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I might be a 7ny bit sick of networks input eg sha1

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Unformatted text preview: of both! Property Symmetric Public Key Key Hard– share secret Easier– publish Distribu7on per pair of users public key per user Run7me Fast– good for Performance high data rate Slow– few, small, messages Computer Networks 17 Winning Combina7on •  Alice uses public key encryp7on to send Bob a small private message –  It’s a key! (Say 256 bits.) •  Alice and Bob send large messages with symmetric encryp7on –  Using the key they now share •  The key is called a session key –  Generated for short- term use Computer Networks 18 9 12/3/13 Introduc7on to Computer Networks Message Authen7ca7on (§8.2- 8.3, §8.4.2- 8.4.3) Computer Science & Engineering Goal and Threat Model •  Goal is to let Bob verify the message came from Alice and is unchanged –  This is called integrity/authen7city •  Threat is Trudy will tamper with messages –  Trudy is an ac7ve adversary (interferes) I♥networks Alice Computer Networks ???? Trudy Bob 20 10 12/3/13 Encryp7on Issues •  What will happen if Trudy flips some of Alice’s message bits? –  Bob will receive an altered message Um?? yuiE#[email protected] Bob Trudy Computer Networks 21 Encryp7on Issues •  What if Trudy reorders message? –  Bob will receive altered message OK! BUY NOW! DO NOT STOP 4 5 2 3 1 Trudy Bob –  Should have been (Woops) •  “STOP DO NOT BUY NOW” Computer Networks 22 11 12/3/13 Message Digest or Cryptographic Hash •  Digest/Hash is a secure checksum –  Determinis7cally mangles bits to pseudo- random output (like CRC) –  Can’t find messages with same hash –  Acts as a fixed- length descriptor of message – very useful! I might be a...
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