DES Flashcards
How big is the key in DES?
64 bit, 56 bits are used in the encryption process and the other 8 are parity bits.
Is DES considered secure by today’s standards?
No, as there are only 2 ^ 56 possible keys, this can be brute forced in 2 days
How big are the blocks used in DES?
64 bit
Give an overview of what happens in DES encryption
Initial permutation
Block is split into 2 32 bit parts
16 Rounds of encryption
Final permutation (inverse of the initial permutation)
What happens during the 16 rounds in DES?
i starts at 1 and ends at 15. Each round 4 bits are taken. Ri-1 becomes Li. Li-1 is XOR’d with the output of the feistel function. R16 is taken as L16 and vice versa, without modifying the bits.
What is the feistel function in DES?
Expand the right 32 bit block to 48 bits by doubling 16 bits and permuting them. XOR this 48 bit block with the round key. Split this new 48 bit block into 8 6 bit blocks. Each of the 6 bit blocks is but through the corresponding substitution box, which outputs a 4 bit block. The 8 4 bit blocks are combined together then another permutation is applied.
How does the key schedule in DES work?
Each round uses a different 48 bit round key. The 56 bit encryption key is split into 2 halves, left and right. Both halves are rotated left by one or two bits, depending on the round number. Then 24 bits are selected from each half by permuted choice 2 (permuted choice 1 gets the 56 bits from the 64 bit key)
How does decryption work in DES?
Decryption works the same as encryption, but the round numbers run from 16 to 1.
Why is running DES twice not that much more secure than running it once?
Despite the doubled key size, if you have a valid plaintext-ciphertext pair it only takes 2 ^ 57 iterations to brute force:
- Run DES encryption on the plaintext using every possible key and store all of the results.
- Try decrypting the ciphertext using every possible key, each time checking if the result matches one of the ciphertexts from the first round.
What is 3DES?
3DES is running DES three times, using three different 56 bit keys. The first run is encryption, then decryption, then encryption again.
How many iterations does it take to brute force 3DES?
2 ^ 118 as meet in the middle attacks between the different runs of DES are still possible
