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![Substitution-Permutation Ciphers
• Substitution-permutation (S-P) networks [Shannon, 1949]
• modern substitution-transposition product cipher
• These form the basis of modern block ciphers
• S-P networks are based on the two primitive
cryptographic operations
• substitution (S-box)
• permutation (P-box)
• provide confusion and diffusion of message](https://image.slidesharecdn.com/mscs2-lec4networksecurity-171021174805/85/Cryptography-and-Network-security-Lecture-4-35-320.jpg)
![Decryption
• The same algorithm as
encryption.
• Reversed the order of key
(Key16, Key15, … Key1).
• For example:
• IP undoes IP-1 step of
encryption.
• 1st round with SK16 undoes
16th encrypt round.
[1]](https://image.slidesharecdn.com/mscs2-lec4networksecurity-171021174805/85/Cryptography-and-Network-security-Lecture-4-49-320.jpg)
![Substitution-Permutation Ciphers
• Substitution-permutation (S-P) networks [Shannon, 1949]
• modern substitution-transposition product cipher
• These form the basis of modern block ciphers
• S-P networks are based on the two primitive
cryptographic operations
• substitution (S-box)
• permutation (P-box)
• provide confusion and diffusion of message](https://image.slidesharecdn.com/mscs2-lec4networksecurity-171021174805/75/Cryptography-and-Network-security-Lecture-4-35-2048.jpg)
![Decryption
• The same algorithm as
encryption.
• Reversed the order of key
(Key16, Key15, … Key1).
• For example:
• IP undoes IP-1 step of
encryption.
• 1st round with SK16 undoes
16th encrypt round.
[1]](https://image.slidesharecdn.com/mscs2-lec4networksecurity-171021174805/75/Cryptography-and-Network-security-Lecture-4-49-2048.jpg)
Uploaded byKabul Education University
Cryptography and Network security # Lecture 4
The document provides an overview of cryptography and its history, including various types of ciphers such as Caesar, monoalphabetic, and one-time pads. It discusses symmetric key encryption, asymmetric key principles, and modern encryption standards like DES and AES. The presentation emphasizes the importance of secure communication in contemporary contexts, highlighting the shift from trust-based communication to secure mechanisms in electronic commerce.
In this document
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Introduction to the lecture on Cryptography and Network Security by Mr. Islahuddin Jalal. Research focus is on the 3C-CSIRT Model for Afghanistan.
Outlines the subjects to be discussed, including various types and classifications of cryptography.
A quote by Henry Ward Beecher emphasizing the understanding of secrets in cryptography.
Cryptography is defined as 'secret writing', combining Greek terms for 'secret' and 'writing'.
Classification based on the number of keys, encryption type, and processing methods: hash functions, secret and public key cryptography.
An overview of the history and origin of cryptography, its purpose in secure communication over insecure channels.
Comparison between old paradigms (trusted communication) and new paradigms requiring protection against fraud and involving trusted third parties.
Discussion on laws governing how law enforcement intercepts traffic and key recovery methods.
Important terms such as plaintext, ciphertext, encipher, decipher, cryptography, cryptanalysis, and cryptology.
Introduction to different types of encryption methods.
Symmetric encryption using the same key for both encryption and decryption, including the Caesar cipher as an example.
Detailed explanation of the Caesar cipher, its transformation, and methods to decrypt it.
Explanation of monoalphabetic ciphers and their security challenges due to language characteristics.
Introduction to transposition ciphers which rearrange letter orders to conceal messages.
Explains the one-time pad cipher concept as unbreakable if a truly random key of equal length is used.
Description of the Rail Fence cipher which encodes messages by arranging letters diagonally over rows.
Discussion on product ciphers that combine multiple ciphers for enhanced security.
Introduction to modern symmetric ciphers.
Differences between block ciphers and stream ciphers and their processing methods.
Overview of block cipher principles and the substitution-permutation (S-P) networks that form their basis.
The importance of confusion and diffusion in achieving security in cryptographic systems.
Explains the structure of the Feistel cipher, including its rounds and operations.
Introduction to the Data Encryption Standard (DES) as a symmetric key encryption algorithm.
DES utilizes confusion and diffusion techniques to protect data.
Chronology of the DES development from IBM's LUCIFER algorithm to its adoption as a standard.
