Application Research of AES Algorithm in PVR Set-Top Box
1 Introduction
The AES algorithm is an advanced encryption algorithm selected by the National Institute of Standards and Technology (NIST) in 3 years and after 2 rounds of evaluation. All the design principles of AES are open and there are no patent rights issues. Anyone can obtain and use this algorithm through normal channels. As a new generation of data encryption standards, AES brings together the advantages of strong security, high performance, high efficiency, ease of use, and flexibility. The encryption speed is fast and the memory requirements are low. It is very suitable for application on embedded platforms. Based on the analysis of the principle of AES encryption algorithm, this paper focuses on the implementation steps of the algorithm, and implements the encryption / decryption operation of AES algorithm completely in C language, and implements the application in PVR set-top boxes.
2 AES encryption / decryption algorithm principle introduction
The AES algorithm is a block cipher algorithm, and its input packet, output packet, and intermediate packet length for encryption / decryption are all 128 b. There are three key lengths: 128, 192, and 256 b.
The AES algorithm is based on permutation and substitution operations. Replacement is to rearrange the data, and substitution is to replace one data unit with another. The input of the AES-128 algorithm used in this article is 128-bit data, and the key length is also 128 bits. Each round requires an extended key with the same length as the input packet to participate.
The AES algorithm is mainly composed of three parts: round change, number of turns and key expansion. This article introduces in detail the implementation steps of each part of the AES encryption / decryption algorithm; combined with the C language, the application of the advanced encryption algorithm AES in the PVR digital TV set-top box is realized. The flow chart of the encryption / decryption module program of the AES-128 algorithm used is shown in Figure 1.
2.1 Encryption transformation
Each round of encryption of the AES encryption algorithm uses substitution and aliasing to process the entire data packet state in parallel. It is mainly realized through the cyclic call of 5 sub-modules SubBytes, ShiftRows, MixColumns, AddRounke and GetNewEncrypt.
Subbyte transformation, or s box replacement, is the only nonlinear transformation in the AES algorithm. It replaces each byte in the state with S-box. The first 4 bits of each byte specify the row value of the s box, the last 4 bits specify the column value of the S box, and then take the corresponding row and column elements in the S box as output to replace the byte.
ShiftRow completes row-based cyclic shift operations. It cyclically shifts the lines in the state according to different offsets. In general, the 0th line does not move, the 1st line rotates left by 1 byte, and the 2nd line rotates left by 2 bytes. The 3 lines rotate left by 3 bytes.
MixColumn performs obfuscation transformation on the status column. Take the transformation result of ShiftRow as a polynomial in the GF (28) domain, multiply by c (x) = 03x3 + 01x2 + 01x + 02, and then modulo the irreducible polynomial x8 + x4 + x3 + x + 1 In this paper, according to the GF (28) finite field operation rules, the column transform is improved into simple shift and XOR operations. Set 1 byte as (b7b6b5b4b3b2b1b0), then:
AddRounkey is a simple XOR logical operation by byte and the result of mixing columns and subkeys.
GetNewEncrypt generates the key needed for the next round of transformation. It is grouped by the columns of the matrix. The generation method is as follows: If the i-th column is not a multiple of 4, that is (i% 4 == 1), then the i-column is as follows Formula determination:
W (i) = W (i-4) XORW (i-1).
If the i-th column is a multiple of 4, ie (i% 4 == 0), then the i-th column is determined by the following equation:
W (i) = W (i- + 4) XOR T [W (i-1)]
Where T [W (i-1)] is a conversion form of w (i-1), and let W (i-1) = [a, b, c, d]. The implementation is as follows:
Shift the element of W (i-1) to the left, that is, [b, C, d, a]. Enter these 4 bytes into the S box to get new 4 bytes [e, f, g, h]. Calculate the new constant r (i) = 2 (i-4) / 4. Generate the converted column [e XOR r (i), f, g, h].
The key generation columns of the i-th round are W (4i), W (4i + 1), W (4i + 2), W (4i + 3), so the key matrix of the i + 1th round is obtained.
When calculating the round constant, when the number of encryption rounds is greater than or equal to 9, 8 bits are not enough to indicate its value. At this time, mod (x8 + x4 + x3 + x + 1), the value is 0x1b, and the subsequent value is 0x1b. multiple. That is, when r (i) is greater than or equal to 256, the value of r (i) is 0x1b * (turn-8).
The AddRounkey transformation XORs the result of the MixColumn transformation and the result of the GetNewEncrypt transformation by corresponding bytes, and the obtained result is used as the input for the next round of transformation.
The above five steps complete a round of AES encryption transformation, and repeat the above process according to the number of encryption rounds until the final round of transformation. The difference of the last round of transformation is that there is no column aliasing transformation MixCol-umn. The AddRounkey transformation directly XORs the result of ShiftRow transformation and the result of GetNewEncrypt transformation according to the corresponding bytes. Text.
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