02_symmetriccrypto - 1 Symmetric Encryption Peter Sjödin...

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Unformatted text preview: 1 Symmetric Encryption Peter Sjödin psj@kth.se 2 Acknowledgements • Many people have contributed to the course material • Former teachers – Alberto Escudero Pascal, Jan-Olov Vatn, Björn Knutsson • We are particularly thankful to Prof. Vitaly Shmatikov, The Univ of Texas at Austin, for letting us use his material 3 Outline • Symmetric encryption – Basics • Modes of operation – How to deal with large pieces of data 4 Symmetric Encryption • Also known as: – Shared key encryption – Secret key encryption • Same key for encryption and decryption • Sender and receiver need to agree on a key ----- ----- ----- Plaintext input Encryption Decryption ----- ----- ----- Plaintext output Ciphertext Shared, secret key 5 One-Time Pad = 10111101… ----- ----- ----- = 00110010… 10001111… ⊕ 00110010… = ⊕ 10111101… Key is a random bit sequence as long as the plaintext Encrypt by bitwise XOR of plaintext and key: ciphertext = plaintext ⊕ key Decrypt by bitwise XOR of ciphertext and key: ciphertext ⊕ key = (plaintext ⊕ key) ⊕ key = plaintext ⊕ (key ⊕ key) = plaintext 6 Advantages of One-Time Pad • Easy to compute – Encryption and decryption are the same operation – Bitwise XOR is very cheap to compute • As secure as theoretically possible – Given a ciphertext, all plaintexts are equally likely, regardless of attacker’s computational resources – “Cipher achieves perfect secrecy if and only if there are as many possible keys as possible plaintexts, and every key is equally likely” (Claude Shannon) – …as long as the key sequence is truly random • True randomness is expensive to obtain in large quantities – …as long as each key is same length as plaintext • But how does the sender communicate the key to receiver? 7 Problems with One-Time Pad • Key must be as long as plaintext – Impractical in most realistic scenarios – Still used for diplomatic and intelligence traffic...
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This note was uploaded on 02/16/2011 for the course ICT 2 taught by Professor 2 during the Spring '11 term at Kungliga Tekniska högskolan.

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02_symmetriccrypto - 1 Symmetric Encryption Peter Sjödin...

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