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Introduction to cryptography

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Amir Neziri
Amir Neziri

This document is an introduction to cryptography presented by Amir Neziri and Jurlind Budurushi. It discusses the history and goals of cryptography, as well as symmetric and asymmetric cryptographic algorithms like AES, RSA, and digital signatures. It also covers cryptographic concepts such as public key encryption, message authentication codes, digital certificates, and concludes with a demo of encryption tools.

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English for Comp. Science I Introduction to Cryptography Amir Neziri & Jurlind Budurushi 2/3/2011
Introduction  Let’s make the introduction... 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 2
Introduction 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 3
Content  History of Cryptography  Goals of Cryptography  Symmetric Cryptography  Message Authentication Codes 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 4
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
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 -> DPMLU 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 6
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
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
Symmetric Algorithms  Stream ciphers  Block ciphers  Data Encryption Standard (DES)  Advanced Encryption Standard (AES)  Triple DES (3DES)  Blowfish 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 9
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 MAC 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 10
Asymmetric Cryptography  The key for encryption and decryption are different  Key exchange problem solved 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 11
Asymmetric Algorithms  Rivest, Shamir and Adleman (RSA)  Elgamal  Chor-Rivest  Rabin 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 12
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 algorithms 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 13
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 n 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 14
Public key encryption 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 15
Digital Signatures  Allow to test authenticity and integrity of documents  Non-repudiation: a third party can be convinced of authenticity 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 16
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 n 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 17
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
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
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
Questions??? 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 21
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
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
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
2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 25
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/P 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 26

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Introduction to cryptography

  • 1. English for Comp. Science I Introduction to Cryptography Amir Neziri & Jurlind Budurushi 2/3/2011
  • 2. Introduction  Let’s make the introduction... 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 2
  • 3. Introduction 2/3/2011 | 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 Codes 2/3/2011 | 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 -> DPMLU 2/3/2011 | 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)  Blowfish 2/3/2011 | 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 MAC 2/3/2011 | 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 solved 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 11
  • 12. Asymmetric Algorithms  Rivest, Shamir and Adleman (RSA)  Elgamal  Chor-Rivest  Rabin 2/3/2011 | 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 algorithms 2/3/2011 | 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 n 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 14
  • 15. Public key encryption 2/3/2011 | 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 authenticity 2/3/2011 | 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 n 2/3/2011 | 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/P 2/3/2011 | Department 20 | B.IT Nieh | Introduction to Cryptography | A. Neziri & J. Budurushi 26