Network Security. Omer Rana

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1 Network Security Omer Rana CM0255 Material from: Cryptography Components Sender Receiver Plaintext Encryption Ciphertext Decryption Plaintext Encryption algorithm: Plaintext Ciphertext Cipher: encryption or decryption algorithms (or categories of algorithms) Key: a number (set of numbers) that the cipher (as an algorithm) operates on. To encrypt a message: Encryption algorithm Encryption key Plaintext Ciphertext 1

or decryption algorithms (or categories of algorithms) Key: a number (set of numbers) that the cipher

2 Symmetric-key Cryptography Alice shared secret key Bob Three types of keys: Secret Key Public Key Private Key Plaintext Encryption Ciphertext Decryption Same key used by both parties Key used for both encryption and decryption Keys need to be swapped beforehand using a secure mechanism Plaintext Asymmetric-key Cryptography To everyone (public) Bob s public key Alice Bob Bob s private key Plaintext Encryption Ciphertext Decryption Plaintext Differentiate between a public key and a private key Symmetric-Key Cryptography Traditional ciphers: Character oriented Two main approaches: Substitution ciphers or Transposition Ciphers Substitution Ciper: Substitute one symbol with another Mono-alphabetic: a character (symbol) in plaintext is changed to the same character (symbol) in ciphertext regardless of its position in the text If L O, every instance of L will be changed to O Plaintext: HELLO Ciphertext: KHOOR Poly-alphabetic: each occurrence can have a different substitute. Relationship between a character in plaintext to ciphertext is one-to-many (based on position being in beginning, middle or end of text). Divide text into group of characters and use a set of keys. THISLECTUREISREALLYINTERESTING Plaintext: HELLO Ciphertext: ABNZF 2

Plaintext Encryption Ciphertext Decryption Plaintext Differentiate between a public key and a private key Symmetric-Key Cryptography Traditional ciphers: Character oriented Two main approaches:

3 Other Traditional Ciphers Shift/Caesar Cipher Plaintext and Ciphertext contains upper case characters Encryption: shift <x> characters up Decryption: shift <x> characters down <x> is the key Use shift cipher with key = 15 to encrypt message HELLO Solution: Each character is shifted 15 times: H W E T HELLO WTAAD L A O D Transposition Cipher Change location of characters from plaintext to cipher text Permute symbols in a block of symbols Key is a mapping between the position of symbols in plaintext to cipher text Example: Plaintext: ; Cipher text: position Encryption using the above Cipher: Message HELLO MY DEAR Remove spaces: HELLOMYDEAR Divide into block of four: HELL OMYD EARZ note addition of Z Ciphertext: ELHLMDOYAZER 3

plaintext to cipher text Permute symbols in a block of symbols Key is a mapping between the position of symbols in plaintext to cipher text Example: Plaintext: 1 2 3 4; Cipher text: 2 4 1 3

4 Currently used Ciphers Traditional ciphers are character-oriented Modern ciphers are bit-oriented Can deal with text, numbers, video, audio, images, etc Convert data into bit stream then apply cipher Symmetric cipher: Use a combination of ciphers rather than just one Ciphers may be applied in rounds Examples: XOR, Rotation Cipher, Substitution Cipher (S-box), Transposition Cipher (P-box) Standards: Data Encryption Standard (DES) Advanced Encryption Standards (AES) Others: Int. Data Encryption Algorithm (IDEA), Blowfish, CAST- 128, RC5 A B C XOR Modern Ciphers 1 Use of data (plaintext) and key Size of data, key and ciphertext are the same Interesting property: encryption and decryption are the same Plaintext: Key: Ciphertext: Key: Recovered Plaintext: Rotation Cipher Key may be specifically defined or assumed Keyless rotation: number of rotations (left/right) is pre-defined If plaintext size is N; then after N rotations, we get original Useless to apply more than N-1 rotations Decryption: use same key but opposite direction of rotation 4

Standards: Data Encryption Standard (DES) Advanced Encryption Standards (AES) Others: Int.

