Net Security Note 03 DES Overview

DES overview

• Block encryption algorithm: plaintext, ciphertext 64-bit packet length
• Symmetric Algorithms: Encryption and decryption key arrangements are different, but use the same algorithm
• Key length: 56 bits - each 8th bit is a parity bit
• The key is any 56-digit number, and there is a weak key , which is easy to avoid
• A combination of Chaos and Diffusion, each combination replaces first and then replaces , 16 rounds
• Use only standard arithmetic and logic operations, easy to time

Other encryption algorithms

Triple DES Triple DES
IDEA
RC5
RC6
AES
Some other more practical algorithms, such as Blowfish, CAST, and RC2, etc.

encryption process

1. Input 64bit plaintext data
2. Initial Replacement $IP$
3. 16 iterations under key control
4. Swap left and right 32bit
5. Initial inverse permutation $I{P}^{-1}$

$$L_0{R}_0\leftarrow IP(64bit输入法)$$
$$L_i\leftarrow R_{i-1}$$
$$L_i\leftarrow L_i\oplus f(R_{i-1},k_1)$$
$$64bitciphertext\_\leftarrow I{P}^{-1}(R_{16}{L}_{16})$$

i is the number of iterations, $\oplus$ is the bitwise mod2 summation, f is the encryption function

In this way, the decryption process is launched

$$(R_{16}{L}_{16})\leftarrow IP(64bit密文)$$
$$R_{i-1}\leftarrow L_i$$
$$L_i\leftarrow R_{i-1}\oplus f(L_{i-1},k_1)$$
$$64bitplaintext\_\leftarrow I{P}^{-1}(L_0{R}_0)$$

replace, extend, replace

Initial Replacement $IPand$ initial inverse permutation$I{P}^{-1}$

The initial replacement IP indicates that the original data at the 58th position (8 rows and 2 columns) is placed in the first row and the first position

Extended permutation E combination - 32 extended to 48 bits

Divide 32 bits into 4 columns

1. The original first column --> columns 2 and 6
2. The original 4th column --> the new 5th column and the first column, note that they have moved one bit forward relative to the row

Compression instead of S-box - 48 bits compressed to 32 bits

Find a box with 4 rows and 16 columns

48 bits become 6-bit binary
$$a_1{a}_2{a}_3{a}_4{a}_5{a}_6$$
Take the first and last digits to form
$a_1{a}_6$As a row , the remainder is $a_2{a}_3{a}_4{a}_5$As a column selection, get the corresponding 32-bit

Security Analysis

1. DES other algorithms linear, S-box nonlinear
2. S-boxes are not easy to analyze, providing better security
3. S-box is the key

How to construct an S-box

1. The only determines the security strength of the algorithm
2. The obfuscation effect necessary for cryptographic algorithms
3. S- and cipher strength, S-box grouping design
4. Nonlinearity, differential uniformity, strict avalanche, reversibility, no trapdoor

Construction of permutation p-boxes

Provide avalanche effect
plaintext, key changes cause ciphertext changes

key

Key Replacement Algorithm Guidelines

1. Subkey statistical independence, flexibility
2. Simple
3. speed
4. But there is no simple relationship (given two seed keys with some relationship, the relationship between their wheel keys can be predicted)
5. The seed key has the same effect on all subkeys
6. It is more difficult for subkeys to obtain other subkey bits
7. no weak keys

Operating mode

1. ECB (electronic codebook mode) ECB (electronic codebook mode)
2. Cipher block chaining CBC (cipher block chaining)
3. Cipher feedback CFB (cipher feedback)
4. Output feedback OFB (output feedback)

ECB

Each plaintext is encrypted separately

$$C_i=E_K(P_i)P_i=D_K(C_i)$$

1. simple and effective
2. parallelizable
3. Mode information cannot be hidden from plaintext
4. The same plaintext generates the same ciphertext, and the same information appears multiple times causing leakage
5. Active attack on plaintext - replace, retake, delete, replay
6. Error transmission: bad ciphertext -> corresponding plaintext damage
7. Send SMS

CBC

$$C_i=E_K(P_i\oplus C_{i-1})P_i=D_K\left(C_i\right)\oplus C_{i-1}$$

1. There is no known parallel implementation of the algorithm
2. Mode information can be hidden

• Common Initialization Vector $IV$
• Same plaintext generates different ciphertext
• The initialization vector IV can be used to change the first block

