QMCS 490 - Class Today

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QMCS 490 - Class Today

• Finish the other presentation
• Cipher Modes

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Straight Crypto

• Just apply the key to the plaintext
• Block after block after block

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Penguin using Straight Crypto

• BEFORE

• AFTER

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What We Want

• BEFORE

• AFTER

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One Approach CBC

• Kind of Rube Goldberg-ish
• Each block of plaintext is mixed with the
  previous block of ciphertext before encryption
• The initialization vector

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CBC Decryption

• Start with the initialization vector (IV)
• XOR with decrypted ciphertext to yield plaintext
• Error extension - how do errors propagate?
• Can we mix and match blocks?

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Another Idea Autokey (OFB)

• The key stream is independent of the data stream
• Sort of like a stream cipher - can work bit by
  bit
• Requires an IV to start things up

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OFB Decryption

• Basically identical to the encryption operation
• Start with the initialization vector (IV)
• Generates the exact same key stream

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Yet Another Counter Mode (CTR)

• Like OFB, but uses a counter instead of chaining
• Nonce is a random data value counter
  increments
• Like OFB, simple XOR to encrypt

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CTR Decryption

• Basically identical to the encryption operation
• Start with the initialization vector (IV)
• Generates the exact same key stream

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In Class group exercise

• Four groups, 4 problems
• What if you swap two blocks in CBC?
• What if the ciphertext flips 1 bit in CBC?
• What if you swap two blocks in CTR?
• What if the ciphertext flips 1 bit in CTR?

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Thats it

• Questions?
• The images all came from the Wikipedia entry on
  Block Cipher Modes
• The Penguin image was produced by
  lewing_at_isc.tamu.edu and The GIMP if someone asks.
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