IDEA encryption and decryption 04/26 Update SLTechnology News&Howtos

IDEA encryption and decryption

2025-04-26 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > Network Security >

Shulou(Shulou.com)06/01 Report--

I. introduction to idea algorithm

IDEA (International Data Encryption Alogrithm) was proposed by XueJiaLai and James L.Massey of the Federal Polytechnic University of Zurich in Switzerland in 1991. IDEA uses a 128-bit key, and the whole algorithm is similar to DES, which divides plaintext into 64-bit data packets, and then obtains 64-bit ciphertext after several iterations and a transformation.

IDEA maps two 16-bit values to a 16-bit value. These operations are:

● semi-addition operation, or "XOR" operation, is represented by the symbol "⊕". The so-called semi-addition operation is to add only, not carry, when performing binary operations.

The addition operation (i.e. mod 65536) of ● module 216is denoted by "+".

The multiplication of ● module 2161is denoted by the symbol "⊙".

In fact, ⊙ is the result of multiplying two input numbers and then calculating the result according to module 216. For such an operation, it should be noted that any binary data involved in the operation, if all 0, is represented by nasty 1-bit data, with the highest bit being 1, and the rest are all 0.

In order to understand the above three operations, we use 2-digit numbers to represent the above three relationships, as shown in Table 2-3-1.

Table 2-3-1 the relationship among the three IDEA operations

X ⊙ Y

X ⊕ Y

Decimal system

Binary system

Decimal system

Binary system

Decimal system

Binary system

Decimal system

Binary system

Decimal system

Binary system

one

two

ten

two

ten

three

eleven

two

ten

three

eleven

three

eleven

two

ten

three

eleven

one

two

ten

one

two

ten

three

eleven

two

ten

three

eleven

one

three

eleven

three

eleven

two

ten

two

ten

two

ten

three

eleven

two

ten

two

ten

one

three

eleven

two

ten

three

eleven

two

ten

two

ten

two

ten

three

eleven

one

three

eleven

three

eleven

two

ten

three

eleven

three

eleven

one

three

eleven

two

ten

three

eleven

two

ten

one

three

eleven

three

eleven

two

ten

2. Idea algorithm encryption process

1.IDEA iterative process

The IDEA encryption algorithm takes eight iterations, as shown in figure 2-3-1:

Figure 2-3-18 iterations

The data generated by the 64-bit key is divided into 8 sub-blocks, each with 16 bits. Each iteration is shown in figure 2-3-2:

Figure 2-3-2 single iterative process

⊕ represents the XOR operation, the addition operation of module 216and the multiplication operation of module 2161in the illustration diagram.

As the input of the first iteration, X1 and X2, X3 and X4, each iteration is an XOR operation between 4 sub-blocks and 16-bit subkeys, and module 216 does addition operation and module (216) multiplication operation.

The iterative steps are as follows:

(1) multiply X1 and the first sub-key block.

(2) X2 and the second subkey block do the addition operation.

(3) X3 and the third subkey block do addition operation.

(4) multiplication is performed between X4 and the fourth subkey block.

(5) the results of (1) and (3) are calculated by XOR.

The results of (6) (2) and (4) are calculated by XOR.

The result of (7) (5) and the fifth sub-key block are multiplied.

The results of (8) (6) and (7) are added.

The result of (9) (8) is multiplied with the sixth sub-key block.

The results of (10) (7) and (9) are added.

The results of (11) (1) and (9) are calculated by XOR.

XOR operations are performed on the results of (12) (3) and (9).

XOR operations are performed on the results of (13) (2) and (10).

XOR operations are performed on the results of (14) (4) and (10).

Each round completed the above 14 operations, a total of 8 rounds, and then the final output transformation, as shown in figure 2-3-3. After 8 rounds of iterative operation, W81memeW82recentW83recoverW84 and Z48recorder Z49recollection Z50memZ51, respectively, Y1memeY2menyy3 and Y4 were obtained. The methods are as follows:

Figure 2-3-3 output transformation after 8 rounds of iterative transformation

2.IDEA key generation process

As can be seen in figure 2-3-3, there are 52 sub-key blocks involved in the encryption process, each of which is 16 bits long. These 52 key blocks are generated by 128-bit keys, and we record these 52 key blocks as Z0Magi Z1. , Z51 . The first 8 sub-keys Z0J Z1,... Z7 is directly from the user input, Z0 is the first 16 bits of the user input key, Z1 is the second 16 bits of the user input key, and Z7 is the last 16 bits of the user input key. So the total length of keys from Z0 to Z7 is 128 bits.

IDEA uses six subkeys in each iteration, each with 16 bits, which means that only 96 bits in the key are used in one iteration. The initial six consecutive subkeys (Z0 to Z5) are directly used in the first iteration, then the 128-bit key is cyclically moved to the left by 25 bits, and then the first 96 bits of the key are taken as the six subkeys in the next round. And so on, until all eight iterations are completed.

The relationship between 3.IDEA decryption algorithm and its encryption

The decryption processing of IDEA is basically the same as its encryption processing, but the input of the decryption process is ciphertext, and the selected key is not the same, but there is also a certain connection. Its relationship to the encryption key is as follows.

The first four keys of the first round of the decryption process are the same as those of the (10muri) round of the encryption process, and finally replaced as the ninth round. The first and fourth rounds of the decryption process are the modular multiplication operations corresponding to the first and fourth rounds of the encryption process, and the second and third rounds of the decryption process correspond to the addition operations of the third and second rounds of the encryption process.

In the first 8 rounds of operations, the last two sub-key blocks of the decrypted round I are equal to the last two sub-key blocks of round 9-I in the encryption process. The sub-key relationships for each round of encryption and decryption are shown in Table 2-3-2.

Table 2-3-2 Sub-key relationships for encryption and decryption

Encryption and decryption rounds

Encryption key for each round

Bits corresponding to the original key

The first round

Z0Z1Z2Z3Z4Z5

Z48-1-Z49-Z50Z51-1Z46Z47

The second round

Z6Z7Z8Z9Z10Z11

Z42-1-Z44-Z43Z45-1Z40Z41

The third round

Z12Z13Z14Z15Z16Z17

Z36-1-Z38-Z37Z39-1Z34Z35

The fourth round

Z28Z19Z20Z21Z22Z23

Z30-1-Z32-Z31Z33-1Z28Z29

The fifth round

Z34Z25Z26Z27Z28Z29

Z24-1-Z26-Z25Z27-1Z22Z23

The sixth round

Z30Z31Z32Z33Z34Z35

Z18-1Z20-Z19Z21-1Z18Z17

The seventh round

Z46Z37Z38Z39Z40Z41

Z12-1Z14-Z13Z15-1Z10Z11

The eighth round

Z42Z43Z44Z45Z46Z47

Z6-1-Z8-Z7Z9-1Z4Z5

The final replacement

Z48Z49Z50Z51

Z0-1-Z1-Z2Z3-1

The relationships between Zj and Zj-1 and between-Zj and Zj are as follows:

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