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Chapter 5 Cryptography Protecting principals communication in systems.

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Presentation on theme: "Chapter 5 Cryptography Protecting principals communication in systems."— Presentation transcript:

1 Chapter 5 Cryptography Protecting principals communication in systems

2 Cryptography Security engineering meets math Cryptography science and art of designing ciphers Cryptanalysis science and art of breaking them Cryptology is both Input is plaintext output is ciphertext

3 Historical background Early stream cipher Vigenere Early block cipher Playfair One-Way functions Protect integrity and authenticity or message Test key Asymmetric primitives Public and Private key

4 Random Oracle Model Elf is in a box with following items: Scroll (infinite length) to store previously provided results Die for randomness

5 Random Function Accepts input string of any length, outputs a random string of fixed length Useful for storing passwords Creates a message digest (hash value) Useful for sending digital signature, since digital signature is long, it can stand for the signature. Same as hashing as learned in database Same string always produces same output string

6 Random function One-way Given string can produce output string Given hash value very difficult to produce original image To attack must keep feeding in input strings until get lucky and match output string, even then not definate. Collisions can occur but hard to find in a true pseudorandom function

7 Random Generator Stream Cipher Short input, long output Also know as key stream Go to key stream generator, enter a key, get a long string of characters to xor with Good for encrypting back-up data for instance Must know key to get proper key stream Do not re-use key, or can decrypt Can prevent this by using a seed with each subsequent message

8 Random permutations Block Ciphers Input output fixed size Given plaintext and key output cipher text Given Cipher text and key output plaintext Given plaintext and cipher text do nothing

9 Public key Encryption Elf will encrypt message for anyone, but will decrypt only for key owner. So I can give away my public key and anyone can encrypt to me, but only I can decrypt.

10 Digital signature Can be created by only one person, but checked by anyone. So these are the basic primitives of symmetric crypto schemes

11 5.4 Symmetric crypto primitives Block ciphers confusion and diffusion S-box Maps numbers (look-up table) Cipher must be wide enough Must have enough “rounds” S-boxes of good design Advanced Encryption Standard (AES)

12 DES Used widely for banking government etc 56 bits key Always a weakness 14,000 Pentium machines on the net broke a challenge in 4 months Machine built that can do it in 3 days Currently inadequate

13 Modes of operation Electronic code book (ECB) Cipher Block Chaining (CBC) Output feedback (OFB) Cipher Feedback (CFB)

14 Asymmetric Cypto Primitives Public key encryption Digital signatures Based on number theory Prime numbers RSA current algorithm based on factoring Used in SSL

15 Asymmetric Cypto Primitives PGP Government systems Based on discrete logarithms DSA Digital Signature Algorithm AKA Digital Signature Standard (DSS)

16 Certification We can do public key encryption and digital signatures Now must bind keys to users CA Certification Authority can do that Signs users public encryption Verifies signature Third party trusted source

17 Discussion topics Breaks of Rijndael Current uses of PGP Current uses of certificates and digital signatures.

18 List of resources Cryptography Random Oracle Model Public Key Block ciphers 68 68

19 List of resources S boxes AES ption_Standard ption_Standard DES p?id=2226 p?id=2226

20 List of resources Modes of operation faq/part01/ faq/part01/ See 5.14 graphy) graphy) ion/0,,sid14_gci344947,00.html ion/0,,sid14_gci344947,00.html

21 List of resources Asymmetric ion/0,,sid14_gci836964,00.html ion/0,,sid14_gci836964,00.html DSA DSS p?id=2239 p?id=2239 Certificates services/security-services/index.html services/security-services/index.html

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