Four main steps involved in the DES algorithm including initial permutation and rounds.
Details about the processes involved in the DES algorithm, including how expansion and XOR operations work.
Overview of the decryption process in DES, which is essentially the reverse of encryption.
Historical instances of DES being cracked within days using advanced computational methods.
Explanation of the Triple DES method using three iterations of DES with different keys.
Details on 3DES utilizing the original DES block cipher with enhanced effective key length.
Discussion on AES development and its introduction following DES and Triple DES limitations.
Overview of AES characteristics, including block size and security features involving S-boxes.
Concluding thank you note for the audience.
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Cryptography and Network security # Lecture 4
- 1. Lec-4: Cryptography &Network Security Mr. Islahuddin Jalal MS (Cyber Security) – UKM Malaysia Research Title – 3C-CSIRT Model for Afghanistan BAKHTAR UNIVERSITY باخترپوهنتون د Bakhtar University 1
- 2. Outlines to bediscussed…. Today • Cryptography • Classification of Cryptography • History of Cryptography • Basic Terminology of Cryptography • Symmetric cryptography • Ceasor cipher • Monoalphabetic cipher • Vienegar cipher • One time pad cipher • Rail fence cipher • Product cipher • Modern symmetric cipher • Block cipher • Stream cipher • Fiestal cipher Bakhtar University 2
- 3. To know thatone has a secret is to know half the secret itself (Henry Ward Beecher) Bakhtar University 3
- 4. Cryptography • Combination ofGreek words i.e. Crypto+Graphy • Crypto = “Secret” and Graphy = “Writing” • Cryptography means secret writing 10/21/2017 Bakhtar University 4
- 5. Classification of Cryptography •Number of keys used • Hash functions: no key • Secret key cryptography: one key • Public key cryptography: two keys - public, private • Type of encryption operations used • substitution / transposition / product • Way in which plaintext is processed • block / stream • The way in which the plaintext is processed (block cipher processes the input one block of elements at a time, producing an output block for each input block. Stream Cipher processes the input elements continuously, producing output one element at a time, as it goes along
- 6. History of Cryptography 10/21/2017Bakhtar University 6
- 7. History of Cryptography 10/21/2017Bakhtar University 7
- 8. Origin of Cryptography 10/21/2017Bakhtar University 8 Two secure end systems communicate over an insecure channel The enemy is an outsider listening to traffic
- 9. Old Paradigm • Aand B communicate over an insecure channel. • A and B trust each other. • Intruder can read, delete, and insert messages. • With cryptography, A and B construct a secure logical channel over an insecure network 10/21/2017 Bakhtar University 9 A B intruder
- 10.
- 11. New Paradigm • Electroniccommerce: A and B are customer and merchant; they do not “trust” each other. • We want protection against insider fraud as much as protection against outsiders. • Trusted Third Parties help settle disputes. 10/21/2017 Bakhtar University 11 A B TTP
- 12. Law Enforcement • Inmany countries laws regulate how a law enforcement agency (LEA) can intercept traffic. • Key recovery makes cryptographic keys available to their owner. • Key escrow makes keys available to a LEA. 10/21/2017 Bakhtar University 12 A B LEA
- 13. 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
- 14. Terminology of Cryptography •Encryption: plaintext (clear text) x is converted into a ciphertext under the control of a key K. • We write eK(x). • Decryption with key K computes the plaintext from the ciphertext y. • We write dK(y). • Symmetric ciphers: the decryption key is essentially the same as the encryption key. • Asymmetric ciphers: it is computationally infeasible to derive the private decryption key from the corresponding public encryption key. 10/21/2017 Bakhtar University 14
- 15. Types of Encryption 10/21/2017Bakhtar University 15
- 16. Symmetric Encryption • Itis a form of cryptosystem in which encryption and decryption are performed using the same key. It is also known as conventional encryption. • Having five components Plaintext Encryption algorithm (e.g AES, DES) Secret keys Ciphertext Decryption algorithm (reverse of encryption algorithm) 10/21/2017 Bakhtar University 16
- 17. Symmetric Encryption Model 10/21/2017Bakhtar University 17
- 18. Caesar Cipher • Theearliest and simplest cipher • Developed by Julius Caesar • Replacing each letter of the alphabet with the letter standing three places 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 • Cipher: 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 10/21/2017 Bakhtar University 18
- 19. Caesar Cipher • Definetransformation 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 0 1 2 3 4 5 6 7 8 9 10 11 12 n o p q r s t u v w x y Z 13 14 15 16 17 18 19 20 21 22 23 24 25 • Then have Caesar cipher as: C = E(p) = (p + k) mod (26) p = D(C) = (C – k) mod (26)
