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# Cryptography

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### Cryptography

1. 1. Post graduate and researchdepartment of computer application
2. 2. PRESENTED BY:
3. 3. CONTENT:
4. 4. cryptography: Cryptography is the study of mathematical techniques related to aspects of information security such as confidentiality, data integrity, entity authentication, and data origin authentication. Cryptography is not the only means of providing information security, but rather one set of techniques. Cryptography is the process of writing,using various method(“ciphers”)to keep message secret
5. 5. Crytographic goals: CONFIDENTIALITY: Confidentiality is a service used to keep the content of information from all but those authorized to have it. Secrecy is a term synonymous with confidentiality and privacy. There are numerous approaches to providing confidentiality, ranging from physical protection to mathematical algorithms which render data unintelligible. DATA INTEGRITY: Data integrity is a service which addresses the unauthorized alteration of data. To assure data integrity, one must have the ability to detect data manipulation by unauthorized parties. Data manipulation includes such things as insertion, deletion, and substitution.
6. 6. AUTHENTICATION: Authentication is a service related to identification. This function applies to both entities and information itself. Two parties entering into a communication should identify each other. Information delivered over a channel should be authenticated as to origin , date of origin, data content, time sent, etc. For these reasons this aspect of cryptography is usually subdivided into two major classes: entity authentication & data origin authentication. Data origin authentication implicitly provides data integrity NON-REPUDIATION : Non-repudiation is a service which prevents an entity fromdenying previous commitments or actions. When disputes arise due to an entity denying that certain actionswere taken, a means to resolve the situation is necessary.
7. 7. History of cryptography: Cryptography was concerned solely with message confidentiality(encryption). Conversion of message from a comprehensible form into an incomprehensible one, rendering it unreadable by interceptors without secret knowledge(decryption). Encryption was used to ensure secrecy in communication , such as military leaders & diplomats. In recent days , it has expanded beyond confidentiality concern include , digital signature, identity authentication, interactive proof & secure computation.
8. 8. Classic cryptography: The earliest forms of secret writing required little more than pen & paper analogs , as most people could not read. The main classical cipher types are transposition cipher , which arrange the order of letters in a message. Transposition cipher eg : hello world becomes “ehlol owrdl”. substitution ciphers, which systematical replace letters or group of letters. substitution cipher eg : fly at once becomes “gmz bu podf”
9. 9. SECURITY OF ALGORITHM: TOTAL BREAKS: A cryptanalyst finds the key k ,such that D k(c)=p. K is a key factor GLOBAL DEDUCTION: A cryptanalyst finds the alternative algorithm , a equivalent to D k( c) INSTANCE OR DEDUCTION: A cryptanalyst finds the plain text of an intercepted cipher text INFORMATION DEDUCTION: A cryptanalyst gains the information about the key or plain text. this information could be a few bit of the key.
10. 10. TYPES OF ATTACKS TO THE CIPHER TEXT: There are 4 attacks , namely ! Cipher text only attack !! Known plaintext attack !!!Cryptanalyst chosen cipher textA. CIPHER TEXT-ONLY ATTACK: c1=e k(p1)….c2=e k(p2)…….c i=e k(pi) reduce : c i+1=e k(pi+1)B. KNOWN PLAINTEXT ATTACK: p1,c1=e k(p1),p2,c2=e k(p2)….pi c i=e k(pi) reduce: pi+1 from ci+1=e k(pi+1)C. cryptanalyst chosen plaintext attack: pi , ci = ek (p1),pi , ci = ek (pi) reduce: pi+1 from ci+1=ek(pi+1)
11. 11. PROCEDURE OF ATTACKS:DATA COMPLEXITY: The amount of data needed as input to the attackPROCESSING: The time needed to perform the attack . This is often called as work factor.STORAGE REQUIREMENT: The amount of memory needed to attack
12. 12. Information security &cryptography To introduce cryptography, an understanding of issues related toinformation security in general is necessary. Information security manifests itself in many ways according to thesituation and requirement. Regardless of who is involved, to one degree or another, allparties to a transaction must have confidence that certain objectives associated withinformation security have been met.
13. 13. Over the centuries, an elaborate set of protocols and mechanismshas been created to deal with information security issues when theinformation is conveyed by physical documents. Often the objectives of information security cannot solely beachieved through mathematical algorithms and protocols alone, butrequire procedural techniques and abidanceof laws to achieve the desired result.
14. 14. Basic terminology and concepts:Sender and Receiver: Suppose a sender wants to send a message to a receiver. Moreover, this sender wants to send the message securely: She wants to make sure an eavesdropper cannot read the message. Messages and Encryption: A message is plaintext (sometimes called clear text). The process of disguising a message in such a way as to hide its substance is encryption. An encrypted message is cipher text. The process of turning Cipher text back into plaintext is decryption. (If you want to follow the ISO 7498-2 standard, use the terms “encipher” and “decipher.” It seems that some cultures find the terms “encrypt” and “decrypt” offensive, as they refer to dead bodies.)
15. 15. The art and science of keeping messages secure is cryptography,and it is practiced by cryptographers. Cryptanalysts are practitioners of cryptanalysis, the art andscience of breaking cipher text. The branch of mathematics encompassing bothcryptography and cryptanalysis is cryptology and its practitioners arecryptologists. Modern cryptologists are generally trained in theoreticalmathematics—they have to be.
16. 16. Encryption and Decryption. Plaintext is denoted by M, for message, or P, for plaintext. It can be a stream of bits, a text file, a bitmap, a stream of digitized voice, adigital video image. As far as a computer is concerned, M is simply binary data.The plaintext can be intended for either transmission or storage. In any case, M is the message to be encrypted. Cipher text is denoted by C. It is also binary data: sometimes the same size as M, sometimeslarger. (By combining encryption with compression, C may be smaller than M.However, encryption does not accomplish this.)
