Classical Encryption Techniques

Classical Encryption Techniques

• Brute force attack treats a cipher like a black box – does not analyze the cipher's inner workings
• Major weakness of substitution ciphers – each plaintext symbol always maps to the same ciphertext symbol
• Statistical properties of plaintext are preserved in ciphertext
• The frequency distribution of any language is relatively stable even for short encrypted pieces of text
• Most frequent letters are easily spotted in ciphertexts
• In English, E is the most frequent letter (about 13%), T is second (about 9%), A is third (about 8%)
• Analyzing combinations of letters allows for breaking, such as 'Q' is almost always followed by 'U'.
• Frequent short words like 'THE', 'AND' in English can be exploited to decipher plaintext
• Good ciphers should hide statistical properties of the plaintext
• Ciphertext symbols should appear random
• A large key space alone is not sufficient for strong encryption
• Cryptanalysis is the science of breaking cryptosystems
• Ciphertext-only Attack: The adversary has only access to the ciphertext.
• Known-plaintext Attack: The adversary also knows some pieces of the plaintext.
• Chosen-plaintext Attack: The adversary chooses the plaintext that is being encrypted and has access to the corresponding ciphertext.
• Chosen-ciphertext Attack: The adversary can choose ciphertexts and obtains the corresponding plaintexts.
• Cryptanalysis attacks rely on the algorithm's nature and general characteristics of the plaintext
• Brute-force Attack: An attacker tries every possible key until an intelligible translation is found.
• On average, half of all possible keys must be tried for success
• Side-channel analysis can be used to extract a secret key by observing the behaviour of a cryptographic implementation
• Examples of side-channel analysis include electric power consumption, electromagnetic radiation and timing behavior of the CPU
• Software side-channels: The adversary controls one process to learn secret values such as cryptographic keys from another process by exploiting timing behavior or cache access patterns.
• Social engineering attacks can include bribing, blackmailing, tricking, and classical espionage.
• Security by Obscurity aims to appear secure by keeping implementation details hidden.
• However, experience and history have shown that such systems are often easily broken.
• Kerckhoffs’ Principle: This principle states that a cryptosystem should be secure even if the attacker knows all the details of the system except the secret key.
• Shift Cipher Encryption: y = ek x ≡ x + k mod 26
• Shift Cipher Decryption: x = dk y ≡ y − k mod 26
• The Shift Cipher has only 26 different keys, thus a brute-force attack is feasible.
• Letter frequency analysis can be applied to Shift Cipher to break it easily.
• Affine Cipher Encryption: y = ek x ≡ ax + b mod 26
• Affine Cipher Decryption: x = dk y ≡ a−1 (y − b) mod 26
• Affine Cipher has only 312 different keys, thus a brute-force attack is feasible.
• Letter frequency analysis can be used to break the Affine Cipher
• Polyalphabetic Substitution Cipher (Vigenère Cipher) uses multiple shifts derived from a secret code word.
• Each letter of the code word corresponds to a number 0, … , 25, representing the shift positions.
• Vigenère Cipher Encryption: yj ≡ xj + k(j mod 𝑙) mod 26
• Vigenère Cipher Decryption: xj ≡ yj − k(j mod 𝑙) mod 26
• The Vigenère Cipher obscures plaintext letter frequency information due to multiple ciphertext symbols
• The repeated sequences in the ciphertext can be used to guess the keyword length.
• If the keyword length is 𝑚, then the cipher consists of 𝑚 monoalphabetic substitution ciphers.
• The periodic nature of the keyword can be eliminated by using a nonrepeating keyword as long as the message.
• Transposition Cipher is based on permuting plaintext letters, making it a more secure choice than substitution
• Rail fence technique is an example of transposition ciphers.
• The Scytale was a transposition cipher tool used by the Spartans.
• Columnar Transposition cipher uses a key to determine the order of columns in the transposition.
• Pure transposition cipher has the same letter frequencies as the original plaintext, making it recognizable.
• Cryptanalysis of columnar transposition involves laying out the ciphertext in a matrix and experimenting with column positions.
• Digram and trigram frequency tables can be utilized for cryptanalysis.
• Multi-stage transposition makes the transposition cipher more secure with a complex permutation that is difficult to reconstruct
• Double transposition: performing two transposition steps on the ciphertext
• An attacker always targets the weakest link in a cryptosystem - the weakest link could be the algorithms or implementation techniques such as social engineering or implementation attacks.
• Strong cryptosystems should adhere to Kerckhoffs’ Principle.

Conclusion

• It is critical to choose strong algorithms that are robust and implement appropriate security measures to prevent social engineering and implementation attacks to secure data, systems, and information.

Test your understanding of classical encryption techniques, including substitution ciphers and the vulnerabilities they present. This quiz covers the significance of frequency analysis and the principles of cryptanalysis. Challenge yourself to identify the weaknesses and features of different encryption methods.