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Nov 03, 2014





Slide 1


Made by:-

Shray Jali


Cryptology is the science of secure communications

Formed from the Greek words krypts, "hidden", and logos, "word".

Cryptography is a framework of methodologies used to ensure the CIA triad for our information

C Confidentiality

I Integrity

A -Authenticity

Cryptography was known anciently as Encryption which means: Hiding the information from unauthorized entities.

It could be implemented manually, mechanically or even electronically


Process by which a message is transformed into another message using a mathematical function and a special encryption password.

If we are using symmetric encryption, then we will encrypt the clear message with one key and decrypt it with the same key

symmetric encryption

We will use 9 different private keys for achieving bidirectional communication between XYZ and ABC

We need to define a way by which we can exchange these private keys in a secure manner between distant entities

DES, 3DES, Blowfish, IDEA, RC5, Safer, Serpent and AES are the well known symmetric encryption algorithms

If we are using asymmetric encryption, then we will encrypt the clear message with one key and decrypt it with a different key

asymmetric encryption

asymmetric encryption

ABC and XYZ have their public keys distributed over each other, anyone needs to talk to the other will use the others public key to encrypt the traffic and the other will use his own private key to decrypt the traffic, X will use As public key to encrypt clear traffic A will receive the cipher to decrypt it using his own private key

RSA is the famous asymmetric key encryption algorithm

Early Cryptography

3500 BC: Sumerians

Cuneiform writings

1900 BC: Egypt

Early Cryptography

Substitution cipher

Used by Hebrew

60 50 BC: Julius Caesar

Shift letters by X positions

E.g. X = 3: A -> D, B -> E,

C ->F, ..

500- 600 BC

486 BC: Greece

Medieval Cryptography

1845: Morse code

1918: The ENIGMA

26 letter keyboard for input & each lamp for a key


Medieval Cryptography

Substitution cipher

substitutes each letter by another letter to diguise it

plain text: a b c d e f g h i j k l m n o p q r s t u v w x y z

Cipher text: q w e r t y u i o p a s d f g h j k l z x c v b n m

Transposition Cipher

The Ciphertext is formed by re-arranging the Plaintext in the form of a matrix and then transposing it.

modern Cryptography

1918: ADFGVX Cipher


modern Cryptography

1976: Diffie-Hellman Key Exchange

Private key: This key must be know only by its owner.

Public key: This key is known to everyone

modern Cryptography

1977: RSA: Rivest, Shamir & Adleman Algorithm

Both public and private key are interchangeable

Variable Key Size (512, 1024, or 2048 bits)

Most popular public key algorithm

modern Cryptography

1991: Pretty Good Privacy

e-mail security

Uses a block cipher called IDEA

Uses Digital Signatures

Uses Web Trust

E.g. X.509 certificates in SSL

modern Cryptography

Stream Ciphers

Generates a keystream & combines with plaintext to form ciphertext, e.g. RSAs RC4

Suitable for online encryption of data, e.g. Encrypting Voice Comms

Much faster than block ciphers for online work

Block Ciphers

Symmetric-key encryption algorithm that changes a fixed length of block text into same length of Cipher text

Encryption works by means of key and Decryption is the reverse of encryption process using the same key

The Data Encryption Standard (DES) and the Advanced Encryption Standard (AES) are block cipher designs which are used across a wide range of applications, from ATM encryption to e-mail privacy and secure remote access

They use different keys for encryption and decryption.

Encryption operates on plaintext using the encryption key and the modulus to produce cipher text

Decryption operates on cipher text using the decryption key and the modulus to produce plaintext

modern Cryptography

Public key cryptography

Quantum Cryptography

relies on the laws of physics to ensure that eavesdroppers are unable to successfully gain access to the key while it is in transit


64-bit Plain text

Iteration 1

Iteration 2

Initial transposition

Iteration 16


Inverse transposition

64-bit cipher text


To ensure the integrity of a message

Hashing is an irreversible process with no keys, the clear message is the only input for the hashing process

MD5 and SHA are the most used hashing algorithms , SHA is more secure than MD5


They take a message of any length as input, and output a short, fixed length hash which can be used in a digital signature.

MD4 is a long-used hash function which is now broken; MD5, a strengthened variant of MD4, is also widely used but broken in practice

The U.S. National Security Agency developed the Secure Hash Algorithm series of MD5-like hash functions: SHA-0 was a flawed algorithm that the agency withdrew; SHA-1 is widely deployed and more secure than MD5, but cryptanalysts have identified attacks against it

Digital signature

It is a mechanism that ensures that the data is received from a secure and a known source.

Document to be signed is sent through a complex mathematical computation that generates a hash.

Hash is encoded with the owners private key then stored.

To prove future ownership, stored hash is decoded using the owners public key and that hash is compared with a current hash of the document.

If the two hashes agree, the document belongs to the owner.


Original Message

Scrambled Message

Scrambled Message

Private Keysender

Original Message


Public Keysender




Digital signature

Symmetric key signature

Public key signature


A, KA ( B, RA, t, P)


KB ( a, RA, t, P, Ksir ( A,t, P))


Shrays pvt key


Sumits public key


Sumits pvt key


Shrays public key









World Wide Web transactions

Virtual private networks

Electronic mail

Client-server applications

Banking transactions

Digital certificates

Certificates are issued by a certificate authority (CA). A CA is either specialized software on a company network or a trusted third party.

Applications of digital certificates

Advantages of cryptography



Data integrity



Access control


Applications of cryptography

Military and diplomatic services

Protecting confidential company information, telephone calls