E-CES, 212-81, Module 5, Cryptanalysis

Brute Force

Trying every possible key to break a cipher. For many types of encryption this is completely impractical because of the amount of time it would take to try every possible key.

Cryptanalysis

Using a method (other than brute force) to derive the key of a cipher. In some cases cryptographic techniques can be used to test the efficacy of a cryptographic algorithm. Frequently used to test hash algorithms for collisions.

Frequency Analysis

The most basic tool for breaking most classical ciphers. Not effective against modern ciphers.

Kasiski

A method if attacking polyalphabetic substitution ciphers. Can be used to deduce the length of the keyword used in the polyalphabetic substitution cipher. Developed in 1863 by Friedrich *******.

Chosen Plaintext Attack

A method for cracking modern cryptography. The attacker obtains the cipher texts corresponding to a set of plain texts of own choosing. Allows the attacker to attempt to derive the key. Difficult but not impossible.

Ciphertext-only and related-key Attack

A method for cracking modern cryptography. Attacker only has access to a collection of cipher texts. Much more likely than known plain text, but also more difficult. The attack is completely successful if the corresponding plain texts can be deduced or, even better, the key. The ability to gain any information about the underlying plain text is considered a success.

Linear Cryptanalysis

Based on finding the approximations to the action of a cipher. Commonly used on block ciphers. A known plain text attack that uses linear approximation to describe the behavior of the block cipher. Given enough pairs of of plain text and corresponding cipher text, bits of information about the key can be obtained. The more pairs of plain text and cipher text the greater chance of success.

Differential Cryptanalysis

Form of cryptanalysis applicable to symmetric key algorithms. The examination of differences in an input and how that affects the resultant difference in output. Originally only worked w/ chosen plain text.

Integral Cryptanalysis

An attack that is particularly successful against block ciphers based on substitution-permutation networks. For a block size b, holds b-k bits constant and runs the other k through all 2k possibilities. For k=1, this is just deferential cryptanalysis, but with k>1 it is a new technique.

Cryptanalysis Resources

Regardless of technique used, there are always three present:
Time - number of primitive operations which must be performed
Memory - Amount of storage required for the attack
Data - the quantity of plain texts and cipher texts required.

Total Break (Cryptanalysis Success)

A cryptanalysis success where the attacker deduces the secret key.

Global Deduction (Cryptanalysis Success)

A cryptanalysis success where the attacker discovers a functionally equivalent algorithm for encryption and decryption, but without key learning.

Instance (Local) Deduction - (Cryptanalysis Success)

A cryptanalysis success where the attacker discovers additional plain texts (or cipher texts) not previously known.

Information Deduction (Cryptanalysis Success)

A cryptanalysis success where the attacker gains some Shannon information about plain texts (or cipher texts) not previously known.

Distinguishing Algorithm (Cryptanalysis Success)

A cryptanalysis success where the attacker can distinguish the cipher from a random permutation.

Rainbow Tables

A password cracker that works with per-calculated hashes of all passwords available withing a character space. Useful against trying to crack hashes. Best way to crack a hash is by finding a match sine they are one-way. Used by popular tools like Ophcrack.