4-Modes-Operations-RC4

4-Modes-Operations-RC4 - Block Cipher Modes of Operation...

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Block Cipher Modes of Operation and Stream Ciphers CSE 651: Introduction to Network Security
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Abstract We will discuss How to use block ciphers? RC4: a widely used stream cipher Problems with WEP’s use of RC4 2
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Modes of Operations
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How to use a block cipher? Block ciphers encrypt fixed-size blocks e.g. DES encrypts 64-bit blocks We need some way to encrypt a message of arbitrary length e.g. a message of 1000 bytes NIST defines several ways to do it called modes of operation 4
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Five Modes of Operation Electronic codebook mode (ECB) Cipher block chaining mode (CBC) – most popular Output feedback mode (OFB) Cipher feedback mode (CFB) Counter mode (CTR) 5
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Message Padding The plaintext message is broken into blocks, P 1 , P 2 , P 3 , . .. The last block may be short of a whole block and needs padding. Possible padding: Known non-data values (e.g. nulls) Or a number indicating the size of the pad Or a number indicating the size of the plaintext The last two schemes may require an extra block. 6
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Electronic Code Book (ECB) The plaintext is broken into blocks, P 1 , P 2 , P 3 , . .. Each block is encrypted independently: C i = E K (P i ) For a given key, this mode behaves like we have a gigantic codebook, in which each plaintext block has an entry, hence the name Electronic Code Book 7
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Remarks on ECB Strength: it’s simple. Weakness: Repetitive information contained in the plaintext may show in the ciphertext, if aligned with blocks. If the same message (e.g., an SSN) is encrypted (with the same key) and sent twice, their ciphertexts are the same. Typical application: secure transmission of short pieces of information (e.g. a temporary encryption key) 8
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( 29 ( 29 1 2 3 1 The plaintext is broken into blocks: , , , . .. Each plaintext block is XORed chained with the previous ciphertext block before encryption (hence the name): E i K i i P P P C C P - = ( 29 0 1 IV Use an Initial Vector IV to start the process. Decryption :
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4-Modes-Operations-RC4 - Block Cipher Modes of Operation...

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