CPSC CPSC 3730 Cryptography Chapter 2 Classical Encryption Techniques

CPSC Fig 2.1 Model of Conventional Encryption

CPSC Brute Force Attack/ Key Search Table 2.2 Average Time Required for Exhaustive Key Search Key Size (bits)Number of Alternative Keys Time required at 1 decryption/µs Time required at 10 6 decryptions/µs = 4.3  µs= 35.8 minutes2.15 milliseconds = 7.2  µs= 1142 years10.01 hours = 3.4  µs= 5.4  years5.4  years = 3.7  µs= 5.9  years5.9  years 26 characters (permutation) 26! = 4   µs= 6.4  years6.4  10 6 years

CPSC Caesar Cipher (substitution technique) Replace each letter of the alphabet with the letter standing three places further down the alphabet. a --> D, b -->E, …, and so on plain: meet me after the party cipher: PHHW PH DIWHU WKH SDUWB

CPSC Caesar Cipher (substitution technique) if we assign a numerical equivalent to each letter (a=0, b=1, …, z=25), the algorithm can be expressed as follows. For each plaintext letter p, the cipher letter c is c=E(p)=(p+3) mod (26)

CPSC Caesar Cipher (substitution technique) Since a shift may be any amount, so the general Caesar algorithm is: c=E k (p)=(p+k) mod (26) The decryption algorithm is: p=D k (p)=(c-k) mod (26) Key (k) is the range from 1 to 25

CPSC English Letter Frequencies

CPSC Row Transposition Ciphers a more complex transposition write letters of message out in rows over a specified number of columns then reorder the columns according to some key before reading off the rows Key: Plaintext: a t t a c k p o s t p o n e d u n t i l t w o a m x y z Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ

CPSC Row Transposition Ciphers Key: Plaintext: a t t a c k p o s t p o n e d u n t i l t w o a m x y z Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ Column Permutation: 3 -> 1, 4->2, 2->3, 1-> 4, 5->5, 6->6, 7->7