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Rijndael is a block cipher, designed by Joan Daemen and Vincent Rijmen as acandidate algorithm for the AES. AES stands for Advanced Encryption Standard. AESis a symmetric key encryption technique which will replace the commonly usedData Encryption Standard (DES). The Advanced Encryption Standard algorithmapproved by NIST in December 2001 uses 128-bit blocks.

The cipher currently supports key lengths of 128, 192, and 256 bits. Each

encryption key size causes the algorithm to behave slightly differently, so theincreasing key sizes not only offer a larger number of bits with which you canscramble the data, but also increase the complexity of the cipher algorithm.

International Data Encryption Algorithm (IDEA):

Secret-key cryptosystem written by Xuejia Lai and James Massey, in 1992 andpatented by Ascom; a 64-bit SKC block cipher using a 128-bit key. Also availableinternationally.

IDEA stands for International Data Encryption Algorithm. IDEA is a symmetric

encryption algorithm that was developed by Dr. X. Lai and Prof. J. Massey to replacethe DES standard. Unlike DES though it uses a 128 bit key. This key length makes it

impossible to break by simply trying every key. It has been one of the best publicly

known algorithms for some time. It has been around now for several years, and no

practical attacks on it have been published despite of numerous attempts to

analyze it. IDEA is resistant to both linear and differential analysis.

Blowfish

Blowfish is a symmetric encryption algorithm designed in 1993 by Bruce Schneieras an alternative to existing encryption algorithm. Blowfish has a 64-bit block size

and a variable key length - from 32 bits to 448 bits. It is a 16-round Feistel cipherand uses large key-dependent S-boxes. While doing key scheduling, it generateslarge pseudo-random lookup tables by doing several encryptions. The tablesdepend on the user supplied key in a very complex way. This approach has beenproven to be highly resistant against many attacks such as differential and linearcryptanalysis. Unfortunately, this also means that it is not the algorithm of choicefor environments where a large memory space is not available. Blowfish wasdesigned in 1993 by Bruce Schneier as a fast, free alternative to existing encryptionalgorithms. Since then it has been analyzed considerably, and it is slowly gainingacceptance as a strong encryption algorithm. Blowfish is unpatented and license-free, and is available free for all uses.

The only known attacks against Blowfish are based on its weak key classes. Asymmetric 64-bit block cipher invented by Bruce Schneier; optimized for 32-bitprocessors with large data caches, it is significantly faster than DES on aPentium/PowerPC-class machine. Key lengths can vary from 32 to 448 bits in length.Blowfish, available freely and intended as a substitute for DES or IDEA, is in use inover 80 products.

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CAST

CAST stands for Carlisle Adams and Stafford Tavares, the inventors of CAST. CAST isa popular 64-bit block cipher which belongs to the class of encryption algorithmsknown as Feistel ciphers.CAST-128 is a DES-like Substitution-Permutation Network (SPN) cryptosystem. It hasthe Feistel structure and utilizes eight fixed S-boxes. CAST-128 supports variablekey lenghts between 40 and 128 bits.CAST-128 is resistant to both linear and differential cryptanalysis. Currently, there isno known way of breaking CAST short of brute force. CAST is now the default cipherin PGP.

Q.2 Explain three each types of stream cipher and blockcipher models?

Ans. Stream ciphers

In cryptography, a stream cipher is a symmetric keycipher where plaintext bitsare combined with a pseudorandom cipher bit stream (keystream), typically by anexclusive-or (xor) operation. In a stream cipher the plaintextdigits are encryptedone at a time, and the transformation of successive digits varies during theencryption. An alternative name is a state cipher, as the encryption of each digit is

dependent on the current state. In practice, the digits are typically single bits orbytes.

Stream ciphers represent a different approach to symmetric encryption from blockciphers. Block ciphers operate on large blocks of digits with a fixed, unvaryingtransformation. This distinction is not always clear-cut: in some modes of operation,a block cipher primitive is used in such a way that it acts effectively as a streamcipher. Stream ciphers typically execute at a higher speed than block ciphers andhave lower hardware complexity.

Stream ciphers can be extremely fast compared with block ciphers although someblock ciphers working in certain modes (such as DES in CFB or OFB) effectively

operate as stream ciphers. Stream ciphers operate on small groups of bits, typicallyapplying bitwise XOR operations to them using as a key a sequence of bits, knownas a keystream. Some stream ciphers are based on what is termed a LinearFeedback Shift Register (LFSR), a mechanism for generating a sequence of binarybits.

