PROJECT AIM
Design Encryption and Decryption scheme for modes of operation on Advanced Encryption Standard (AES-128) in Verilog
Introduction
As the number of applications of IOT and wireless communication arises, the need of encryption algorithms increases which are fast and secure (as the hardware has to limited) and also to remove errors during communication. Manufacturers know that encryption must be the silver bullet to protect their products and customers.
In short, AES is a symmetric type of encryption, as it uses the same key to both encrypt and decrypt data. It also uses the SPN (substitution permutation network) algorithm, applying multiple rounds to encrypt data. These encryption rounds are the reason behind the impenetrability of AES, as there are far too many rounds to break through.
To code, we have used a sample image that has been encoded to bitstream of 128 bit using python code. This bitstream is encoded using following modes.
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Electronic Code Book Mode
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Cipher Block Chaining Mode
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Cipher Feedback Mode
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Output Feedback Mode
Then it is decoded and the bits are converted to image using python code.
The key expanded into an array of forty-four 32-bit words, w[i]. Four distinct words (128 bits) serve as a round key for each round.
Four different stages are used, one of permutation and three of substitution
| Substitute bytes: | Uses an S-box to perform a byte-by-byte substitution of the block |
| ShiftRows: | A simple permutation |
| MixColumns: | A substitution that makes use of arithmetic over |
| AddRoundKey: | A simple bitwise XOR of the current block with a portion of the expanded key |
Once it is established that all four stages are reversible, it is easy to verify that decryption does recover the plaintext.
Substitute Bytes Transformation
AES defines a matrix of byte values, called an S-box, that contains a permutation of all possible 256 8-bit values the 128-bit plaintext is converted to a 4x4 matrix where each element is of 8 bits or two hex numbers. Each individual byte of State is mapped into a new byte using the tables where first four bits represent the row and last four bits represent the column.
In Verilog, an array of registers is used to store the S box and inverse S box and based on the .8-bit text the new byte is returned via the function.
ShiftRows Transformation
The matrix of 4x4 for forward transformation left shift is performed and for backward transformation right shift is performed. For first row no shift, for 2nd row 1 byte shift, for 3rd row 2-byte shift. so on.
In Verilog, this has been implemented by giving hard coded values between 1and 128, range of input.
MixColumns Transformation [6]
The matrix of 4x4 is transformed by performing array multiplication. All operations are performed on 8-bit bytes. In particular, the arithmetic operations of addition, multiplication, and division are performed over the finite field. So, each multiplication with a number represents a special operation[2]. After this operation addition is done i.e., XOR in GF.
For encryption.
For decryption
<<<<<<< HEAD Method 1 :
Galois Multiplication Lookup Tables
Method 2: Mathematical application (architecture 1) [14] {0D}* M(x) = (m0 + m5 + m6) + (m1 + m5 + m7) x + (m0 + m2 + m6) x2 + (m0 + m1 + m3 + m5 + m6 + m7) x3 + (m1 + m2 + m4 + m5 + m7) x4+ (m2 + m3 + m5 + m6) x5 + (m3 + m4 + m6 + m7) x6 + (m4 + m5 + m7) x7.
