Cryptography

Cryptography The science of writing in secret

code

The Science of Writing In Secret Code

• Cryptography is an ancient art - first documented use of cryptography in writing dates back to circa 1900 B.C. when

an Egyptian scribe used non-standard hieroglyphs in an inscription

• New forms of cryptography came soon after the widespread development of computer communications

Secure Communication

• Crypto is the secure communication in the presence of third parties (adversaries)

• Generally: about constructing and analyzing protocols that overcome such adversaries

• MAIN GOAL: SECURITY!

DISCIPLINES• Modern crypto intersects disciples of Math,

Computer Science, and Electrical Engineering

• APPLICATIONS: ATM cards, computer passwords, electronic commerce, etc.

Related to various aspects in information security such as data confidentiality, data integrity, authentication

Specific Security Requirements

• Authentication: process of proving one’s identity

• Privacy/confidentiality: ensuring that no one can read the message except the intended receiver

• Integrity: assuring the intended receiver that the original message has not been altered in any way

• Non-repudiation: mechanism to prove that sender really sent the message

Cryptographic Schemes

• Initial unencrypted date is referred to as plaintext• It is then encrypted into ciphertext• Then decrypted into usable plaintext

PLAINTEXT CIPHERTEXT PLAINTEXT

Types of Cryptography

• Secret Key Cryptography (SKC)• Public Key Cryptography (PKC)

•Hash Functions

Crypto

SECRET KEY ENCRYPTIONAlso known as private key & symmetric key

Secret Key Encryption

PUBLIC KEY ENCRYPTION

Public Key Encryption

Why Three?

• Each scheme is optimized for some specific applications

• Secret Key Cryptography is suited for encrypting messages as it provides privacy and

confidentiality• Public Key Cryptography is ideal for non-

repudiation and user authentication• Hash functions ensure data integrity

Caesar Cipher

Roman ruler Julius Caesar used a very simple cipher for secret communication

Involves the use of a “displacement” concept

Caesar cipher is the simplest to solve, since there are only 25 possible combinations

Caesar Cipher

Outside wheel: The Alphabet

Inner wheel: can be rotated so that any letter on one wheel can be aligned with any letter on the other wheel

- The inner wheel becomes the key

CAESAR CIPHER

A lock is only as strong as its weakest link