Unformatted text preview: a key in two parts: a public part (widely known), and a private part (only owner knows) – Encryp7on is based on mathema7cs (e.g., RSA is based on diﬃculty of factoring) Computer Networks 12 6 12/3/13 Symmetric (Secret Key) Encryp7on • Alice and Bob have the same secret key, KAB – Anyone with the secret key can encrypt/decrypt Plaintext I♥networks Alice Plaintext I♥networks Encrypt Hi there Decrypt Ciphertext Secret key KAB KAB Bob Secret key Computer Networks 13 Public Key (Asymmetric) Encryp7on • Alice and Bob each have public/private key pair (KB / KB 1) – Public keys are well known, private keys are secret to owner Plaintext I♥networks Alice Plaintext I♥networks Encrypt Bob’s public key Computer Networks Hi there Decrypt Ciphertext KB KB 1 Bob Bob’s private key 14 7 12/3/13 Public Key Encryp7on (2) • Alice encrypts with Bob’s public key KB; anyone can send • Bob decrypts with his private key KB 1; only he can do so Plaintext I♥networks Alice Plaintext I♥networks Encrypt Hi there Decrypt Ciphertext Bob’s public key KB Computer Networks KB 1 Bob Bob’s private key 15 Key Distribu7on • This is a big problem on a network! – OCen want to talk to new par7es • Symmetric encryp7on problema7c – Have to ﬁrst set up shared secret • Public key idea has own diﬃcul7es – Need trusted directory service – We’ll look at cer7ﬁcates later Computer Networks 16 8 12/3/13 Symmetric vs. Public Key • Have complementary proper7es – Want the best...
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 Fall '09
 Cryptography, Alice 00, Bob 00, Trudy 00

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