Network Security Lecture 2 PDF

Network Security Lecture 2: Symmetric encryption Chapter #2 in the text book (Network Security Essentials : Applications and Standards -William Stallings) Prepared and modified by: Dr. Hossam Mahmoud Moftah Associate professor – Faculty of computers and artificial intel...

Network Security Lecture 2: Symmetric encryption Chapter #2 in the text book (Network Security Essentials : Applications and Standards -William Stallings) Prepared and modified by: Dr. Hossam Mahmoud Moftah Associate professor – Faculty of computers and artificial intelligence – Beni-Suef University Network Security Essentials Chapter 2 Wei Chen [email protected] 189-5189-6489 (Based on Lecture slides by Lawrie Brown) Outline  Symmetric encryption  Block encryption algorithms  Stream ciphers  Cipher Block Modes 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 public-key in 1970’s  and by far most widely used Crypto  Cryptology  The art and science of making and breaking “secret codes”  Cryptography  making “secret codes”  Cryptanalysis  breaking “secret codes”  Crypto  all of the above (and more) Some Basic Terminology  plaintext - original message  ciphertext - 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) - study of principles/ methods of deciphering ciphertext without knowing key  cryptology - field of both cryptography and cryptanalysis  Simple Substitution  Plaintext: fourscoreandsevenyearsago  Key: a b c d e f g h i j k l m n o p q r s t u v wx y z Plaintext DE F G H I J K L MN O P Q R S T U V WX Y Z A B C Ciphertext  Ciphertext: IRXUVFRUHDAGVHYHABHDUVDIR  Shift by 3 is “Caesar’s cipher”  Ceasar’s Cipher Decryption  Suppose we know a Ceasar’s cipher is being used  Ciphertext: VSRQJHEREVTXDUHSDQWU a b c d e f g h i j k l m n o p q r s t u v wx y z Plaintext DE F G H I J K L MN O P Q R S T U V WX Y Z A B C Ciphertext  Plaintext: spongebobsquarepants Not-so-Simple Substitution  Shift by n for some n  {0,1,2,…,25}  Then key is n  Example: key = 7 a b c d e f g h i j k l m n o p q r s t u v wx y z Plaintext HI J K L MN O P Q R S T U V WX Y Z A B C D E F G Ciphertext Cryptanalysis I: Try Them All  A simple substitution (shift by n) is used  But the key is unknown  Given ciphertext: CSYEVIXIVQMREXIH  How to find the key?  Only 26 possible keys  try them all!  Exhaustive key search  Solution: key = 4 Even-less-Simple Substitution  Key is some permutation of letters  Need not be a shift  For example a b c d e f g h i j k l m n o p q r s t u v wx y z Plaintext J I C A X S E Y V D K WB Q T Z R H F MP N U L G O Ciphertext  Then 26! > 288 possible keys! Cryptanalysis II: Be Clever  We know that a simple substitution is used  But not necessarily a shift by n  Can we find the key given ciphertext: PBFPVYFBQXZTYFPBFEQJHDXXQVAPTPQJKTOYQWIPBVWLXTOXBTF XQWAXBVCXQWAXFQJVWLEQNTOZQGGQLFXQWAKVWLXQWAEBIPB FXFQVXGTVJVWLBTPQWAEBFPBFHCVLXBQUFEVWLXGDPEQVPQGV PPBFTIXPFHXZHVFAGFOTHFEFBQUFTDHZBQPOTHXTYFTODXQHFTD PTOGHFQPBQWAQJJTODXQHFOQPWTBDHHIXQVAPBFZQHCFWPFHPB FIPBQWKFABVYYDZBOTHPBQPQJTQOTOGHFQAPBFEQJHDXXQVAVX EBQPEFZBVFOJIWFFACFCCFHQWAUVWFLQHGFXVAFXQHFUFHILTTA VWAFFAWTEVOITDHFHFQAITIXPFHXAFQHEFZQWGFLVWPTOFFA Cryptanalysis II  Can’t try all 288 simple substitution keys  Can we be more clever?  English letter frequency counts… 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 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  Cryptanalysis II  Ciphertext: PBFPVYFBQXZTYFPBFEQJHDXXQVAPTPQJKTOYQWIPBVWLXTOXBTFXQWAXB VCXQWAXFQJVWLEQNTOZQGGQLFXQWAKVWLXQWAEBIPBFXFQVXGTVJVW LBTPQWAEBFPBFHCVLXBQUFEVWLXGDPEQVPQGVPPBFTIXPFHXZHVFAGFOT HFEFBQUFTDHZBQPOTHXTYFTODXQHFTDPTOGHFQPBQWAQJJTODXQHFOQP WTBDHHIXQVAPBFZQHCFWPFHPBFIPBQWKFABVYYDZBOTHPBQPQJTQOTOG HFQAPBFEQJHDXXQVAVXEBQPEFZBVFOJIWFFACFCCFHQWAUVWFLQHGFXVA FXQHFUFHILTTAVWAFFAWTEVOITDHFHFQAITIXPFHXAFQHEFZQWGFLVWPTO FFA  Decrypt this message using info below Ciphertext frequency counts: A B C D E F G H I J K L MN O P Q R S T U V WX Y Z 21 26 6 10 12 51 10 25 10 9 3 10 0 1 15 28 42 0 0 27 4 24 22 28 6 8 Comparison 60 50 40 30 Ser i es1 20 10 0 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 0. 