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Types of stream ciphers

A stream cipher generates successive elements of the keystream based on aninternal state. This state is updated in essentially two ways: if the state changesindependently of the plaintext or ciphertext messages, the cipher is classified as a

synchronous stream cipher. By contrast, self-synchronising stream ciphers updatetheir state based on previous ciphertext digits.

Synchronous stream ciphers

In a synchronous stream cipher a stream of pseudo-random digits is generatedindependently of the plaintext and ciphertext messages, and then combined withthe plaintext (to encrypt) or the ciphertext (to decrypt). In the most common form,binary digits are used (bits), and the keystream is combined with the plaintext usingthe exclusive or operation (XOR). This is termed a binary additive stream cipher.

In a synchronous stream cipher, the sender and receiver must be exactly in step for

decryption to be successful. If digits are added or removed from the messageduring transmission, synchronisation is lost. To restore synchronisation, variousoffsets can be tried systematically to obtain the correct decryption. Anotherapproach is to tag the ciphertext with markers at regular points in the output.

If, however, a digit is corrupted in transmission, rather than added or lost, only asingle digit in the plaintext is affected and the error does not propagate to otherparts of the message. This property is useful when the transmission error rate ishigh; however, it makes it less likely the error would be detected without furthermechanisms. Moreover, because of this property, synchronous stream ciphers arevery susceptible to active attacks if an attacker can change a digit in theciphertext, he might be able to make predictable changes to the corresponding

plaintext bit; for example, flipping a bit in the ciphertext causes the same bit to beflipped in the plaintext

Self-synchronizing stream ciphers

Another approach uses several of the previous N ciphertext digits to compute thekeystream. Such schemes are known as self-synchronizing stream ciphers,asynchronous stream ciphers or ciphertext autokey (CTAK). The idea of self-synchronization was patented in 1946, and has the advantage that the receiver willautomatically synchronise with the keystream generator after receiving Nciphertext digits, making it easier to recover if digits are dropped or added to themessage stream. Single-digit errors are limited in their effect, affecting only up to Nplaintext digits.

An example of a self-synchronising stream cipher is a block cipher in cipherfeedback (CFB) mode.

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Linear feedback shift register-based stream ciphers

Linear feedback shift registers (LFSRs) are popular components in stream ciphers as

they can be implemented cheaply in hardware, and their properties are well-

understood.

Binary stream ciphers are often constructed using linear feedback shift registers(LFSRs) because they can be easily implemented in hardware and can be readilyanalysed mathematically. The use of LFSRs on their own, however, is insufficient toprovide good security. Various schemes have been proposed to increase thesecurity of LFSRs.

Block ciphers

Block ciphers convert a fixed-length block of plain text into cipher text of the samelength, which is under the control of the secret key. Decryption is effected using thereverse transformation and the same key. For many current block ciphers the blocksize is 64 bits, but this is likely to increase.

Plain text messages are typically much longer than the particular block size anddifferent techniques, or modes of operation, that are used. Examples of such modes

are electronic codebook (ECB), cipher block chaining (CBC) or cipher feedback(CFB). ECB simply encrypts each block of plain text, one after another, using thesame key; in CBC mode, each plain text block is XORed with the previous ciphertext block before being encrypted, thus adding a level of complexity that can makecertain attacks harder to mount. Output FeedBack mode (OFB) resembles CBCmode although the quantity that's XORed is generated independently. CBC is widelyused, for example in DES (qv) implementations, and these various modes arediscussed in depth in appropriate books on technical aspects of cryptography. Notethat a common vulnerability of roll-your-own cryptosystems is to use somepublished algorithm in a simple form rather than in a particular mode that givesadditional protection.

Iterated block ciphers are those where the process of encryption has severalrounds, thus improving security. In each round, an appropriate transformation maybe applied using a subkey derived from the original secret key that uses a specialfunction. Inevitably, this additional computing requirement has an impact on thespeed at which encryption can be managed, therefore there is a balance betweensecurity needs and speed of execution. Nothing is free and in cryptography; aselsewhere, part of the skill in applying appropriate methods is derived from

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understanding the tradeoffs that need to be made and how these relate to thebalance of requirements.

Block ciphers include DES, IDEA, SAFER, Blowfish, and Skipjack -- this last being thealgorithm used in the US National Security Agency (NSA) Clipper chip.

Cipher feedback (CFB)

The cipher feedback(CFB) mode, a close relative of CBC, makes a block cipherinto a self-synchronizing stream cipher. Operation is very similar; in particular, CFBdecryption is almost identical to CBC encryption performed in reverse:

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In notation, where Si is the ith state of the shift register, a

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Each output feedback block cipher operation depends on all previous ones, and socannot be performed in parallel. However, because the plaintext or ciphertext isonly used for the final XOR, the block cipher operations may be performed inadvance, allowing the final step to be performed in parallel once the plaintext orciphertext is available.

