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# Introduction to cryptography

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• 1. English for Comp. Science IIntroduction to CryptographyAmir Neziri & Jurlind Budurushi2/3/2011
• 2. Introduction  Let’s make the introduction...2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 2
• 3. Introduction2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 3
• 4. Content  History of Cryptography  Goals of Cryptography  Symmetric Cryptography  Message Authentication Codes2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 4
• 5. Content  Asymmetric Cryptography  Public Key Encryption  Digital Signatures  Digital Certificates  Demo(CrypTool)2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 5
• 6. History of Cryptography  Substitution ciphers:  a->Z, b->T, c->E,…  Encryption determined by Permutation of alphabet  Special case of Caesar Cipher:  a->D, b->E, c->F,…  Example: amir -> DPMLU2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 6
• 7. Goals of Cryptography  Confidentially  Encryption  Integrity  MAC / Digital Signatures  Authenticity  MAC / Digital Signatures  Non-Repudiation  Digital Signatures  Others...2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 7
• 8. Symmetric Cryptography  The key for encryption and decryption is the same  Key exchange problem!!!2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 8
• 9. Symmetric Algorithms  Stream ciphers  Block ciphers  Data Encryption Standard (DES)  Advanced Encryption Standard (AES)  Triple DES (3DES)  Blowfish2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 9
• 10. Message Authentication Codes (MAC)  Allow to test the integrity of a document  3 algorithms: key generation, tagging, verification  Verification is usually done by re-computing the MAC2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 10
• 11. Asymmetric Cryptography  The key for encryption and decryption are different  Key exchange problem solved2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 11
• 12. Asymmetric Algorithms  Rivest, Shamir and Adleman (RSA)  Elgamal  Chor-Rivest  Rabin2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 12
• 13. RSA  Frist practical asymmetric encryption algorithm proposed by Rivest, Shamir and Adleman  Security is based on the intractability of factoring large numbers  Slower than symmetric algorithms2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 13
• 14. RSA  Key generation:  Public key: (n, e), Private key: d  Encryption:  c = Enc(m, e) = m^e mod n, where m is a message  Decryption  m = Dec(m, d) = c^d mod n2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 14
• 15. Public key encryption2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 15
• 16. Digital Signatures  Allow to test authenticity and integrity of documents  Non-repudiation: a third party can be convinced of authenticity2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 16
• 17. RSA signatures: “signing by decryption”  RSA has a commutative encryption/decryption operations:  Enc(Dec(m)) = Dec(Enc(m)) = m, where m := Message  Key generation  Similar to RSA, public key: (n, e), private key: d  Uses a collision-resistant hash function H  Signing  s = [H(m)]^d mod n, where s := Signature  Verification  Test if, H(m) ≡ s^e mod n2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 17
• 18. Digital Certificates  How can I be sure that a public key really belongs to the right person?  Solution: Digital Certificates  A digital certificate is a message signed by a trusted party that contains the identity of a person and a public key  Contents of a digital certificate: name of issuer, date, expiration date, public key, name of a person, ...2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 18
• 19. X.509 Digital Certificates  Centralized architecture  Hierarchical system  Each department has its own certificatioin authority (CA)  The public key of this CA is signed by a “higher” CA  “Root certificates”2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 19
• 20. Demo (CrypTool)  Symmetric Algorithms  XOR  Triple DES(ECB)  ….  Asymmetric Algorithms  RSA  ….  Others…2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 20
• 21. Questions???2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 21
• 22. Discussion(1)  Q: Give some other Goals of Cryptography.  A: Availability, Anonymity, Untraceability, Unlinkability, Being dated…2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 22
• 23. Discussion(2)  Q: Can you give an example as a solution to the key exchange problem for the symmetric cryptography?2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 23
• 24. Discussion(3)  Q: Why do we use a collision-resistant hash function H by signing?  A: We sign the hash value of a message, because it is faster than signing the whole message and even more secure.2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 24
• 25. 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 25
• 26. References  Trusted System lecture (WS10/11)  http://www.seceng.informatik.tu-darmstadt.de/ts10/  C. Eckert. IT-Sicherheit: Konzepte - Verfahren – Protokolle. Oldenbourg, 2009.  J. Buchmann. Einführung in die Kryptographie. Springer, 2010  Public Key Infrastructures lecture (SS10)  http://www.cdc.informatik.tu-darmstadt.de/lehre/SS10/vorlesung/PKI/P2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 26