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Public Key Cryptography

Public Key Cryptography. Cryptography Lecture 3: Chantilly Academy Poorvi Vora Department of Computer Science George Washington University. Get the key to Amazon. Suppose you want to buy something from amazon.com online Pay with your credit card

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Public Key Cryptography

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  1. Public Key Cryptography Cryptography Lecture 3: Chantilly Academy Poorvi Vora Department of Computer Science George Washington University

  2. Get the key to Amazon • Suppose you want to buy something from amazon.com online • Pay with your credit card • You don’t want anyone to be able to snoop the number on the line, so you encrypt it, using AES (hey, it’s a new secure standard) • How do you get the key to amazon? Chantilly Academy Crypto Lecture 2: Spring 07

  3. Public Key Cryptography • In the late 70’s, a famous paper by Diffie and Hellman • Suppose two keys • One made public the other kept private • Anything encrypted with one can be decrypted by the other, and with no other key • It is not possible determine the private key through encrypted messages • Take amazon’s public key to encrypt your credit card number • amazon will figure the number out by using it’s private key Chantilly Academy Crypto Lecture 2: Spring 07

  4. RSA (Rivest Shamir Adleman) • RSA is the most popular public-key encryption scheme • Developed in mid-70s • Turns out it was developed two years earlier by a British intelligence agent who could not make it public. • This was revealed in the early 2000s Chantilly Academy Crypto Lecture 2: Spring 07

  5. Public Key Cryptography for non-repudiation • Suppose you want an electronic IOU from a friend • You need the friend to “sign” it so he cannot later deny he agreed to it • How about if he encrypts it with his private key? • Nobody else has his private key, so nobody else could do it • YOU can decrypt it with his public key • Instead of encrypting the whole message, he can encrypt a message digest Chantilly Academy Crypto Lecture 2: Spring 07

  6. Message Digest • Shortened version of message such that • it is difficult to get a message, or any part of it, back from its digest • It is difficult to get another message that has the same digest • It is difficult to find two messages with the same digest • Instead of encrypting a big long IOU, your friend can encrypt a digest of the IOU • SHA-1 provides such a digest Chantilly Academy Crypto Lecture 2: Spring 07

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