CRYPTOGRAPHY

INTRODUCTION

Cryptography (from Greek kryptós, "hidden", + gráphein, "to write").

Cryptography is the practice and study of techniques for secure communication in the presence of third parties (called adversaries).

It is about constructing and analysing protocols that

overcome the influence of adversaries.

Cipher: the algorithm that does the encryption.

Ciphertext: the encrypted (scrambled) version of the message. Message altered to be unreadable by anyone except the

intended recipients.

Cryptanalysis: the science of breaking cryptographic algorithms.

Cryptanalyst: a person who breaks cryptographic codes; also referred to as “the attacker”.

Cryptosystem – The combination of algorithm, key, and key management functions used to perform cryptographic operations.

Decryption: the process of converting ciphertext back to the original plaintext.

Encryption: scrambling a message or data using a specialized cryptographic algorithm.

Initialization Vector – Random values used with ciphers to ensure no patterns are created during encryption.

Key – Sequence that controls the operation and behavior of

the cryptographic algorithm.

Keyspace – Total number of possible values of keys in a crypto

algorithm.

Plaintext – A message in its natural format readable by an attacker.

Authentication

ensures that whoever supplies or accesses sensitive data is an

authorized party.

Confidentiality

assures that only authorized parties are able to understand

the data.

Integrity

ensures that when a message is sent over a network, the

message that arrives is the same as the message that was

originally sent.

Nonrepudiation

ensuring that the intended recipient actually received the

message & ensuring that the sender actually sent the

message.

THERATES

There are several size of encryption keys:

64 bit encryption

128 bit encryption

256 bit encryption

512 bit encryption

1024 bit encryption

Number indicates the size of the key used to encrypt the message.

Symmetric

o Even if an attacker captures the data, the attacker will not be

able to manipulate it in any meaningful way.

o Symmetric algorithms use a single key shared by two

communicating parties.

o The same key is used for both encryption and decryption.

Asymmetric

o Mathematically related key pairs for encryption and

decryption.

o Public and private keys.

Hybrid

o Combines strengths of both methods.

o Asymmetric distributes symmetric key.

10

Asymmetric Cryptography

Symmetric Cryptography

SUBSTITUTION CIPHER

A Substitution technique is one in which the letters/numbers/symbols

of plaintext are replaced by other letters/numbers/symbols.

Ex. A D , T z

2 5, 3 6

TRANSPOSITION CIPHER

In this transposition technique the position of letters/numbers/symbols is changed with one another.

Plain Text : MEET ME AFTER PARTY

Cipher Text : TEMEEMEFAPTRYRPT

Key Used:421635

1 2 3 4 5 6

M E E T M E

A F T E T P

A R T Y

4 2 1 6 3 5

T E M E E M

E F A P T R

Y R P T

COMPARISON

SYMMETRIC CRYPTOGRAPHY

The same algorithm with the same

key is used for encryption and

decryption.

The Key must be kept secret.

It may be possible or at least

impractical to decipher a

message if no other information is

available.

ASYMMETRIC

CRYPTOGRAPHY

One algorithm is used for

encryption and decryption with a

pair of keys , one for encryption

and one for decryption.

One of the two keys must be kept

secret.

It may be possible or at least

impractical to decipher a

message if no other information is

available.

Types of Symmetric Cryptography:

Stream-based Ciphers

Mixes plaintext with key stream.

Good for real-time services.

Block Ciphers

Substitution and transposition.

E.g.: DES, AES.

Public Key Cryptography

In public-key cryptosystems, the public key may be

freely distributed, while its paired private key must

remain secret and is used for decryption.

Public-key cryptography can be used for

implementing digital signature schemes.

Quantum cryptography describes the use of quantum

mechanical effects (in particular quantum

communication and quantum computation) to

perform cryptographic tasks or to break cryptographic

systems.

The advantage of quantum cryptography is that it allows

the completion of various cryptographic tasks that are

proven or conjectured to be impossible using only

classical communication.

For example, quantum mechanics guarantees that

measuring quantum data disturbs that data. This can be

used to detect eavesdropping in quantum key

distribution.

Digital signatures can be permanently tied to the content of the

message being signed. They cannot then be 'moved' from one

document to another, for any attempt will be detectable.

RSA and DSA are two of the most popular digital signature

schemes.

In digital signature schemes, there are two

algorithms: one for signing, in which a secret key is

used to process the message and one for

verification, in which the matching public key is

used with the message to check the validity of the

signature.

APPLICATION

Defense services

Secure data manipulation

E-commerce

Business Transaction

Internet payment system

User identification system

Access control

Data security