Information Systems Security
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1 Information Systems Security Dr. Ayman Abdel-Hamid College of Computing and Information Technology Arab Academy for Science & Technology and Maritime Transport Chapter 2 Classical Encryption Techniques ISS Dr. Ayman Abdel-Hamid 1
2 Outline Classical Encryption Techniques ISS Dr. Ayman Abdel-Hamid 2
3 Symmetric Encryption or conventional / private-key / single-key sender and recipient share a common key all classical encryption algorithms are private-key was only type prior to invention of publickey in 1970 s ISS Dr. Ayman Abdel-Hamid 3
4 Basic Terminology plaintext - the original message ciphertext - the coded message cipher - algorithm for transforming plaintext to ciphertext key - info used in cipher known only to sender/receiver encipher (encrypt) - converting plaintext to ciphertext decipher (decrypt) - recovering ciphertext from plaintext cryptography - study of encryption principles/methods cryptanalysis (codebreaking) - the study of principles/ methods of deciphering ciphertext without knowing key cryptology - the field of both cryptography and cryptanalysis ISS Dr. Ayman Abdel-Hamid 4
5 Symmetric Cipher Model ISS Dr. Ayman Abdel-Hamid 5
6 Requirements 1/2 two requirements for secure use of symmetric encryption: a strong encryption algorithm (if you know the algorithm, and have access to one or more ciphertexts, you are unable to decipher or figure out key, stronger?) a secret key known only to sender / receiver Y = E K (X) X = D K (Y) assume encryption algorithm is known implies a secure channel to distribute key ISS Dr. Ayman Abdel-Hamid 6
7 Requirements 2/2 ISS Dr. Ayman Abdel-Hamid 7
8 Cryptography can characterize by: type of encryption operations used substitution / transposition / product (multiple stages) number of keys used single-key or private or secret-key (symmetric) two-key or public (asymmetric) way in which plaintext is processed block (1 output block for each input block) Stream (one element at time) ISS Dr. Ayman Abdel-Hamid 8
9 How to attack secret-key Encryption? Cryptanalysis Relies on nature of algorithm and may be knowledge of properties of plain text or even knowledge of sample plaintext-ciphertext pairs Brute-force Attack Attempt every possible key on a ciphertext until an intelligible translation into plaintext is obtained ISS Dr. Ayman Abdel-Hamid 9
10 Types of Cryptanalytic Attacks ciphertext only only know algorithm / ciphertext, statistical (general idea about type of plaintext), can identify plaintext known plaintext know/suspect plaintext & ciphertext to attack cipher chosen plaintext select plaintext and obtain ciphertext to attack cipher chosen ciphertext select ciphertext and obtain plaintext to attack cipher chosen text select either plaintext or ciphertext to en/decrypt to attack cipher ISS Dr. Ayman Abdel-Hamid 10
11 More Definitions unconditional security no matter how much computer power is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext (no matter how much ciphertext is available) computational security Cost of breaking the cipher exceeds the value of encrypted information Time required to break the cipher exceeds the useful lifetime of the information ISS Dr. Ayman Abdel-Hamid 11
12 Brute Force Search always possible to simply try every key most basic attack, proportional to key size assume either know / recognise plaintext DES AES 3 DES ISS Dr. Ayman Abdel-Hamid 12
13 Classical Substitution Ciphers where letters of plaintext are replaced by other letters or by numbers or symbols or if plaintext is viewed as a sequence of bits, then substitution involves replacing plaintext bit patterns with ciphertext bit patterns ISS Dr. Ayman Abdel-Hamid 13
14 Caesar Cipher 1/2 earliest known substitution cipher by Julius Caesar first attested use in military affairs replaces each letter by 3rd letter on example: meet me after the toga party PHHW PH DIWHU WKH WRJD SDUWB ISS Dr. Ayman Abdel-Hamid 14
15 Caesar Cipher 2/2 can define transformation as: 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 mathematically give each letter a number 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 then have Caesar cipher as: C = E(p) = (p + k) mod (26) p = D(C) = (C k) mod (26) ISS Dr. Ayman Abdel-Hamid 15
16 Cryptanalysis of Caesar Cipher only have 26 possible ciphers A maps to A,B,..Z (A to A is not really secure) could simply try each in turn a brute force search (encryption/decryption algorithms known, only 25 keys to try, and language of plaintext is know and recognized) given ciphertext, just try all shifts of letters do need to recognize when have plaintext e.g., break ciphertext "GCUA VQ DTGCM" ISS Dr. Ayman Abdel-Hamid 16
17 Monoalphabetic Cipher rather than just shifting the alphabet could shuffle (jumble) the letters arbitrarily Monoalphabetic substitution cipher (a single cipher alphabet is used per message) hence key is 26 letters long Plain: abcdefghijklmnopqrstuvwxyz Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN Plaintext: ifwewishtoreplaceletters Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA ISS Dr. Ayman Abdel-Hamid 17
18 Monoalphabetic Cipher Security now have a total of 26! = 4 x keys with so many keys, might think is secure but would be!!!wrong!!! problem is language characteristics ISS Dr. Ayman Abdel-Hamid 18
19 Language Redundancy and Cryptanalysis human languages are redundant eg "th lrd s m shphrd shll nt wnt" letters are not equally commonly used in English e is by far the most common letter then T,R,N,I,O,A,S other letters are fairly rare cf. Z,J,K,Q,X have tables of single, double & triple letter frequencies ISS Dr. Ayman Abdel-Hamid 19
20 English Letter Frequencies ISS Dr. Ayman Abdel-Hamid 20
21 Use in Cryptanalysis key concept - monoalphabetic substitution ciphers do not change relative letter frequencies discovered by Arabian scientists in 9 th century calculate letter frequencies for ciphertext compare counts/plots against known values ISS Dr. Ayman Abdel-Hamid 21
22 Example Cryptanalysis given ciphertext: UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ count relative letter frequencies (see text) guess P & Z are e and t guess ZW is th and hence ZWP is the proceeding with trial and error finally get: it was disclosed yesterday that several informal but direct contacts have been made with political representatives of the viet cong in moscow ISS Dr. Ayman Abdel-Hamid 22
23 Transposition Ciphers now consider classical transposition or permutation ciphers these hide the message by rearranging the letter order without altering the actual letters used can recognise these since have the same frequency distribution as the original text ISS Dr. Ayman Abdel-Hamid 23
24 Rail Fence cipher write message letters out diagonally over a number of rows then read off cipher row by row e.g., write message out as: m e m a t r h t g p r y e t e f e t e o a a t giving ciphertext MEMATRHTGPRYETEFETEOAAT ISS Dr. Ayman Abdel-Hamid 24
25 Row Transposition Ciphers a more complex scheme 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 ISS Dr. Ayman Abdel-Hamid 25
26 Product Ciphers ciphers using substitutions or transpositions are not secure because of language characteristics hence consider using several ciphers in succession to make harder, but: two substitutions make a more complex substitution two transpositions make more complex transposition but a substitution followed by a transposition makes a new much harder cipher this is a bridge from classical to modern ciphers ISS Dr. Ayman Abdel-Hamid 26
27 Steganography an alternative to encryption hides existence of message using only a subset of letters/words in a longer message marked in some way using invisible ink hiding in LSB in graphic image or sound file has drawbacks high overhead to hide relatively few info bits ISS Dr. Ayman Abdel-Hamid 27
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1 2 3 4 5 Basic Terminology plaintext - the original message ciphertext - the coded message cipher - algorithm for transforming plaintext to ciphertext key - info used in cipher known only to sender/receiver
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