[EN] Symmetric Algorithm - feistel vs SPN

☝Symmetric-Key Block Cipher

Designed to satisfy properties such as diffusion that hide the relationship between plaintext and ciphertext and confusions that hide the relationship between ciphertext and keys.

P-box(Permutation box) 

1. Straight P-box :  n=m

2. Expansion P-box :  n<m

3.Compression P-box :  n>m

(n-number of input bit // m-number of output bit)

(p-box is used for expansion of number of bits for many cases)

(no inverse function exists)

S-box(Substitution box)

n=m or n<>m(inverse function could be possibly existed or not)

1. shift

2. swap

3. split

4. combine

5. exclusive-OR

☝Product Cipher 
Product cipher is combine of the components of a p-box and s-box in order to cover two properties(Diffusion + Confusion).

1. Feistel Cipher 

There's two set : ant sets(working), grasshopper set(resting).
Only doing XOR and take it to swap is what grasshopper set is doing,
Taking half bits into Function and doing encryption is set ants.
BUT this set will be cross and sent it to next round.

(process of Encryption = process of Decryption )
DES,SEED(korean encryption standard)
ROUND>=3 + even number

Considerations for implementing symmetric block passwords

Block size : more than 128bit
Key size : 128bit
number of rounds : more than 16
Subkey Generation Algorithm
Round Function

2. SPN NETWORK 

(Substitution-Permutation Network)

After dividing the input value into blocks + processing the S-box.

Take it to permutation(p-box) process with processed S-box.(process of Encryption = process of Decryption )
AES,ARIA
*available for Parallel processing

💁Possible attack for Block Cipher

1. differential Cryptanalysis

This type of attack comes from the idea which is how could CipherText would be distinguished if you changed a part of the PlainText "CPA ATTACK"
CPA = chosen plaintext attack

2. Linear Cryptanalysis

Investigate the probability that several revisions of the plaintext and ciphertext bits of result which is zero, after XOR progress (Masui)  "KPA ATTACK"
KPA = known plaintext attack
After XOR process, probability for 0 will be 1/2

Investigating bits which is not in this 1/2 probability can help to get information about key.

Which means 

For linear detoxification, if you have found an approximate linear relationship,
This allows the transition from KPA to CPA.

3. exhaustive key search

4. statistical analysis

5. mathematical analysis

Stream Cipher

1. Synchronized stream cipher  

The key stream is independent of the plain text or cipher text.
Plaintext or ciphertext between key bits are irrelevant at all

ex)

👀 OTP(one time pad) 

Mathematically proofed that it is impossible to decipher by Shannon.
Literally a single-use pad, and every time you do an encoding, you're going to be able to create a random keystroke.
Unconditionally safe and theoretically unreadable.
Cryptographic and Decryption algorithms use Expensive ORs (reversely related to each other).

👀FSR(Feedback Shift Register) 

👀LFSR(Linear Feedback Shift Register) 

👀NLFSR(Non-Linear Feedback Shift Register) 

2. Asynchronized stream cipher 

The key stream is dependent on the plaintext or ciphertext.
This one method used to create different operating modes in block cipher which is
 cipher feedback mode (CFB), actually generates stream passwords.

✊Special features of strong stream cipher

1. No repeatable value inside the keystream value for a long period of time (pseudorandom) 
2. Key stream is not linearly related to key. >(Finding the key stream value does not mean knowing the key value)






✊Considering the design of Stream Cipher

1. The continuation of Encryption should have a long cycle.
The pseudo-random number generator will eventually use a function that produces a bit stream that is repeated one day.
The longer iteration cycles mean that deciphering the code is as difficult.

2. Key stream is as similar as pseudo-random numbers.


3. You must be long enough to respond to an enterprise-wide attack.