Cryptanalysis and Attack Types

Cryptanalysis

• Cryptanalysis is the study of methods for obtaining the meaning of encrypted text without knowing the key.
• Different attack types exist, categorized by the available information to the attacker.

Types of Attacks in Practice

• Exhaustive search: Trying all possible keys.
• Exhaustive search in restricted keyspace: Limited key search space due to poor practice.
• Exploit malpractice: Key management or storage vulnerabilities.
• Steal keys: Direct unauthorized access to keys.
• Spoofing: Deceiving systems into accepting false information.
• Weaknesses of ciphers: Exploiting flaws in the encryption algorithm itself.

Types of Attacks in Theory

• Ciphertext only: Only the ciphertext is available.
• Known plaintext: Ciphertext and corresponding plaintext are known.
• Chosen plaintext: Plaintext of choice is encrypted to obtain the ciphertext.
• Chosen ciphertext: Ciphertext of choice is decrypted to obtain plaintext.

Ciphertext Only Attack

• The attacker has access to one or more ciphertexts.
• This attack method is considered the most difficult.
• It's the most common scenario in practice due to potential easy retrieval via an unsecured connection.

Known Plaintext Attack

• The attacker has access to one or more ciphertexts and their corresponding plaintexts.
• This type of attack gives the attacker more information compared to a ciphertext-only attack.
• Less common than a ciphertext-only attack.

Chosen Plaintext Attack

• Similar to a known plaintext attack, but the attacker has the possibility to choose the input plaintext to obtain corresponding ciphertext.

Chosen Ciphertext Attack

• Similar to a chosen plaintext attack but the attacker chooses the ciphertext input to obtain corresponding plaintext.

Recap

• Classical Ciphers: Transposition and Substitution Ciphers
• Modern Ciphers: Symmetric and Asymmetric Ciphers

Attack on a Transposition Cipher

• Hand-based attacks on transposition ciphers are difficult, hence skipped.

Attack on a Monoalphabetic Substitution Cipher - Letter Frequencies

• Letter frequencies are used as a comparison for English and German.
• The presentation includes charts with letter frequencies in both languages.

Attack on a Monoalphabetic Substitution Cipher - Other Properties of Natural Language

• Frequency of certain letters at the beginning/end of words.
• Bi- and Trigrams are important to consider.

Attack on a Monoalphabetic Substitution Cipher - Beginning and Ending of Words

• Lists frequencies of letters at beginnings and ends of words in text.

Attack on a Monoalphabetic Substitution Cipher - Bi- and Trigrams

• Provides example bigram and trigram frequencies.
• Tables include frequencies of two letter combinations (bigrams) and three letter combinations (trigrams).

Attack on a Polyalphabetic Substitution Cipher - Prerequisites

• Statistical properties of natural language are lost when using polyalphabetic ciphers, but the keyword length (n) leaves repeating sequences.
• This allows for multiple puzzles to solve for the key.
• The method to determine n is discussed later.

Attack on a Polyalphabetic Substitution Cipher - The Kasiski Test

• The attacker searches for repetitions in the ciphertext.
• The approach looks for repetitions in fragment sequences to estimate the keyword length (n).
• The likelihood of finding a repeated sequence within a distance n is higher than a random occurrence.

Attack on a Symmetric Cipher

• Symmetric ciphers can be attacked using linear, differential, and integral cryptanalysis.

Linear Cryptanalysis

• Based on linear equations relating plaintext, ciphertext, and key.
• Linear equations combined with known plaintext-ciphertext pairs are used to derive key bits.

Differential Cryptanalysis

• Based on differences between two ciphertexts (or plaintexts) often using XOR operations.
• The pair of differences (differential) of two ciphertexts and their corresponding plaintext is used for analysis.
• Usually used with a chosen plaintext attack.

Integral Cryptanalysis

• Based on sets of plaintexts.
• A section of selected plaintexts is constant and the next is variable for analysis against sets of corresponding ciphertexts.
• The sum of the elements of each set is analyzed.

Attack on an Asymmetric Cipher

• It's very hard to mount a general attack on asymmetric encryption ciphers.
• The difficulty stems from the use of mathematical principles with rigorous proofs.

Explore the intriguing world of cryptanalysis, focusing on various methods of breaking encrypted text and the different types of attacks used in practice and theory. This quiz delves into exhaustive searches, spoofing, and other key management vulnerabilities. Test your knowledge on these critical concepts in cryptography.