5 Substitution: S-box Modern Ciphers 2 Similar to traditional substitution cipher for characters Plaintext: stream of length N Ciphertext: stream of length M, N<>M Transposition: P-box (Permutation box) Similar to traditional transposition cipher for characters Performs transposition at bit level E.g.: Straight (N N), Expansion (N M, M>N), Compression (N M, M<N) Round Ciphers Ciphers involve multiple rounds Each round is a complex cipher made up of simple ciphers Each round has a key which is a subset/variation of the general key ( round key ) Key generator If N rounds N keys are generated one for each round 5

: Straight (N N), Expansion (N M, M>N), Compression (N M, M<N) Round Ciphers Ciphers involve multiple rounds Each round is a complex cipher made up of

6 Types Keys used for Encryption and Decryption Same Different Symmetric-Key Cryptography DES AES Asymmetric-Key Cryptography RSA Diffie-Hellman IDEA Blowfish CAST-128 RC5 Data Encryption Standard (DES) from IBM 64-bit plaintext Initial Permutation Round 1 Round 16 Final Permutation 64-bit ciphertext k1 k16 Round Key Generator Symmetric key cipher divide plaintext into blocks (use same key to encrypt/decrypt block) 64-bit plaintext, 64-bit key Two p-boxes, 16 complex (repeated) round ciphers with different key Initial and final permutations are keyless straight permutations (inverse of each other) 6

ciphertext k1 k16 Round Key Generator Symmetric key cipher divide plaintext into blocks (use same key to encrypt/decrypt block) 64-bit plaintext,

7 DES function for the i th Round 32 bits 32 bits Li Ri Ri 32 bits f(ri,ki) Outcome + XOR Li+1 Ri+1 32 bits 32 bits Key (Ki) 48-bits Expansion Permutation 48 bits + S S S S S S S Straight Permutation Outcome 48 bits 32 bits 32 bits (Ki) S-boxes Decryption Round 32 bits 32 bits Li Ri Ri 32 bits f(ri,ki) Outcome + XOR Li+1 Ri+1 32 bits 32 bits Key (Ki) 48-bits Expansion Permutation 48 bits + S S S S S S S Straight Permutation Outcome 48 bits 32 bits 32 bits (Ki) S-boxes 7

S-boxes Decryption Round 32 bits 32 bits Li Ri Ri 32 bits f(ri,ki) Outcome + XOR Li+1 Ri+1 32 bits 32 bits Key

8 Triple DES (3DES) To improve on the limited size of DES key use of three DES blocks Encryption block: Encryption-Decryption-Encryption combination Decryption block Decryption-Encryption-Decryption combination Two version: 2 keys and 3 keys 64-bit plaintext Triple DES (3DES) Encrypt DES Decrypt DES Key 1 Key 2 2 keys version: Key 1 = Key 3 Text encrypted by a Single DES block can Be decrypted by the new 3DES Encrypt DES 64-bit ciphertext Key 3 Key sizes: 112 bits and 168 bits 8

2 keys and 3 keys 64-bit plaintext Triple DES (3DES) Encrypt DES Decrypt DES Key 1 Key 2 2 keys version: Key 1 = Key 3

9 Advanced Encryption Standard (AES) from NIST Overcome limitation with small DES keysize 3DES increases key size but slow Uses Rijndael algorithm Uses a complex round cipher with 3 key sizes 128 bits (10 rounds), 192 bits (12 rounds), 256 bits (14 rounds) data block: 128 bits Other variations proposed which differ in either size of block, key, number of rounds, or function used 10 Blocks identical (except 10), but each uses a different key Each Round (except 10) is a cipher with four operations that are invertible. Last round has only three operations Each operation uses a complex cipher AES (10 rounds, 128-bit key) 128-bit plaintext 128-bit data Round i + k0 Byte Substitution (SubByte) Round 1 k1 Round Key Generator Byte Permutation (ShiftRow) Round 10 (different from other rounds) k16 Complex Operation -- except Round 10 (MixColumn) 128-bit ciphertext 128-bit key ki bit data 9

identical (except 10), but each uses a different key Each Round (except 10) is a cipher with four operations that are invertible.

