Cryptography - SM4 Encryption Algorithm

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The SM4 method is the first commercial block cipher algorithm, published by China in 2006 under the name SMS4. The SM4 method is used in the Chinese National Standard for Wireless LAN WAPI (Wired Authentication and Privacy Infrastructure). And the WAPI became China's mandatory national standard.

From the standpoint of security and communication performance, WAPI is an excellent solution for communication infrastructure. This work uses the SM4 algorithm in an Ethernet encryption system.

Structure of SM4

The SM4 algorithm represents a standard Feistel structure. The input, output, and key of the SM4 algorithm are 128 bits. And they are divided into four sections, each of which has 32 bits known as words. The SM4 algorithm consists of 32 identical rounds. The structure for encryption and decryption is the same. The only operations implemented are XOR, circular shifts, and S-box applications. The keys used for decryption are identical to those used for encryption, but in reverse order.

The SM4 method is easily implemented in both software and hardware. It also has a wide range of applications.

A fresh f8 key stream generator that uses SM4 as its fundamental algorithm can be safely used in a 3G communication system. The SM4 algorithm is co-designed in hardware and software in an embedded processor of the MicroBlaze soft core to enable flexibility when designing cryptographic communication equipment. The SMS4 algorithm is also devised and implemented on Java cards.

Keys and key parameters

Encryption keys have a length of 128 bits and are expressed as MK=(MK₀, MK₁, MK₂, MK₃), where MK_i(i=0, 1, 2, 3)} is a 32-bit word. The round keys are represented by (rk₀, rk₁,...., rk₃₁), with each rk_i (i=0,..., 31)} representing a word. It is generated using the encryption key and the following parameters −

• FK=(FK₀, FK₁, FK₂, FK₃)
• CK=(CK₀, CK₁,..., CK₃₁)

FK_i and CK_i are words used to generate the round keys.

Round

Each round is computed based on the four previous round outputs. X_i,X_i+1,X_i+2,X_i+3 Such that −

X_i+4 = X_i⊕ F(X_i+1oplus X_{i + 2}⊕ X_{i + 3}⊕ rk_i), Where F is a substitution function composed of a non-linear transform, the S-box, and linear transform L.

S-box

S-box is fixed for 8-bit input and output, denoted as Sbox(). The S-box, like AES, uses the multiplicative inverse over GF(28). The affine transforms and polynomial bases differ from those of AES, however due to affine isomorphism, they can be calculated quickly using an AES Rijndael S-box.

How it Works?

The SM4 algorithm scrambles data, makes it impossible to understand without a special key. It operates with 128-bit blocks, and the key used to scramble and unscramble the data is also 128 bits. These blocks and keys are divided into four sections, each with 32 bits.

The method scrambles the information by mixing and modifying bits in 32 cycles. This process is the same for scrambling and unscrambling, but the order of the stages varies for unscrambling.

The method only does three things: combine bits using XOR (similar to a digital "either-or" operation), shift them in circles, and use a particular function known as an S-box.

Here are some key terms −

• Words and bytes − They are just terms used to describe clusters of bits.
• S-box − It works similarly to a secret code, converting one set of bits into another.
• Fundamental operations − These are the algorithm's core functions, such as combining and moving bits.

The scrambling key, or MK, is made up of four components. Another type of key, known as round keys, is made from MK and used throughout the scrambling process. These keys help to mix up the bits.

The round function is the algorithm's main component. It scrambles 32 bits at a time, utilising the key for each round.

After scrambling the information, we can reverse the process to unscramble it.

Below is a brief explanation of how it works −

• We begin with plain text (the information we want to protect).
• We divided it into blocks of 128 bits.
• We scramble each block 32 times using the key and several more methods.
• We get scrambled text as output.
• To read the text, we use the same steps but in reverse order.

Example

Consider we have a simple message that we want to encrypt with the SM4 method. The message is "HELLO", and the secret key is "KEY1234567890123". This is how to encrypt it using SM4.

Convert the message and key into binary −

HELLO becomes

KEY1234567890123 becomes

Separate the message and key into blocks −

Message blocks (128 bits) −

Block 1: 01001000 01000101 01001100

Block 2: 01001111 (padded with zeros to produce 128 bits)

Each key block contains 128 bits.

• Block 1: 01001011 01000101 01011001 00110001.
• Block 2: 00110010 00110011 00110100 00110101.
• Block 3: 00110110 00110111 00111000 00111001.
• Block 4: 00110000 00110001, 00110010, 00110011.

Encrypt each block − Apply the encryption process to each message block, using the round keys received from the key blocks.

Combine the encrypted blocks − Combine the encrypted blocks to form the final encrypted message.

Send an encrypted message − The encrypted communication is now ready to be transmitted across the network or stored securely.