Chapter 5 Cryptography Protecting principals communication in systems.

Chapter 5

Cryptography

Protecting principals communication in systems

Cryptography

Security engineering meets mathCryptography science and art of

designing ciphersCryptanalysis science and art of

breaking themCryptology is bothInput is plaintext output is ciphertext

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

Random Oracle Model

Elf is in a box with following items:Scroll (infinite length) to store previously

provided resultsDie for randomness

Random Function

Accepts input string of any length, outputs a random string of fixed lengthUseful for storing passwordsCreates 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 databaseSame string always produces same output

string

Random function

One-wayGiven string can produce output stringGiven hash value very difficult to produce

original imageTo 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

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

Random permutations

Block Ciphers Input output fixed sizeGiven plaintext and key output cipher textGiven Cipher text and key output plaintextGiven plaintext and cipher text do nothing

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.

Digital signature

Can be created by only one person, but checked by anyone.

So these are the basic primitives of symmetric crypto schemes

5.4 Symmetric crypto primitives

Block ciphers confusion and diffusionS-box

Maps numbers (look-up table)Cipher must be wide enoughMust have enough “rounds”S-boxes of good designAdvanced Encryption Standard (AES)

DES

Used widely for banking government etc56 bits keyAlways a weakness14,000 Pentium machines on the net

broke a challenge in 4 monthsMachine built that can do it in 3 daysCurrently inadequate

Modes of operation

Electronic code book (ECB)Cipher Block Chaining (CBC)Output feedback (OFB)Cipher Feedback (CFB)

Asymmetric Cypto Primitives

Public key encryptionDigital signaturesBased on number theory

Prime numbersRSA current algorithm based on

factoringUsed in SSL

Asymmetric Cypto Primitives

PGPGovernment systems

Based on discrete logarithmsDSA Digital Signature Algorithm

AKA Digital Signature Standard (DSS)

Certification

We can do public key encryption and digital signatures

Now must bind keys to usersCA Certification Authority can do that

Signs users public encryptionVerifies signatureThird party trusted source

Discussion topics

Breaks of RijndaelCurrent uses of PGPCurrent uses of certificates and digital

signatures.

List of resources

Cryptography http://en.wikipedia.org/wiki/Cryptography

Random Oracle Model http://en.wikipedia.org/wiki/Random_oracle_model http://www-cse.ucsd.edu/users/mihir/papers/ro.pdf

Public Key http://en.wikipedia.org/wiki/Public-key_cryptography

Block ciphers http://www.rsasecurity.com/rsalabs/node.asp?id=21

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List of resources

S boxeshttp://en.wikipedia.org/wiki/S-box

AEShttp://en.wikipedia.org/wiki/Advanced_Encry

ption_StandardDES

http://www.rsasecurity.com/rsalabs/node.asp?id=2226

List of resources

Modes of operationhttp://www.faqs.org/faqs/cryptography-faq/

part01/See 5.14

http://en.wikipedia.org/wiki/Padding_(cryptography)

http://searchsecurity.techtarget.com/sDefinition/0,,sid14_gci344947,00.html

List of resources

Asymmetrichttp://searchsecurity.techtarget.com/

sDefinition/0,,sid14_gci836964,00.htmlDSA DSS

http://www.rsasecurity.com/rsalabs/node.asp?id=2239

Certificateshttp://www.verisign.com/products-services/

security-services/index.html