lecture3 - Lecture 3 Public Key Cryptography CS 392/6813...

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Lecture 3: Public Key Cryptography CS 392/6813: Computer Security Fall 2010 Nitesh Saxena * Adopted from Previous Lectures by Nasir Memon 9/23/2010 Lecture 3: Pubic Key Cryptography 2 Course Administration HW Groups formed HW2 posted – due Sept 30. Solution to HW1 was provided (emailed) Regarding programming portions of the homework Submit the whole modified code that you used to measure timings Comment the portions in the code where you modified the code Include a small “readme” for us to understand this
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9/23/2010 Lecture 3: Pubic Key Cryptography 3 Course Administration Good/Bad News HW#1 will not be graded! Not a trick – just for the benefit of this course Next two lectures (Sep 30 and Oct 07) I am traveling; attending two conferences (Ubicomp 2010 and CCS 2010) TA (Jon) will cover Secure Design Principles Threat Modeling Regular lectures; important – please do attend 9/23/2010 Lecture 3: Pubic Key Cryptography 4 Course Administration TA Sessions Moved to Mondays from 5-6pm in JAB 474. Starting the coming Monday (Sept 27). Emails bouncing: [email protected] [email protected]
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9/23/2010 Lecture 3: Pubic Key Cryptography 5 Outline of Today’s Lecture Public Key Crypto Overview Number Theory Background Public Key Encryption RSA ElGamal Public Key Signatures (digital signatures) RSA DSS 9/23/2010 Lecture 3: Pubic Key Cryptography 6 Recall: Private Key/Public Key Cryptography Private Key : Sender and receiver share a common (private) key Encryption and Decryption is done using the private key Also called conventional/shared-key/single-key/ symmetric-key cryptography Public Key : Every user has a private key and a public key Encryption is done using the public key and Decryption using private key Also called two-key/asymmetric-key cryptography
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9/23/2010 Lecture 3: Pubic Key Cryptography 7 Private key cryptography revisited. Good : Quite efficient (as you’ll see from the HW#2 programming exercise on AES) Bad : Key distribution and management is a serious problem – for N users O(N 2) keys are needed Message Source Cryptanalyst Key Source Destination X X X ^ K ^ Y Secure Channel K Encryption Algorithm Decryption Algorithm 9/23/2010 Lecture 3: Pubic Key Cryptography 8 Public key cryptography model Good : Key management problem potentially simpler Bad : Much slower than private key crypto (we’ll see later!) Plaintext input Public key directory Transmitted ciphertext Plaintext output Encryption algorithm Alice Decryption algorithm Bob Joy Mike Ted Bob Bob's public key Bob 's private key
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9/23/2010 Lecture 3: Pubic Key Cryptography 9 Public Key Encryption Two keys: public encryption key e private decryption key d Encryption easy when e is known Decryption easy when d is known Decryption hard when d is not known We’ll study such public key encryption schemes; first we need some number theory.
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