@Yuan Xue CS 285 Network Security Cryptography Overview and Classical Encryption Algorithms Fall 2012 Yuan Xue

@Yuan Xue Cryptography Overview Cryptography is a strong tool against many kinds of security threats. Encryption (Symmetric/Asymmetric) Hash Function Message Authentication Code Digital Signature

@Yuan Xue Let’s start with encryption What encryption does. Introduce the basic concepts in encryption and study the encryption model. How encryption works. Examine the basic principles to design encryption algorithms. Start with two building blocks of encryption algorithms Build stronger and practical encryption algorithms How encryption can fail. Discuss cryptanalysis Show how encryption algorithms can be broken. How encryption can be applied. Study how encryption algorithms can be used as building blocks with protocols and structures to perform security services.

@Yuan Xue Model for Encryption Start with an example Suppose Alice wants to send a message to Bob. She wishes the message not to be understood by others. “secret codes”  substitute a letter for each letter in the original message.  The codes must be agreed by Bob Bob Alice

@Yuan Xue Caesar Cipher

@Yuan Xue Simple Encryption Model Desired Cryptosystem: P = D(E(P))

@Yuan Xue Symmetric Encryption Model The “key” is the key. The encryption/decryption algorithms (E and D) are published The key K is kept secret

@Yuan Xue Asymmetric Encryption Model The encryption/decryption algorithms (E and D) are published One key is published, the other is kept secret

@Yuan Xue Cryptography vs. Cryptanalysis Cryptography Studies the encryption and decryption schemes. Cryptanalysis studies how to “break the code”, i.e., how to decrypt the ciphertext without the knowledge of the encryption details.  Based on the knowledge of the attacker  Ciphertext only attack encryption algorithm and ciphertext  Known plaintext attack  More details later

@Yuan Xue Classical Encryption Algorithms How they work Why they work Any design principles behind?

@Yuan Xue Mono-Alphabetic Ciphers Caeser cipher How many possible substitutions? Bruce-force attack  Enumerate all possible keys Allow arbitrary substitution How many possible substitutions? How to represent the key? How to derive the key?

@Yuan Xue Attacking Encryption Scheme Goal break a single message deduce the key in order to break the subsequent messages. Approach Brute-force attack tires every possible key on a piece of cihphertext until an intelligible translation into plaintext is obtained. Cryptanalysis exploits the characteristics of the algorithm and the traces of structure or pattern in the plaintext that survive encryption  How ? -- Use statistical tools and properties of languages.

@Yuan Xue Frequency Analysis

@Yuan Xue Multiple-letter Ciphers Key idea: Treat consecutive letters in the plaintext as a single unit (block) and translate the unit into ciphertext. Benefit: Lessen the extent to which the structure of the plaintext survives in the ciphertext, thus are stronger against frequency analysis. Issue: How to specify a mapping (substitution)?

@Yuan Xue Single letter Bigram

@Yuan Xue Playfair Cipher Use a table to specify the mapping

@Yuan Xue Playfair Cipher Playfair cipher was invented in 1854 by Charles Wheatstone, but bears the name of Lord Playfair who promoted the use of the cipher. Playfair cipher takes two-letter combinations (digrams) as single units for encryption. The encryption algorithm takes a 5*5 matrix of letters as a key table to translate digrams. The key table is constructed by a keyword.

@Yuan Xue Playfair Cipher Details To encrypt a message, 1. If both letters are the same (or only one letter is left), add an ``x'' (any uncommon letter will do) after the first letter. For example, ``balloon'' would be treated as ``ba lx lo on''. 2. If the letters appear on the same row of the table, replace them with the letters to their immediate right respectively (the table wraps around). For example, ar is encrypted as RM. 3. If the letters appear on the same column of the table, replace them with the letters immediately below respectively. For example, mu is encrypted as CM. 4. If the letters are not on the same row or column, replace them with the letters on the same row respectively but at the other pair of corners of the rectangle defined by the original pair. For example, hs is encrypted as BP, and ea is encrypted as IM (or JM).