1) Five different Stream vs Block Stream | Block | * One symbol of plaintext is immediately convert into its symbol in cipher text * E.g. Caesar cipher, Playfair cipher. | * A group of plaintext symbols is converted into its equivalent group of cipher text symbols * E.g. DES, AES. |

2) 4 mod operation in block cipher * In cryptography, a block cipher operates on blocks of fixed length, often 64 or 128 bits. * Because messages may be of any length, and because encrypting the same plaintext under the same key always produces the same output.

* Several modes of operation have been invented which allow block ciphers to provide confidentiality for messages of arbitrary length. * The 4 modes of operation are * Electronic Code Book (ECB) * Cipher Block Chaining (CBC) * Output Feedback (OFB) and * Cipher Feedback (CFB)

3) Briefly describe the working of DES * most widely used block cipher in world adopted in 1977 by NBS (now National Institute of Standards and Technology NIST)as Federal Information Processing Standard 46 (FIPS PUB 46) * encrypts 64-bit data using 56-bit key * has widespread use * The algorithm itself is referred to as the Data Encryption Algorithm (DEA) * IBM developed Lucifer cipher by team led by Feistel in late 60’s used 64-bit data blocks with 128-bit key * then redeveloped as a commercial cipher with input from NSA and others * in 1973 NBS issued request for proposals for a national cipher standard * IBM submitted their revised Lucifer which was eventually accepted as the DES

4) Advantage of TRIPLE DES * Triple DES uses longer bits of key compared to DES (i.e. 112 bits or 168bits depending whether the key is broken down into 2 or 3 sub-keys), thus providing for better security. * Since it is based on the DES algorithm, it is very easy to modify existing software to use Triple DES. * The procedure for encryption is exactly the same as regular DES, but it is repeated three times, hence the name Triple DES. * The data is encrypted with the first key, decrypted with the second key, and finally encrypted again with the third key. * Triple DES runs three times slower than standard DES.

5) Briefly explain digital signature with example
A digital signature or digital signature scheme is a mathematical scheme for demonstrating the authenticity of a digital message or document.
A valid digital signature gives a recipient reason to believe that the message was created by a known sender, and that it was not altered in transit.
Commonly used for software distribution, financial transactions, and in other cases where it is important to detect forgery or tampering.
A digital signature scheme typically consists of three algorithms: a. A key generation algorithm that selects a private key uniformly at random from a set of possible private keys. The algorithm outputs the private key and a corresponding public key. b. A signing algorithm that, given a message and a private key, produces a signature. c. A signature verifying algorithm that, given a message, public key and a signature, either accepts or rejects the message's claim to authenticity.

6) Explain 3 properties of zero knowledge proof
In cryptography, a zero-knowledge proof or zero-knowledge protocol is a method by which one party (the prover) can prove to another party (the verifier) that a given statement is true, without conveying any additional information apart from the fact that the statement is indeed true. * Completeness: if the statement is true, the honest verifier (that is, one following the protocol properly) will be convinced of this fact by an honest prover. * Soundness: if the statement is false, no cheating prover can convince the honest verifier that it is true, except with some small probability. * Zero-knowledge: if the statement is true, no cheating verifier learns anything other than this fact .

7) Conventional encryption and public key encryption

8) What do u understand by interactive proof protocol
In an interactive proof system there are two parties d. An (all powerful) Prover, often called Peggy (a randomizedalgorithm using a private random number generator); e. A (little (polynomially) powerful) Verifier, often called Vic (a polynomialtime randomized algorithm using a private random number generator).
Prover knows some secret, or a knowledge, or a fact about a specific object, and wishes to convince Vic, through acommunication with him, that he has the above knowledge.
For example, both Prover and Verifier possesan input x and Prover wants to convince Verifier that x has a certain properties and that Prover knows howto proof that.
The interactive proof system consists of several rounds. In each roundProver and Verifier alternatively do the following. 1. Receive a message from the other party. 2. Perform a (private) computation. 3. Send a message to the other party.
Communication starts usually by a challenge of Verifier and a response by Prover.
Atthe end, Verfier either accepts or rejects Prover's attempts to convinceVerifier.

9) Row transposition cipher

10) Signcryption * Signcryption is a public-key primitive that simultaneously performs the functions of both digital signature and encryption. * Signcryption is a relatively new cryptographic technique that is supposed to fulfill the functionalities of digital signature and encryption in a single logical step and can effectively decrease the computational costs and communication overheads in comparison with the traditional signature-then-encryption schemes. * Signcryption provides the properties of both digital signatures and encryption schemes in a way that is more efficient than signing and encrypting separately.

Any signcryption scheme should have the following properties: * Correctness: Any signcryption scheme should be correctly verifiable. * Efficiency: The computational costs and communication costs of a signcryption scheme should be smaller than those of the best known signature-then-encryption schemes with the same provided functionalities. * Security: A signcryption scheme should simultaneously fulfill the security attributes of an encryption scheme and those of a digital signature.

11) 5 advantage of AES and DES * Advantages of AES Encryption Algorithm

* Advanced Encryption Standard not only assures security but also improves the performance in a variety of settings such as smartcards, hardware implementations etc. * AES is federal information processing standard and there are currently no known non-brute-force direct attacks against AES. * AES is strong enough to be certified for use by the US government for top secret information * Performance that falls short of your requirements * Higher installation costs * Higher maintenance costs * Shorter operating life

Advantages des * DES has been around a long time (since 1977), even now no real weaknesses have been found: the most efficient attack is still brute force. * DES is an official United States Government standard; the Government is required to re-certify, DES every five years and ask it be replaced if necessary. * DES has been re-certified in 1983, 1987, 1992. * DES is also an ANSI and ISO standard - anybody can learn the details and implement it. * Since DES was designed to run on 1977 hardware, it is fast in hardware and relatively fast in software.