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# Cryptography

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• 1. CONTENTSo Introductiono Need of Cryptographyo Types of Attackso Techniques of Cryptographyo Encryption Algorithm • Symmetric • Asymmetrico Digital Signatureo Conclusion
• 2. INTRODUCTION What is Cryptography?  “Hidden Writing”  Mainly used to protect Information. Goal of Cryptography  Ensure security of communication over insecure medium  Communicate even with possibility of adversary
• 3. NEED OF ENCRYPTION Confidentiality Integrity Authentication Nonrepudiation Access Control Availability
• 4. TYPES OF ATTACKS A General View A Technical View A Practical Side of Attacks Programs that Attack
• 5. PLAIN PLAIN TEXT TEXT ENCRYPTION DECRYPTION CIPHER TEXTSENDER RECEIVER BASIC BLOCK DIAGRAM
• 6. BASIC TERMINOLOGIES Encryption  Encryption is the process of encoding a message so that its meaning is not obvious Decryption  Decryption is the reverse process, transforming an encrypted message back into its normal, original form Cryptosystem  A system for encryption and decryption is called a cryptosystem.
• 7. BASIC TERMINOLOGIES Plaintext Cipher text Key –  key refers to a sequence of symbols or a numerical value used by an algorithm to alter information & making that information secure Encryption algorithm  The cryptosystem involves a set of rules for how to encrypt the plaintext and how to decrypt the cipher text. Cryptanalysis  Cryptanalysis is an attempt to break the cipher text.
• 8. TECHNIQUES OF CRYPTOGRAPHY Substitution Technique  Caesar Cipher  Monoalphabetic Cipher  Homophonic Cipher  Polyalphabetic Cipher Transposition Technique  Rail Fence Technique  Vernam Cipher(One -time Pads)  Simple Columnar Cipher
• 9. ENCRYPTION ALGORITHM Symmetric  Same key for encryption and decryption  Key distribution problem Asymmetric  Key pairs for encryption and decryption  Public and private keys
• 10. SYMMETRIC ALGORITHM It is also called as Secret Key Cryptography  Single key used for both encrypt & decrypt  Key must be known to both the parties Key Original Plaintext Ciphertext Plaintext Encryption Decryption Symmetric Cryptosystem
• 11. ASYMMETRIC ALGORITHM Private keys are used for decrypting. Public keys are used for encrypting encryption plaintext ciphertext public key decryption ciphertext plaintext private key
• 12. COMPARISON Secret Key (Symmetric) Public Key (Asymmetric)Number of Key 1 2Protection of Key Must be kept secret One key must be kept secret & other can be freely exposedBest Uses secrecy and integrity of Key exchange, authentication dataKey Distribution Problematic SaferSpeed Fast Slow; typically, 10,000 times slower than secret key
• 13. SYMMETRIC ALGORITHM Data Encryption Standard (DES): 56 bits key Advance Encryption Standard (AES): 128, 192 or 256 bits key International Data Encryption Algorithm(IDEA): 128 bits key
• 14. DATA ENCRYPTION STANDARD Developed by IBM and it is known as the Data Encryption Standard It is also known as Data Encryption Algorithm The DES algorithm is a careful and complex combination of two fundamental building blocks of encryption:  Substitution and  Transposition DES uses only standard arithmetic and logical operations on numbers up to 64 bits long
• 15. Plain text (64 bits) Initial Permutation (IP) LPT RPTkey 16 rounds 16 rounds key Final Permutation Cipher text (64 bits) BROAD LEVEL STEPS IN DES
• 16. DATA ENCRYPTION STANDARD 1st 64 bit plain text is handed over to initial permutation function. IP is performed over the plain text. IP produces two halves of the permuted blocks left plain text (LPT) & right plain text (RPT). Now LPT & RPT goes 16 rounds of encryption process, each with its own key. Now LPT & RPT are rejoined and FINAL PERMUTATION (FP) is performed on the combined block. The result is 64 bit cipher text.
• 17. DETAILS OF ONE ROUND IN DES Key Transformation Expansion Permutation S- box Substitution P- box Permutation XOR and Swap
• 18. ADVANTAGES OF DES:o DES is also an ANSI and ISO standard - anybody can learn the details and implement it.o Since DES was designed to run on hardware, it is fast in hardware.o Hard to crack.DISADVANTAGES OF DES:o Hardware implementations of DES are very fast; DES was not designed for software and hence runs relatively slowly.
• 19. ASYMMETRIC ALGORITHM Rivest Shamir Adleman (RSA) Encryption: Based on factoring the product of large prime numbers. Knapsack Algorithm: If M1,M2…., Mn are given values & S is the sum, S=b1M1+b2M2….+bnMn where, bi can be 0 or 1
• 20. RSA It is named after its three inventors Rivest Shamir and Adleman This algorithm was introduced in 1978 and to date remains secure. RSA has been the subject of extensive cryptanalysis, and no serious flaws have yet been found. The encryption algorithm is based on the underlying problem of factoring large numbers.
• 21. RSA p and q are two large prime numbers n=p.q m = (p-1)(q-1) a is such that 1 < a < m and gcd (m , a) = 1. b is such that ( a. b) mod m = 1
• 22. RSA ENCRYPTION & DECRYPTION Message M < n. Encryption key = (a , n). Decryption key = (b , n). Encrypt => E = Ma mod n. Decrypt => M = Eb mod n.
• 23.  Advantage  Individuals can post their public key on their Web site.  The number of the keys is only twice of the number of user. Disadvantage  It is slower than symmetric algorithms.  The cipher text may be larger than plain text.
• 24. DIGITAL SIGNATURE  When an author signs a document, it cannot be changed.  When you send a document electronically, you can also sign it.  Digital signature can be done in two ways:  You can sign the whole document  You can sign a digest of the document
• 25. Signing the whole document You can not provide these aspects of security using the secret key. The method provides authentication & non-repudiation.
• 26. Signing the digest The two most common hash functions are:  Message digest 5 (MD5)  Secure hash algorithm (SHA-1) The properties of hash function  One-way: the digest can only be created from the message, but not vice versa  One-to-one: be very difficult to find two messages that create the same digest.
• 27. Sender site