{0E}* M(x) = (m5 + m6 + m7) + (m0 + m5) x + (m0 + m1 + m6) x2 + (m0 + m1 + m2 + m5 + m6) x3 + (m1 + m2 + m3 + m5) x4+ (m2 + m3 + m4 + m6) x5 + (m3 + m4 + m5 + m7) x6 + (m4 + m5 + m6) x7
{0B}* M(x)= (m0 + m5 + m7) + (m0 + m1 + m5 + m6 + m7) x + (m1 + m2 + m6 + m7) x2 + (m0 + m2 + m3 + m5) x3 + (m1 + m3 + m4 + m5 + m6 + m7) x4+ (m2 + m4 + m5 + m6 + m7) x5 + (m3 + m5 + m6 + m7) x6 + (m4 + m6 + m7) x7
{09}* M(x) = (m0 + m5) + (m1 + m5 + m6) x + (m2 + m6 + m7) x2 + (m0 + m3 + m5 + m7) x3 + (m1 + m4 + m5 + m6) x4+ (m2 + m5 + m6 + m7) x5 + (m3 + m6 + m7) x6 + (m4 + m7) x7
{03}* M(x) = (m0 + m7) + (m0 + m1 + m7) x + (m1 + m2) x2 + (m2 + m3 + m7) x3 + (m3 + m4 + m7) x4+ (m4 + m5) x5 + (m5 + m6) x6 + (m6 + m7) x7. (7)
{02}* M(x) = (m7) + (m0 +m7) x + (m1) x2 + (m2 + m7) x3 + (m3 + m7) x4+ (m4) x5 + (m5) x6 + (m6) x7. (8)
{01}* M(x) = M(x) (9)
Method 3 :
According to the two architectures previous we were able to achieve other method based on the Properties of the binary calculation that have for goal the easiness the use of this operation to the material level you find the manner and the stages that we followed in order to calculate the multiplication mixcolumn below:
Multiplication by 01 (00000001 in binary):
The number remains unaltered Multiplication by 02 (00000010 in binary):
The bits of the number are baffled toward the left:
N = 10101110 = > 2N = 101011100
Since the operations make themselves in a number finished of the values (field of Galois GF (28)
of 256 values), the MSB of 2N must be omitted:
2N = 101011100
2N = 01011100
If the MSB was (that we have just omitted) a '0', then 2N are the final result of the multiplication:
2N = 01011100
If the MSB was (that we have just omitted) a '1', what is the case in this example, then it is
necessary to add (XOR) the binary number again 00011011 (1B) to 2N in order to compensate
the loss of the MSB caused (provoked) by the shift:
2N XOR 00011011 = 01011100 XOR 00011011 = 01000111 2N = 01000111
For the Multiplications (03; 09; 0B; 0D; 0E):
we take The MSB like a mask and one calculates the operations (temp1, temp2 and temp3) of
shift on the left to add by the number (1B) in order to compensate the loss of the MSB caused
(provoked) by the shift.
and_mask := m(7) & m(7) & m(7) & m(7) & m(7) & m(7) & m(7) & m(7);
temp1:= m(6 downto 0) & '0' xor (("00011011") and and_mask);
and_mask := temp1(7) & temp1(7) & temp1(7) & temp1(7) & temp1(7) & temp1(7) & temp1(7) & temp1(7);
temp2:= temp1(6 downto 0) & '0' xor (("00011011") and and_mask);
and_mask := temp2(7) & temp2(7) & temp2(7) & temp2(7) & temp2(7) & temp2(7) & temp2(7)
& temp2(7);
temp3:= temp2(6 downto 0) & '0' xor (("00011011") and and_mask);
Multiplication par 03 (00000011 en binaire) :temp1 xorm
Multiplication par 09 (00001001 en binaire) : temp3 xor m
Multiplication par 0B (00001011 en binaire) : temp1 xor temp3 xor m
Multiplication par 0D (00001101 en binaire) : temp2 xor temp3 xorm
Multiplication par 0E (00001110 en binaire) : temp1 xor temp2 xor temp3
Note => &: Operators of concatenation
AddRoundKey Transformation
In the forward and backward add round key transformation, called AddRoundKey, the 128 bits of State are bitwise XORed with the 128 bits of the round key. As XOR with same number two times returns the same number.
Key Expansion Algorithm
The AES key expansion algorithm takes as input a four-word (16-byte) key and produces a linear array of 44 words (176 bytes). This is sufficient to provide a four-word round key for the initial AddRoundKey stage and each of the 10 rounds of the
cipher. The pseudocode on the next page describes the expansion.
| PSUEDO CODE (units in bytes) | Subworld Function |
|---|---|
 |
 |
1. Rot Word performs a one-byte circular left shift on a word. This means that an
input word is transformed into.
2. SubWord performs a byte substitution on each byte of its input word, using the
S-box (Table 5.2a).
3. The result of steps 1 and 2 is XORed with a round constant.
Overall Pipelined Architecture :
All of these can be easily implemented in Verilog using functions.
ECB Mode or Electronic CodeBlock:
A key is Xor-ed with text to generate cipher. Doing this again returns us the plain text.
