[EN] DES, 3DES, AES

☝DES(Data Encryption Standard)

1. Purpose : data encryption standard

2. method : substitution + permutation 

3. Number of Round : 16회전( for each P-box,S-box )

4. Reason to be crack : Key is way too short

5. key size : 56bit 

==> modified structure of feistel network 

Plaintext size : 64bit // key size : 56bit // round : 16

Components of DES : 2 of permutation box (Initial permutation, Final permutation)

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16 of feistel round function

Each round key will be newly generated 48 bit key with Round-key generator ( with initial 56 bit cipher key )

<<How each round function works>>

components of DES function

1. Expansion P-box 

2. key XOR

3. 8 different S-boxs

4. Straight P-box

quick explanation : input 64 bit (block) will be divided into 32 bits as L,R
With Expansion permutation, 32 bit is changed into 48 bit.
Then proceed Exclusice or with reformed 48 key bit.
After Exclusive or, S-box is going to do it's work as changing the set of output bit with input bit. 
8*6=48bit ==> 8*4=32bit 
FInal step is straight permutation.
This is one round

For Decryption of DES, round key (K1~K16) should be applied reverse order

Analysis of DES fucntion

1. S-BOX :   Designed to satisfy confusion from each round to the next.

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Non-Linear Function

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One bit change in input will be result in 2 bits change in output 

2. P-BOX : Using Expansion permutation for diffusion

weak point of DES

🙅DES has a very large avalanche effect where small changes in the plain or key make a big difference in the cryptogram. Also, it is very difficult to infer a plain sentence from a cryptogram because each bit of a cryptogram is proven to be complete and dependent on many bits of the plain.

2DES is vulnerable for

=> meet in the middle attack

man in the middle attack

☝3DES

1. DES-EDE2

2. DES-EDE3

DES-EDE2 

PlainText==> Encryption==>Decryption==>Encryption==>CipherText

                                 K1                    K2                K1                        

if  K2=K1 then DES-EDE2 could be compatible with 3DES

DES-EDE3

PlainText==> Encryption==>Decryption==>Encryption==>CipherText

                                   K1                    K2                K3                        

For DES-EDE3 is used by applications such as PGP 

difference between DES-EDE2, DES-EDE3: number of KEY 

NUMBER OF ROUND FOR DES = 16

NUMBER OF ROUND FOR 3DES = 16*3 = 48

velocity comparison

3DES<DES<AES<RC4(stream cipher)

☝AES(Advanced Encryption Standard)

non-feistel algorithm 

==>using SPN Network 

Rijndael 

Input bit : 128 bit 

number of round : 10/12/14

key size : 128/192/256

AES-128

AES-192

AES-256

components of AES ROUND 1 

1. substitution bytes 

2. ShiftRows  

3. MixColumns : *excludes last round

4. AddRoundKey 

✋Some cipher methods that follows SPN algorithm 

ex) SAFER, SHARK, SQUARE, CRYPTON RIJNDAEL, SAFER+,PRESENT, SERPENT