14 0. 12 0. 10 0. 08 0. 06 0. 04 0. 02 0. 00 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 15 Try by yourself https://www.youtube.com/watch?v=Hd2gY1cPDUw 16 Try by yourself https://www.youtube.com/watch?v=Hd2gY1cPDUw 17 Try by yourself https://www.youtube.com/watch?v=Hd2gY1cPDUw 18 Try by yourself 19 https://www.youtube.com/watch?v=Hd2gY1cPDUw Try by yourself https://www.youtube.com/watch?v=Hd2gY1cPDUw 20 Try by yourself https://www.youtube.com/watch?v=Hd2gY1cPDUw 21 Try by yourself 22 https://www.youtube.com/watch?v=Hd2gY1cPDUw Try by yourself 23 https://www.youtube.com/watch?v=Hd2gY1cPDUw Try by yourself 24 https://www.youtube.com/watch?v=Hd2gY1cPDUw Try by yourself https://www.youtube.com/watch?v=Hd2gY1cPDUw 25 Symmetric Cipher Model Requirements  two requirements for secure use of symmetric encryption: – a strong encryption algorithm – a secret key known only to sender / receiver  mathematically have: Y = E(K, X) X = D(K, Y)  assume encryption algorithm is known  implies a secure channel to distribute key Cryptography  can characterize cryptographic system by: – type of encryption operations used » substitution » transposition » product – number of keys used » single-key or private » two-key or public – way in which plaintext is processed » block » stream Cryptanalysis  objective to recover key not just message  general approaches: – cryptanalytic attack – brute-force attack  if either succeed all key use compromised Cryptanalytic Attacks  ciphertext only only know algorithm & ciphertext, is statistical, know or can identify plaintext  known plaintext know/suspect plaintext & ciphertext  chosen plaintext select plaintext and obtain ciphertext  chosen ciphertext select ciphertext and obtain plaintext  chosen text select plaintext or ciphertext to en/decrypt computationally secure encryption scheme  An encryption scheme: computationally secure if – The cost of breaking the cipher exceeds the value of information – The time required to break the cipher exceeds the lifetime of information Brute Force Search  always possible to simply try every key  most basic attack, proportional to key size  assume either know / recognise plaintext µs means microsecond (one million of second) Key Size (bits) Number of Alternative Time required at 1 Time required at 106 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 26! = 4  1026 2  1026 µs = 6.4  1012 years 6.4  106 years (permutation) Feistel Cipher  A Feistel cipher is a symmetric structure used in the construction of block ciphers, named after the German-born physicist and cryptographer Horst Feistel who did pioneering research while working for IBM (USA); Feistel Cipher Algorithm Adapted from William Stalling, Network Security Essentials, Applications and Standards, Fifth Edition Symmetric Block Cipher Algorithms  DES (Data Encryption Standard)  3DES (Triple DES)  AES (Advanced Encryption Standard) Data Encryption Standard (DES)  most widely used block cipher in world  adopted in 1977 by NBS (now NIST) – as FIPS PUB 46  encrypts 64-bit data using 56-bit key  has widespread use  has considerable controversy over its security Time to Break a DES Code (assuming 106 decryptions/s) DES algorithm  Description of the algorithm: – Plaintext is 64 bits in length – Key is 56 bits in length – Structure is a minor variation of the Feistel network – There are 16 rounds of processing – Process of decryption is essentially the same as the encryption process  The strength of DES: – Concerns fall into two categories » The algorithm itself  Refers to the possibility that cryptanalysis is possible by exploiting the characteristics of the algorithm » The use of a 56-bit key Adapted from William Stalling, Network Security Essentials, Applications and Standards, Fifth Edition Thank you