Cryptography
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This document provides an overview of cryptography concepts including symmetric and asymmetric encryption algorithms, hashing, digital signatures, and public key infrastructure (PKI). Symmetric algorithms like AES use a shared secret key for encryption and decryption while asymmetric algorithms like RSA use separate public and private keys. Digital signatures combine hashing and asymmetric encryption to provide data integrity, non-repudiation and authentication. PKI establishes trust in public keys through a system of digital certificates issued by a trusted certification authority.
Cryptography
- 1. Cryptography A Tutorial on Cryptography By: Er. Chetan Goyal Contact for more tutorials on other topics of ethical hacking [email protected] [email protected] Or visit www.windowwares.blogspot.com
- 2. Cryptography • This module includes Related terms Symmetric key cryptography Asymmetric Key cryptography PKI and Digital signatures Demonstration: how SSL works.. 2
- 3. Cipher • Cipher is the actual algorithm. • We can use cipher and algorithm interchangeably. 3
- 4. More definitions • Cryptosystem • Encompasses all of the necessary components of encryption and decryption » Softwares » Protocols » Algorithms • Plain text – readable data • Cipher text – encrypted data 4
- 5. Caesar Cipher • Julius Caesar, 49 BC – Securely communicate with friends • Simple Substitution cipher – Shift alphabet 3 characters. 5
- 6. Caesar Cipher Example • Substitution 6 Plaintext: ET TU BRUTE Shift Algorithm 3 characters Cipher text: HW WX EUXWH
- 7. Symmetric Encryption • Single key – - Shared secret • Examples Data Encryption Standard (DES) – - Block Cipher, 56 bit key – - Triple DES 112 bit key Advanced Encryption Standard (AES) - Rijndael Algorithm • Belgian cryptographers, Joan Daemen and Vincent Rijmen. - 128, 192, 256 bit keys 7
- 8. Symmetric Key Encryption • Symmetric key encryption is based on substitution and transposition • Each algorithm consist many substitution and transposition “rounds” • Symmetric key encryption is mostly used for bulk or mass encryption. 8
- 9. Key and Algorithm Relationship • Algorithm consist many static equations (functions) or rounds these rounds are basically mathematical formulas – substitution and transposition. • Key will decide which function/s or block/s, their order, values should be used to encrypt or decrypt data. 9
- 10. Key Size • Does key size really matter ? • Why and how • Key space – total number of bits available for key generation • So for 64 bits we have 2 power 64. • For 128 bits we have 2 power 128 • So the larger the key space, the more values an attacker has to bruteforce. 10
- 11. Types of Symmetric Cipher • Block Cipher – text is divided into blocks, and each block is encrypted individually • Eg: AES, DES, 3DES etc. • Stream cipher – stream is used instead of blocks, synchronization of encryption and decryption ends is needed • Eg: RC4, RC5 11
- 12. Symmetric Encryption Example 12 Dear Bob: How about coming over to my place at 1:30? Love, Alice Dear Bob: How about coming over to my place at 1:30? Love, Alice Alice Bob decryptencrypt 011100111001001 110011100111001 001110000111111 ciphertext
- 13. Symmetric cipher downfalls • Key management • Does not provide non repudiation and digital signature facilities 13
- 14. Asymmetric Key Cryptography • Two mathematically related keys – Unable to derive one from the other – Encrypt with one – decrypt with other • Public Key Cryptography – One (public) key published for all to see – Other (private) key kept secret • Algorithms – RSA - Integer Factorization (large primes) – Diffie-Hellman - Discrete Logarithms – ECES - Elliptic Curve Discrete Logarithm 14
- 15. Asymmetric Encryption Example 15 Dear Carol: I think Alice is having an affair with Bob. I need to see you right always. Love, Ted Dear Carol: I think Alice is having an affair with Bob. I need to see you right always. Love, Ted Mohan Jagmohan encrypt decrypt Jagmohan's Private KeyJagmohan's Public Key 011100111001001 110011100111001 001110000111111 ciphertext
- 16. Asymmetric Advantages • No shared secret key • Public key is public – Can be freely distributed or published – Key management is much easier • Private key known ONLY to owner – Less vulnerable, easier to keep secret • Supports Non-repudiation – Sender can not deny sending message 16