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It is possible to obtain an OFB mode keystream by using CBC mode with a constantstring of zeroes as input. This can be useful, because it allows the usage of fasthardware implementations of CBC mode for OFB mode encryption.

Using OFB mode with a partial block as feedback like CFB mode reduces theaverage cycle length by a factor of 232 or more. A mathematical model proposed by

Davies and Parkin and substantiated by experimental results showed that only withfull feedback an average cycle length near to the obtainable maximum can beachieved. For this reason, support for truncated feedback was removed from thespecification of OFB.

Counter (CTR)

counter mode turns a block cipher into a stream cipher. It generates the nextkeystream block by encrypting successive values of a "counter". The counter can beany function which produces a sequence which is guaranteed not to repeat for along time, although an actual counter is the simplest and most popular. The usage

of a simple deterministic input function used to be controversial; critics argued that"deliberately exposing a cryptosystem to a known systematic input represents anunnecessary risk. By now, CTR mode is widely accepted, and problems resultingfrom the input function are recognized as a weakness of the underlying block cipherinstead of the CTR mode. Nevertheless, there are specialized attacks like aHardware Fault Attack that is based on the usage of a simple counter function asinput.CTR mode has similar characteristics to OFB, but also allows a random accessproperty during decryption. CTR mode is well suited to operation on a multi-processor machine where blocks can be encrypted in parallel.

Note that the nonce in this graph is the same thing as the initialization vector (IV) inthe other graphs. The IV/nonce and the counter can be concatenated, added, or

XORed together to produce the actual unique counter block for encryption.

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Q.3 Explain the principle of security? What is attack and

what kind of attaks?

Ans. Security principles

For over twenty years, information security has held confidentiality, integrity andavailability (known as the CIA triad) to be the core principles of information security.

There is continuous debate about extending this classic trio. Other principles suchas Accountability have sometimes been proposed for addition - it has been pointedout that issues such as Non-Repudiation do not fit well within the three coreconcepts, and as regulation of computer systems has increased (particularlyamongst the Western nations) Legality is becoming a key consideration for practicalsecurity installations.

In 2002, Donn Parker proposed an alternative model for the classic CIA triad that hecalled the six atomic elements of information. The elements are confidentiality,possession, integrity, authenticity, availability, and utility. The merits of theParkerian hexad are a subject of debate amongst security professionals.

Confidentiality

Confidentiality is the term used to prevent the disclosure of information tounauthorized individuals or systems. For example, a credit cardtransaction on theInternet requires the credit card number to be transmitted from the buyer to themerchant and from the merchant to a transaction processing network. The systemattempts to enforce confidentiality by encrypting the card number during

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transmission, by limiting the places where it might appear (in databases, log files,backups, printed receipts, and so on), and by restricting access to the places whereit is stored. If an unauthorized party obtains the card number in any way, a breachof confidentiality has occurred.

Breaches of confidentiality take many forms. Permitting someone to look over your

shoulder at your computer screen while you have confidential data displayed on itcould be a breach of confidentiality. If a laptop computer containing sensitiveinformation about a company's employees is stolen or sold, it could result in abreach of confidentiality. Giving out confidential information over the telephone is abreach of confidentiality if the caller is not authorized to have the information.

Confidentiality is necessary (but not sufficient) for maintaining the privacy of thepeople whose personal information a system holds.

Integrity

In information security, integrity means that data cannot be modified withoutauthorization. This is not the same thing as referential integrity in databases.Integrity is violated when an employee accidentally or with malicious intent deletesimportant data files, when a computer virusinfects a computer, when an employeeis able to modify his own salary in a payroll database, when an unauthorized uservandalizes a web site, when someone is able to cast a very large number of votes inan online poll, and so on.

There are many ways in which integrity could be violated without malicious intent.In the simplest case, a user on a system could mis-type someone's address. On alarger scale, if an automated process is not written and tested correctly, bulkupdates to a database could alter data in an incorrect way, leaving the integrity of

the data compromised. Information security professionals are tasked with findingways to implement controls that prevent errors of integrity.

Availability

For any information system to serve its purpose, the information must be availablewhen it is needed. This means that the computing systems used to store andprocess the information, the security controls used to protect it, and thecommunication channels used to access it must be functioning correctly. Highavailability systems aim to remain available at all times, preventing servicedisruptions due to power outages, hardware failures, and system upgrades.Ensuring availability also involves preventing denial-of-service attacks.

Authenticity

In computing, e-Business and information security it is necessary to ensure that thedata, transactions, communications or documents (electronic or physical) aregenuine. It is also important for authenticity to validate that both parties involvedare who they claim they are.

