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  • 8/10/2019 crypto finaale.ppt







  • 8/10/2019 crypto finaale.ppt





    The word CRYPTOGRAPHY is derived from Greek word,


    which means "hidden, secret"; and , grph,meaning "I

    write.It may be defined as the practice and study of hiding


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    Cryptology:All-inclusive term used for the study of securecommunication over non-secure channels and relatedproblems.

    Cryptography: The process of designing systems to realize

    secure communications over non-secure channels.

    Cryptoanalysis: The discipline of breaking the cryptographicsystems.

    Coding Theory: Deals with representing the informationusing codes. It covers: compression, secrecy, and error-correction.

    Recently, it is predominantly associated with error-correcting

    codes which ensures the correct transmissions over noisy-channels.

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    The Aspects of Cryptography

    Modern cryptography heavily depends on mathematicsand

    the usage of digital systems.

    It is a inter-disciplinary study of basically three fields:MathematicsComputer ScienceElectrical Engineering

    Without having a complete understanding of

    cryptoanalysis(or cryptoanalytic techniques) it isimpossible to design good(secure, unbreakable)cryptographic systems.

    It makes use of other disciplines such as error-

    correcting codes compression.

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    Overview of Cryptography

    People want and need privacy and security whilecommunicating

    In the past, cryptography was heavily used formilitary applications to keep sensitive information

    secret from enemies (adversaries).Eg: Julius Caesar used a simple shift cipher to

    communicate with his generals in the battlefield.

    Nowadays, with the technological progress, our

    dependency on electronic systems has increasedso we need more sophisticated techniques.

    Cryptography provides most of the methods andtechniques for a secure communication.

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    Cryptographic process

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    Cryptographic Methods


    Symmetr ic

    Same key for encryption and decryption

    Key distribution problem

    Asymmetr ic Mathematically related key pairs for encryption and


    Public and private keys

    Hybr id Combines strengths of both methods

    Asymmetric distributes symmetric key

    Symmetric provides bulk encryption

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    Encryption is a process in which the senderencrypts/scrambles the message in such a waythat only the recipient will be able to decrypt/

    descramble the message. Encryption is the conversion of data into a

    form, called a cipher text, that cannot be easilyunderstood by unauthorized people.

    Decryption is the process of convertingencrypted data back into its original form, so itcan be understood.

    Encryption/decryption is especially important in

    wireless communications.

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    Private-Key Cryptography

    traditional private/secret/single keycryptographyuses onekey

    shared by both sender and receiver

    if this key is disclosed communications are

    compromised also is symmetric, parties are equal

    hence does not protect sender from receiverforging a message & claiming is sent by sender

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    How it works????

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    Key distribution is an awkward process.

    Key distribution for multiple keys can be an


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    Stream ciphers

    Stream ciphers operate on a single bit (byte orcomputer word) at a time and implement some form offeedback mechanism so that the key is constantly

    changing. Block cipher

    It encrypts one block of data at a time using the samekey on each block.

    In general, the same plaintext block will alwaysencrypt to the same ciphertext when using the samekey in a block cipher whereas the same plaintext willencrypt to different ciphertext in a stream cipher.

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    Secret key cryptography algorithms

    Data Encryptio n Standard (DES)

    DES was designed by IBM in the 1970s and adoptedby the National Bureau of Standards (NBS) [now theNational Institute for Standards and Technology

    (NIST)] in 1977 for commercial and unclassifiedgovernment applications. DES is a block-cipheremploying a 56-bit key that operates on 64-bit blocks.DES has a complex set of rules and transformations.

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    Secret key cryptography


    Variants of DES Trip le-DES (3DES):A variant of DES that employs up to

    three 56-bit keys and makes three encryption/decryption

    passes over the block. DESX:A variant devised by Ron Rivest. By combining

    64 additional key bits to the plaintext prior to encryption,

    effectively increases the keylength to 120 bits. Here, the

    input plaintext is bitwise XORed with 64 bits of additionalkey material before encryption with DES and the output

    is also bitwise XORed with another 64 bits of key


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    Secret key cryptography


    Advanced Encrypt ion Standard (AES)

    NIST develop a new secure cryptosystem for U.S.

    government applications i.e. the Advanced Encryption

    Standard. It became the official successor to DES inDecember 2001. AES uses an SKC scheme

    called Rijndael, a block cipher designed by Belgian

    cryptographers Joan Daemen and Vincent Rijmen.

    The algorithm can use a variable block length and keylength; the latest specification allowed any

    combination of keys lengths of 128, 192, or 256 bits

    and blocks of length 128, 192, or 256 bits.
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    Secret key cryptography


    Rivest Ciphers (akaRon's Code)Named for Ron Rivest, a series of SKC algorithms.

    RC1:Designed on paper but never implemented.

    RC2:A 64-bit block cipher using variable-sized keys designed to replace

    DES. It's code has not been made public although many companies havelicensed RC2 for use in their products.

    RC3:Found to be breakable during development.

    RC4:A stream cipher using variable-sized keys; it is widely used incommercial cryptography products, although it can only be exported usingkeys that are 40 bits or less in length, but keys this small can be broken

    easily by criminals, amateurs and govts. RC5:A block-cipher supporting a variety of block sizes, key sizes, and

    number of encryption passes over the data.

    RC6:An improvement over RC5, RC6 was one of the AES Round 2algorithms.

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    Secret key cryptography


    Internat ional Data Encrypt ion A lgo ri thm


    Secret-key cryptosystem written by Xuejia Lai

    and James Massey, in 1992 and patented byAscom. It offers strong encryption using a 128-

    bit key to encrypt 64-bit blocks, which makes it

    resistant to brute-force attacks.

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    Public-Key Cryptography

    PKC depends upon the existence of so-called one-way functions, ormathematical functions that are easy to computer whereas theirinverse function is relatively difficult to compute. Here are two simpleexamples:

    Multiplication vs. factorization:Suppose that I have two numbers, 9 and16, and that I want to calculate the product; it should take almost no

    time to calculate the product, 144. Suppose instead that I have anumber, 144, and I need to find which pair of integers I multipliedtogether to obtain that number. I will eventually come up with thesolution but whereas calculating the product took milliseconds,factoring will take longer because it first need to find the 8 pair ofinteger factors and then determine which one is the correct pair.

    Exponentiation vs. logarithms:Suppose I tell you that I want to take thenumber 3 to the 6th power; again, it is easy to calculate 36=729. But if Itell you that I have the number 729 and want you to tell me the twointegers that I used,xand yso that logx 729 = y, it will take you longerto find all possible solutions and select the pair that I used.

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    Public-Key Cryptography

    probably most significant advance in the 3000

    year history of cryptography

    employs twokeys that are mathematically

    relateda public & a private key

    asymmetricsince parties are notequal

    complements rather thanreplaces private key


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    How it works????

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    Public-Key Characteristics

    Public-Key algorithms rely on two keys where:

    it is computationally infeasible to find decryption key

    knowing only algorithm & encryption key

    it is computationally easy to en/decrypt messageswhen the rel