mirror of
https://github.com/XTXMarkets/ternfs.git
synced 2026-05-01 15:19:14 -05:00
Francesco Mazzoli
68 lines
1.7 KiB
Python
68 lines
1.7 KiB
Python
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# Rijndael's poly, see <https://en.wikipedia.org/wiki/Finite_field_arithmetic#Rijndael's_(AES)_finite_field>
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POLY = 0x11B # x^8 + x^4 + x^3 + x + 1, or 100011011
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# See <https://en.wikipedia.org/wiki/Finite_field_arithmetic#Generator_based_tables>
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GENERATOR = 0x03 # x + 1, or 00000011
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print('// generated with gf_tables.py')
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print('#include <stdint.h>')
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print()
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def gf_mul(x, y):
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out = 0
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# Multiply and add the factor
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for i in range(8):
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if (y>>i)&1 != 0:
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out ^= x<<i
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# Compute the modulo -- shift and subtract the polynomial
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for i in range(14,7,-1):
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if (out>>i)&1 != 0:
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out ^= POLY<<(i-8)
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return out
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# Exp table
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print('extern const uint8_t gf_exp_table[256] = {', end='')
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exp_table = [0]*256
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x = 1
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for i in range(256):
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exp_table[i] = x
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if i % 16 == 0:
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print('\n ', end='')
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print(f'{x:#04x}, ', end='')
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x = gf_mul(x, GENERATOR)
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print('\n};\n')
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# Log table
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log_table = {}
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log_table[0] = 0
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for a, b in enumerate(exp_table):
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log_table[b] = a
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print('extern const uint8_t gf_log_table[256] = {', end='')
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for i in range(256):
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if i % 16 == 0:
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print('\n ', end='')
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print(f'{log_table[i]:#04x}, ', end='')
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print('\n};\n')
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# Inv table
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inv_table = [0]*256
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print('extern const uint8_t gf_inv_table[256] = {', end='')
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for i in range(256):
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if i == 0:
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x = 0
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else:
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x = exp_table[255 - log_table[i]]
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inv_table[i] = x
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if i % 16 == 0:
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print('\n ', end='')
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print(f'{x:#04x}, ', end='')
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print('\n};\n')
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# Verify
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for i in range(1, 256):
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assert gf_mul(i,inv_table[i]) == 1
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for j in range(1, 256):
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tmp = log_table[i] + log_table[j]
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ixj = exp_table[(tmp - 255) if tmp > 254 else tmp]
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assert gf_mul(i, j) == ixj
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