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Non-repudiation

In law, non-repudiation implies one's intention to fulfill their obligations to acontract. It also implies that one party of a transaction cannot deny having receiveda transaction nor can the other party deny having sent a transaction.

Cryptographic attacks

A cryptographic attack is a method for circumventing the security of acryptographic system by finding a weakness in a code, cipher,cryptographic protocol or key management scheme. This process is alsocalled "cryptanalysis".

Types of attacks

A useful means of classifying security attacks, used both in X.800 and RFC 2828, isin terms of passive attacks and active attacks. A passive attack attempts to learn ormake use of information from the system but does not affect system resources. Anactive attack attempts to alter system resources or affect their operation.

Passive Attacks

Passive attacks are in the nature of eavesdropping on, or monitoring of,transmissions. The goal of the opponent is to obtain information that is beingtransmitted. Two types of passive attacks are release of message contents andtraffic analysis.

The release of message contents is easily understood. A telephone conversation, anelectronic mail message, and a transferred file may contain sensitive or confidentialinformation. We would like to prevent an opponent from learning the contents ofthese transmissions.

Passive Attacks

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A second type of passive attack, traffic analysis, is subtler. Suppose that we had away of masking the contents of messages or other information traffic so thatopponents, even if they captured the message, could not extract the informationfrom the message. The common technique for masking contents is encryption. If wehad encryption protection in place, an opponent might still be able to observe thepattern of these messages. The opponent could determine the location and identityof communicating hosts and could observe the frequency and length of messagesbeing exchanged. This information might be useful in guessing the nature of thecommunication that was taking place.

Passive attacks are very difficult to detect because they do not involve anyalteration of the data. Typically, the message traffic is sent and received in anapparently normal fashion and neither the sender nor receiver is aware that a third

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party has read the messages or observed the traffic pattern. However, it is feasibleto prevent the success of these attacks, usually by means of encryption. Thus, theemphasis in dealing with passive attacks is on prevention rather than detection.

Active Attacks

Active attacks involve some modification of the data stream or the creation of afalse stream and can be subdivided into four categories: masquerade, replay,modification of messages, and denial of service.

A masquerade takes place when one entity pretends to be a different entity. Amasquerade attack usually includes one of the other forms of active attack. Forexample, authentication sequences can be captured and replayed after a validauthentication sequence has taken place, thus enabling an authorized entity withfew privileges to obtain extra privileges by impersonating an entity that has thoseprivileges.

Active Attacks

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KeySize: 16, 24, 32 Byte (corresponds to 128 to 256 Bit)*

RC6

Developed by :Ron Rivest, Robshaw, Sidney, Yin (RSA Labs/MIT, USA)

Description: For AES RC6 was defined as a 20-rounds (encryption Depth 3)iterative Cipher. It was developed from RC5 (and fully parameterized). RC6 uses sixbasic 32-bit operations to mix the data in every round. These operations areaddition, subtraction, bitwise XOR, integer multiplications as well as left and rightrotations of 32 bit words.

Cipher Mode: ECB,CBC,CFB,OFB,CTR,CCM,OCB,COA

Blocksize: 16

KeySize: 16, 24, 32 Byte (corresponds to 128 to 256 Bit)*

Depth: Defines the encryption-deep. For AES a depth of 3 was defined, thiscorresponds to 20 encryption rounds. The depth can be adjusted continuously from3 to 64.

Blowfish

Developed by : B. Schneier, 1994

Description: Blowfish is a 64 bit block, 16 rounds Feistel Blockcipher with avariable key length. It is optimized for data encryption which needs no key change,since the key change operation is slow. Blowfish uses four very large 8* 32 bit key-dependend lookup tables for the data substitution. Every round consists of a key-dependent permutation and a key and data-dependent substitution. The outputfunction uses additions and XOR operations. Blowfish uses a great number ofsubkeys (4168 bytes)Particular keys can lead in Blowfish to the generation of weak S-Boxes, which canlead to attacks in a reduced round version. However, this attack is completelyinefficient in the defined 16 rounds version. Through that no successful attacks are

known on Blowfish up to now .

Cipher Mode: ECB,CBC,CFB,OFB,CTR,CCM

Blocksize: 8 Byte

KeySize: Actual Password length until a maximum of 448 Bit.

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- Relative Strength Comparisons of Algorithms-

Type SecurityLevel*

Implementation

Speed

Idea MilitaryGrade

128 bit SharedSecret

Fast

Blowfish

MilitaryGrade

256 to 448 bitShared Secret

Fastest

DES Low 40 to 56 bitShared Secret

Fast

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