- 17. Asymmetric Disadvantage • 1000 times slower than Symmetric encryption so can’t use for bulk encryption. 17
- 18. Non-repudiation • Since only the sender knows their private key, only the sender could have sent the message. • Authentication mechanism • Basis for Digital Signature 18
- 19. Hash Function • Hash Function are like CRC functions but are more then CRC (mathematically) • Hash provide integrity check • Hash functions generate finger prints for a given data, a little change to data will result completely different fingerprint. (non guessable) • When hash function generate same fingerprint for different data, known as collision or birthday attack • Hash functions are one way functions 19
- 20. Examples of Hash Functions • MD5 • SHA - 1 • SHA - 256 20
- 21. Digital Signature • Combines one-way secure hash functions with public key cryptography – Hash function generates fixed length value – No two documents produce the same hash value – Secure Hash Algorithm 1 (SHA-1) • Characteristics – Data Integrity - hash value – Non-repudiation – encrypted with private key – Does NOT provide confidentiality 21
- 22. Digital Signature Creation 22 Dear Mr. Ted: We have asked the Court to issue a restraining order against you to stay away from Carol. Sincerely, Sue Yew Dewey, Cheatam & Howe, Law Firm Dear Mr. Ted: We have asked the Court to issue a restraining order against you to stay away from Carol. Sincerely, Sue Yew Dewey, Cheatam & Howe, Law Firm encrypt Sue's Private Key Hash Function Sue 0F47CEFF AE0317DB AA567C29 Hash Value 0101011110000110101 1011110101111010111 Digital Signature
- 23. Digital Signature Validation 23 Dear Mr. Ted: We have asked the Court to issue a restraining order against you to stay away from Carol. Sincerely, Sue Yew Dewey, Cheatam & Howe, Law Firm 0101011110000110101 1011110101111010111 Sue's Public Key decrypt 0F47CEFF AE0317DB AA567C29 0F47CEFF AE0317DB AA567C29 Signature is valid if the two hashes match
- 24. Source of Public Key • Can be on a server (common server) • With PGP - embedded into mail 24
- 25. But • How do you know for sure who is the owner of a public key? 25
- 26. Public Key Infrastructure • Public Key Infrastructure (PKI) provides the means to bind public keys to their owners and helps in the distribution of reliable public keys in large heterogeneous networks. NIST 26
- 27. Public Key Certificates • Digital Certificates – Binds a public key to it's owner – Issued and digitally signed by a trusted third party – Like an electronic photo-id • Follows X509 V3 standard – RFC 2459 27
- 28. X509 V3 Basic Fields • Owner's X.500 distinguished name (DN) – C=US;O=GOV;O=NIH;OU=CIT;CN=Mark Silverman • Owner's public key • Validity period • Issuer's X.500 distinguished name 28
- 29. PKI Components • Certification Authority (CA) • Registration Authority (RA) • Repository • Archive • Users 29
- 30. Certification Authority (CA) • TRUSTED third party • Issues Certificates – Creates and signs them – Publishes current certificates • Issues Certificate Revocation Lists (CRLs) – List of invalid (revoked) certificates – Online Certificate Status Protocol (OCSP) • Maintains archives of status information • May retain copy of data encryption private key, for purposes of key recovery – government requirement 30
- 31. Registration Authority (RA) • Verify certificate contents for CA – Identity proofing – RA's public key known to CA • A CA may have multiple RAs 31
- 32. Certificate Policy (CP) • A high level document that describes the security policy for issuing certificates and maintaining certificate status information. • Describes operation of the CA. • Defines user's responsibilities for requesting, using and handling certificates and keys. 32
- 33. Certification Practice Statements (CPS) • A highly detailed document that describes how a CA implements a specific CP. • Specifies the mechanisms and procedures that are used to achieve the security policy. • Effectively the CA's operations manual. 33
- 34. Conclusion • Enabling technology for E-Gov – Data Confidentiality – Data Integrity – Non-repudiation • Technology is complicated – But not unmanageable • Difficulty is in establishing trust – 20% technology – 80% policy 34