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Asymmetric Encryption Prof. Ravi Sandhu

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2 © Ravi Sandhu PUBLIC KEY ENCRYPTION Encryption Algorithm E Decryption Algorithm D Plain- text Plain- text Ciphertext INSECURE CHANNEL B's Public Key B's Private Key RELIABLE CHANNEL A A B B

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3 © Ravi Sandhu PUBLIC KEY CRYPTOSYSTEM solves the key distribution problem provided there is a reliable channel for communication of public keys requires reliable dissemination of 1 public key/party scales well for large-scale systems

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4 © Ravi Sandhu PUBLIC KEY ENCRYPTION confidentiality based on infeasibility of computing B's private key from B's public key key sizes are large (512 bits and above) to make this computation infeasible

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5 © Ravi Sandhu SPEED OF PUBLIC KEY VERSUS SECRET KEY Public key runs at kilobits/second think modem connection Secret key runs at megabits/second and even gigabits/second think LAN or disk connection This large difference in speed is likely to remain independent of technology advances

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6 © Ravi Sandhu RSA public key is (n,e) private key is d encrypt: C = M e mod n decrypt: M = C d mod n

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7 © Ravi Sandhu GENERATION OF RSA KEYS choose 2 large (100 digit) prime numbers p and q compute n = p * q pick e relatively prime to (p-1)*(q-1) compute d, e*d = 1 mod (p-1)*(q-1) publish (n,e) keep d secret (and discard p, q)

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8 © Ravi Sandhu PROTECTION OF RSA KEYS compute d, e*d = 1 mod (p-1)*(q-1) if factorization of n into p*q is known, this is easy to do security of RSA is no better than the difficulty of factoring n into p, q

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9 © Ravi Sandhu RSA KEY SIZE key size of RSA is selected by the user casual384 bits commercial512 bits military1024 bits

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