uses same key for encryption, decryption classical, conventional, single-key encryption

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1 CEN 448 Security and Internet Protocols Chapter 2 Classical Encryption Techniques Dr. Mostafa Hassan Dahshan Computer Engineering Department College of Computer and Information Sciences King Saud University Symmetric Encryption Symmetric uses same key for encryption, decryption Other names classical, conventional, single-key encryption Oldest type of encryption Was the only type until 1970 s Remains most widely used 2

2 Ingredients Plaintext original intelligible message Encryption algorithm performs substitutions, transformations input: plaintext, key. output: ciphertext Secret Key different keys different outputs, substitutions and transformations 3 Ingredients Cipher text unintelligible scrambled message depend on plaintext and key Decryption algorithm encryption algorithm run in reverse input: ciphertext, key. output: plaintext 4

3 Simplified Model 5 Simplified Model 6

4 Requirements Impractical to decrypt by only knowing ciphertext and algorithm Algorithm need not to be kept secret allows widespread use low-cost manufacturing 7 Notation Y = E(K, X) or Y = E K (X) X = D(K, Y) or X = D K (Y) X^: estimate of plaintext K^: estimate of key 8

5 Characterization Type of operation substitution: each element of plaintext (bit, character) mapped to another element transposition: plaintext elements rearranged Number of keys used symmetric: same key for sender, receiver asymmetric/public-key: different keys Processing method stream cipher: element by element (bit, byte) block cipher: block transformed as a whole 9 Encryption Attacks Cryptanalysis exploit characteristics of algorithm to deduce plaintext or encryption key may use pairs of plaintext, ciphertext Brute-force attack try all possible keys on ciphertext on average, half of possible keys tried 10

6 Encryption Security Types Unconditional security unbreakable cipher no matter how much time is available only one algorithm: one-time pad Computational security time required to break cipher exceeds the time data is useful cost of breaking cipher exceeds value of data 11 Cryptanalysis Attacks Attempt to deduce specific plaintext or key Rely on nature of algorithm some knowledge of plaintext characteristics Examples some file types have common header exploit statistics of human language power consumed by encryption algorithm 12

7 Cryptanalysis Attacks 13 Brute-Force Attacks Try all possible keys On average, half of keys are attempted Must be able to recognize plaintext Human language header of known file type file format, checksum, 14

8 Brute Force Attacks Key size (bits) Number of alternative keys Time required at 1 decryption/ s Time required at 10 6 decryption/ s = 4.3 x s = 35.8 minutes 2.15 milliseconds = 7.2 x s = 1142 years hours = 3.4 x s = 5.4 x years 5.4 x years = 3.7 x s = 5.9 x years 5.9 x years 26 characters (permutation) 26! = 4 x x s = 6.4 x years 6.4 x 10 6 years 15 Substitution Techniques Letters in plaintext is replaced by other letters numbers symbols Plaintext bit-sequence is replaced by a ciphertext sequence 16

9 Substitution Techniques Caesar cipher Monoalphabetic ciphers Polyalphabetic ciphers 17 Caesar Cipher Ciphertext letter = plaintext letter + 3 Letters wrap around, Z is next after A a b c d e f g h i j k l m n o p q r s t u v w x y z D E F G H I J K L M N O P Q R S T U V W X Y Z A B C 18

10 Caesar Cipher C = E(3, p) = (p + 3) mod 26 If shift is different from 3 C = E(k, p) = (p + k) mod 26 p = D(k, C) = (C - k) mod Brute Force Attack Encryption and decryption algorithms are known Only 25 keys to try Plaintext language is known 20

11 Monoalphabetic Cipher Arbitrary substitution of letters Number of keys = 26! (Over ) Regularities in the language can be exploited 21 Monoalphabetic Example 22

12 Monoalphabetic Example Frequency of letters P e, Z t Frequency of twoletter combinations ZW th 23 Relative Frequency of Letters 24

13 Polyalphabetic Ciphers Different monoalphabetic substitutions for different parts of plaintext Set of monoalphabetic substitution rules Key determines rule used for each part Flatter letter frequency, harder cryptanalysis Best known example: Vigenère cipher 25 Vigenère Cipher 26 Caesar ciphers Using keys 0 to 25 Each denoted by key letter (0=a, 1=b, ) Arranged in a matrix (Vigenère tableau) Key constructed from keyword Repeated to match length of plaintext Ciphertext letter = intersection of: row of plaintext letter column of key letter 26

14 Vigenère Tableau 27 Example Keyword: deceptive Plaintext: we are discovered save yourself Keyword Key Plaintext Ciphertext deceptive deceptivedeceptivedeceptive wearediscoveredsaveyourself ZICVTWQNGRZGVTWAVZHCQYGLMGJ 28

15 Transposition Techniques Perform some permutations on plaintext letters Transposition matrix 29 Transposition Matrix Write message in rectangle, row by row Read message off, column by column Permute order of columns Order of columns is the key 30

16 Transposition Matrix Original order of letters After transposition Somewhat regular structure 31 Transposition Matrix More than one stage of transposition After second transposition Much less structured 32

17 Reading Assignment Textbook chapter Caesar Cipher Monoalphabetic Ciphers Polyalphabetic Ciphers Transposition matrix 33 Additional References Polyalphabetic Substitution Ciphers, (how to crack polyalphabetic ciphers), Cryptool 2, Encryption Examples, 34