3. Hard to actively attack plaintext

• Information blocks cannot be easily replaced, rearranged, deleted, or retransmitted
• Error propagation: bad ciphertext -> two bad files

4. Safer than ECB
5. Suitable for transmitting messages with a length greater than 64 bits , and can also perform user authentication – SSL or IPSec

CFB

$block\; cipher\Rightarrow stream\; cipher$

assumed

1. $S_i$Is a shift register, transfer jbit
2. Encrypt $C_i=P_i\oplus E_k(S_i)的高j位$
   $$S_{i+1}=(S_i<<j)|C_i$$
3. Decrypt $P_i=C_i\oplus E_k\left(S_i\right)的高j位$
   $$S_{i+1}=(S_i<<j)|C_i$$

features

1. $block\; cipher\Rightarrow stream\; cipher$
2. Algorithms without parallel implementation
3. hidden plaintext
4. Common Shift Register Initial Value $IV——$ different information, unique IV
5. Error Propagation: One Unit Damage Affects Multiple Units

OFB

$block\; cipher\Rightarrow stream\; cipher$

Assume the same as CFB
but encrypt as follows
$C_i=P_i\oplus E_k(S_i)的高j位$
$$S_{i+1}=(S_i<<j)|(E_k\left(S_i\right)highjbit)$$
(S change is different from CFB)

Decrypt
$P_i=C_i\oplus E_k\left(S_i\right)的高j位$
$$S_{i+1}=(S_i<<j)|(E_k\left(S_i\right)highjbits)$$

Features
The first four points are the same as CFB

1. $block\; cipher\Rightarrow stream\; cipher$
2. Algorithms without parallel implementation
3. hidden plaintext
4. Common Shift Register Initial Value $IV——$ different information, unique IV

start at fifth

5. Error Propagation: Damage to one unit only affects the corresponding unit
6. Active attacks on plaintext are possible – replaced, rearranged, deleted, replayed
7. Not as safe as CFB

Multiple DES

safety

1. F-function (S-Box) design principle unknown
2. The key length debate
3. Deciphering of DES
4. Weak and semi-weak keys

Length problem
$2^{56}\approx 10^{17}$

Keys can now be cracked in a day

weak key, semi-weak key

Weak key:
$E_K\cdot E_K=IDES$
has 4 weak keys
$$p=E_k(E_k(P))$$

Semi-weak key:
$E_K=E_{K2}$
There are at least 12 semi-weak keys
$$C=I{k}_1(P)=E_{k2}\left(P\right)$$

Use conventional encryption for confidential communication

Targeting vulnerable locations
with link encryption and end-to-end encryption

Link Layer Encryption

for a communication link

The message is encrypted before being transmitted , and the received message is decrypted at each node , and then the key of the next link is used to cancel the key of a link to encrypt the message before transmission

advantage

• All intermediate transmission node messages are decrypted and re-encrypted
• The data on the link appears in ciphertext
• Mask the source and destination of the message
• The use of padding technology->The message frequency and length are covered to prevent the analysis of communication services

shortcoming

• It is necessary to synchronize the encryption devices at both ends of the link, which will bring side effects to network performance and manageability
• In the network nodes, link encryption only provides security on the communication link, and the node is plaintext——needs physical security of the node
• Symmetric encryption system needs to establish a dedicated network, but the distribution of network nodes is wide , and the continuous distribution of keys is difficult

Node Encryption

Similar to link encryption. Intermediate node decryption and re-encryption

Difference: Messages are not allowed to exist in plain text at the node , and after decryption, they are encrypted with another key——in another security module of the node

Security: Header and routing information are transmitted in clear text, powerless for attackers to analyze communication traffic

end-to-end encryption

Always exist in ciphertext during transmission from source to destination

Also known as off-line encrypted packet encryption, it will not be decrypted until the end. Node corruption will not result in message leaks

advantage

• Cheaper, more reliable, easier to design, implement, and maintain
• Avoid synchronization problems - packets are independently encrypted, and packet transmission errors do not affect subsequent
• End-to-end encryption is more natural and does not affect other users of the network

shortcoming

1. Encryption of the destination address is not allowed
2. Does not cover up the source and destination of the message, and is powerless to the attacker to analyze the communication business