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UNIT-5 Cryptography

UNIT-5 Cryptography. 1.Principles Of Cryptography 2.Cryptography Definition 3.Cipher Methods 4.Encrption Algorithms 5.Public Key Infrastructure 6.Attacks on Cryptosystems. 1. Principles of Cryptography.

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UNIT-5 Cryptography

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  1. UNIT-5 Cryptography 1.Principles Of Cryptography 2.Cryptography Definition 3.Cipher Methods 4.Encrption Algorithms 5.Public Key Infrastructure 6.Attacks on Cryptosystems Firewalls & Network Security, 2nd ed. - Chapter 9

  2. 1. Principles of Cryptography • Encryption: the process of converting an original message into a form that cannot be understood by unauthorized individuals • Cryptology, the science of encryption, encompasses two disciplines: • Cryptography: describes the processes involved in encoding and decoding messages so that others cannot understand them • Cryptanalysis: the process of deciphering the original message (plaintext) from an encrypted message (ciphertext) without knowing the algorithms and keys used to perform the encryption Firewalls & Network Security, 2nd ed. - Chapter 9

  3. 2.Encryption Definitions • Algorithm: the mathematical formula or method used to convert an unencrypted message into an encrypted message • Cipher: the transformation of the individual components (characters, bytes, or bits) of an unencrypted message into encrypted components • Ciphertext or cryptogram: the unintelligible encoded message resulting from an encryption • Cryptosystem: the set of transformations necessary to convert an unencrypted message into an encrypted message Firewalls & Network Security, 2nd ed. - Chapter 9

  4. Contd… • Decipher: to decrypt or convert ciphertext to plaintext • Encipher: to encrypt or convert plaintext to ciphertext • Key or cryptovariable: the information used in conjunction with the algorithm to create the ciphertext from the plaintext; it can be a series of bits used in a mathematical algorithm or the knowledge of how to manipulate the plaintext • Keyspace: the entire range of values that can possibly be used to construct an individual key Firewalls & Network Security, 2nd ed. - Chapter 9

  5. Contd… • Plaintext: the original unencrypted message that is encrypted and results from successful decryption • Steganography: the process of hiding messages, usually within graphic images • Work factor: the amount of effort (usually expressed in units of time) required to perform cryptanalysis on an encoded message Firewalls & Network Security, 2nd ed. - Chapter 9 5

  6. Contd… M represents original message; C represents ciphertext; E represents encryption process; D represents the decryption process; K represents a key So… E(M) = C encrypting a message results in cyphertext D(C) = M and D[E(M)] = M E(M,K) = C specifies encrypting the message with a key; keys can be annotated K1, K2, etc. in the case of multiple keys Firewalls & Network Security, 2nd ed. - Chapter 9

  7. 2.Cipher Methods • Plaintext can be encrypted through bit stream or block cipher method • Bit stream: each plaintext bit transformed into cipher bit one bit at a time • Block cipher: message divided into blocks (e.g., sets of 8- or 16-bit blocks) and each is transformed into encrypted block of cipher bits using algorithm and key Principles of Information Security, 3rd edition

  8. Contd… • Cryptosystems typically made up of algorithms, data handling techniques, and procedures • Substitution cipher: substitute one value for another • Monoalphabeticsubstitution: uses only one alphabet • Polyalphabetic substitution: more advanced; uses two or more alphabets • Vigenère cipher: advanced cipher type that uses simple polyalphabeticcode; made up of 26 distinct cipher alphabets Principles of Information Security, 3rd edition

  9. Principles of Information Security, 3rd edition

  10. 2.1 Deciphering using the Vigenere Square • Using the VS, decipher the following message: HQ UEDY Hint: Use the most basic approach presented in the text. Q: Is the VS cipher monoalphabetic or polyalphabetic? Q: VS is an example of a _______________ cipher. • Transposition Cipher Create cyphertext for the message “Accountants”, using a transposition cipher based a four letter displacement. Principles of Information Security, 3rd edition

  11. Contd… • Transposition cipher: rearranges values within a block to create ciphertext • Exclusive OR (XOR): function of Boolean algebra; two bits are compared • If two bits are identical, result is binary 0 • If two bits not identical, result is binary 1 Principles of Information Security, 3rd edition

  12. Table 8-1 Exclusive OR Operations Perform an XOR cipher on the following bits. Message 01100001 01100010 01100011 Cypher Key 01111111 01111111 01111111 Principles of Information Security, 3rd edition

  13. 01100001 = a • 01100010 = b • 01100011 = c • Message 01100001 01100010 01100011 • Key 01111111 01111111 01111111 • Cypher text 00011110 00011101 00011100 Principles of Information Security, 3rd edition

  14. 2.2 Hash Functions • Mathematical algorithms that generate message summary/digest to confirm message identity and confirm no content has changed • Hash algorithms: publicly known functions that create hash value • Use of keys not required; message authentication code (MAC), however, may be attached to a message • Used in password verification systems to confirm identity of user Principles of Information Security, 3rd edition