- 20. 20 The Caesar cipher •Encrypt “go cavaliers” • Translate to numbers: g = 6, o = 14, etc. • Full sequence: 6, 14, 2, 0, 21, 0, 11, 8, 4, 17, 18 • Apply the cipher to each number: f(6) = 9, f(14) = 17, etc. • Full sequence: 9, 17, 5, 3, 24, 3, 14, 11, 7, 20, 21 • Convert the numbers back to letters 9 = j, 17 = r, etc. • Full sequence: jr wfdydolhuv • Decrypt “jr wfdydolhuv” • Translate to numbers: j = 9, r = 17, etc. • Full sequence: 9, 17, 5, 3, 24, 3, 14, 11, 7, 20, 21 • Apply the cipher to each number: f-1(9) = 6, f-1(17) = 14, etc. • Full sequence: 6, 14, 2, 0, 21, 0, 11, 8, 4, 17, 18 • Convert the numbers back to letters 6 = g, 14 = 0, etc. • Full sequence: “go cavaliers”
- 21. Cryptanalysis of CaesarCipher • Only have 25 possible ciphers • A maps to B,..Z • Given ciphertext, just try all shifts of letters • Do need to recognize when have plaintext • E.g., break ciphertext "GCUA VQ DTGCM"
- 22. Monoalphabetic Cipher • Ratherthan just shifting the alphabet • Could shuffle (jumble) the letters arbitrarily • Each plaintext letter maps to a different random ciphertext letter • Key is 26 letters long Plain: abcdefghijklmnopqrstuvwxyz Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN Plaintext: ifwewishtoreplaceletters Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA
- 23. Monoalphabetic Cipher Security •Now have a total of 26! = 4 x 1026 keys • Is that secure? • Problem is language characteristics • Human languages are redundant • Letters are not equally commonly used
- 24.
- 25. Transposition Ciphers • Nowconsider 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
- 26. One-Time Pad • Proposedby Gilbert S. Vernam of AT&T in 1917 • Effective for short message, such as nuclear launch codes • If a truly random key as long as the message is used, the cipher will be secure - One-Time pad • Unbreakable since ciphertext bears no statistical relationship to the plaintext • For any plaintext, it needs a random key of the same length • Hard to generate large amount of keys • Have problem of safe distribution of key
- 27. One time pad •There 26 characters in English alphabets so assign a single number for each character in a sequence. Starting from A=1, B=2……….Z=26. • Example • Plaintext: DINOSAUR • Replace the text with the numbers, so the numbers for DINOSAUR is 4,9,14,15,19,1,21,18 • Key: 6,2,21,14,7,8,12,9 • Now add up the key with corresponding plaintext number Bakhtar University 27
- 28. Continued…. • So thecorresponding cipher numbers for DINASOUR is 10,11,35,29,26,9,33,27 • Note: • One time pad for English has a 26 position maximum, so that is why, the concept of modular arithmetic is used. • After modular arithmetic • The cipher numbers for DINASOUR become: 10,11,9,3,0,9,7,1 • Now mapping the cipher numbers with corresponding character of the English alphabet. • Ciphertext: JKICZIGA Bakhtar University 28
- 29. Decryption of Onetime Pad • Reverse the process • Note: keep one thing in mind that when ever you are subtracting the key from the cipher numbers and it give you negative value, then add 26 with the number and then subtract the key. Bakhtar University 29
- 30. 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
- 31. Product Ciphers • Ciphersusing 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 bridge from classical to modern ciphers
- 32. Modern Symmetric Ciphers BakhtarUniversity 32
- 33. Block vs StreamCiphers • Block ciphers process messages in into blocks, each of which is then en/decrypted • Like a substitution on very big characters • 64-bits or more • Stream ciphers process messages a bit or byte at a time when en/decrypting • Many current ciphers are block ciphers, one of the most widely used types of cryptographic algorithms
- 34. Block Cipher Principles •Most symmetric block ciphers are based on a Feistel Cipher Structure • Block ciphers look like an extremely large substitution • Would need table of 264 entries for a 64-bit block • Instead create from smaller building blocks • Using idea of a product cipher
- 35. Substitution-Permutation Ciphers • Substitution-permutation(S-P) networks [Shannon, 1949] • modern substitution-transposition product cipher • These form the basis of modern block ciphers • S-P networks are based on the two primitive cryptographic operations • substitution (S-box) • permutation (P-box) • provide confusion and diffusion of message
- 36. Confusion and Diffusion •Cipher needs to completely obscure statistical properties of original message • More practically Shannon suggested S-P networks to obtain: • Diffusion – dissipates statistical structure of plaintext over bulk of ciphertext • Confusion – makes relationship between ciphertext and key as complex as possible
- 37. Feistel Cipher Structure •Feistel cipher implements Shannon’s S-P network concept • based on invertible product cipher • Process through multiple rounds which • partitions input block into two halves • perform a substitution on left data half • based on round function of right half & subkey • then have permutation swapping halves
- 38.
