IS Unit 1_Conventional Encryption_Classical Encryption Techniques
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The document discusses classical encryption techniques such as the Caesar cipher, monoalphabetic substitution cipher, polyalphabetic ciphers like the Vigenère cipher, and the Playfair cipher. It explains the basic concepts of encryption including plaintext, ciphertext, encryption algorithms, decryption algorithms, and symmetric key cryptography. It also covers cryptanalysis techniques like frequency analysis that can be used to break some classical ciphers. The document is intended to introduce basic concepts of encryption as a precursor to studying modern cryptography.
IS Unit 1_Conventional Encryption_Classical Encryption Techniques
- 1. Chapter 1:Chapter 1:Chapter 1:Chapter 1:---- Conventional Encryption: Classical Encryption Techniques By:- Sarthak Patel (www.sarthakpatel.in)
- 2. Outline Conventional Encryption Model Steganography Classical EncryptionTechniques Sarthak Patel (www.sarthakpatel.in)2
- 3. Classical encryption techniques As opposed to modern cryptography Goals: to introduce basic concepts & terminology of encryption to prepare you for studying modern cryptography Sarthak Patel (www.sarthakpatel.in)3
- 4. Principles of Security Security Goals: Confidentiality Integrity Authentication Non-repudiation Sarthak Patel (www.sarthakpatel.in)4 Non-repudiation Access Control Availability
- 5. Loss of Confidentiality • SecretA B Sarthak Patel (www.sarthakpatel.in)5 C Attack:-Interception
- 6. Absence of Authentication BA I am User A Sarthak Patel (www.sarthakpatel.in)6 C Attack:-Fabrication
- 7. Loss of Integrity • Ideal RouteA B Sarthak Patel (www.sarthakpatel.in)7 C Attack:-Modification
- 8. Non-repudiation A B I never sent that message, which you claim to have received Sarthak Patel (www.sarthakpatel.in)8 A B
- 9. Access Control The principles of access control determines who should be able to access what. Access control is broadly related to two areas: role management and rule management. Sarthak Patel (www.sarthakpatel.in)9
- 10. Loss of Availability A B Sarthak Patel (www.sarthakpatel.in)10 A B Attack:-Interruption C
- 11. Basic terminology Plaintext: original message to be encrypted Ciphertext: the encrypted message Enciphering or encryption: the process of converting plaintext into ciphertext Sarthak Patel (www.sarthakpatel.in)11 plaintext into ciphertext Encryption algorithm: performs encryption Two inputs: a plaintext and a secret key
- 12. Symmetric Cipher Model Sarthak Patel (www.sarthakpatel.in)12
- 13. Contd… Deciphering or decryption: recovering plaintext from ciphertext Decryption algorithm: performs decryption Two inputs: ciphertext and secret key Sarthak Patel (www.sarthakpatel.in)13 Secret key: same key used for encryption and decryption Also referred to as a symmetric key
- 14. Contd… Cipher or cryptographic system : a scheme for encryption and decryption Cryptography: science of studying ciphers Sarthak Patel (www.sarthakpatel.in)14 Cryptanalysis: science of studying attacks against cryptographic systems Cryptology: cryptography + cryptanalysis
- 15. Ciphers Symmetric cipher: same key used for encryption and decryption Block cipher: encrypts a block of plaintext at a time (typically 64 or 128 bits) Sarthak Patel (www.sarthakpatel.in)15 128 bits) Stream cipher: encrypts data one bit or one byte at a time Asymmetric cipher: different keys used for encryption and decryption
- 16. Symmetric Encryption or conventional / secret-key / single-key sender and recipient share a common key all classical encryption algorithms are symmetric The only type of ciphers prior to the invention of asymmetric-key ciphers in 1970’s Sarthak Patel (www.sarthakpatel.in)16 asymmetric-key ciphers in 1970’s by far most widely used
- 17. Symmetric Encryption Mathematically: Y = EK(X) or Y = E(K, X) X = DK(Y) or X = D(K, Y) X = plaintext Y = ciphertext K = secret key Sarthak Patel (www.sarthakpatel.in)17 K = secret key E = encryption algorithm D = decryption algorithm Both E and D are known to public
- 18. Cryptanalysis Objective: to recover the plaintext of a ciphertext or, more typically, to recover the secret key. Kerkhoff’s principle: the adversary knows all details about a cryptosystem except the secret key. Sarthak Patel (www.sarthakpatel.in)18 about a cryptosystem except the secret key. Two general approaches: brute-force attack non-brute-force attack (cryptanalytic attack)
- 19. Brute-Force Attack Try every key to decipher the ciphertext. On average, need to try half of all possible keys Time needed proportional to size of key space Key Size (bits) Number of Alternative Keys Time required at 1 decryption/µs Time required at 106 decryptions/µs Sarthak Patel (www.sarthakpatel.in)19 Keys decryption/µs decryptions/µs 32 232 = 4.3 × 109 231 µs = 35.8 minutes 2.15 milliseconds 56 256 = 7.2 × 1016 255 µs = 1142 years 10.01 hours 128 2128 = 3.4 × 1038 2127 µs = 5.4 × 1024 years 5.4 × 1018 years 168 2168 = 3.7 × 1050 2167 µs = 5.9 × 1036 years 5.9 × 1030 years 26 characters (permutation) 26! = 4 × 1026 2 × 1026 µs = 6.4 × 1012 years 6.4 × 106 years
- 20. Cryptanalytic Attacks May be classified by how much information needed by the attacker: Ciphertext-only attack Known-plaintext attack Sarthak Patel (www.sarthakpatel.in)20 Known-plaintext attack Chosen-plaintext attack Chosen-ciphertext attack
- 21. Ciphertext-only attack Given: a ciphertext c Q: what is the plaintext m? An encryption scheme is completely insecure if it cannot resist ciphertext-only attacks. Sarthak Patel (www.sarthakpatel.in)21
- 22. Known-plaintext attack Given: (m1,c1), (m2,c2), …, (mk,ck) and a new ciphertext c. Q: what is the plaintext of c? Q: what is the secret key in use? Sarthak Patel (www.sarthakpatel.in)22
- 23. Chosen-plaintext attack Given: (m1,c1), (m2,c2), …, (mk,ck), where m1,m2, …, mk are chosen by the adversary; and a new ciphertext c. Q: what is the plaintext of c, or what is the secret key? Sarthak Patel (www.sarthakpatel.in)23 Q: what is the plaintext of c, or what is the secret key?
- 24. Example: chosen-plaintext attack In 1942, US Navy cryptanalysts discovered that Japan was planning an attack on “AF”. They believed that “AF” means Midway island. Pentagon didn’t think so. Sarthak Patel (www.sarthakpatel.in)24 Pentagon didn’t think so. US forces in Midway sent a plain message that their freshwater supplies were low. Shortly, US intercepted a Japanese ciphertext saying that “AF” was low on water. This proved that “AF” is Midway.
- 25. Chosen-ciphertext attack Given: (m1,c1), (m2,c2), …, (mk,ck), where c1,c2, …, ck are chosen by the adversary; and a new ciphertext c. Q: what is the plaintext of c, or what is the secret key? Sarthak Patel (www.sarthakpatel.in)25
- 26. Classical Ciphers Plaintext is viewed as a sequence of elements (e.g., bits or characters) Substitution cipher: replacing each element of the plaintext with another element. Sarthak Patel (www.sarthakpatel.in)26 Transposition (or permutation) cipher: rearranging the order of the elements of the plaintext. Product cipher: using multiple stages of substitutions and transpositions
- 27. Caesar Cipher Earliest known substitution cipher Invented by Julius Caesar Each letter is replaced by the letter three positions further down the alphabet. • Plain: 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 Sarthak Patel (www.sarthakpatel.in)27 • Plain: 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 Cipher: D E F G H I J K L M N O P Q R ST UVW XY ZA B C Example: ohio state RKLRVWDWH
- 28. Caesar Cipher Mathematically, map letters to numbers: a, b, c, ..., x, y, z 0, 1, 2, ..., 23, 24, 25 Then the general Caesar cipher is: c = E (p) = (p + k) mod 26 Sarthak Patel (www.sarthakpatel.in)28 c = EK(p) = (p + k) mod 26 p = DK(c) = (c – k) mod 26 Can be generalized with any alphabet.
- 29. Cryptanalysis of Caesar Cipher Key space: {0, 1, ..., 25} Vulnerable to brute-force attacks. E.g., break ciphertext “KHOOR“ Answer is:“HELLO” Sarthak Patel (www.sarthakpatel.in)29 Answer is:“HELLO”
- 30. Monoalphabetic Substitution Cipher Shuffle the letters and map each plaintext letter to a different random ciphertext letter: Plain letters: abcdefghijklmnopqrstuvwxyz Cipher letters: DKVQFIBJWPESCXHTMYAUOLRGZN Sarthak Patel (www.sarthakpatel.in)30 Cipher letters: DKVQFIBJWPESCXHTMYAUOLRGZN Plaintext: ifwewishtoreplaceletters Ciphertext:WIRFRWAJUHYFTSDVFSFUUFYA What does a key look like?
