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Symmetric Cryptography

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Presentation on theme: "Symmetric Cryptography"— Presentation transcript:

1 Symmetric Cryptography
CS 5323 Symmetric Cryptography Prof. Ravi Sandhu Executive Director and Endowed Chair Lecture 7 © Ravi Sandhu World-Leading Research with Real-World Impact!

2 Basic Concepts World-Leading Research with Real-World Impact!
© Ravi Sandhu World-Leading Research with Real-World Impact!

3 Cryptographic Technology
SECRET KEY Symmetric Key Single Key Conventional PUBLIC KEY Asymmetric Key Public-Private Key © Ravi Sandhu World-Leading Research with Real-World Impact! 3

4 Cryptographic Technology
Secret-key encryption Secret-Key message authentication codes (MAC) Public-key encryption Public-key digital signatures Public-key key agreement Message digests (hash functions) Public-key certificates Challenge-response authentication © Ravi Sandhu World-Leading Research with Real-World Impact! 4

5 ATMs run on secret-key technology
Cryptographic Technology Secret-key encryption Secret-Key message authentication codes (MAC) Public-key encryption Public-key digital signatures Public-key key agreement Message digests (hash functions) Public-key certificates Challenge-response authentication SSL uses all of these ATMs run on secret-key technology © Ravi Sandhu World-Leading Research with Real-World Impact! 5

6 Traditional formulation
Cryptographic Services confidentiality traffic flow confidentiality integrity authentication non-repudiation Traditional formulation © Ravi Sandhu World-Leading Research with Real-World Impact! 6

7 Cryptographic Services
confidentiality crypto keys leak profusely via side channels integrity + authentication no point having one without the other non-repudiation requires asymmetric cryptography Stronger form on integrity + authentication replay protection beyond integrity? Important insights © Ravi Sandhu World-Leading Research with Real-World Impact! 7

8 Safe Cryptography Secret-key cryptography Public-key cryptography
128 bit or higher Public-key cryptography 2048 bit or higher Message digests 256 bit or higher These numbers keep increasing © Ravi Sandhu World-Leading Research with Real-World Impact! 8

9 Symmetric Encryption World-Leading Research with Real-World Impact!
© Ravi Sandhu World-Leading Research with Real-World Impact!

10 Secret-Key Encryption
INSECURE CHANNEL Plain- text Plain- text Ciphertext Encryption Algorithm E Decryption Algorithm D A B Secret Key shared by A and B K K SECURE CHANNEL Confidentiality Integrity © Ravi Sandhu World-Leading Research with Real-World Impact! 10

11 Secret-Key Encryption
confidentiality depends only on secrecy of the key size of key is critical secret key systems do not scale well with N parties we need to generate and distribute N*(N-1)/2 keys A and B can be people or computers © Ravi Sandhu World-Leading Research with Real-World Impact! 11

12 Master Keys and Session Keys
master keys, lifetime is couple of years prolonged use increases exposure session keys short-term keys communicated by means of master secret keys public key technology © Ravi Sandhu World-Leading Research with Real-World Impact! 12

13 Cryptanalysis ciphertext only known plaintext chosen plaintext
cryptanalyst only knows ciphertext known plaintext cryptanalyst knows some plaintext-ciphertext pairs chosen plaintext chosen ciphertext © Ravi Sandhu World-Leading Research with Real-World Impact! 13

14 Known-Plaintext Attack
40 bit key requires 239  5 * 1011 trials on average (exportable from USA, early 1990’s) trials/second time required ,000 years years days minutes seconds © Ravi Sandhu World-Leading Research with Real-World Impact! 14

15 Known-Plaintext Attack
56 bit key requires 255  3.6 * 10^16 trials on average (DES, 1977) trials/second time required years years years year hours © Ravi Sandhu World-Leading Research with Real-World Impact! 15

16 Known-Plaintext Attack
80 bit key requires 279  6 * 1023 trials on average (SKIPJACK, mid-1990s) trials/second time required years years years years years © Ravi Sandhu World-Leading Research with Real-World Impact! 16

