March R. Smith - University of St Thomas - Minnesota QMCS Class Today Finish the other presentationFinish the other presentation Cipher ModesCipher Modes

March R. Smith - University of St Thomas - Minnesota “Straight” Crypto Just apply the key to the plaintextJust apply the key to the plaintext Block after block after blockBlock after block after block

March R. Smith - University of St Thomas - Minnesota Penguin using Straight Crypto BEFORE:AFTER:

March R. Smith - University of St Thomas - Minnesota What We Want BEFORE:AFTER:

March R. Smith - University of St Thomas - Minnesota One Approach: CBC Kind of Rube Goldberg-ishKind of Rube Goldberg-ish Each block of plaintext is mixed with the previous block of ciphertext before encryptionEach block of plaintext is mixed with the previous block of ciphertext before encryption The “initialization vector”The “initialization vector”

March R. Smith - University of St Thomas - Minnesota CBC Decryption Start with the initialization vector (IV)Start with the initialization vector (IV) XOR with decrypted ciphertext to yield plaintextXOR with decrypted ciphertext to yield plaintext “Error extension” - how do errors propagate?“Error extension” - how do errors propagate? Can we “mix and match” blocks?Can we “mix and match” blocks?

March R. Smith - University of St Thomas - Minnesota Another Idea: Autokey (OFB) The key stream is independent of the data streamThe key stream is independent of the data stream Sort of like a ‘stream cipher’ - can work bit by bitSort of like a ‘stream cipher’ - can work bit by bit Requires an IV to start things upRequires an IV to start things up

March R. Smith - University of St Thomas - Minnesota OFB Decryption Basically identical to the encryption operationBasically identical to the encryption operation Start with the initialization vector (IV)Start with the initialization vector (IV) Generates the exact same key streamGenerates the exact same key stream

March R. Smith - University of St Thomas - Minnesota Yet Another: Counter Mode (CTR) Like OFB, but uses a counter instead of chainingLike OFB, but uses a counter instead of chaining “Nonce” is a random data value; counter increments“Nonce” is a random data value; counter increments Like OFB, simple XOR to encryptLike OFB, simple XOR to encrypt

March R. Smith - University of St Thomas - Minnesota CTR Decryption Basically identical to the encryption operationBasically identical to the encryption operation Start with the initialization vector (IV)Start with the initialization vector (IV) Generates the exact same key streamGenerates the exact same key stream

March R. Smith - University of St Thomas - Minnesota In Class group exercise Four groups, 4 problemsFour groups, 4 problems 1.What if you swap two blocks in CBC? 2.What if the ciphertext flips 1 bit in CBC? 3.What if you swap two blocks in CTR? 4.What if the ciphertext flips 1 bit in CTR?

March R. Smith - University of St Thomas - Minnesota That’s it Questions?Questions? The images all came from the Wikipedia entry on Block Cipher Modes The Penguin image was produced by and The GIMP if someone asks. Creative Commons License This work is licensed under the Creative Commons Attribution-Share Alike 3.0 United States License. To view a copy of this license, visit or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA.