quickly if the individual has a large enough sample of the encoded text.
4. To protect against this, modern algorithms attempt to remove the repetitive and predictable sequences of characters from the ciphertext.
5. Occasionally, an attacker may obtain duplicate texts, one in ciphertext and one in plaintext, which enable the individual to reverse-engineer the encryption algorithm in a known-plaintext attack scheme.
6. Alternatively, an attacker may conduct a selected-plaintext attack by sending the potential victim a specific text that he or she is certain the victim will forward on to others.

Man-in-the-Middle Attack
1. A man-in-the-middle attack is designed to intercept the transmission of a public key or even to insert a known key structure in place of the requested public key.
2. From the perspective of the victims of such attacks, their encrypted communication appears to be occurring normally, but in fact the attacker is receiving each encrypted message and decoding it and then encrypting and sending it to the originally intended recipient.
3. Establishment of public keys with digital signatures can prevent the traditional man-in-the-middle attack, as the attacker cannot duplicate the signatures.

Correlation Attacks
1. Correlation attacks are a collection of brute-force methods that attempt to deduce statistical relationships between the structure of the unknown key and the ciphertext that is the output of the cryptosystem.
2. Differential and linear cryptanalysis, both of which are advanced methods of breaking codes, have been used to mount successful attacks on block cipher encryptions such as DES.
3. The only defense against this kind of attack is the selection of strong cryptosystems that have stood the test of time, thorough key management, and strict adherence to the best practices of cryptography in the frequency of changing keys.

Dictionary Attacks
1. In a dictionary attack, the attacker encrypts every word in a dictionary using the same cryptosystem as used by the target.
2. Dictionary attacks can be successful when the ciphertext consists of relatively few characters, as in, for example, files that contain encrypted usernames and passwords.
3. After a match is located, the attacker has essentially identified a potential valid password for the system under attack.

Timing Attacks
1. In a timing attack, the attacker eavesdrops during the victim’s session and uses statistical analysis of the user’s typing patterns and inter-keystroke timings to discern sensitive session information.
2. While timing analysis may not directly result in the decryption of sensitive data, it can be used to gain information about the encryption key and perhaps the cryptosystem in use.
3. Once the attacker has successfully broken an encryption, he or she may launch a replay attack, which is an attempt to resubmit a recording of the deciphered authentication to gain entry into a secure source.

Defending Against Attacks
1. No matter how sophisticated encryption and cryptosystems have become, however, they have retained the same flaw that the first systems contained thousands of years ago: If you discover the key, you can determine the message.
2. Thus, key management is not so much the management of technology, but rather the management of people.