Flashcards
Blockciphers & Modes of Operation
Cards in this deck (21)
- 1Chosen plaintext attack?
- 2What is exhaustive search?Goes through all keys in the keyspace
- 3What is indistinguishability?
- 4What is the birthday bound?In the ideal world we pick a random ciphertext for each new message and at some point we'll receive the same one which is a collision: M /=/ M' but E(M) = E(M')
Expected number of collisions is q^2/|C| - 5What is nonce-based encryption?Splitting up to blockciphers where some are the same leaks patterns. It's encryption not enciphering so M /=/ C
Enc takes K,N & M, Dec takes N,C & K - 6Nonce-based indistinguishability?No repeat nonces but can choose nonce and message. In an ideal world random ciphertexts need to have the correct length.
- 7What is CTR mode?Generate a large & unique keystream that's the same amount of blocks as the message. For the ith block of the keystream combined with i encoded with nonce. The nonce is l/2 bits so is the counter.
- 8What is ECB mode?
- 9Advantages of CTR?Paralizable, Ek(X[i]) can be evaluated before result of E(X[i - 1])
Can precompute keystream before the message is known
Deciphering shares enciphering function so they can share circuitry - 10What are reductions?If X is A secure it's B secure
If X is B insecure it's A insecure
If there's a successful B adversary against X there's a successful A adversary against X - 11Reduction for kr-1kca to ow-pas?B must simulate A's one time oracle and turn key guess into a message guess. If A recovers challenge key, B can use it to decrypt
- 12What is CBC mode?
- 13What does lg|Perm(M)| represent?Amount of bits to represent drawing uniformally at random
- 14How to show probability of guessing?2^-lg|K|
- 15How to create an indistinguishable adversary attack?Try to get somethhing in the real world that you wouldn't get in the ideal world
- 16How many bits to defeat exhaustive search?Near 128
- 17What's diffusion?That a small change in the message should result in a large change in ciphertext so find examples based on i
- 18Good way of showing Shannon's Theorem?Like + show that it's a group and therefore invertible
- 19What is ow-cpa?The same as ow-pas but it has it's own oracle it. The adversary has control over the oracle and can choose the N,M pair to get the correct ciphertext under the key we chose.
Then, since the nonce we used is universally available, he can get the correct ciphertext. Then using the oracle he can get the real M.
- 20What is CFB?Cipher Feedback Mode
- 21What is OFB ?Output Feedback Mode
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- Cryptography Flashcards on Blockciphers & Modes of Operation, created by Philip Masters on 05/01/2020.
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- Philip Masters
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