- 31. Monoalphabetic Cipher Security Now we have a total of 26! = 4 x 1026 keys. With so many keys, it is secure against brute-force attacks. But not secure against some cryptanalytic attacks. Problem is language characteristics. Sarthak Patel (www.sarthakpatel.in)31 Problem is language characteristics.
- 32. Language Statistics and Cryptanalysis Human languages are not random. Letters are not equally frequently used. In English, E is by far the most common letter, followed byT,A, R, N, I, O, S. Sarthak Patel (www.sarthakpatel.in)32 byT,A, R, N, I, O, S. Other letters like Z, J, K, Q, X are fairly rare. There are tables of single, double & triple letter frequencies for various languages
- 33. English Letter Frequencies Sarthak Patel (www.sarthakpatel.in)33
- 34. Statistics for double & triple letters In decreasing order of frequency Double letters: to he an in re on, … Sarthak Patel (www.sarthakpatel.in)34 Triple letters: the and for nab, …
- 35. Use in Cryptanalysis Key concept: monoalphabetic substitution does not change relative letter frequencies To attack, we Sarthak Patel (www.sarthakpatel.in)35 To attack, we calculate letter frequencies for ciphertext compare this distribution against the known one
- 36. Example Cryptanalysis Given ciphertext: UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ Count relative letter frequencies (see next page) Guess {P, Z} = {e, t} Sarthak Patel (www.sarthakpatel.in)36 Guess {P, Z} = {e, t} Of double letters, ZW has highest frequency, so guess ZW = th and hence ZWP = 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
- 37. Letter frequencies in ciphertext P 13.33 H 5.83 F 3.33 B 1.67 C 0.00 Z 11.67 D 5.00 W 3.33 G 1.67 K 0.00 S 8.33 E 5.00 Q 2.50 Y 1.67 L 0.00 U 8.33 V 4.17 T 2.50 I 0.83 N 0.00 Sarthak Patel (www.sarthakpatel.in)37 U 8.33 V 4.17 T 2.50 I 0.83 N 0.00 O 7.50 X 4.17 A 1.67 J 0.83 R 0.00 M 6.67
- 38. Polyalphabetic Substitution Ciphers A sequence of monoalphabetic ciphers (M1, M2, M3, ..., Mk) is used in turn to encrypt letters. A key determines which sequence of ciphers to use. Each plaintext letter has multiple corresponding ciphertext letters. Sarthak Patel (www.sarthakpatel.in)38 ciphertext letters. This makes cryptanalysis harder since the letter frequency distribution will be flatter.
- 39. Example(Poly): Vigenère Cipher Simplest polyalphabetic substitution cipher Consider the set of all Caesar ciphers: { Ca, Cb, Cc, ..., Cz } Key: e.g. security Encrypt each letter using C , C , C , C ,C , C , C , C in Sarthak Patel (www.sarthakpatel.in)39 Encrypt each letter using Cs, Ce, Cc, Cu,Cr, Ci, Ct, Cy in turn. Repeat from start after Cy. Decryption simply works in reverse.
- 40. Example of Vigenère Cipher Keyword: deceptive key: deceptivedeceptivedeceptive plaintext: wearediscoveredsaveyourself Sarthak Patel (www.sarthakpatel.in)40 plaintext: wearediscoveredsaveyourself ciphertext: ZICVTWQNGRZGVTWAVZHCQYGLMGJ
- 41. Key Plain Text Sarthak Patel (www.sarthakpatel.in)41
- 42. Playfair Cipher Not even the large number of keys in a monoalphabetic cipher provides security. One approach to improving security is to encrypt multiple letters at a time. The Playfair Cipher is the best known such cipher. Sarthak Patel (www.sarthakpatel.in)42 The Playfair Cipher is the best known such cipher. Invented by CharlesWheatstone in 1854, but named after his friend Baron Playfair.
- 43. Playfair Key Matrix Use a 5 x 5 matrix. Fill in letters of the key (w/o duplicates). Fill the rest of matrix with other letters. E.g., key = MONARCHY. Sarthak Patel (www.sarthakpatel.in)43 E.g., key = MONARCHY. MM OO NN AA RR CC HH YY BB DD EE FF GG I/JI/J KK LL PP QQ SS TT UU VV WW XX ZZ
- 44. Encrypting and Decrypting Plaintext is encrypted two letters at a time. 1. If a pair is a repeated letter, insert filler like 'X’. 2. If both letters fall in the same row, replace each with the letter to its right (circularly). 3. If both letters fall in the same column, replace each with the the Sarthak Patel (www.sarthakpatel.in)44 3. If both letters fall in the same column, replace each with the the letter below it (circularly). 4. Otherwise, each letter is replaced by the letter in the same row but in the column of the other letter of the pair.
