1. BZUPAGES.COM Cryptography Cryptography is the technique of converting a message into unintelligible or non-understandable form such that even if some unauthorized or unwanted person intercepts the message he/she would still not be able to make any sense out of it.

2. BZUPAGES.COM Techniques of Cryptography Substitution Techniques used for cryptography Substitution In substitution we replace each letter in the message with another to make the message non-understandable. For example, each letter “a” in the message can be replaced with letter “d” and letter “b” with letter “e” and so on. Transposition Transposition It is based on scrambling the characters in a message. A transposition system may first write a message into a table row by row then the message can be read and rewritten column by column to make it scrambled

3. BZUPAGES.COM Transposition IWILL NOTCO METDA Y.TEL LCR INMYLWOE.CITTTRLCDELOAL

4. BZUPAGES.COM Cryptography is a collection of mathematical techniques used to ensure confidentiality of information. Encryption The process of scrambling a message with the help of a key is called Encryption. Decryption The process of unscrambling a message using an appropriate key is called decryption. Keys are numbers or characters that are randomly generated with the help of some cryptographic algorithm. Pretty Good Privacy (PGP) is the name of a popular cryptographic system which is available for general public use.

5. BZUPAGES.COM Plain Text Encryption Decryption Cipher Text Key Encryption Algorithm

6. BZUPAGES.COM Types of Cryptography Symmetric Cryptography In symmetric cryptography same keys are used for encryption and decryption. Asymmetric or Public Key Cryptography In this type a pair of public and private keys is used for encryption and decryption.

7. BZUPAGES.COM Original Text Cipher Text Recipients public Key Original Text Encryption Algorithm Decryption Algorithm Recipients Secret Key

8. BZUPAGES.COM Digital/electronic signature An electronic signature means any letters, numbers, symbols, images, characters or any combination thereof in electronic form applied to an electronic document which can ensure authenticity, integrity and non-repudiation. It uses public key cryptography. Authenticity means that the message is from a particular source/individual. Integrity means that the message has not been altered during transmission. Non-repudiation means that the execution of the digital signatures cannot be denied by the one who is alleged to be the executor of those signatures

9. BZUPAGES.COM Digital certificates These are the certificates in electronic form which establish whether or not a public key belongs to the purported owner. A digital certificate at least comprises a public key, certification information (name, ID etc.) and electronic signatures of a certification authority. Digital certificates are prepared according to a generally accepted format called X.509 standard format.

10. BZUPAGES.COM SYMMETRIC KEY ALGORITHMS Cryptographic algorithms are measured in terms of key length. Following is the list of some popular symmetric key algorithms: DES (Data Encryption Standard) – 56 bits IDEA (International Data Encryption Algorithm (IDEA) – 128 bits RC2 – (block cipher) 1-2048 bits RC4 (stream cipher) – 1-2048 bits Rinjdael – 128-256 bits

11. Attacks on Symmetric Key Algorithms Following attacks have been reported on symmetric key algorithms: Key Search Attacks Cryptanalysis System-based Attacks

12. Key Search (Brute Force) Attacks In this type of attack an attempt is made by the attacker to decrypt the message with every possible key. Thus, the greater the key length, the more difficult it is to identify the key.

13. Cryptanalysis Encryption algorithms can be defeated by using a combination of sophisticated mathematics and computing power so that many encrypted messages can be deciphered without knowing the key. Such type of an attack is called cryptanalysis.

14. System-Based Attacks In it the attack is made on the cryptographic system that uses the cryptographic algorithm without actually attacking the algorithm itself.

15. Public Key Algorithms Following is the list some popular public key algorithms: DSS – Digital Signature Standard based on DSA (Digital Standard Algorithm) – key length is between 512-1024 bits RSA Elliptic Curves

16. Attacks on Public Key Algorithms Key Search Attacks Analytical Attacks

17. Key Search Attacks The public key and its corresponding private key are linked with each other with the help of a large composite number. These attacks attempt to derive the private key from its corresponding public key using that number. According to an estimate 1024 bit RSA public key may be factored due to fast computers by 2020. Note that both symmetric and asymmetric algorithms are based on different techniques. In case of asymmetric algorithms the increase in key length does not much increase the difficulty level for the attacker as compared to symmetric algorithms. Thus, a 128-bit RC2 symmetric key may prove to be much stronger than a 1024 bit RSA asymmetric public key.

18. Analytical Attacks Such attacks use some fundamental flaw in the mathematical problem on which the encryption system itself is based so as to break the encryption. Quantum computing is the branch of computer science that deals with the development of cryptographic algorithms. It can also be used to find flaws in the cryptographic system/algorithms and to launch attacks.