CRYPTOGRAPHYCRYPTOGRAPHY

Gayathri V.R. Kunapuli

OUTLINEOUTLINEHistory of CryptographyNeed for cryptographyPrivate Key CryptosystemsPublic Key CryptosystemsComparison between Public and

Private Key CryptosystemsPEMFuture Work

History of History of Cryptography[2]Cryptography[2]Ceaser CiphersTransposition CipherSubstitution CipherVigene`re CipherEnigma Machine

Need for Cryptography[1]Need for Cryptography[1]Authentication of the

Communicating PrincipalsAuthenticated message carries a

Digital Signature

Private Key Private Key Cryptosystems[1,2]Cryptosystems[1,2]Also called Symmetric

CryptographyEncryption algorithm E turns plain

text message M into a cipher text CC=E(M)Decrypt C by using decryption

algorithm D which is an inverse function of E

M=D(C)

Private Key Cryptosystems Private Key Cryptosystems cont[1,2]cont[1,2]Algorithm decomposed into

Function(public) and Key(secret)Encrypted using the key Ke and

decrypted using the key KdM=DKd(EKe(M))

• A function and a variable number of keys constitute a class of algorithms indexed by the keys.

Cont…Cont…The encryption function is

-One-to-one injective mapping

-One way function • The secrecy rests on the keys

rather than on algorithms. • The key should be of sufficient

length in bits.

DES(Jan,1977)[1,2]DES(Jan,1977)[1,2]Encryption consists of 3 stages of

Transposition and 16 stages of Substitution of bits.

Easy to implement on VLSIThe 56-bit length key was found

insufficient and easy to breakRepetitions in cipher text give

clues to eavesdroppersSpurious data can be injected

Contd…Contd…Private Key systems require

[n*(n-1)]/2 keys for ‘n’ principals in a system

The conversation key must be agreed upon beforehand

Management of the keys is a function of the Key Distribution Server(KDS)

Public Key Cryptographic Public Key Cryptographic SystemsSystems(Need)[1](Need)[1]Also called as the Asymmetric

CryptographyTo avoid the need to transmit

secret keys andTo reduce the key requirement to

2n, the public key systems are used

Public Key Cryptosystems Public Key Cryptosystems ContContIntroduced by Diffie and HellmanEach principal keeps a set of encryption

keys (Ke & Kd)Encryption algorithm E is public and so is

the key KeDecryption algorithm D and decryption

key Kd is kept privateData sent to a principal is encrypted using

that corresponding KeE and D can be made public if Ke and Kd

are chosen such that it is impossible to infer Kd from Ke.

RSA(Aug,1977)[1,2,5]RSA(Aug,1977)[1,2,5]The algorithms E and D are inversesPlain text messages are limited to a

size is limited to ‘k’Integer k is chosen such that 2k < NN =p * q where p & q are LARGE

prime numbersKp (public encyrption key) and Ks

(private decryption key) are derived from p & q

ContdContdThe robustness of RSA algorithm

relies on the computational complexity in factoring a large number upon which the keys are based.

The authenticity of the sender can also be verified.

Comparison between the Comparison between the cryptosystems[1]cryptosystems[1]Private Key DES is

computationally efficientPublic Key RSA is

computationally expensivePossible best use is RSA for

short/important data and DES for long or less critical

PEM[1,5]PEM[1,5]Provides mechanism for the mail

users to specify the cryptographic algorithm and parameters to be used for mail messages.

Essential data fields in PEM are◦DEK◦IK◦MIC

Extended Works[4]Extended Works[4]To prevent the Denial-of-

Decryption To reduce the time taken for the

authentication of the digital signatures

Self Generated Certificate Public Key Cryptography

ReferencesReferences1. Chow, Randy; Johnson, Theodore;

Distributed Operating Systems & Algorithms, 19982. Aiden A.Bruen,Mario A.Forcinito; Cryptography,

Information theory and Error-correction,20053.www.wikipedia.org/history of cryptography4. Self generated certificate public key

cryptography and certificateless signature/Encryption scheme in the standard model

ASIACCS’07, March 20-22, 2007, Singapore.5.http://www.cybercrimes.net/

Cryptography/Articles/Hebert.html (April 2007)