3 - Modern Symmetric Cryptography Flashcards
Confusion ( in terms of ciphertext and key)
Hiding the relation between ciphertext and key
Diffusion
Spreading the info; any small change in plaintext should produce a large change in ciphertext.
Each input bit affects all output bits
AES typical block length
64-256bits
When should you use smaller vs larger keys.
Hint: think about data uses
Smaller keys can be used for volatile data. ie, data that will be useless by the time it is broken
4 benefits of modern symmetric block ciphers
Easy to analyse
Speed
Ease of implementation
Low memory/power requirements
DES
Data Encryption Standard
DES bits
Blocks of 64 bits, key of 56bits (or even 40bits)
Triple DES
Encrypt using one key, then using another then using the first again
AES
Advanced Encryption Standard
AES’s original name was…
Rijndael cipher.
J Daemen and V Rijmen, Belgium
AES block length
128 bits
AES key bits
Variable key length: 128, 192, 256
AES substitution permutation network
Several rounds, each round consists of combination of substitutions, permutations and XORing with the round key
AES blocks are arranged in..
a 4x4 array of bytes called the state.
Successive bytes are written column-wise in the array
How many rounds of encryption for 128bit, 192 and 256bit keys?
10 - 128
12 - 192
14 - 256
AES 1. Substitute bytes
Take each byte and substitute using an S-box.
This substitution table is fixed in the standard. NOT part of the key.
AES 2. Shift Rows
Shift each row circularly to the left
row 0 by 0 bytes
row 1 by 1 bytes
row 2 by 2 bytes
row 3 by 3 bytes
…
AES 3. Mix Columns
Each byte is replaced by a new value which depends on the 4 bytes in the column.
(“How is this done?” Is optional.)
AES 4. Add Round key
Bitwise XOR between the current state and the key of this round
AES Round keys are generated from…
Generated from the original key
Mode of operation list
- ECB (Electronic codebook)
- CBC (Cipher Block Chaining)
- CTR (Counter Mode)
- GCM (Galois/counter mode)
- OFB (Output feedback), CFB (Cipher feedback) etc
ECB
Electronic Codebook
Do not do it.
- Each block is encoded independently using the same key
- Used for short mesages
- Vulnerable in long messages where blocks may be repeated.
Should you just encrypt each block sequentially
No,
Blocks will have the same data in some cases and then it is eassier to determine decryption
CBC
Cipher BlockChain
Each plaintext before encryption is XORed with the last ciphertext (or the initialisation vector)
CTR
Counter mode
Transform a block cipher into a stream.
Start with some 128bit(etc) counter value and then encrypt to make a key stream. More needed? +1 counter and repeat.