CBC Mode or Cipher Block Chaining:
Input to the encryption algo is the Xor of the next 128-bit plaintext and the preceding 128 bit of cipher text.
CFB Mode Cipher Feedback Block: Input is processed s bits at a time. Preceding ciphertext is used as input to the encryption algorithm to produce pseudorandom output, which is Xored with plaintext to produce next unit of ciphertext.
OFB Mode or Output Feedback:
Similar to CFB, except that input to encryption algorithm is the preceding encryption output, and full blocks are used.
Conclusion
The basic AES structure remains same in all the modes which makes the system very robust.
For lengthy message ECB is not secure. As if the message has repetitive elements an analyst can predict the possible word. It is good for transmitting encrypted key.
For CBC mode the input to encryption algorithm is the XOR of the current plain text block and preceding ciphertext block and the same key is used for each block, so the repeating patterns are not exposed.
The potential problem is that a bit error occurred during transmission will get propagated easily. Hence protective measures like crcgenerator[3] and retransmission[4] will have to be considered.
The CFB mode can operate in real time with s bit input. The advantage of OFB is that bit errors in transmission do not propagate. The disadvantage is that OFB vs CFB is that OPF is more vulnerable to message stream attack than in CFB. Complementing a bit in cipher text complements a bit in the recovered plaintext.
For wrong key, initial value the output plaintext / here the final image changes drastically.
All the given modes have advantages and their disadvantage as some are more secure and some might prove to be more power and hardware efficient. So, depending on the system either mode can be used or the encryption module needs to standardised for all systems for applications of IOT to increase.
For FPGA implementation all the components need to be perfectly timed and implementation has to be real time for its future applications to increase.
RTL schematic black box design of overall project in which Data in , key acts as basic input for process along with that CE , K EN,DIN , ENC DEC are Clock Enable ,CLK, Key Enable , Data Input status, Encryption decryption mode selection respectively acts as system control inputs. DATA_OUT is output either cipher text or plain text depend on status of ENC DEC mode.
Future Endeavours
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To design RTL model for the code written and increasing the THROUGHPUT
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Using the same architecture for 64 bit and 32-bit cipher text.
References
[1] Chapter 5, Crytography And Network Security By William Stallings
[2]GALOIS FIELD MULTIPLICATION https://arxiv.org/ftp/arxiv/papers/1209/1209.3061.pdf
[3] https://www.geeksforgeeks.org/modulo-2-binary-division/
[4] https://en.wikipedia.org/wiki/OSI_model
[5] M. Biglari, E. Qasemi and B. Pourmohseni, "Maestro: A high performance AES encryption/decryption system," The 17th CSI International Symposium on Computer Architecture & Digital Systems (CADS 2013), 2013, pp. 145-148, doi: 10.1109/CADS.2013.6714255.
[6] Design and Implementation A different Architectures of mixcolumn in FPGA http://dx.doi.org/10.5121/vlsic.2012.3402
File Structure
For cbc,cfb,ecb,ofg=mode
mode/binary_mode_decode: Binary decoded bits in after testbench execution
mode/binary_mode_encode: binary encoded bits after testbenchexecution
mode/t_mode_decode: code for decoding in test bench
mode/t_mode_encode: code for encoding in test bench
image_sample: output waveform of various test benches and hierarchy
general / binary.txt : Encode image in form of bits
general / decrypt.v : Verilog module code that constitutes the decryption module in AES
general / encrypt.v : Verilog code that constitutes the encryption module in AES
general / expand_key.v : Verilog moule to convert 16 bytes to 44 bytes
general / img2txt.py : Python code to convert image to txt , written in 1 and 0
general / mix_columns .v: Verilog module code for mix_columns module in AES
general / s_box.v: Verilog module code for s_box module in AES
general / shift_rows.v: Verilog module code for shift_rows module in AES
general / t_cipher_128.v : Testbench to test the encryption and decryption on 128 bit test case
general / t_key_expansion : Testbench to test key_expansion module
general / t_mix_columns : Testbench to test mix_columns module
general / t_read_image : Testbench code to read and write from .txt file to read its path
general / t_s_box: Testbench code to test s box module
general / t_shift_rows : Testbench code to test shift rows module
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