  15. 3.Cryptographic Algorithms • Often grouped into two broad categories, symmetric and asymmetric; today’s popular cryptosystems use hybrid combination of symmetric and asymmetric algorithms • Symmetric and asymmetric algorithms distinguished by types of keys used for encryption and decryption operations Principles of Information Security, 3rd edition

  16. Contd… • Symmetric encryption: uses same “secret key” to encipher and decipher message • Encryption methods can be extremely efficient, requiring minimal processing • Both sender and receiver must possess encryption key • If either copy of key is compromised, an intermediate can decrypt and read messages Principles of Information Security, 3rd edition

  17. Figure 8-3 Symmetric Encryption Example Principles of Information Security, 3rd edition

  18. Contd… • Data Encryption Standard (DES): one of most popular symmetric encryption cryptosystems • 64-bit block size; 56-bit key • Adopted by NIST in 1976 as federal standard for encrypting non-classified information • Triple DES (3DES): created to provide security far beyond DES • Advanced Encryption Standard (AES): developed to replace both DES and 3DES Principles of Information Security, 3rd edition

  19. Contd… • Asymmetric encryption (public-key encryption) • Uses two different but related keys; either key can encrypt or decrypt message • If Key A encrypts message, only Key B can decrypt • Highest value when one key serves as private key and the other serves as public key Principles of Information Security, 3rd edition

  20. Figure 8-4 Using Public Keys Principles of Information Security, 3rd edition

  21. 3.1 Encryption Key Size • When using ciphers, size of cryptovariable or key is very important • Strength of many encryption applications and cryptosystems measured by key size • For cryptosystems, security of encrypted data is not dependent on keeping encrypting algorithm secret • Cryptosystem security depends on keeping some or all of elements of cryptovariable(s) or key(s) secret Principles of Information Security, 3rd edition

  22. Principles of Information Security, 3rd edition

  23. 4.Public Key Infrastructure • Public Key Infrastructure (PKI): integrated system of software, encryption methodologies, protocols, legal agreements, and third-party services enabling users to communicate securely • PKI systems based on public-key cryptosystems; include digital certificates and certificate authorities (CAs) Principles of Information Security, 3rd edition

  24. Contd… • PKI protects information assets in several ways: • Authentication • Integrity • Privacy • Authorization • Nonrepudiation Principles of Information Security, 3rd edition

  25. 4.1 Digital Signatures • Encrypted messages that can be mathematically proven to be authentic • Created in response to rising need to verify information transferred using electronic systems • Asymmetric encryption processes used to create digital signatures Principles of Information Security, 3rd edition

  26. 4.2 Digital Certificates • Electronic document containing key value and identifying information about entity that controls key • Digital signature attached to certificate’s container file to certify file is from entity it claims to be from Principles of Information Security, 3rd edition

  27. Figure 8-5 Digital Signatures Principles of Information Security, 3rd edition

  28. Principles of Information Security, 3rd edition

  29. 4.3 Hybrid Cryptography Systems • Except with digital certificates, pure asymmetric key encryption not widely used • Asymmetric encryption more often used with symmetric key encryption, creating hybrid system • Diffie-Hellman Key Exchange method: most common hybrid system; provided foundation for subsequent developments in public-key encryption Principles of Information Security, 3rd edition

  30. Figure 8-7 Hybrid Encryption Example Principles of Information Security, 3rd edition

  31. 4.4 Steganography • Process of hiding information; in use for a long time • Most popular modern version hides information within files appearing to contain digital pictures or other images • Some applications hide messages in .bmp, .wav, .mp3, and .au files, as well as in unused space on CDs and DVDs Principles of Information Security, 3rd edition

  32. 5. Attacks on Cryptosystems • Attempts to gain unauthorized access to secure communications have typically used brute force attacks (ciphertext attacks) • Attacker may alternatively conduct known-plaintext attack or selected-plaintext attach schemes

  33. 5.1 Man-in-the-Middle Attack • Designed to intercept transmission of public key or insert known key structure in place of requested public key • From victim’s perspective, encrypted communication appears to be occurring normally, but in fact attacker receives each encrypted message, decodes, encrypts, and sends to originally intended recipient • Establishment of public keys with digital signatures can prevent traditional man-in-the-middle attack

  34. 5.2 Correlation Attacks • Collection of brute-force methods that attempt to deduce statistical relationships between structure of unknown key and ciphertext • Differential and linear cryptanalysis have been used to mount successful attacks • Only defense is selection of strong cryptosystems, thorough key management, and strict adherence to best practices of cryptography in frequency of changing keys

  35. 5.3 Timing Attacks • Attacker eavesdrops during victim’s session; uses statistical analysis of user’s typing patterns and inter-keystroke timings to discern sensitive session information • Can be used to gain information about encryption key and possibly cryptosystem in use • Once encryption successfully broken, attacker may launch a replay attack (an attempt to resubmit recording of deciphered authentication to gain entry into secure source)

  36. 5.4 Defending Against Attacks • No matter how sophisticated encryption and cryptosystems have become, if key is discovered, message can be determined • Key management is not so much management of technology but rather management of people

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