Chapter 2 Basic Encryption and Decryption (part B)

csci5233 computer security & integrity Transpositions (Permutations) P.47 Transposition: an encryption in which the letters of the message are rearranged Also known as permutations Compare the goals: –Substitution  confusion –Transposition  diffusion

csci5233 computer security & integrity 3 Confusion vs Diffusion Confusion: making it difficult to determine how a message and key were transformed into ciphertext. Diffusion: spreading the information from the message or the key out widely across the ciphertext See p.59 for more discussions. Note: The definition of diffusion in the book seems to change depending on the context of discussion, leading to contradictory statements regarding whether ‘transposition’ methods have diffusion or not. See p.47 (1 st paragraph) and p.59 (last 2 nd paragraph).

csci5233 computer security & integrity 4 Columnar Transpositions A rearrangement of the plaintext characters into columns. The ciphertext is generated from the columns. Example: p.47

csci5233 computer security & integrity 5 Complexity of Columnar Transpositions Time: proportional to the length of the message, that is, O(n) or at the order of function n. Space: depends on the length of the message. Output cannot be produced until all characters of the message have been read. Initial delay varies, depending on the length of the message. C.f., constant initial delay in the previous (substitution) algorithms.

csci5233 computer security & integrity 6 Digrams, Trigrams, & Other Patterns Digrams: groups of two letters Trigrams: groups of three letters Table 2-8 (p.49): Frequencies of digrams Note: not counting digrams that consist of the last letter of one word and the first letter of the next word Exercise: What’s the frequency of digrams BE, RF, and WY?

csci5233 computer security & integrity 7 Cryptanalysis by Digram Analysis 1. To compute the letter frequencies oClue: The fact that all letters appear with their normal frequencies implies that a transposition has been performed. 2. To find where in the ciphertext a pair of adjacent columns lies (that is, to determine the width of a row in the original table used for encryption) oThe ‘moving window’ method: Fig. 2-9 (p.50)

csci5233 computer security & integrity 8 The ‘moving window’ method 1. Pick a window size, say n. 2. Compare every C i, 1  i  n,in the window to C i+n and determine if the two form a common digram by checking their frequency (table 2-8, p.49) 3. Do most of the digrams look reasonable? Compute their mean and std. deviation Example: Table 2-9, p.51

csci5233 computer security & integrity 9 Double transposition (P.51) Involves two columnar transpositions An example of product ciphers, in which one encryption is applied to the result of another C = E 2 (E 1 (P) ) Example: Table 2-10 and 2-11 Cryptanalysis P i  C column * ( (i-1) mod row ) + (i-1) div row + 1 Note: correction of the formula!

csci5233 computer security & integrity 10 Double transposition Example: p.52 1 st transposition: # of rows = 10, # of columns = 5 example 1: P 8  C 10*((8-1) mod 5) + (8-1) div = C 22 example 2: P 14  C 10*((14-1) mod 5) + (14-1) div = C 33 2 nd transposition: # of rows = 8, # of columns = 7 example 1: C 22  C’ 8*((22-1) mod 7) + (22-1) div = C’ 4 example 2: C 33  C’ 8*((33-1) mod 7) + (33-1) div = C’ 37 So, P 8  C’ 4 and P 14  C’ 37

csci5233 computer security & integrity 11 Analysis of double transposition ciphers 1. Locating pairs of ciphertext letters that probably appear together in the plaintext (chosen plaintext attack, probable plaintext attack)  p Inferring a mathematical relationship from those pairs of letters 3. Verifying the relationship on other ciphertext letters to see if they produce probable digrams

csci5233 computer security & integrity 12 Fractionated Morse A keyed monoalphabetic cipher Uses Morse code (Table 2-12, p.53) as its base Double encodings (P  Morse code  P’) 3 steps: 1.Convert P to Morse code, using separator(s) between letters and words. 2.Divide the Morse code messages into blocks of 3 symbols. 3.Each block of symbols is encoded as the letter corresponding to that 3-symbol pattern (see Table 2-13, p.55).

csci5233 computer security & integrity 13 Stream versus Block Ciphers Stream ciphers: The plaintext characters are encoded by the sender letter-by-letter as sent to the receiver. –Example: substitution ciphers Block ciphers: Blocks of plaintext are encoded into ciphertext before being sent. –Example: columnar transposition

csci5233 computer security & integrity 14 Stream Ciphers + Fast + Little storage space + Low error propagation, meaning that encoding errors affect just one character in the ciphertext - low diffusion, meaning that individual characters in the ciphertext can be analyzed using frequency distribution, digram analysis, IC and the Kasiski method - Susceptibility to malicious insertions and modifications

csci5233 computer security & integrity 15 Block Ciphers - Slow - Require more storage space - Error propagation + High diffusion + High immunity to insertions

csci5233 computer security & integrity 16 4 cryptanalysis cases & 5 approaches 1.Ciphertext only  Ciphertext-only attack 2.Full or partial plaintext  Known plaintext attack  Probable plaintext analysis 3.Ciphertext of any plaintext  Chosen plaintext attack 4.Algorithm + Ciphertext  Chosen ciphertext attack

csci5233 computer security & integrity 17 Summary Two basic methods of encryption: substitutions and transposition Common cryptanalytic tools: Frequency distribution, Digram/trigram study, IC, Repeated patterns, Probable letters Four cryptanalysis cases & 5 approaches: Next: Pf, Ch 3 (Cryptosystems)