10 Applying the Cipher Electronic Code Block Plaintext divided into N blocks, Ciphertext has N blocks If Plaintext blocks 1,2,3 are identical Ciphertext blocks 1,2,3 are identical Blocks are considered independent of each other error in one block is not propagated to another Cipher Block Chaining Use previous cipher block in the preparation of the current block (Plaintext and Ciphertext still N blocks) Identical blocks in Plaintext are not identical in Ciphertext Blocks have dependencies errors propagated across blocks An initiation vector used to bootstrap the process Various other approaches are also possible Rivest, Shamir and Adleman (RSA) Two keys (e,d); e:public; d:private Keys related Selecting keys by Bob Chose two large prime numbers: p, q n=p * q; f=(p-1) * (q-1) Chose a random integer e d * e = 1 mod f Public: e,n Private: f,d Size of Plaintext block < n Alice sends message to Bob: C = P e (mod n) Bob receives message from Alice: P = C d (mod n) 10

blocks) Identical blocks in Plaintext are not identical in Ciphertext Blocks have dependencies errors propagated across blocks An initiation vector used to bootstrap the process Various other

11 RSA Example Bob chooses: p=7; q=11 n=p*q=7*11=77 f=(7-1) * (11-1) = 60 Bob chooses: e=13; and calculates: d=37 Public key = 13 Private key = 37 Alice: Plaintext = 5 C= 5 13 = 26 mod 77 Bob receives ciphertext 26 Bob: Ciphertext=26 P= = 5 mod 77 Plaintext = 5 RSA 2 Slow if message is long Useful for small message digests Useful for Digital signatures Useful for encrypting a symmetric key Diffie-Hellman Used for key exchange Bob and Alice create a symmetric session key They do not need to remember or store the key They do not have to meet to agree on the key 11

message is long Useful for small message digests Useful for Digital signatures Useful for encrypting a symmetric key Diffie-Hellman Used for

12 Bob and Alice: Diffie-Hellman Choose two numbers: p, g these are public p: large prime number (1024 bits); g: random number Bob: chooses y ; Alice: chooses x (x,y: large random numbers) Secret key has three parts: (p,g),x,y: (g,p=public; Bob knows y; Alice knows x). Each adds their own part to calculate the shared key Alice R1=g x mod p R2=g y mod p Bob R1 R2 K=(R2) x mod p K=(R1) y mod p Shared secret key K=g xy mod p (g x mod p) y mod p = (g y mod p) x mod p = g xy mod p Both reach the same value for the key, without Bob knowing x, and Alice knowing y Simple Diffie-Hellman Example Assume: g=7; p=23 Alice: chooses x=3; R1=7 3 mod 23 = 21 Bob: chooses y=6; R2=7 6 mod 23 = 4 Alice 21 Bob 4 Bob Alice: calculate symmetric key (K) K = 4 3 mod 23 = 18 Bob: calculate symmetric key (K) K = 21 6 mod 23 = 18 Alice g xy mod p = 7 (3*6) mod 23 = 18 12

Each adds their own part to calculate the shared key Alice R1=g x mod p R2=g y mod p Bob R1 R2 K=(R2) x mod p K=(R1) y mod p Shared secret key K=g xy mod p (g x mod p) y mod p = (g y mod p) x mod p =

13 Man-in-the-middle attack Diffie-Hellman is a sophisticated symmetric-key creation algorithm If x,y are large numbers; difficult for another party to find the key (knowing only p,q) Even if R1 and R2 intercepted, intruder still needs to know x,y Finding x from R1; or y from R2 are difficult tasks Alternative attack: Intruder can fool Alice and Bob by creating two keys (one between each party) Intermediate party (Eve) can fool Alice and Bob into believing they are communicating directly Eve can also change/modify message before forwarding Alice Eve Bob R1=g x mod p R1 R2=g z mod p R2 R2 R3=g y mod p K1=(R2) x mod p K1=(R1) z mod p K2=(R3) z mod p R3 K2=(R2) y mod p Alice-Eve: K1= g xy mod p Eve-Bob: K2= g zy mod p 13

two keys (one between each party) Intermediate party (Eve) can fool Alice and Bob into believing they are communicating directly Eve can also change/modify message before

Network Security. Abusayeed Saifullah. CS 5600 Computer Networks. These slides are adapted from Kurose and Ross 8-1

Network Security. Abusayeed Saifullah. CS 5600 Computer Networks. These slides are adapted from Kurose and Ross 8-1 Network Security Abusayeed Saifullah CS 5600 Computer Networks These slides are adapted from Kurose and Ross 8-1 Goals v understand principles of network security: cryptography and its many uses beyond