17 Known-Plaintext Attack
128 bit key requires 2127  2 * 1038 trials on average (AES-128, 2001) trials/second time required years years years years years © Ravi Sandhu World-Leading Research with Real-World Impact! 17

18 AES Advanced encryption standard, 2001 3 key sizes: 128, 192, 256
DES, 1977: designed by IBM. Blessed by NSA. SKIPJACK, early 1990s: designed by NSA, declassified 1998 AES, 2001: designed by open international competition, winner was a European team 3 key sizes: 128, 192, 256 Block size: 128 Previously most (e.g. DES) used 64 bit block size 128 bit block size is safer due to birthday attack © Ravi Sandhu World-Leading Research with Real-World Impact! 18

19 Secret-Key Encryption
INSECURE CHANNEL Plain- text Plain- text Ciphertext Encryption Algorithm E Decryption Algorithm D A B Secret Key shared by A and B K K SECURE CHANNEL Confidentiality Integrity © Ravi Sandhu World-Leading Research with Real-World Impact! 19

20 A B Password Derived Keys INSECURE CHANNEL Plain- text Plain- text
Ciphertext Encryption Algorithm E Decryption Algorithm D A B K K Secret Key shared by A and B Key Derivation Function Key Derivation Function Password (salted) Password (salted) © Ravi Sandhu World-Leading Research with Real-World Impact! 20

21 A B Password Derived Keys Dictionary attack INSECURE CHANNEL Plain-
text Plain- text Ciphertext Encryption Algorithm E Decryption Algorithm D A B K K Secret Key shared by A and B Key Derivation Function Key Derivation Function Dictionary attack Password (salted) Password (salted) © Ravi Sandhu World-Leading Research with Real-World Impact! 21

22 Perfect Secrecy Vernam one-time pad Plain- text Ciphertext Plain- text
+ + Mi Mi A B AB Secret Key Ki Ki Vernam one-time pad SECURE CHANNEL © Ravi Sandhu World-Leading Research with Real-World Impact! 22

23 Double DES Intermediate Ciphertext Plain- text Cipher- text E E K1 K2 effective key size is only 57 bits due to meet-in-the-middle attack © Ravi Sandhu World-Leading Research with Real-World Impact! 23

24 Triple DES Cipher- text Plain- text E D E K1 K2 K1 effective key size is 112 bits due to meet-in-the-middle attack © Ravi Sandhu World-Leading Research with Real-World Impact! 24

25 Electronic Code Book (ECB) Mode
128 bit Data block 128 bit key 128 bit key D E 128 bit encrypted Data block 128 bit Data block OK for small messages identical data blocks will be identically encrypted © Ravi Sandhu World-Leading Research with Real-World Impact! 25

26 Cipher Block Chaining (CBC) Mode
128 bit Data block + 128 bit previous ciphertext block 128 bit key 128 bit key D E  is the exclusive OR operation 128 bit previous ciphertext block + 128 bit Data block © Ravi Sandhu World-Leading Research with Real-World Impact! 26

27 Message Authentication Code (MAC)
Secret-Key Message Authentication Code (MAC) © Ravi Sandhu World-Leading Research with Real-World Impact!

28 Message Authentication Code
INSECURE CHANNEL Plaintext + MAC Plain- text Yes/No MAC Algorithm M Verification Algorithm V A B K K © Ravi Sandhu World-Leading Research with Real-World Impact! 28

29 Does not provide non-repudiation
Message Authentication Code INSECURE CHANNEL Plaintext + MAC Plain- text Yes/No MAC Algorithm M Verification Algorithm V A B K K Does not provide non-repudiation © Ravi Sandhu World-Leading Research with Real-World Impact! 29

30 Message Authentication Code
Symmetric Encryption Based Message-Digest Based © Ravi Sandhu World-Leading Research with Real-World Impact! 30

31 Will revisit after discussing message digests
Message Authentication Code Symmetric Encryption Based Message-Digest Based Will revisit after discussing message digests © Ravi Sandhu World-Leading Research with Real-World Impact! 31

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