- 45. Example of Playfair Cipher Key: MONARCHY Plaintext: BALLOON MM OO NN AA RR CC HH YY BB DD EE FF GG I/JI/J KK LL PP QQ SS TT UU VV WW XX ZZ Sarthak Patel (www.sarthakpatel.in)45 oBA LX LO ON Ciphertext: IB SU PM NA UU VV WW XX ZZ
- 46. Security of Playfair Cipher Security is much improved over the simple monoalphabetic cipher. Was widely used for many decades eg. by US & British military inWW1 and earlyWW2 Sarthak Patel (www.sarthakpatel.in)46 Once thought to be unbreakable. Actually, it can be broken, because it still leaves some structure of plaintext intact.
- 47. Rotor Cipher Machines Before modern ciphers, rotor machines were most common complex ciphers in use. Widely used inWW2. Used a series of rotating cylinders. Sarthak Patel (www.sarthakpatel.in)47 Implemented a polyalphabetic substitution cipher of period K. With 3 cylinders, K = 263 =17,576. With 5 cylinders, K = 265 =12 x 106. What is a key? If the adversary has a machine If the adversary doesn’t have a machine
- 49. German secret setting sheets Sarthak Patel (www.sarthakpatel.in)49 Date Which rotors to use (there were 10 rotors) Ring setting Plugboard setting
- 50. The Rotors Sarthak Patel (www.sarthakpatel.in)50
- 51. Enigma Rotor Machine Sarthak Patel (www.sarthakpatel.in)51
- 52. Enigma Rotor Machine Sarthak Patel (www.sarthakpatel.in)52
- 53. Transposition Ciphers Also called permutation ciphers. Shuffle the plaintext, without altering the actual letters used. Example: i) ColumnarTransposition Ciphers ii) Rail FenceTechnique Sarthak Patel (www.sarthakpatel.in)53 ii) Rail FenceTechnique
- 54. Columnar Transposition Ciphers Plaintext is written row by row in a rectangle. Ciphertext: write out the columns in an order specified by a key. C O M E H O Sarthak Patel (www.sarthakpatel.in)54 Key: 3 4 2 1 5 6 Plaintext: Ciphertext: MTOR EOWN OERE CMRT HMAO OOFA C O M E H O M E T O M O R R O W A F T E R N O O
- 55. Rail Fence Technique Rail fence technique involves writing plain text as sequence of diagonals and then reading it row-by-row to produce cipher text. PlainText: COME HOMETOMORROW C M H M T M R O Sarthak Patel (www.sarthakpatel.in)55 C M H M T M R O O E O E O O R W CipherText: CMHMTMRO OEOEOORW
- 56. Product Ciphers Uses a sequence of substitutions and transpositions Harder to break than just substitutions or transpositions This is a bridge from classical to modern ciphers. Sarthak Patel (www.sarthakpatel.in)56
- 57. Steganography Hide a message in another message. Invisible ink,Tiny pin punctures or minute variations between handwritten characters, pencil marks etc. E.g., hide your plaintext in a graphic image Sarthak Patel (www.sarthakpatel.in)57 Each pixel has 3 bytes specifying the RGB color The least significant bits of pixels can be changed w/o greatly affecting the image quality So can hide messages in these LSBs
- 58. Summary Have considered: classical cipher techniques and terminology monoalphabetic substitution ciphers cryptanalysis using letter frequencies Sarthak Patel (www.sarthakpatel.in)58 cryptanalysis using letter frequencies Playfair cipher polyalphabetic ciphers transposition ciphers product ciphers and rotor machines Steganography
- 59. Symmetric Asymmetric Symmetric cryptography uses the same secret (private) key to encrypt and decrypt its data Asymmetric uses both a public and private key Symmetric requires that the secret key be known by the party encrypting the data and the party Asymmetric allows for distribution of your public key to anyone with which they can encrypt the Sarthak Patel (www.sarthakpatel.in)59 by the party encrypting the data and the party decrypting the data key to anyone with which they can encrypt the data they want to send securely and then it can only be decoded by the person having the private key Faster than Asymmetric The issue with asymmetric is that it is about 1000 times slower than symmetric encryption which makes it impractical when trying to encrypt large amounts of data Less Security compare to Asymmetric Stronger Security compare to Symmetric