1. DISSERTATION ON
CRYPTOGRAPHY

2. 1 Introduction
Derived from Greek kryptós "hidden," and the gráfo "write"
The study of message secrecy
central contributor to several fields: information security and related issues, authentication, and access control
Primary purposes is hiding the meaning of messages
Study of ways to convert information from its normal form into an unreadable without special knowledge
Applications : the security of ATM cards, computer passwords, and electronic commerce
Examples include digital signatures and digital cash, digital rights management for intellectual property protection, and securing electronic commerce

3. 2 History of cryptography
Before the modern era cryptography was concerned solely with message confidentiality
In recent decades it has included techniques for authentication of message integrity or sender/receiver identity, digital signatures, interactive proofs, and secure computation
The main classical cipher types are :
Transposition ciphers (e.g. 'help me' becomes 'ehpl em' in a trivially simple rearrangement scheme)
Substitution ciphers (e.g., 'fly at once' becomes 'gmz bu podf' by replacing each letter with the one following it in the alphabet)

4. Types of cryptography 1.Classical cryptography
2. Medieval Cryptography
3. World War II cryptography

5. Types of Cryptographic Algorithms

6. Symmetric-key cryptography
Public-key cryptography
Hash Functions

7. Symmetric-key cryptography
This was the only kind of encryption publicly known until 1976
both the sender and receiver share the same key (or, less commonly, in which their keys are different, but related in an easily computable way)
Symmetric-key algorithms can be divided into stream ciphers and block ciphers
Stream ciphers encrypt the bits of the message one at a time
block ciphers take a number of bits and encrypt them as a single unit

8. Stream cipher
a stream cipher is a symmetric cipher in which the plaintext digits are encrypted one at a time
And the transformation of successive digits varies during the encryption
An alternative name is a state cipher, as the encryption of each digit is dependent on the current state
Stream ciphers typically execute at a higher speed than block ciphers and have lower hardware complexity
However, stream ciphers can be susceptible to serious security problems if used incorrectly

9. Block cipher
A block cipher is a symmetric key cipher which operates on fixed-length groups of bits with an unvarying transformation
The exact transformation is controlled using a second input — the secret key
An early and highly influential block cipher design was the Data Encryption Standard
A successor to DES, the Advanced Encryption Standard (AES)

10. Data Encryption Standard
A method for encrypting information
Selected as an official Federal Information Processing Standard (FIPS) for the United States in 1976
DES is now considered to be insecure for many applications
This is due to the 56-bit key size being too small; DES keys have been broken in less than 24 hours
56-bit key is divided into eight 7-bit blocks and an 8th odd parity bit is added to each block (i.e., a "0" or "1" is added to the block so that there are an odd number of 1 bits in each 8-bit block)
In recent years, the cipher has been superseded by the Advanced Encryption Standard (AES).
Data Encryption Standard

11. Advanced Encryption Standard
is a block cipher adopted as an encryption standard by the U.S. government
It became effective as a standard on May 26, 2002
As of 2006, AES is one of the most popular algorithms used in symmetric key cryptography

12. Speed
Symmetric-key algorithms are generally much less computationally intensive than asymmetric key algorithms
This means that a quality asymmetric key algorithm is hundreds or thousands of times slower than a quality symmetric key algorithm

13. Limitations
The requirement of a shared secret key, with one copy at each end
They need to be changed often and kept secure during distribution and in service
The consequent requirement to choose, distribute and store keys without error and without loss, known as key management, is difficult to reliably achieve
The symmetric-key algorithms can't be used for authentication or non-repudiation purposes

14. Public Key Cryptography
Also known as asymmetric cryptography
Is a form of cryptography in which a user has a pair of cryptographic keys - a public key and a private key
The private key is kept secret, while the public key may be widely distributed
A message encrypted with the public key can only be decrypted with the corresponding private key

15. The two main branches of public key cryptography
Public key encryption
Digital Signature

16. Public key encryption
A message encrypted with a user's public key cannot be decrypted by anyone except the user possessing the corresponding private key
Anyone can encrypt using the public key, but only the holder of the private key can decrypt. Secrecy depends on the secrecy of the private key

17. Digital Signature
Digital signature is used to mean a cryptographically based signature assurance scheme
An analogy for digital signatures is the sealing of an envelope with a personal wax seal
The message can be opened by anyone, but the presence of the seal authenticates the sender
There are several digital signature schemes; most establish two complementary algorithms, one for signing and the other for checking the signature at some later time

18. Hash Functions
A cryptographic hash function is a hash function with certain additional security properties
It makes it suitable for use as a primitive in various information security applications, such as authentication and message integrity
A hash function takes a long string (or 'message') of any length as input and produces a fixed length string as output

19. SUMMARY & CONCLUSIONS
The history of cryptography dates back thousands of years
There are several ways of classifying cryptographic algorithms:
Secret Key Cryptography (SKC): Uses a single key for both encryption and decryption
Public Key Cryptography (PKC): Uses one key for encryption and another for decryption
Hash Functions: Uses a mathematical transformation to irreversibly "encrypt" information