1. Copyright 1999 S.D. Personick. All Rights Reserved. Telecommunications Networking II Lecture 41b Cryptography and Its Applications

2. Copyright 1999 S.D. Personick. All Rights Reserved. Basic Cryptography

3. Copyright 1999 S.D. Personick. All Rights Reserved. Cryptography Using mathematically-based methods to protect information from being read and/or modified by unauthorized persons

4. Copyright 1999 S.D. Personick. All Rights Reserved. The concept of a cipher Four score and seven years ago … Gpvs tdpsf boe tfwfo zfbst bhp...

5. Copyright 1999 S.D. Personick. All Rights Reserved. Cryptography The simple substitution cipher is easy to “break” We need a much more secure approach for real-world applications

6. Copyright 1999 S.D. Personick. All Rights Reserved. Cryptography The most secure approach to cryptography is to use a “one time pad”… However, in most applications it is not practical to use the “one time pad” method

7. Copyright 1999 S.D. Personick. All Rights Reserved. Cryptography Most cryptographic methods are based on -A cryptographic algorithm that is assumed to be widely known (the algorithm itself is not secret) -A secret cryptographic “key” that is known only to those who are authorized to have the secret key

8. Copyright 1999 S.D. Personick. All Rights Reserved. Cryptography EncrpytDecrpyt Secret Key

9. Copyright 1999 S.D. Personick. All Rights Reserved. Desired Properties of an Encryption Algorithm It should be very difficult (computationally) to decrypt a message without having the secret key It should be reasonably easy to encrypt and decrypt a message, if you have the secret key

10. Copyright 1999 S.D. Personick. All Rights Reserved. Details The secret key is usually a binary sequence (1s and 0s) that is at least 56 bits long, and preferably 128 bits long (or longer) Key management... E.g., distributing secret keys to people who are authorized to have them, without making them accessible to unauthorized persons … is always a challenge

11. Copyright 1999 S.D. Personick. All Rights Reserved. Details Nobody knows for sure how “hard” it is to “break” modern encryption methods … however mathematicians are able to make statements about the comparative difficulty of breaking one method vs. another Increasing computing power makes brute force methods feasible… leading to the need for longer keys

12. Copyright 1999 S.D. Personick. All Rights Reserved. Details The ability to break many encryption methods is closely related to the ability to “factor” a large number … thus you may read about competitions among people working in the field of cryptography to come up with efficient computational schemes for factoring large numbers

13. Copyright 1999 S.D. Personick. All Rights Reserved. Public-key Cryptography In the 1970’s cryptographic researchers came up with some amazing results/concepts that have had a remarkable impact on the ability to build practical cryptographic systems These results/concepts helped address the key management problem

14. Copyright 1999 S.D. Personick. All Rights Reserved. The concept of a 1-way function A one-way function is one for which it is easy to compute y = f(x), where y and x are sequences of binary digits (1s and 0s) … … but it is very “hard” to compute what x is, given that you have access to y A one way function is analogous to a padlock: I can easily snap it shut, but I can’t open it (without a key or a combination)

15. Copyright 1999 S.D. Personick. All Rights Reserved. The Concept of Public-key Cryptography Public key encryption EncrpytDecrpyt Public Key Private (secret) Key

16. Copyright 1999 S.D. Personick. All Rights Reserved. The Concept of Public Key Encryption A public key is a sequence of binary digits (1s and 0s) that is accessible to anyone who wishes to know what it is (I.e., its published in a publicly accessible directory) The corresponding private (secret) key is only known to authorized persons

17. Copyright 1999 S.D. Personick. All Rights Reserved. The Concept of Public Key Encryption A public key is used to apply a one-way function, I.e. to encrypt the red information. Anyone with a message to send to a particular recipient, or set of recipients can use the recipient’s public key to do this The corresponding private (secret) key is used by the authorized recipient(s) to decrypt messages that have been sent to them

18. Copyright 1999 S.D. Personick. All Rights Reserved. Details It is computationally difficult (and correspondingly slow) to utilize public key cryptography Therefore, in practice, public key cryptography is often used as a secure method for exchanging private keys; and then private key cryptography is used to exchange information

19. Copyright 1999 S.D. Personick. All Rights Reserved. A key exchange protocol Client Server Obtain server’s public key Use server’s public key to send ID info to server Receive/decrypt session key Use session key ----- Receive/decrypt message Obtain client’s public key Send session key to client - Use session key

20. Copyright 1999 S.D. Personick. All Rights Reserved. Digital Signatures Problem -How can I be sure that a message with your name associated with it: really came from you hasn’t been altered since you sent it

21. Copyright 1999 S.D. Personick. All Rights Reserved. Digital Signatures Message HashSignature Encrypt Sender’s private key

22. Copyright 1999 S.D. Personick. All Rights Reserved. Digital Signatures The hash is a summary of my message Given the message, anyone can compute the hash When I encrypt the hash and my signature, using my secret key, anyone can decrypt it using my public key

23. Copyright 1999 S.D. Personick. All Rights Reserved. Digital Signatures However, no one can change the message without producing a mismatch between the hash derived from the changed message, and the hash that I sent in my encrypted hash/signature file Furthermore, no one can create a fake hash/ signature file that will decrypt properly with my public key

24. Copyright 1999 S.D. Personick. All Rights Reserved. Certificates When I send you a message claiming to be Prof. Stewart Personick of Drexel University, encrypted with my private key, and it decrypts properly with my public key …how do you know that the public key you obtained from the public key directory really belongs to Prof. Stewart Personick of Drexel University? Who certifies this?

25. Copyright 1999 S.D. Personick. All Rights Reserved. Certificate Authority A well known/trusted “certificate authority” can provide me with an electronically signed certificate (encrypted with the certificate authority’s private key) vouching for the fact that a particular public key has, in fact, been issued to Professor Stewart Personick of Drexel University One can create a hierarchy of certificate authorities