- 39. What is DES? SymmetricKey algorithm for Encryption
- 40. DES - Basics DES uses the two basic techniques of cryptography - confusion and diffusion. At the simplest level, diffusion is achieved through numerous permutations and confusion is achieved through the XOR operation.
- 41. History of DES In1971, IBM developed an algorithm, named LUCIFER which operates on a block of 64 bits, using a 128-bit key Walter Tuchman, an IBM researcher, refined LUCIFER and reduced the key size to 56-bit, to fit on a chip.
- 42. History of DES In1977, the results of Tuchman’s project of IBM was adopted as the Data Encryption Standard by NSA (NIST).
- 43. How Data EncryptionStandard (DES) work?
- 44. Illustration of DESalgorithm • There are four Steps involved in the DES 1. Initial Permutation (IP) 2. 16 Feistal Rounds 3. Left right Swapping 4. Final Permutation (FP) which is reverse permutation (IP- )
- 45.
- 46. What is doneinside F function?
- 47. How Expansion isdone?
- 48. 0 1 10 0 1 0 1 K0 R0L0 K1 R1L1 Kn Rn Ln XOR f(Kn,Rn) Li+1 = Ri Ri+1 = Li XOR f ( Ri , Ki ) C = (Rn+1,Ln+1) m C
- 49. Decryption • The samealgorithm as encryption. • Reversed the order of key (Key16, Key15, … Key1). • For example: • IP undoes IP-1 step of encryption. • 1st round with SK16 undoes 16th encrypt round. [1]
- 50. 0 1 10 0 1 0 1 Kn Kn-1 Rn-1Ln-1 K0 L0 R0 Ri = Li+1 Li = Ri+1 XOR f ( Li+1, Ki ) m = (L0 , R0) Ln XOR f(Kn,Rn) c m
- 51. DES Cracked • In1998, a DES-encrypted message was cracked in 3 days • In 1999, a network of 10,000 desktop systems cracked a DES- encrypted message in less than 1 day
- 52. Triple DES • DESalgorithm used three times • K1 is used to encrypt a message (p) resulting in C1 cipher text • K2 is used to decrypt C! resulting in C2 cipher text • K3 is used to encrypt C2 resulting in C3 cipher text
- 53. 3DES Technical Details •Still using the DES block cipher with 56 bit keys • Applying three different keys yields effective key length of 168 bits
- 54. 7/3/01 Advanced Encryption Standard •DES cracked, Triple-DES slow: what next? • 1997: AES announced, call for algorithms • August 1998: 15 candidate algorithms • August 1999: 5 finalists • October 2000: Rijndael selected • Two Belgians: Joan Daemen, Vincent Rijmen • May 2001: Comment period ended • Summer 2001: Finalized, certified until ‘06
- 55. 7/3/01 AES Technical Details •Similar to DES: block cipher (with different modes), but 128-bit blocks • 128-bit, 192-bit, or 256-bit key • Mix of permutations, “S-boxes” • S-boxes based on modular arithmetic with polynomials: • Non-linear • Easy to analyze, prove attacks fail
- 56. Thank You For YourPatience Bakhtar University 56