17. 17. The encryption function E, operates on M to produce C. Or, inmathematical notation: E(M) = C In the reverse process, the decryption function D operates onC to produce M: D(C) = M Since the whole point of encrypting and then decrypting amessage is to recover the original plaintext, the following identitymust hold true: D(E(M)) = M
18. 18. Notation: p is the plaintext. This is the original readable message(written in some standard language like english , french , hindi). c is ciphertext .this is the output of some encryption scheme , and is not readable by humans. E is the encryption function. E(P)=C EG. to mean that appling the encryption process E to the plaintext P produces the ciphertext C. D is the decryption function. Eg D(C)=P. NOTE: D(E(P))=P & E(D(C))=C
19. 19. The encryption key is the piece of data that allows thecomputation of E. similarly we have the decryption key . These may or may not be same. they also may not besecret. To attack a cipher is to attempt unauthorized reading ofplaintext , or to attempt unauthorized transmission of cipher text
20. 20. Cryptography techniques: Cryptanalysis is the study of mathematical techniques forattempting to defeat cryptographic techniques, and, more generally, informationsecurity services A cryptanalyst is someone who engages in cryptanalysis Cryptology is the study of cryptography and cryptanalysis A cryptosystem is a general term referring to a set ofcryptographic primitives used to provide information security services. Most often the term is used in conjunction with primitivesproviding confidentiality, i.e., encryption Cryptographic techniques are typically divided into two generic types: symmetric-key cryptography public-key cryptography
21. 21. Symmetric key cryptography: Let a={ a,b,c………x ,y,z}, be the english alphabet , let m & c be the set of string of length five over A .the key E is chosen to be permutation A. To encrypt , an english message is broken up into groups each having five letter (appropriate padding , if the length of the message is not a multiple of five & permutation E is applied to each letter one at a time). To decrypt , the inverse permutation D=E/1,is applied to each letter of the cipher text , E is choosen to the permutation E= A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z. D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z,A,B,C
22. 22. PUBLIC KEY CRYTOGRAPHY: Each user has an encryption function & a decryption function. Alice makes her encryption function Ea publicly known , but keeps her decryption function Da secret Bob wants to send alice a message P ,so he computes C=Ea(P) and sends it to her Alice receives C and computes P=Da(C) The point is that the encryption function & a decryption function are set up so that Da is very difficult to compute only knowing Ea. Thus even if an attacker knows Ea ,he can’t compute Da and hence can’t read bob message
23. 23. PROTOCOLS: A cryptographic protocol (protocol) is a distributed algorithm defined by a sequence of steps precisely specifying the actions required of two or more entities to achieve a specific security objective Remark: (protocol vs. mechanism) As opposed to a protocol, a mechanism is a more general term encompassing protocols, algorithms (specifying the steps followed by a single entity) non-cryptographic techniques (eg : hardware protection and procedural controls) to achieve specific security objectives
24. 24. Protocols play a major role in cryptography and are essentialin meeting cryptographic Goals Encryption schemes, digital signatures, hash functions, andrandom number generation are among the primitives which may be utilizedto build a protocol. Secret sharing : alice,bob,carol,….yanni,&zeke each have a piece ofinformation that is part of a commonly held secret S. If N or more of them meet and combine their knowledge,then S canbe reconstructed. But if less than N get together, S cannot be reconstructed.
25. 25. Example: (a simple key agreement protocol) Alice and Bob have chosen a symmetric-key encryption scheme to usein communicating over an unsecured channel. To encrypt information they require a key. The communication protocol is the following: 1. Bob constructs a public-key encryption scheme and sends his public key toAlice over the channel. 2. Alice generates a key for the symmetric-key encryption scheme. 3. Alice encrypts the key using Bob’s public key and sends the encrypted key toBob. 4. Bob decrypts using his private key and recovers the symmetric (secret) key. 5. Alice and Bob begin communicating with privacy by using thesymmetric-key system and the common secret key.
26. 26. Uses of protocols: Today we use cryptography for a lot more than just sendingsecret message Authentication: Alice receives cipher text from bob. How can she be sure that the message originated from bob? How can she be sure that the message wasn’t corrupted? Key exchange: Over an instance channel , Alice & bob exchange two piece of data that allows them tocompute a common encryption/decryption key . But any attacker who intercepts the transmissions can’t recoverthe key.
27. 27. Remark (causes of protocol failure): Protocols and mechanisms may fail for a number of reasons,including: 1. weaknesses in a particular cryptographic primitive which may beamplified by the protocol or mechanism; 2. claimed or assumed security guarantees which are overstated ornot clearly understood; 3. the oversight of some principle applicable to a broad class ofprimitives such as encryption.
28. 28. Classes of attacks and security models: Over the years, many different types of attacks on cryptographic primitives and protocols have been identified. The roles of an active and a passive adversary were discussed. The attacks these adversaries can mount may be classified as follows:. 1. A passive attack is one where the adversary only monitors the communication channel. A passive attacker only threatens confidentiality of data. 2. An active attack is one where the adversary attempts to delete, add, or in some other way alter the transmission on the channel. An active attacker threatens data integrity and authentication as well as confidentiality.
29. 29. One Application of Cryptography:ELECTRONIC MONEY: The definition of electronic money (also called electronic cash or digital cash) is a term that is still evolving. It includes transactions carried out electronically with a net transfer of funds from one party to another, which may be either debit or credit and can be either anonymous or identified. There are both hardware and software implementations. Encryption is used in electronic money schemes to protect conventional transaction data like account numbers and transaction amounts, digital signatures can replace handwritten signatures or a credit-card authorizations, and public-key encryption can provide confidentiality.